Table of Contents
Acknowlegements
History
Introduction
Working Area and Equipment
Types of Plaster
Types of Mixing Vessels
Mixing Plaster
Additives and Their Preparation
Pouring Plaster
Some Pouring Suggestions
Using Armatures
Fillers Added to Castings
Plaques Requiring Hangers
Excessive Weight In Casing
Setting-Time Control
Care After Casting
Removing Castings from Molds
Repairing New Castings
Hollow Castings
Release Agent
Removing Air Bubbles from Heavy Castings with a Vibrator
Drying Castings
Painting
Paint Mixing Guide
Making Molds
Plastic Molds
Preparation of Model for Rubber Mold
Fins
Rubber
Synthetic Rubber Mold Compound
Applying Natural Latex Rubber
Plaster Back-up for Rubber Molds
Marking Molds
Removing Shell and Mold from Model
Waste Mold
Large Molds with Glass Cloth and Resin
Obtaining a Copyright
Where to Sell Your Product
Buyer’s Guide Section
ACKNOWLEGEMENTS

This book could have been written only with the unselfish cooperation of many people. The author wishes to thank the following close friends for their technical assistance: R. E. Hallagan and K.E. Beverly, editing; C.W. Reynolds, illustrations; Joseph Gach, cover art; Linda Tytar and Dorothy Kehrle, typing.

HISTORY

The product called plaster has been used in making statuary items for over five thousand years. Even in the Pyramids of Egypt, archaeologists have found relics containing plaster. It is a derivative of the common mineral gypsum which is found in abundance throughout the world, but it was not until about 1750 that it was produced commercially. It was first mined in the suburbs of Paris, France, and it therefore became known as “Plaster of Paris.” Later, each manufacturer improved the basic material for more diversified applications and each type was given a different name. For statuary work the most common products are Art Plaster, Casting Plaster, Molding Plaster, HYDROCAL® Gypsum Cements, and HYDROSTONE® Gypsum Cement. There are major differences between these products in hardness, strength and water requirements, but each has its place in the field of statuary work.

The technical definition for plaster is: A FINE WHITE POWDER CONSISTING ESSENTIALLY OF HEMIHYDRATE CALCIUM SULFATE or CaSO4 * ½ H2O. It is made from gypsum rock, using a process which consists essentially of heat-treating and grinding. In the processing, other chemicals may be added to produce various types for the many special uses needed today. One of the largest applications is in construction of buildings. In general the products used for building have no place in the field of plaster casting. Incidentally, products on the market still known as “Plaster of Paris” may be completely unsuitable for plaster casting.

After many years of slow progress the plaster casting art has improved with the development of new mold making materials such as plastics and rubber. The first artisans made molds of clay, sand, glue, wax and similar materials which would seem very crude today. With modern mold making materials it is possible to produce almost anything, large or small, with perfect detail.

INTRODUCTION

This book is written primarily for those interested in the art of casting plaster objects in a home workshop. It is not intended for those already in the business, but rather for people seeking information for starting a hobby or vocation in the field of plaster figurines and statuary. A description will be given as to what is essential with an attempt to reduce the technical aspect to as simple terms as possible. However, the experienced person may also obtain some valuable information.

Most creative people enjoy making objects with their hands. This can be just a pastime or could prove to be one the most interesting vocations ever experienced. There are no limits to the possible types of plaster castings which can be produced. Carve a model out of wood, mold one of clay or have a sculptor produce some special pieces. Another approach is to purchase small items in stores or novelty shops that lend themselves to duplication.

From these various items molds can be made from which plaster duplicates can be cast. The raw casting made from these molds can be sold unpainted, or you may wish to apply your talents and paint them to be sold as finished items. Make sure the item duplicated for sale is not copyrighted. This can be determined by checking for a small “c” in a circle and the creator’s name engraved on the product. Another possibility is to buy molds from one of the companies listed in this book and start casting a product without spending time to make molds.

There are hobbies or businesses that can be started which require such a small investment of capital as is required for plaster casting.

There are four basic requirements that must be met in order to produce a saleable, attractive and durable plaster casting:

  1. A model of practical design;
  2. Correctly constructed molds;
  3. The proper grade of plaster;
  4. Proper finish (paint) on the casting.

Later in the book each one of these requirements will be covered in detail.

Casting with plaster can be a most satisfying hobby, whether you buy just a single mold and make a few castings or go into production for a profit. It is one of the oldest arts in the world, fascinating enough to interest a child or captivate the most talented person. If you are looking for an interesting hobby or a way to make some extra money with little investment, plaster casting holds a great potential.


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WORKING AREA AND EQUIPMENT

The best space arrangement for the beginner is to section off an area in the basement or garage. Allow enough space for a work table four or five feet long as well as space for storing castings and bags of plaster. A beginner can dry castings by setting them up off the ground where air can circulate around each piece on a warm sunny day or by building a rack near a furnace with a small fan to circulate the air. For drying purposes during inclement weather, volume producers may wish to build a drying room adjacent to the work area (see section on drying room).

The equipment required for plaster casting is not expensive. Here are a few items that are necessary to make the work pleasant and help produce castings on an efficient basis: A large glass, plastic, or porcelain-coated steel water pitcher for a mixing pot; an old-fashioned, long-handled wood or stainless steel spoon or a wood paddle for stirring; a twenty-five pound scale, which can be purchased at most hardware stores; and two plastic one-half gallon bowls for measuring water and plaster (To see figures 1 & 2, click on the numbers).


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TYPES OF PLASTER

The selection of plaster to be used for casting will depend on the item selected for production and on the personal choice of the individual making the cast. For those who intend to start their own business it is desirable to be familiar with all grades of plaster available. This will permit selection of different plaster products for special items or for special strength requirements.

The most common materials in use for plaster casting are USG White Art Plaster, USG Casting Plaster and USG Hobby Plaster. These three products produce castings with good detail and with good surface finish for painting. Other materials which may be available locally should be avoided, since not all plaster products are suitable for plaster casting work. If there is any doubt about the suitability of a particular plaster product for casting, consult the manufacturer or some reliable person familiar with the field of plaster casting.

For harder, stronger casts, one of the HYDROCAL Gypsum Cements can be used. HYDROCAL White Gypsum Cement is a good choice for all types of castings including those made on an armature. HYDROSTONE Gypsum Cement is an extra-hard material which is normally used for solid casting work. It is not as desirable for armature work or hollow castings as it does not have the property of progressive thickening required for these applications. The cost of the HYDROCAL Gypsum Cements and the HYDROSTONE Gypsum Cement is two to three times the cost of USG White Art Plaster, USG Casting Plaster, and USG Hobby Plaster.

All of the plaster products and gypsum cements are sold by the manufacturer through dealers and they are available online at www.plaster.com or call 1-800-USG-4YOU. They may also be available from other companies who repackage and sell under their own brand names. The United States Gypsum Company sells its products in convenient 1, 2 and 6-gallon pails. Smaller quantities are available from the companies that are listed in the Buyers Guide.

If the local dealer contacted does not carry plaster for casting it may be because there is not enough demand for it. This should be new in one way because it means there is no competition in the area. Most dealers are willing to stock materials which will be used; and if your business is successful, then supply won’t be a problem. For those who are just starting for the first time it may be necessary to pay more for the plaster to bring ti in from a distant point until a local supply can be established. This is particularly true for those who start their business in a small community since the most time-consuming problem is likely to be getting a source for the proper type of plaster.

The main difference between the plasters and gypsum cement is hardness, strength and the ratio of water to plaster or gypsum cement. The products taking the smallest amounts of water will be harder, stronger and heavier than those which require more water. USG White Art Plaster, USG Casting Plaster and USG Hobby Plaster are most commonly used for general work, but they are not as strong or hard as gypsum cement products.

For castings with slender protrusions or thin sections it is good practice to use one of the gypsum cement products. Keep in mind that the smaller the quantity of water the hotter the plaster or gypsum cement will become while setting. This is particularly true when the castings are fairly large or thick and when they are of the hollow or stand-up type of casting. The mold will tend to trap and hold the heat and the casting may become hot enough to be uncomfortable to the touch. This is particularly true where the accelerator or potash solution mentioned in the section on additives is used.

The heat development of the special hard plasters may soften plastic molds if they are used for large quantities of castings. It is desirable to have a few extra plastic molds when using these hard plasters for many castings so that time can be allowed for cooling of the molds.

Regardless of the type of plaster or gypsum cement used, always follow the recommended water to plaster ratio for each product as outlined in this booklet or by the manufacturer. Any deviations can produce results which may be unsatisfactory.

Always store plaster in bags, off the floor and in a clean, dry place to avoid deterioration of the material or contamination from outside sources.


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TYPES OF MIXING VESSELS

Usually those who are experienced in plaster casting use either a large glass water pitcher, bowl, or a porcelain-coated steel pan for a mixing vessel. The choice depends on the operator and type of casting being made. The author’s choice happens to be the large glass pitcher because it is a handy size, is easy to clean, never gets rough inside, and it is easy to see when it is clean. However, plastic or steel does have the advantage of being less susceptible to breakage during use. The average glass pitcher holds five pints, which is sufficient volume for most castings. It holds enough when casting large items because one batch can be soaking in one pitcher while pouring another. Figure 3 shows the correct procedure of adding the water first and then sifting the plaster into the water in the mixing vessel.


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MIXING PLASTER

Mixing plaster starts with learning the quantities of both water and plaster needed. To arrive at these quantities:

  1. Fill the mold or molds with water, weigh the water in the regular water vessel. This weight is the weight of dry plaster required.
  2. After weighing out the plaster as stated above, dispose of one third of the water from the water vessel.
  3. Put the remaining water in the mixing vessel and sift the plaster into the water.
  4. Let the mixture stand for one or two minutes, shake to settle, and double check with figure 4. Plaster may vary in the amount of water to plaster required.
  5. If HYDROCAL White Gypsum Cement is used, the amount of water required would be approximately one-half the weight of the plaster.
  6. If HYDROSTONE Gypsum Cement is used, the mixture would be 35% water to 100% plaster by weight.

HYDROCAL Gypsum Cement and HYDROSTONE Gypsum Cement are types of plaster for special purposes and will not be considered in general instructions unless specifically mentioned. When these two types are used, a 25% mixture of art or casting plaster added to them will improve the surface finish for painting.

After adding the plaster to the water, let it soak four or five minutes before stirring. This will allow the plaster to become completely wet and to drive out the air bubbles. After the batch has been soaking one or two minutes, the water should just cover the plaster but no more than 1/8 inch maximum (see Figure 4). This will give a double check on the first measurements and if more water is needed, add it; or if there is too much water over the plaster, add more plaster to maintain the strength of the final casting. If the batch is allowed to soak as described above, it will require only a few seconds’ stirring to remove all the lumps before pouring. Once stirring begins, do not add either plaster or water to the batch. If by mistake too much plaster has been added and the batch starts to thicken too fast, it is better to dispose of that batch and start over. If water is added at this point, it would kill the chemical action of the plaster and result in a soft casting. Stirring too long will have the same effect. If the mix is too thick when poured, it will neither fill out the mold details nor float the air bubbles to the top of the mold. Once the proper water-to-powder ratio has been determined it should be carefully followed.


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ADDITIVES AND THEIR PREPARATION

Additives should be used in plaster only after the individual is completely familiar with the handling of the plaster without additions. If additives are used they should be added with extreme caution as serious problems can result from uncontrolled additions of any material to plaster.

To hasten the setting time of plaster, potassium sulfate can be used. This is a white crystalline material which is available from most drug stores. It is best used in liquid form in plaster casting and the solution can be prepared by dissolving the crystals in warm or hot water. Dissolve as much material in the water as it will take and hold in suspension. Stir the water solution to facilitate dissolving the crystals and then let the remainder settle. Store the solution in a rustproof container (glass or plastic) during use.

A cheaper, fertilizer-grade potassium sulfate or potash is available where fertilizers are sold. However, this material will require straining after is dissolved as it contains a high level of impurities which would be harmful to the plaster casting. The choice of this material compared to the purer grade will depend on availability and personal preference. Use no more than one ounce of potash water to each quart of plaster mixing water. This is the maximum potash to use in plaster casting to be painted. Higher amounts will cause the paint to peel.

To slow the setting time of plaster, either sodium citrate or citric acid can be used. Because of the pronounced effect of these materials on the set time of the plaster, great care should be taken in using them. The solution which is prepared from these two products should consist of 99 parts of water to one part of either citric acid or sodium citrate by weight. The resulting solution should be used in quantities of no more than 0.25 or 0.5 ounce per quart of mixing water. If citric acid is used, mix only enough for a few days at a time since the solution is subject to bacterial growth if kept too long.

Household borax also can used to slow the setting of some types of plaster. Prepare this material the same as the potash solution but don’t mix any more than can be used in a few weeks. As with other retarders, a little goes a long way. Before mixing any quantity of the borax solution, it should be tested for its retardive effect on the particular plaster being used.

In all cases some experimenting will be necessary to determine the exact quantity of retarder or potash solution that is needed for a given job. The best approach is to use only the smallest amount that is required to do the job.


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POURING PLASTER

Before pouring plaster, check the bottom of the mold for small pockets such as rabbit or dog ears. Pour a small amount of plaster into the mold and jar or bounce the mold on the bench to get all of the air out. Jar with one hand and pour with the other (see Figure 5).

Some figures with many small details require pouring the plaster into the mold and then pouring it out again. Repeat this process once or twice before the final pour. This permits all the air to escape before the plaster sets up. Always pour down the side of the mold to let the air out as the plaster goes in (see Figure 6). When pouring a mold of an item such as a statue that has a nose and chin with reverse pockets, it is advisable in some cases to fill the mold approximately half full, then tilt the mold on its side with the nose and chin pointed down to jar out the air in those pockets. While holding the mold in this position, fill it enough to bring the plaster above the air pockets as the mold is returned to the upright position ( see Figure 7). The above technique may be applied with little loss of time once it is learned which molds require special treatment.

Fill small molds just below level full. If it is a little too full, use a straight piece of wood or cardboard to scrape off the excess plaster. Large molds should be filled to about 0.0625 inch below the top. This eliminates most of the trimming operation. A little experience will correct any over or under filling.


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SOME POURING SUGGESTIONS

Here are a few tips which are not vital in making castings but which may eliminate difficulty.

When air bubbles persist while using good plaster and following all the other suggestions, try drawing the water used for mixing plaster into a tub or barrel and letting it stand for a few hours before use. This allows time for any air or gas entrapped in the water to escape. The potash water may be added at this time to eliminate mixing it in each small batch.

When pouring a large number of castings in one day, prepare materials in advance. Weigh the exact amount of plaster for each batch and store in bags as shown in Figure 8. Use a cup that holds the correct amount of water, or mark one so that weighing each time can be eliminated. Draw the water in advance as outlined above.

If running high production, at least three mixing vessels are necessary. Let one batch soak while pouring another. One person can keep two or three vessels working. Another person will have to keep the vessels filled and help remove castings from the molds. Another person may put in the hanger wires and trim castings. This is where the children can help the parent increase production.


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USING ARMATURES

The use of armatures in sculpture incorporates the same principle as adding reinforcement to concrete structures.

When a sculptor models or sculptures in clay, he may use an armature like the one shown in Figure 9.

If castings an item with a slender part like the limb of a tree or an arm protruding that may be easily broken, it is advisable to utilize a metal rod or wire to support the casting; never use wood! Wood will absorb moisture and swell; the plaster will shrink and may cause a cracked casting as it dries. Aluminum clothesline or a coat hanger make a good armature for most small plaster castings.

To keep the wire or rod from showing on the surface when the casting is finished, coat the wire with plaster ahead of time. Either use some of the plaster when pouring another casting or mix a small amount and coat the wire after bending it to the shape you need (see Figure 10). Let it dry but dip in water just before inserting it in the casting. If completely dry, the armatures will absorb too much of the water out of the casting in the area where it is placed.

This process of adding plaster to the wire assures a thickness of plaster between the outside surface of the casting and the armature by keeping it away from the side of the mold (see Figure 11). In some cases, this is not possible and the armature may have to be installed in the mold before it is assembled. If the armature is too large in proportion to the diameter of the section of plaster, the final shrinkage of the plaster may cause the cast section to crack.


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FILLERS ADDED TO CASTINGS

There are two reasons for adding a filler to a casting: one is to save weight and the other is to save plaster. In the first instance, the casting may be too heavy for handling or it may be top heavy and require shifting most of the weight to the bottom.

As an example, in a full-size bust of a man, in order to shift weight to the bottom, install a large styrofoam ball in the head of the figure while the mold is apart. Insert wood match sticks or tooth picks all around the ball to keep it from floating or shifting (see Figure 12). Next install this in the rubber mold and assemble the shell.

Other variations of this technique can be used. Match sticks are not large enough to damage the casting and if one of the ends shows on the surface, it can be cut back after the casting is dry and the spot repaired by adding plaster. Dip the ball (with centering sticks installed) in water before inserting in the mold.

To add weight to the base of a casting, pour the mold full up to base and let it set until the plaster has set sufficiently to hold up some weight. Next add lead, iron or stone and fill the balance of the mold to the top with plaster, which will bond as long as the other plaster is not completely dry (see Figure 13).

Add broken pieces of old plaster in making a large cast to conserve plaster. First break up old castings and soak them in water until ready to use in a new casting. Add these pieces to thick sections where plenty of fresh plaster will be between them and the outside surface. Making the casting hollow is another way to remove weight.


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PLAQUES REQUIRING HANGERS

Plaque-type castings are those that hang on the wall and require a hanger insert in the rear. This is done just before the plaster hardens. Small pieces of wire may be used whose weight and gauge depend on the weight of the casting. Often a paper clip, bobby pin or a piece of electrical bell wire will do. On small plaques, place the prongs of the wire about an inch from the top of the casting (see figure 14). If the casting is large or heavy,it may be better to use two hangers as shown in Figure 15 or one heavy wire. Cooper wire is best for hanger wire because it will not rust.


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EXCESSIVE WEIGHT IN CASTING

When a plaque casting is quite heavy and deep, remove some of the excess weight by scooping out some of the plaster from the center. Do this just before the plaster hardens but after it is too stiff to run. A large tablespoon is suitable for this purpose (see Figure 16).


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SETTING-TIME CONTROL

It is possible to speed up the setting time of plaster with addition of small amounts of potash (see Preparation of Additives).

To retard the setting time, use retarder solution in small quantities. It is used when building the back-up shell for rubber molds by hand lay-up method, or when extra time is needed before the plaster sets up.

The average time required for a casting to harden sufficiently to be removed from the mold (without using any additives), is twenty to thirty minutes. This period is called the heat cycle. This time cycle is based on using the types of plaster recommended for regular casting. The time also depends on how complex the mold is regard to the shape of the casting.

A casting with a limb sticking out that could be broken easily requires letting the mold set a longer time than usual before removing the casting. This time can be shortened by the use potash. The only purpose for the potash is to shorten the setting time of the plaster.

Another factor that changes the setting time of plaster is the temperature of the workroom and the materials being used. The best room temperature for working plaster is 70° to 80° F. Old plaster not cleaned out of the mixing vessel from a previous batch will also quicken the setting time of the new batch.


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CARE AFTER CASTING

After the molds are filled and the air bubbles eliminated, wash out the mixing vessel immediately before the plaster hardens. Keep all the equipment clean, especially mixing vessels and molds. Never pour plaster in the sink as it will clog the plumbing drains. It is best to have a bucket on hand for this purpose. The scrap plaster will settle to the bottom of the container, making it possible to pour the waste water off the top. As the plaster dries, it can be disposed of by tapping on the bottom of the container with a mallet over a trash receptacle.

If a casting breaks while being removed from the mold, do not try to glue the pieces together while they are damp. After the pieces are dry, glue them together with white glue such as Elmer’s or Weldwood.


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REMOVING CASTINGS FROM MOLDS
 
PLASTER MOLDS

To remove a casting from a plastic mold, pop it out by applying thumb pressure to the back of the mold with open (flat) side down over a table (see Figure 17).

RUBBER MOLDS

To remove a casting from a rubber mold, it will be necessary to lubricate the outside of the rubber mold in most cases so it will slide over itself, just like stripping off a rubber glove.

Do not use oil or grease on natural rubber. The only exception to this is castor oil. On small molds, talcum powder may be sufficient but on large molds, soapy water or liquid soap are good lubricants.

For large rubber molds use a liquid soap and brush it on with a paint brush (see Figure 18). Apply the soap on the outside only, never on the inside of the mold! Be sure to wash the inside of the rubber mold before reassembling.

When removing a large or heavy casting from a mold, exercise the following precautions: Place a cushion or pile cloth on the work bench to protect the casting. If the casting has an object projecting out from the main body (such as an arm, or limb of a tree) extra care must be taken when removing the rubber part of the mold by pulling the rubber loose from the casting as shown (in Figure 19).

Follow these precautions when shutting down production for the night with expectations of continuing the next day. Fill the rubber molds with ordinary water and let stand. This has a tendency to freshen the rubber and clean the mold.

If the old molds have stood idle for a time, rejuvenate the rubber by the same method. However, in this case, add a small amount of Epsom salts to the water as a wetting agent and allow soaking. When ready to use, wash out with fresh water.


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REPAIRING NEW CASTINGS

Necessary repair work or trimming that is to be done on the casting, such as filling holes or trimming off the flash left where the fins were on the model, should be done while the casting is still damp. If the casting has completely dried, soak the area to be repaired with water or the new plaster will not stick.

Use a small amount of retarder solution in the water for a repair mixture to keep the plaster from setting until one has time to make larger repairs. Press the mixture into the holes and smooth off the outside with your finger. It may be necessary to enlarge the opening before filling (see Figure 20).

After all repairs are made, let the casting dry completely before sanding. Use a 400 to 600 grit waterproof sandpaper and keep it wet while sanding. Let the casting dry before sealing for paint.

Castings made in plastic molds require trimming only the edges with a dull tool and filling any holes. For removing flash left on the casting from fins or separators, it is best to use a wooden knife or the dull edge of a putty knife; otherwise you may cut too deep and damage the casting. For smoothing off the bottom and rounding the edges, use an open wood rasp with a removable blade. After using the rasp on wet plastic, clean it with a wire brush before the plaster in the opening dries.

If a small casting has a number of pin holes in one or two spots, wet the area, dip a finger in some dry plaster and rub it into the holes by adding more water to make a paste while rubbing. If the casting is large and it has sections which are porous, it can be easily repaired with spackling compounds. Most hardware stores sell spackling compounds in small boxes. Mix a thin mixture of powder and water, use an old paint brush and paint the porous area with the mixture; let it dry and then sand smooth.


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HOLLOW CASTING

The art of hollow casting is performed in one of two ways or a combination of both. One method is to pour each mold partially full of plaster in the regular manner, slush it all over the inside of the mold, and then pour it out. Wait long enough for the layer to thicken, then pour it full and empty again, repeating the process until there is sufficient wall thickness depending on size of casting. For example: A casting the size of a man’s fist may require only 0.125 inch thickness, while a casting the size of a man’s head would need to be 0.5 inch thick. After the final layer, let it set up the usual length of time and remove in the regular way.

After the casting is out of the mold, make a plaster mixture and pour it on a piece of wax paper as shown in Figure 21. Place the casting in the plaster with the opening down, let the plaster harden (this will close the opening), then remove the wax paper and trim off the excess plaster. This method is suggested when working with a quantity of molds so the mixture from one mold may be poured to the other without wasting plaster.

The second method of hollow casting consists of filling the mold one-third full of plaster and covering the opening. Use a piece of glass or put wax paper on a board to seal the opening. The cover may be held by rubber bands or in some instances held by hand.

With the cover in place, rotate the mold in all directions until the plaster sets up. This method accomplishes a completely closed casting. It requires potash in the mixture for quick setting.


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RELEASE AGENT

A release agent is any material used to prevent one object from sticking to another. A release agent is often used in the art of plaster casting and mold making. What to use in each kind of material is important for two reasons:

  1. Some release agents are harmful to rubber
  2. Some release agents that release one material will not release another material

A neutral liquid soap is a good release agent for plaster and rubber, or plaster from plaster.

Plaster and soap are not compatible but soap can be used as a release agent for plaster if the soap is neutral, non-alkaline and non-acid. The plaster surface on the soap will be rough, but not enough to harm its function. Never use a product containing alcohol against fresh plaster.

Never use grease or oil in contact with natural latex rubber; the only exception is castor oil which is harmless to rubber. In some cases, it is a desirable lubricant.

A heavy cup grease can be used to separate plaster from plaster, but never use it on a surface to be painted. There are many release agents sold in pressurized cans but they are too expensive for plaster craft art.

Stearic acid in a liquid form is ideal for releasing plaster from wood; NEVER USE IT ON RUBBER. To make liquid from stearic acid powder, which may be purchased at a drug store, use half (by volume) kerosene and melt the two items together in a double boiler arrangement: Place the vessel containing the stearic acid and kerosene in a pan containing only enough water so that the inside vessel will not float. Do not use an open flame; electric heat is ideal.

While the stearic acid mixture is warm, apply to the surfaces to be separated, using a small brush.


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REMOVING AIR BUBBLES FROM HEAVY CASTINGS WITH A VIBRATOR

A vibrator table is desirable for handling bulk and weight of large models (see Figure 22). The table is custom-made to suit each shop’s needs. The power supply may be purchased (see buyer’s guide).

The two kinds of vibrators are air and electric. Each kind has two types of motion. One produces a shaking action; the other is a piston type with a reciprocating motion. The piston-type vibrator does a better job than a plain vibrator. It achieves the same results as pounding on the bottom of the table or bouncing the mold on top of a table.

The rate of vibration should be adjustable. With the air type, the speed is regulated by changing the air pressure, and with the electric type, it is regulated by adjusting the amount of current supplied to the vibrator. The controls may be purchased from the same company that sells the vibrator (see Buyer’s Guide).


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DRYING CASTINGS

Castings should be dried as soon as possible after they are removed from the mold. Forced drying can be used to speed up the drying time but this requires a special room.

If the weather is warm and dry, put the castings up off the ground outdoors or in the house near the heat. In either case, stack them on a screen or in some manner so the air can circulate around each casting. Do not let castings lie around two or three days before placing them where they will dry. The sooner a casting is dried after removal from the mold, the stronger and harder the final casting.

“NEVER USE A KITCHEN STOVE TO DRY CASTINGS FOR IT MAY RUIN BOTH CASTING AND STOVE.”

If the volume of production is large enough to warrant building a drying room, the method is the same for any size. The shelves should be made with a frame which is covered with either hardware cloth or fine chicken wire (see Figure 23).

The room should have an outside vent to carry out the moist air and an air inlet for circulation. A fan is also needed in the drying room to keep the air moving. A small heater will supply enough heat (see Figure 24).

The air inlet to the room can be a space under the door. If the room is large, the shelves should be removable so they can be loaded or unloaded outside the room. The temperature should be thermostatically controlled if possible. Keep a thermometer in the room and make sure the temperature never exceeds 120° F.


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Painting

The many methods of finishing (painting) plastercast items would require a special addition; however, here are some simple suggestions.

Be sure the casting is thoroughly dry before painting. Start by sealing the surface regardless of how the casting is to be painted. The best sealer for general purposes is a lacquer-type, wood sanding-sealer, thinned to penetrate the surface. Thin shellac may be used but it takes longer to dry. Sealer can be applied either with a hand paint brush or air brush. If the casting needs sanding, it should be done between coats of sealer with wet sandpaper.

For the beginner, either temperature or enamels are most suitable for hand painting.

When using water- or latex-based paint, use paint prepared for figurine painting, or exterior house paint. Do not use interior latex and water-based house paint. The only disadvantage to using the exterior house paint is that there is only a limited number of ready-mixed colors available; however, one can buy white and add the color in tube form and mix a choice of colors (see chart on Page 29).

When using a spray gun (air-brush) for antique finish on raw plaster, apply sealer, then a coat of white lacquer. Allow it to dry at least one hour at or above room temperature, coat with water or latex brown-, black- or chocolate-color paint. Allow it to dry a few seconds then wipe off with a damp sponge, leaving the water coat remaining in the cracks and crevices. Finish with a dry cloth.

NOTE: If the darker paint stain remains in spots, it indicates the lacquer was not completely dry when the water coat was applied.

Warning: If the water-paint coat is not removed almost immediately, it will set and cannot be removed.

After the paint dries, use clear lacquer as a protective coat. A high gloss finish is less desirable than a semi-gloss. To remove or regulate the gloss, use lacquer flattener in the clear lacquer coat. This can be purchased at most automotives paint stores. There are many different finishes to be achieved using the above method. A black basecoat can be made to look like stone by using a white water-base paint for the second coat and wiping off the surface. To get a good wood finish, use light tan lacquer with dark brown water paint.

When imitating wood color, wipe with a damp sponge. Always wipe lightly in the same direction with each stroke (see Figure 25). For different effects, try removing various amounts of the brown coat. The true effect will not show until it dries and the clear lacquer is applied over the top, which brings life to the wood tone.

Try a cream-color base with a tan top for another interesting finish. This procedure requires some practice to acquire the knack of making the finish look professional. The second — or wiping coat — must be one that will not affect the first coat. Both enamel and water-based paint can be used over lacquer but lacquer cannot be used over either one.

Lacquer can be used over shellac or water-based paint but is not suitable for a wiping coat.

Another technique is to paint a solid color such as bronze, silver, gold or pearl. For metallic paints, we recommend buying the powder and mixing with clear lacquer instead of buying the pressurized can because the flakes in pressurized cans are not large enough to give the same effect as loose powder.

Pearl is purchased by the pound in paste form. This is one place where quality is in ratio to price paid; never but a cheap grade (see Buyer’s Guide). Pearl is usually sprayed over white lacquer but by fogging it over other colors, it produces some interesting finishes.

A product on the market, in tubes, called “Rub and Buff” can be used over a solid color to produce an interesting effect.

The factory formula for mixing pearl essence paint is one pound of pearl added to one gallon of clear lacquer and two gallons of thinner. Small quantities can be mixed in the same ratio. All of the solid color finishes should be applied by spraying. The pearl, especially, is most successfully applied with an air-brush.

When purchasing an airbrush, the one recommended is the airbrush made by Binks Mfg. Company. It is not expensive and is ideal for painting statuary. When purchasing an air compressor, consider one that will produce at least fifty pounds of air pressure (piston type) because it requires at least fifty pounds pressure to spray lacquer or enamel paint satisfactorily. Install a pressure regulator and a moisture trap in the line with the airbrush.

For hand-brush painting supplies, if there are no art supply stores or hobby craft shops nearby, write to one of the supply dealers listed in the Buyer’s Guide Section.


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PAINT MIXING GUIDE*
Yellow-Green 2 parts yellow, 1 part blue
Green 1 part yellow, 1 part blue
Blue-Green 1 part yellow, 2 parts blue
Violet 1 part red, 1 part blue
Red-Violet 2 parts red, 1 part blue
Red-Orange 2 parts red, 1 part yellow
Orange 1 part red, 1 part yellow
Yellow-Orange 2 parts yellow, 1 part red
Citron 1 part orange, 1 part green
Olive 1 part green, 1 part violet
Russet 1 part orange, 1 part violet
Brown 1 part yellow, 1 part red, 1 part blue
Pink 1 part red , 1 part white
Charcoal 2 parts blue, 1 part red , 1 part yellow
*Add white for lighter shades of any color

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MAKING MOLDS

Molds may be made of many materials. Two kinds are of rubber, which are natural latex and synthetic materials (see section on rubber), plastic, and plaster. Koroseal, Glass Cloth and Resin are also used for making some types of large molds for plaster castings.

First, two types of castings must be considered in mold making, plus the sizes of the different types.

Plaque-type castings are the flat-back items that hang on the wall. These are poured from the back side of the casting.

The other type casting is called stand-up casting, such as animals or people standing on their feet, busts and statues, where the height is much greater than the diameter. These are poured from the bottom of the casting.

For the best results in stand-up work, natural latex rubber compound is recommended with a plaster back-up for items where the casting weight is under thirty pounds. Rubber or Koroseal with Glass Cloth and Resin back-up is better for larger items. Some large items lend themselves to straight Glass Cloth and Resin molds.

When making molds for small, flat (plaque-type) castings of items such as birds, cats, dogs, or anything up to eight to ten pounds (dry) where fine detail is not important factor, use plastic molds. The plastic molds will wear out after a few hundred pieces, but for the difference in cost, they can be replaced and still be more economical than any other kind of mold.

When it comes to rubber, the difference in the quality of one over the other will not be debated, but 90 percent of industry uses synthetic rubber and 98 percent of the hobbyists and small casters use natural latex rubber compound for their molds, and here is why: in industry, labor cost is high and one can make a mold with synthetic rubber compound by pouring and curing it a few hours. Also, most of these molds are for making one or two castings. It requires more time to complete a mold of natural latex rubber.

There is a vast difference in the types of rubber and each has advantages depending on its function or purpose.

In reading, and talking to old timers in the plaster casting art, mention is often made of other materials; most of them have been outmoded through development of plastic rubber.

Plaster molds are used only by the caster of figurines when making one casting of an object. If the object is complicated, the plaster mold may have to be made in many pieces or cast in tow pieces and destroyed when removing it from the casting. Flat items without undercuts are easy to mold in plaster but fine detail may be lost by application of the parting compound. For high production and good detail, a rubber mold with a plaster shell as a back-up is used. The rubber will pick up the fine detail and the plaster shell cast over the rubber will retain the shape of the rubber in the casting process.

All rubber molds do not require a back-up. Many small rubber molds for children can be purchased are stiff and small enough to require no back-up. They can be suspended between two water glasses or in a jar of water (see page 34).


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PLASTIC MOLDS

There are at least two or three times as many small vacuum-formed plastic molds used in plaster craft as any other type mold, since there is a larger market for this type casting. As long as the casting is shallow and has a good draft angle, the plastic mold will produce hundreds of castings before it fails; but if the casting is more than 12 inches across and it has any deep draws with straight sides, a plastic mold will not last as long as a rubber mold for this type of casting.

The making of plastic molds requires a vacuum forming machine and the manufacturer of these machines will furnish directions for their use. The machines are rather expensive and it is usually more economical to purchase plastic molds from one of the companies listed in this book. They make hundreds of molds and keep machines busy, whereas the small operator would use a machine for only a few molds and it would sit idle most of the time, which would be very uneconomical.

Plastic molds are easy to use and one person with a few hundred molds can produce hundreds of castings a day. The plastic molds do not produce as good a detail as a rubber mold and are usually used for flat type plaque castings. Plastic molds should have a wooden frame stapled to the back (see Figure 26). If the mold is large enough to sag, then a support should be used (see Figure 27). In some cases this frame of wood is superior to using plaster for a back-up on plastic molds. For rubber molds the plaster back-up is better and less expensive than other materials. For a limited number of castings a plastic mold can be imbedded in sand or soft dirt. The wood frame support makes it one piece which is light and protects the plastic when not in use.


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PREPARATION OF MODEL FOR RUBBER MOLD

First examine the model for defects or changes as desired. Be sure it is smooth and the detail well defined.

Always remember that each time any object is duplicated, some detail is lost. The amount of loss may be slight but there is always some loss; therefore make sure the model is as perfect as possible before starting to make a mold. Never hurry with the preparation of a model.

If the model is metal or another hard material, use a non-hardening clay to repair or reshape it. If using a plaster model, repair large holes with plaster and small blemishes with soft clay. Before sanding a plaster model, be sure it is well sealed with wood sanding sealer or thin shellac. Use “wet” or “dry” sandpaper, #400 for coarse and #600 fine.

The model should be mounted on a board. Use 0.125 inch or 0.75 inch plywood, depending on the size of the model. Cut the board one or two inches larger than the model on all sides and mount the model on the end or side from which the casting will be poured.

At this point add any fins or separators needed (see section on “Fins”). Then cover the model and fins with a coat of sealer. Do not use paint on a model if more than one mold is to be made from the same model. Even lacquer will peel off in spots with the rubber. Wood sanding sealer or thin shellac will not peel as easily as lacquer.

Click here to view an illustration on how to properly anchor the model to the board, do not use a rubber-base glue. Use one of the white glues such as Elmer’s or Weldwood; any wood patternmaker’s glue will be satisfactory. If a rubber-base glue is used, the mold rubber will blend with it and pull off with the mold. Mold rubber can be used as a cement when making only one mold of some delicate item that is to be removed from it mounting board without damage as soon as the mold is complete. The rubber used in the mounting operation must be cured in a minimum of 72 hours in a warm room before filling any cracks between the model and board with clay.

Beware of modeling clay which has mineral oil in its mixture because this oil will come to the surface when heated. An example of this is when the model is placed in a drying room; the mineral oil will harm the rubber. The sealer will hold back some of the oil but it is best to use the non-hardening gray-green type which is sold by most art supply stores.

Non-hardening clay is almost a “must” to have on hand when making molds. It can be used to repair blemishes, fill cracks and make changes where desired on the model. The clay can covered with sealer along with the rest of the model.


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FINS

Fins make internal slits or openings in the rubber part of a rubber mold, allowing it to expand. This will assist in removing rubber from the casting without breaking delicate protrusions extending out from the main body and will also relieve air that may be trapped in the end of such protrusions. To add fins or separators, foil, plastic or cardboard may be used.

The figure selected for an illustration on this page would present some of the worst conditions found in producing an object. A model having parts extending down such a person’s arm, a horse’s nose or the tip of a tail, would be poured from the bottom with these objects extending up away from the body. Air would be trapped in the end of these members and they would not fill the plaster. To relieve this condition, put a fin between the member and the body.

Start by cutting the fin to fit the contour of the model and cementing it in place as shown on Figure 28. Use a white wood glue or wood pattern glue; do not use a rubber-base glue. To make a fin fit the contour of the body, use soft copper wire (after heating it red hot and letting it cool) or small soft solder wire as shown in Figure 29. Bend to shape by pressing over the contour of the model. Lay the formed wire on the fin material and mark the outline of its shape; then cut to the outline which may require additional trimming.

The outside plaster shell will hold these inside separations together and let the plaster of the casting fill only the space left by the thin separator material.

The various steps shown on pages 36-38 are continuous steps, as mentioned above. Item “F” is a fin down the back. A fin in this location is common for items like the bust of a person. Since the neck is smaller than the head the rubber would have to stretch twice its size to be removed. The slit made by a fin, from the top of the head to the base or mounting board, will allow the mold to be removed without damage to the figure or mold. Always slope the fin edge toward the opening of the mold (see Items “D” and “E”).

When adding a fin or separator, be sure there is no open space between the fin or separator and the model where the mold rubber can run together from one side of the fin to the other. When making a fin to the contour of the model, there may still be a few openings showing light between the fin and the model after it is glued in place. Fill these with a piece of tin foil and cement or clay. The thinner the section, the better, because the fin will result in extra plaster on the casting which must be removed.

Where the model has openings as in Figure 30, it would be necessary to put a thin divider between the legs and lengthwise under the body to separate the rubber in order to remove it from the casting.

When working with a large object where the fin will not relieve the air fast enough, air relief can be assured by slitting a piece of small plastic tubing and adding it to a edge of the fin (see Figure 31).


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RUBBER

The most common material used for large plaques, small statues and standup figurines, is air-curing latex rubber compound. In all cases except extra small molds, it requires a back-up of plaster or a solid material to maintain the rubber mold’s shape.

The rubber is applied over a model in layers by either painting on with a brush, spraying or dipping. It is built up to a thickness of 0.0625 to 0.125 inch and air cured (see mold making).

Purchase a good grade of rubber latex compound from a reliable source. There are many variations from one brand to another. Some will be thin, some will cure fast and some will be slow to cure. Some will be one color and others will be another. The color is only an identification.

A mold rubber should be thick enough to build a thickness of 0.0625 inch with seven or eight coats. On the other hand, it should be thin enough, about the consistency of mayonnaise, to brush into all the details.

The price paid for mold rubber is not necessarily an indication of its quality for mold making. The dealers listed in this book are but a few of those who handle this product. When purchased in gallon or five-gallon quantities, the rubber should be repackaged in smaller containers. The quantity needed for the job at hand is removed and placed in a small container with an opening large enough for a brush. Then seal up the rest for storage in a cool place.

Mold rubber can be purchased in most hobby stores or by contacting one of the dealers listed in this book. Large cities may have a dealer listed in the yellow pages of the telephone book. Caution should be used when purchasing rubber from the telephone book listing, for it may be listed as mold rubber whereas it could be the type used by industry where heat is required and steel molds used. It may not be hobby craft mold rubber, which is air curing.

The most important identification of the rubber when purchased will be the odor. If it has an ammonia odor, we shall classify it as “A” type, and if it has no ammonia odor it will be the “G” type.

The “G” type rubber as indicated in our Buyer’s Guide section contains no ammonia. “G” type rubber can be used over model of any solid material including brass, bronze, copper or gold, without sealing, as long as the surface is smooth. It has a high rubber content and makes a good mold. The curing time is slow and requires four to five hours to dry at 72° to 82° F. between coats, and 72-hours curing time after the last coat. It has a heavy body and requires fewer coats to make a mold than most other types of mold compound.

The rubber listed as “A” type in the Buyer’s Guide section is also a good mold rubber but the model will require extra care in sealing when it is brass, bronze, copper or gold, whether it is solid or painted. Ammonia attacks these metals and the metal reaction affects the rubber. Most of the “A” type creates more air bubbles and its surface is more porous than the “G” type compound after curing.

Most of the “A” type rubbers on the market are thinner in body and require more coats than the “G” type. Some are thinner than others. This does not necessarily mean that it requires more time to make a mold because the “A” type is a faster drying rubber. A coat can be applied every hour at 75°to 80 ° room temperature. All types of rubber should be cured 72 hours at room temperature after the last coat is applied.

The rubber when cured should be thick enough to be durable and thin enough to be removed from the casting without damage to the casting. This all depends on the size and shape of each item being cast.

Air-curing latex rubber never stops curing. The rate of cure will slow down with time but all latex rubber will deteriorate with age. The age could be ten years; this will depend upon the care and the amount of usage.

These suggestions are based on past experience and may or may not hold true with future compounds. New materials in this field are being developed, as in all other lines of production.

Natural latex rubber has a relatively short shelf life, but this depends somewhat on how and where it is stored. Latex rubber should be stored in a cool, dark place, but not where it may freeze.

As mentioned before, color is only identification for the distributor. The natural color of latex as it comes from the tree is white, and the only time when color adds to the quality is where the mold is exposed to strong sunlight, then color would increase its life.


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SYNTHETIC RUBBER MOLD COMPOUND

There are many different brands of synthetic rubber but this type rubber is not recommended for the type mold the average hobbyist would be making, because it is designed for industrial usage and requires a different releasing agent than used for natural latex. The cost of synthetic rubber is usually double the cost of natural latex rubber.

Synthetic rubber is supplied in two and three-component systems; resins, hardener and inhibitor. The mixture of resin and hardener will gel in five to ten minutes. When longer times to gel are required, the inhibitor must be added to the resin and thoroughly mixed before the hardener is added. The more inhibitor added, the longer the gel time. Instructions are usually printed on the container label.

Cold-setting synthetic rubber may be readily poured or laminated by painting on the model to shape or thickness desired. This rubber also has perfect adhesion to itself so that changes or additions can be made at any time. This rubber may be used over metal, plaster, wood, clay or plastic with good reproduction of detail. Oil or grease can be used for a release agent with synthetic rubber but they should never be used with natural latex rubber.


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APPLYING NATURAL LATEX RUBBER

Use a pure-bristle brush for applying rubber. Store the brush in water with two or three percent household ammonia added (with this exception: use plain water if the model is brass, bronze, aluminum or gold). Do not use soap in the water as mentioned in some instructions. Soap will cause air bubbles in the rubber.

Start each coat of rubber by placing the first brush full on the mounting board around the base of the model. The reason for this is to deposit any lumps of rubber, dirt or water sticking to the brush. This becomes a habit with practice.

When applying rubber, one of the most important items is the elimination of air bubbles between the rubber and model on the first coat. Some brands of rubber use soap as a wetting agent in their compound. Soap creates air bubbles while painting the rubber on the model.

When painting rubber over fins, be sure to cover the edges and leave no opening in the rubber at this point. When applying the second coat, don’t brush too long or it may roll off the first coat. Rubber of the consistency of mayonnaise used on a small dog or cat model four or five inches tall would require five or six coats. A large model the size of a man’s head would require eight to ten coats; however, twelve coats would be the maximum coats of “G” type rubber for any mold. If the rubber is thin like water, it may require many coats for a small mold.

The only difference in the procedure for applying “A” and “G” type rubber is the drying time between coats. When applying “A” type rubber, allow one-half to one hour between coats at room temperature above 72° F., but no longer than six hours. If the one coat cures too long before the next coat is applied, it will result in separation of the layers.

When applying “G” type rubber, allow four to five hours drying time between coats. After the last coat, allow 72 hours curing time the same as for type “A” rubber.

After applying half the estimated coats of rubber on a model, study the model and mold to determine if there are any under-cuts that my lock the back-up shell to keep it from releasing (see Figure 32). Use sponge rubber as a filler. If sponge is needed, dab rubber on the underside of the sponge and fill the undercut. Do not cover the sponge with rubber at this time. Wait until the next time to apply rubber to the entire model, giving the rubber under the sponge time to dry before sealing out of air.

Another method of filling undercuts is the use of inserts made by filling the space with plaster after all the rubber is applied and cured, tapering the outside as shown in Figure 33. Plaster inserts are used when the object to be cast is large with deep undercuts that would require excessive sponge material and make mold removal from the casting a problem. A combination of both sponge and inserts is necessary in some cases.

After the back-up is made, strip the rubber mold off the model (see section on Removing Castings). Leave the rubber wrong side out to cure further while the plaster shell is drying.


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PLASTER BACK-UP FOR RUBBER MOLDS

To make an outer shell or back-up for a rubber mold, it is necessary to give some thought in advance to the back-up while applying the rubber, as mentioned previously.

To make a plaster insert, coat the mold with release agent and mix just enough thick plaster to fill the cavity. When the cavities are filled, smooth off the outside and give it plenty of angle on the surface to release the back-up. The back-up is also referred to as the shell.

When the inserts are dry, file the outside smooth. Study the mold to determine the best possible division of the shell. It may only need two sections but if there are too many hills and valleys, the shell may require being made in three or more pieces (see Figure 34), such as mold for the clown shown on the front cover of this book.

The back-up or shell is only to hold the rubber in shape while pouring the cast. The number of pieces used to make the outside shell has no bearing upon the casting. The reason for making more than one piece is to enable the shell to be removed after the casting has set up hard inside the mold. With more than two pieces it is more complex to assemble and more pieces to account for when disassembled. Paint the inserts a bright color so they will be clearly visible and not thrown out as a scrap piece of plaster. If more than one insert is used, it is advisable to paint each insert a different color and paint the location in the shell to match in order to save time in assembling the mold.

After deciding how and where to split the shell, use an ink marking pen to mark the lines on the rubber mold where to part the shell. Next, use non-hardening clay to build a dummy flange on one side of the dividing line to make the first section (see Figure 35). Lay the mold on its side or lay it so that sections of shell be built is up. Estimate the amount of plaster needed for the one section (there is no easy, accurate way to measure the amount needed), and mix the plaster, using a small amount of retarder solution in the mix to slow the setting time. Paint some release agent on the flat side of the clay flange and side of the mold, then pour as much of the plaster on the mold as will remain in place. Wait until the plaster starts to thicken enough to stay in place and then use a putty knife, spoon or the hands to build up a thickness of 0.5 to 1.0 inch, depending on the size and weight of the mold (see Figure 36).

With extra-large, heavy molds, it is advisable to add some reinforcement to the shell by cutting metal or plastic screen or hardware cloth into strips and building them into the plaster shell. Be sure the edges of the screen are covered with plaster to prevent injury to the hands.

Loose-woven burlap or sisal dipped in plaster and built into the shells will also act as a reinforcement material.

When the first section is completed, allow it to harden and then remove the flange. Where there are more than two sections, of back-up, make another dummy flange alongside of the next line (see Figure 36).

Apply release agent to the new flange and the edge of the first section just completed.

Keep repeating this process until the sections are finished. Where there are only two sections, such as Figure 35, it is only necessary to make a dummy flange one time across the entire mold on one side of the line. It is always best to make a shell in as few pieces as possible so there are fewer pieces to account for and assemble; however, divide the shell into enough pieces for easy removal. Do not make a back-up that requires unnecessary time to remove when the casting is finished.

There are several methods used to make a base for holding the mold in pouring position.

While making the shell, either make a large end which can be filed off for a flat base (see Figure 37) or complete the shell the same thickness all over and make a socket to set the shell into. A socket can be made in one of two ways: First, put release agent on the end of the shell opposite the opening and then place a piece of wax paper on a level table. Pour a quantity of plaster with potash added on the wax paper and set the mold in the mixture. Use a level and keep checking in both directions until the plaster hardens (see Figures 38 and 44).

If the mold is large, make a form to contain the plaster so it takes up less space on the table. A handy size is 8 inches by 2 inches deep, made of 0.125 inch thick hardboard. Tape the box pieces together, coat both the mold release agent. Place the rubber-covered model in the box on its side (see Figure 40).

Use a small “C” clamp to hold the mounting board against the side until the first half is poured. Pour the first half up to the parting line and wait until it is solid. Then take a large drill bit and make a hold by using only the point (see Figure 41).

Next, coat the first half with release agent and pour the second half. When the second half has set, remove the tape and the box. Scrape all the edges of the new shell smooth and part the halves; a putty knife is best for this operation. If the shell breaks, which happens occasionally, let the shell dry and then glue it together with Elmer’s or Weldwood white glue.

The box should be at least 0.5 inch larger than the model on all sides. If making a quantity of the same size mold, it is advisable to make a box held together with 0.1875 inch threaded rod and wing nuts as shown in Figure 42 The small rod will stretch enough to allow for the plaster expansion.


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MARKING MOLDS

When there is more than one rubber mold of the same object, it is necessary to mark the molds and their back-ups (see Figure 43). The rubber can be marked by cutting notches in the rim and the plaster shell marked with a marking pen or crayon.

The plaster back-up of the mold is cast to fit the finished rubber shape, and no two are ever the same, but similar enough to look alike. It is therefore necessary to insure that no parts become intermingled and so make the casting defective.

Mark the name of the object on the plaster shell of the mold; it helps to find the right one quickly.

Where the base and shell (or back-up) are separate, mark the name on both base and shell. Also, mark the location of the top part where it meets the base so it will be easy to find the position where the shell fits the base (see Figure 44).

When a plaster casting requires a hanger wire, mark the location on the outside rim of the mold with a marking pen on plastic or cut a notch in a rubber flange where the hanger should be located. After some molds are poured full of plaster without being properly marked, it is difficult to tell the top from the bottom in hanging position and where the hanger wire should be located.


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REMOVING SHELL AND MOLD FROM MODEL

Use a putty knife when separating the shell the first time. Set it on the crack between the sections and tap it lightly with a hammer until the crack starts to open. Use care and work the shell loose and off. While the shell is still damp, use a knife and remove any spots that appear as though they may be preventing the release of the mold. Remove no more plaster then necessary in any one spot.

Apply release agent on the rubber mold so it will slide over itself and strip it off like a rubber glove. Never try to roll the rubber off. While the rubber is still green, be careful as possible in removing the mold for the first time and do not stretch the rubber any more than necessary. The inside may not be completely cured and if it is stretched too far, it may not return to its original shape.

As soon as the rubber is removed from the model, wash it in clean water. Leave it wrong side out and allow it to cure while the shell is drying.


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WASTE MOLD

Waste molds are most commonly used by sculptors and are made by using the same technique as used to make a two piece shell except that plaster is applied directly over the model instead of making a rubber mold.

Waste molds are used when the object is made of clay or some soft material which is to be reproduced in plaster and when a rough form is wanted in which additional detail can be sculptured. The mold is made in two pieces so it may be separated for cleaning. The model is usually destroyed while removing the mold as no provisions are made to release the mold from irregularities or undercuts. While removing the mold the model is destroyed but the mold is saved. In the next step the mold is destroyed and the casing is saved.

Before starting the mold, the parting line is selected and a false flange is made. Materials used for this type flange is made. Materials used for this type of flange can be clay, plaster, tin foil or cardboard (see Figure 45). Another method is to stick pieces of sheet metal in a soft model. It all depends on the kind of material used in the model, bearing in mind that the flange must be removed after the first half of the mold is made.

Apply a release agent after the flange is completed. Mix only enough plaster to cover the first section about 0.125 inch thick; then add some coloring to the plaster while mixing and make a thin shell over the first half of the model. Allow it to dry and build the next layer of plaster to the full thickness without any coloring, but the more water than normal. Let this dry and repeat the process on the other half.

When the mold is dry, remove the shell by pulling it apart or take out some of the model by removing the board it is mounted on and digging out the inside.

When the halves are separated, clean them inside by brushing all the detail. Coat the inside with a release agent and fit the halves together. Depending on the size of the mold, use rubber bands or rope to hold the halves together.

Now the mold is ready to pour. Mix the plaster and pour as any other mold. When the casting is set up, remove the mold by chipping it off down to the colored plaster. Use extreme care in taking off this last thin shell so as not to damage the casting.


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LARGE MOLDS WITH GLASS CLOTH AND RESIN

There are two methods of making large molds using glass cloth and resin. One method is to prepare the model and mount it on a board as explained before. Then build a rubber mold with ten or twelve coats of rubber compound such as type “G” or twelve or fifteen coats of type “A” rubber compound. The rubber should be 0.0625 to 0.125 inch thick.

If the rubber is too thick, even on a large item, it will be too heavy and hard to remove. It may have to be stretched to get it over a projection on some part of the model.

When the rubber has cured at least 72 hours above 70° F., it is ready for the back-up. First decide where to part the mold and build the dummy flange on one side of the parting line. If the object is two or more feet high, it is advisable to use plaster to make the flange because clay will not have sufficient adhesion. Apply a release agent to the rubber and flange for the first section. Mix the plaster and when there is a large section to cover, use some retarder solution in order to give more time to cover the entire section before the plaster hardens.

The coat of plaster need be just thick enough to cover the rubber since it is only to keep the EPOXICAL from sticking


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OBTAINING A COPYRIGHT

To obtain a copyright on a new item of plaster craft, it is necessary to mark the item with the word “Copyrighted” or a small “c” in a circle, along with the year of issue and the name of the originator of the item.

The first piece and all succeeding pieces must be marked before they are placed on the market. Register the item with the Library of Congress. Forms and instructions can be obtained by requesting them from the Library of Congress, Washington, D. C. 20540; or see your attorney.


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WHERE TO SELL YOUR PRODUCT

Markets exist for finished products in gift shops, novelty shops and department stores. Small unpainted items can be sold in large quantities to arts and crafts departments of the parks and recreation division in most cities.

This provides an economical way for camp directors to furnish entertainment and educational experience to the children during their summer programs. They sell the plaster casting, paint and brushes to the children at cost. Many hospitals provide unpainted plaster castings to their patients to paint as therapy.

Another example: Doctors and dentists use small unpainted plaster castings to give to the children when they visit their offices. It helps to build a friendship with the child and dispels some of the fears children have of visiting a dentist or physician.

In other words, a steady business can be built with a very small investment. The plaster casting business receives little publicity because most of the production comes from thousands of small operators in their home workshops. Some have grown into large operators selling wholesale only to large department and chain stores.

Local markets often have need for a special item like the symbol of an athletic team, such as lion, bear or tiger, or the symbol of the state which can be sculptured in clay as a model and then molds made from the model.

The finished items may be bookends, an ash tray, or a plaque to hang on the wall and sold as a souvenir of that state or city.

Don’t overlook the two major political parties that have animal symbols. Another big market for unpainted and painted plaster items is the fair or carnival, where they are given as prizes for games of chance. Some carnival men contract months in advance with a plaster caster living as near as possible to locations where they are scheduled to show. In some instances, the castings have to be shipped many miles.

Some casters of figurines rent a booth at the County or State Fair and sell their finished products at retail.

The market is much broader than listed here. All these are the usual markets for people starting a plastercraft business; but for those with experience in mold making and the ability to make good molds, there is much larger field.

Most cities have sculptors who enjoy sculpturing with clay but either have to learn mold making or let their clay pieces deteriorate after a few months. Most amateur sculptors are not good at mold making and welcome the opportunity of having a good mold maker duplicate their clay models.

Most items, such as busts of people, should be cast in HYDROCAL Gypsum Cement.

The sale of molds is almost as large a field as the casting business.


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BUYER'S GUIDE SECTION
Companies Selling Hobby Craft Supplies Companies Selling Materials and Supplies

Blue Rapids Supply Company
Blue Rapids, Kansas 66411

Bersted’s Hobby Craft Inc.
Monmouth, Illinois 61462

W. Wooley Company
Box 29
Peoria, Ill 61601

Steece Brothers
Box 177
Sioux Falls, South Dakota 57101

Deep Flex Plastic Molds, Inc.
2740 Lipscomb St.
P.O. Box 11471
Fort Worth, Texas 76110

Chaney’s Plastic Arts
5415 San Jose Blvd.
Jacksonville, FL 32207

R.S. Plouffe Studio
82-A Pleasant St.
Worcester, Mass. 01608

Plaster Supply House
(Plaster working tools only)
3713 Grand Blvd.
Brookfield, IL 60513

Air Brush
Binks Mfg. Co.
3114 Carroll Ave
Chicago, IL 60612

Since there are many other makes of air brushes, see your art supply dealer or check your local phone book for other sources.
Vibrator
Cleveland Vibrator Co.
2828 Clinton Ave.
Cleveland, Ohio 44013
They make both electric and air vibrators

Plaster Mixers (Power)
Plaster Supply House
3713 Grand Blvd.
Brookfield, IL 60513

Sealer for Plaster
The Permalac Corp.
17339 Conant
Detroit, Mich. 48212

Release Agent (neutral liquid soap)
The Perma Flex Mold Company
1919 East Livingston Ave.
Columbus, Ohio 43209


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Companies Selling Mold Rubber
Company Name Types of Mold(s) Company Name Types of Mold(s)
Perma Flex Mold Company
1919 East Livingston Ave.
Columbus, Ohio 43209
G & Synthetic Douglas-Sturgess
730 Bryant St.
San Francisco, Calif. 94100
G
W. Wooley Company
Box 29
Peoria, Illinois 61601
G Bersted’s Hobby Craft Inc.
Monmouth, IL 61462
A
Lasting Products Co.
202 South Franklin Town Ave.
Baltimore, Md. 21223
G Steece Brothers
Box 177
Sioux Falls, S. Dakota 57101
A & G
Jim Robbins Company
Royal Oak, Mich. 48068
G Deep-Flex Plastic Mold
Box 11471
Fort Worth, Texas 76110
A
Latex Laboratories Inc.
2616 West Grand
Chicago, IL 60612
G Chaney Plastic Arts
5415 San Jose Blvd.
Jacksonville, Fla 32207
A
Levin Novelties
Box 296
Leavittsburg, Ohio 44430
G Heveatex Company Div.
Firestone Tire & Rubber Co.
Melrose, Mass. 02176
A
J. Largo’s
2205 Cedar Springs
Dallas, Texas 75200
G Blem Plastics Inc.
608 E. 10 Mile Rd.
Hazel Park, Mich. 48030
Synthetic & A
Polymer Chemical Co.
131 Barron Drive
Cincinnati, Ohio 45215
G Blue Rapids Supply Co.
Blue Rapids, Kansas 66411
A
A-R Products
Box 4312
Whittier, Calif. 90607
G Adhesive Products Corp.
1660 Boone Avenue
New York, N.Y. 10060
A