Definitions

Figure 14-3: Diagram showing the parts of a mold.

The object or model you wish to duplicate is called the master. You surround it with some other material to make a mold, often known as a negative. The mold material typically starts as a liquid or putty which then hardens, leaving a void or impression in the exact shape and form of the master, known as the mold cavity. To cast a piece, the void in the mold is filled with your casting material, which is also a liquid or runny paste that hardens. When the casting material dries or cures, it is removed from the mold, and you have an exact duplicate, or positive of your master. The cast is rarely an exact duplicate because most casting materials shrink a bit as they harden, but the casts themselves are all identical to each other. In most modern materials, the shrinkage is imperceptible on stage or on camera.

A vent is a passageway that allows air to escape the mold cavity as you fill it. A sprue is the spout or channel where you pour your casting material. A gate is the passage that connects the sprue to the mold cavity. “Sprue” also refers to the excess material that solidifies in the sprue and gates that will be removed when the casting material solidifies. Some molds are open at the top because that part of the object (usually the bottom because such molds are cast upside down) is meant to be flat; when the material is filled to the top of such a mold, it will level out, and that open area will solidify into a flat surface. If the sprue is large enough to allow air out as the casting material is poured in, then a vent may not be needed. You want your sprue to lead to the highest point of your mold, otherwise your casting material will not fill the entire mold.

A release agent (sometimes called a mold release or surface separator) keeps the cast from sticking to the mold and keeps the mold pieces from sticking to each other. Some mold releases also seal the pores of the surface; in other cases, you may need a separate “sealing agent” to accomplish that before adding your mold release. You will find more information and a helpful table on choosing the correct release agent on p. 276.

A mold can be either flexible or rigid. Prop builders commonly use silicone rubber for flexible molds and plaster for rigid ones; many other materials can be used, and these will be discussed later in the chapter. We will look at various reasons for choosing a flexible or rigid mold material, but a general rule of thumb is if you want to cast a rigid object, use a flexible mold, and if you want to cast a flexible object, use a rigid mold.

Some objects have areas that will create a recessed area in the mold, preventing it from being pulled straight out. These recessed areas are known as undercuts. If a rigid mold has undercuts and you fill it with a rigid casting material, the cast will become trapped in there when it hardens. If the piece you are molding has undercuts, you must use a flexible mold material, or make a piece mold. A piece mold consists of one or more pieces that can be removed to liberate the positive and then reassembled to create another cast. Note that even flexible molds may need to be made in two or more parts if your object has severe undercuts. If your object has a hole that passes all the way through, you will also need to make a piece mold.

Air Bubbles

Air which gets trapped in your mold will turn into a void or hole in your cast. These can range from visible imperfections to actual weakened parts of your cast. Vents, sprues, and gates need to be placed in your mold to allow all the air to escape. They can be sculpted in clay if you are doing a two-part mold. You can also attach items to your master that will leave passageways while making a mold. For instance, a flexible tube or a dowel will leave you a nice clean tunnel when molded.

Air bubbles can be present in the casting material itself, which may be too viscous or solidify too quickly for the bubbles to rise to the top. Most casting materials require you to stir them to activate; either stirring a powder with water, or stirring a two-part compound together. You can easily stir air into the mixture if you are not careful; keep your stirrer submerged as you rotate it to avoid introducing air.

Figure 14-4: When you pour casting material into your mold, you are not filling an empty mold; you are replacing the air in the mold with casting material. Without vents, air will get trapped and the casting material will not fill those parts of the mold cavity. If the vent exhausts at a point lower than the sprue, then the casting material will exit that point rather than filling the entire mold. If you need a vent for a low point in your mold, carve it out so it leads up to the top of the mold.

Before pouring, tap the sides of your mixing container, or vibrate it a bit to help these air bubbles float to the surface. If you have a whole film of bubbles on top, scrape it off and discard it. When you pour, pour in a thin stream, and aim for a single spot. Let the casting material flow over the mold surface rather than pouring it directly onto every square inch. Empty your container as high as you can above the sprue hole; the air bubbles have an easier time escaping the thin stream that runs from your cup to the mold than from the large mass inside the mold. Tap and vibrate the mold once it is full to shake loose all the bubbles and allow them to rise to the top.

The impression coat or beauty coat is a first coat of a molding or casting material applied carefully to capture all details fully without any air bubbles. It is applied thinly and spread over the entire surface of a mold before pouring the rest of the casting material in. For opaque casting materials, air bubbles that are trapped inside are of less consequence than the ones that stick to the surface and cause visual defects.

One-Piece Box Mold

Figure 14-5: Making a one-piece box mold.

A box mold (or bucket mold) is where you build a box or some other wall around an object and fill it with your molding material. A one-piece box mold is one of the simplest to make.

Figure 14-6 and 14-7: These wood appliqués were molded in a one-piece silicone box mold. Now, more appliqués can be cast in resin, plaster, or other materials.

In the following example, we are making a one-piece plaster box mold, but the idea is the same if you are making a mold out of different materials.

Figure 14-8: With a one-piece box mold, the back or bottom of the piece you are casting must be completely flat. Attach it to a flat surface and build a box around it. The box should be at least ½″ (12 mm) from the edges of your master, and be ½″ taller than the highest point of your master. The box needs to be sealed so that the mold material does not leak out anywhere. The master, box, and base should all be sealed and have the appropriate mold release applied.

Figure 14-9: Mix up your plaster according to its instructions. It may be helpful to pour a little bit in, and then smear that plaster all over the surface with your hands. This coating will help make sure plaster has made its way into the finest cracks and crevices without trapping any air bubbles.

Figure 14-10: Pour the plaster in the lowest part of your mold, and be careful not to let it splash. Allow the plaster to flow over the master as the box fills up, rather than dumping it onto the master. Fill the box until the mold material is ½″ higher than the highest point of the master.

Figure 14-11: Once the mold has set (after around fifteen to thirty minutes), you can take the box apart and demold the master.

Two-Piece Mold

Figure 14-12 and 14-13: Making a two-piece box mold.

For three-dimensional objects, you can make a two-piece box mold. The most common technique is to bury your object up to the seam and pour the first half of your mold. Then, you can pour the second half over the top of the first half.

You cannot pour the material underneath the object because the hydraulic pressure will push the object up, and air bubbles will be trapped underneath as well.

Be sure to use the correct mold releases for every step. Even though silicone rubber sticks to nearly nothing, it does stick to itself. Without mold release, you can easily create one giant block of rubber rather than two separate pieces.

First decide where to place the seam, which is where the two mold halves will join up. If you are making the mold with a rigid material, you must ensure that neither half has any undercuts. For both rigid and flexible molds, you must make sure there are no encapsulated areas that will get trapped.

The seam will also create a line of material that needs to be trimmed. This material is known as flashing, and is caused by a tiny amount of material leaking between the two surfaces of the mold at the parting line, which is where they join together. With most materials, it is easy to trim away this flashing, but for areas of your cast that will get the most visual attention, you will want to keep the flashing away from there to leave those surfaces as pristine as possible. Whenever possible, place your seam along a preexisting line or detail where it will disappear the most.

Before you pour your first half, you want to add what are known as registration marks, keying notches, or just keys. Carve a few little dents or notches in your clay base. These will become raised shapes in your first mold half. When you pour your second mold half, it will have little dents and notches that match the raised shapes in the first half. These will fit together to make sure the two halves are in perfect alignment.

Without keys, your two mold halves may not line up precisely. Carve enough registration marks in your mold to ensure it locks in place; if you are doing a square box mold, one mark in every corner usually does the trick. You can find endless variations of shape, size, and style of registration marks if you watch other mold makers make a mold; there is no “correct” way to lay out and make your registration marks, but as you gain experience, you may develop better ways.

Figure 14-14: For a two-piece silicone rubber box mold, embed the model in clay up to where you want your seam line. The clay should meet up perpendicularly to the seam line as cleanly as possible.

Figure 14-15: Build a box around the clay. The sides should be at least ½″ (12.5 mm) from the edges of the model, and extend ½″ taller than the highest point of the model. Be sure all the seams are sealed well using clay, hot glue, or whatever else suits you. Carve registration marks in the clay to help align the two mold halves when pouring your cast.

Figure 14-16: Fill the box with your silicone rubber. Because this is silicone rubber, you do not need any mold release at this point.

Figure 14-17 and 14-18: When set, take the mold and the master out of the box. Remove the clay, flip the mold and master, and put it back in the box. You should now have a box containing half of your mold and your object. Apply mold release over all of the silicone rubber.

Figure 14-19 and 14-20: Fill the box with more rubber to make the second half of your mold. When this sets, you should be able to peel the two mold halves apart and remove the master from inside. You now have a two-part mold.

Figure 14-21: You can see minor flashing from the mold parting line on the right side of this hand running from the wrist, on top of the thumb, and up the side of the index finger.

Multi-Piece Molds

For a three-piece mold, you will bury all but the piece you wish to mold first. When that mold piece has been made, you keep the object in the mold and bury part of what is left in clay, so that only the second third is exposed. Finally, you can keep the object in the two finished mold pieces and pour your third mold piece. This process can be extended to make molds with however many pieces you need. This gets complicated very quickly, and is rarely worth the time just to create a few props. Often, it is easier to break your object apart and make a bunch of one- or two-piece molds for each part, which are then assembled after being cast.

Figure 14-22: This hippo requires an eight-piece plaster mold to avoid undercuts. If a flexible mold material was used, it would have needed only two or three pieces, but since it is a mold for absorption casting, it needs to be plaster.

Brush-Up Mold

A brush-up mold involves brushing mold material onto a secured object. Often, the first coat, the beauty coat, is put on thinly to capture the finest details and to avoid air bubbles. Subsequent coats are put on thicker, building up the thickness.

Some silicone rubber is specifically formulated for brushing on. Otherwise, it will run off before setting up, leaving just the thinnest of layers on your master. You can also add thickening agents to make your silicone rubber able to hold to a vertical surface. You can do a brush-up mold with plaster as well (check out the section on plaster later in this chapter to find out more).

Brush-up molds can be done in one piece (glove mold) or in multiple pieces, either by brushing on the pieces separately using a dam, or by cutting the mold when finished, usually by building up a “separation line.” A mother mold can be built around the brush-up mold to help support it.

Figure 14-23: This safe dial is duplicated with a one-piece brush-up mold. First, it is secured to a flat surface. Photo courtesy of Anna Warren.

Figure 14-24: One layer of silicone rubber is brushed on. Another layer of silicone rubber, colored pink, is brushed on top to thicken it up. Photo courtesy of Anna Warren.

Figure 14-25: A rigid plastic resin paste is used to make a mother mold to support the rubber mold; it is shown in the last photograph cut in half to illustrate how it fits over the rubber mold. Photo courtesy of Anna Warren.

Two-Part Brush-Up Mold

When doing a two-part brush-up mold, you will not bury your model in clay. Rather, you will use area separators known as fences or mold dividers along where you want to place your seam. These have traditionally been made of a thin wall of clay or thin pieces of metal (or thin pieces of metal with clay on the back to help support them). You can use stiff cardboard as well if you are making the mold out of a two-part compound or the like; a water-based material like plaster of Paris will soak the cardboard and possibly disintegrate it.

If the fence is made of clay, you can carve registration marks in just like with a box mold. If it is metal, you can glue pieces to it to make raised registration marks. Some mold makers use short lengths of metal and zigzag them along the seam, so the two pieces will only fit together the way they are supposed to.

When making a three-piece mold, you will build two fences, one for each seam. When the first mold piece is dry, leave it on and build another fence where the second seam will go. Finally, leave the first two mold pieces on, and simply brush the third one on.

Figure 14-26: This helmet has a clay wall that has been built along where the prop builder wants to place the seam. Tiny hemispheres are carved in along the length to act as registration marks. Notice how the clay wall sticks out completely perpendicular to the object; this ensures the cleanest possible seam. Photograph and prop by Harrison Krix.

Figure 14-27: A layer of silicone rubber has been brushed on. It has been brushed over the clay wall as well, so the two halves will have a nice big flange where they are joined. When the rubber has set, the clay wall is removed, mold release is applied to the silicone rubber along the seam, and a new batch of silicone rubber is brushed on the other half. Photograph and prop by Harrison Krix.

Figure 14-28: For the fence on this piece, aluminum rectangles were cut from soda cans and inserted directly into the clay. They overlap each other to keep the plaster from leaking out.

Mother Molds

Flexible molds often need to be reinforced and stiffened with an outer shell or casing, known as a mother mold (or “jacket”), made of plaster of Paris, fiberglass, or a thick plastic resin.

Figure 14-29 to 14-32: These photographs show a three-piece mother mold that surrounds a one-piece RTV silicone mold that was brushed on. The mold was used to create a bust of Giuseppe Garibaldi out of polyurethane resin. The Intelligent Homosexual's Guide to Capitalism and Socialism with a Key to the Scriptures, the Guthrie Theatre, 2009.

A brush-up mold made of a flexible mold material such as silicone rubber and supported by a rigid mother mold made of plaster uses a lot less material than a box mold.

Other Types of Molds

You may also have a press mold, where you push rigid shapes or stamps into a clay-like material by hand.

A waste mold needs to be destroyed to remove or release the positive cast inside. It is less time-consuming to make and it eliminates seam lines, but it can only be used once. Waste molds (sometimes called temporary or throw-away molds) are frequently used so that a sculptor can sculpt the basic shape in an easy-to-shape material, then cast it in a material that allows further, more detailed refinement.

In a matrix mold, the object is covered in a thickness of clay. A support shell is poured around it. The clay is removed, and the flexible mold material is poured in to fill the gap between the object and the support shell. The thickness of clay determines the thickness of your mold (usually around ½″ for silicone rubbers). This combines the economy of brush-on molds with the speed and convenience of box molds.

Figure 14-33 to 14-35: The above was a quick press mold made in oil-based clay to cast a tongue in silicone rubber. Oil clay is great for press molds because it softens when warm and stiffens when cold. You can push a model into a bed of warmed oil clay to capture all the details, then allow it to cool (or even put it in a refrigerator for faster cooling) to keep it from distorting when you remove the model.

Figure 14-36: For a one-piece matrix mold, the model is first attached to a board. It is covered in about ½″ (12.5 mm) of clay to take up the space of the silicone rubber.

Figure 14-37: The clay has registration marks added along with a piece of PVC to leave room for a pour spout in the plaster. Wooden semicircles were glued to the board to act as registration marks for the plaster, and a box is built around the whole thing.

Figure 14-38: Plaster is poured into the box.

Figure 14-39: When the plaster is set, it is flipped, and the clay is removed. The model remains glued to the board.

Figure 14-40: The plaster is placed back over the model using the registration marks to line it up. You should now have a ½″ void between the plaster and the model. Pour your silicone rubber into the pour spout until this void is full.

Figure 14-41 and 14-42: When the silicone is set, you will have a thin flexible mold supported by a plaster mother mold.

You may be able to save time by purchasing preexisting molds. Craft and hobby stores sell all manner of molds for decorative elements and common shapes. Food molds can be repurposed for prop use. You can also find used or discontinued molds from factories and businesses that may be usable. Finally, you can use ordinary objects as molds, as long as you figure out the proper mold release to use. Plastic bowls can give you interesting dome shapes; muffin tins can give you tapered disks.

Figure 14-43: A plastic Jell-O mold was used to make a permanent wiggly dessert out of silicone rubber.

Release Agents

If you pour plaster over a plaster piece, you will end up with one solid piece of plaster. You need a mold release or release agent, which is a thin film, usually brushed or sprayed on your model, that will prevent the wet plaster of your mold from bonding to the dry plaster of your model. A release agent is also needed when you are making multi-part molds and you need to pour a molding material on top of another mold piece.

Some combinations of molding and casting materials do not need mold releases, but many do. Success with molding and casting depends on knowing the right kind of mold release to use with the molding and casting compounds you are working with.

Mold release is especially vital when dealing with any sort of plastic resin. After all, plastic resin is used as an adhesive for other materials, so you can expect them to stick to almost anything and be nearly impossible to remove once cured.

Traditionally, mold makers used lard and tallow as mold releases, but they leave brush marks that transfer to your cast. Modern materials can create a much finer film.

Wax works well on epoxies, polyurethane foam, and polyester resins. Popular waxes include beeswax, carnauba wax, or paraffin wax. You can find aerosol spray versions as well. Waxes often work well in conjunction with PVA, which will be described later.

Green soap works well when pouring plaster against plaster, such as when you are making a two-piece plaster mold. It is also useful for other gypsum-based products and papier-mâché. It is also known as “tincture of green soap” or “tattoo soap.” A solution of castile soap or Murphy Oil Soap will also work.

Lecithin is one of the few mold releases approved for use with food (obviously, the casting material and the mold itself also need to be approved before you start casting food). Nonstick cooking sprays often use lecithin, such as PAM and Crisco. Lecithin is useful on epoxy resin, urethane, some flexible and semiflexible urethane foams, polyester, and polystyrene, particularly when you need to paint or coat the piece after demolding. Lecithin is useful as a release on plaster, particularly when used with clay and oil-based clay.

Petroleum jelly is often referred to by its most popular brand name, Vaseline. It is good on polyurethane rubber, epoxy resin, polyester resin, silicone rubber, plaster, gypsum-based products, and papier-mâché. As with lard, if you brush it on, the brush marks will show up on the surface of your cast. You can thin it with naphtha, mineral spirits, or mineral oil to minimize brush marks. Alternatively, you can quickly run a heat gun over the petroleum jelly after adding it to level out the brush marks. Pre-formulated sprays especially made for molds can be found.

PVA (polyvinyl alcohol) is effective on many epoxies, polyester resins, and polyurethanes. It is either sprayed or brushed on, and when the alcohol evaporates it leaves a thin film of vinyl behind. PVA is often used in conjunction with a wax, and it usually requires a sealer used underneath. It can be built up with several applications, allowing drying time between coats. When casting plastic resins in plaster molds, whether polyester, polyurethane, or epoxy (including fiberglass work), it is often best to seal the plaster (with shellac, acrylic, or latex paint), wax the sealer, dust it with corn starch or baking soda, and then apply your PVA.

Powdered cornstarch or baking soda dusted over the surface can help in other situations as well. They help break the surface tension of the casting liquid, allowing it to flow more readily into tiny cracks and crevices. Soapy water added to the inside of a silicone rubber mold will do the same thing when casting plaster.

Silicone mold release is one of the newest types of mold releases. When it first appeared, it promised to replace all other mold releases. Silicone, after all, sticks only to silicone. It turned out not to be perfect, though. It transfers to the casting, which prohibits paint and other coatings from sticking. Since it is sprayed in a fine mist, it also travels throughout the air in the shop and gets on everything, contaminating all your surfaces and props. If you take the time to clean your cast afterward, or if your piece needs no further coloring, then silicone mold release can work great.

Finally, thin sheets and films, such as polyethylene film (Saran Wrap) or aluminum foil, can serve as effective mold releases when you do not need a perfect replication of detail between the two surfaces.

Mold releases that are applied every time will build up in a mold and need to be cleaned out periodically. This cleaning wears down the mold. When making multiple castings from a mold, you can skip adding mold release every couple of castings. The buildup of mold release residue can also slowly combine with residues of your casting resin, forming a difficult-to-remove encrustation (known as “carbonization”).

Mold release may remain on your casting when it is fully cured or dried, particularly on polyurethane resins, which are very sticky. They need to be cleaned off prior to painting or coating your cast. Dishwashing detergent and warm water will remove most wax-based release agents. Silicone-based mold releases may require acetone (not paint thinner or mineral spirits) to fully remove all the material.

No single mold release will be appropriate for every situation. You need to consider what types of mold releases are compatible with your process and whether you need to paint or coat your prop further. The chart included on p. 276 will help you with some general guidelines for compatibility of certain mold releases with various molding and casting materials.

One trick for aiding release from your molds is to always cast rigid materials in flexible molds, and flexible materials in hard molds. It makes it easier to pop or peel the piece out of the mold. You may cast rigid materials in a rigid mold provided you have a way to pull it out and there are absolutely no undercuts, but I have seen far too many prop builders end up with a rigid cast permanently trapped inside a rigid mold; a two-piece mold or a flexible mold material is often the better choice. Many rigid materials, particularly the casting plastics, reach a point where they hold their shape but are still a little flexible; it is helpful to demold them at this point and allow them to finish curing to full rigidity outside of the mold.

Sealing Agents

If you are making a mold of a porous object, such as plaster, stone, concrete, or wood, the surface will need to be sealed first. Sealers include shellac, paste wax, and petroleum jelly thinned with mineral spirits. Companies also make proprietary products that are nonintrusive and can be washed off the model after the mold is complete. Do not use shellac when making a mold out of silicone rubber. Many materials can also be sealed with paint if you are able to paint the object. As with mold releases, thick sealing agents will alter the appearance of your object.

Casting Methods

Many different casting methods exist. Prop builders mainly use gravity casting, slush casting, and absorption casting. Processes like injection molding, die casting, and centrifugal casting require far more setup and machinery than prop shops have.

Gravity Casting

In gravity casting, the casting material is simply poured into the top and left to harden. Gravity causes the liquid material to flow all the way to the bottom of the mold. Gravity casting creates a solid piece.

Figure 14-44: Gravity casting with polyurethane resin.

In some molds, it helps to tilt the mold while you are pouring or after you have poured to let the air out. Air floats up in casting material, so if it floats up to a point where it is walled in by the mold material, it is trapped and has nowhere to go.

Choosing Your Materials

Your first decision is what you want your final piece to be cast out of. There's no point buying molding materials if it turns out they can't be used to make a cast out of the material you have chosen. Determine your choice of material as well by whether the prop needs to be solid or hollow. Considerations include what type of finish you want the surface to have (including whether it needs to be transparent or translucent), its weight and strength, whether it needs to be rigid or flexible, and if it will be exposed to water or extreme temperatures.

Further limitations depend on what your original master is made out of. The shape will determine whether it is best to use a flexible or rigid mold, and a single piece or multi-piece mold. It may even be necessary to mold the object in several parts that are assembled together after being cast individually. The materials the original is made out of may also determine what types of molding materials are compatible for the prop's use.

Once you have the possibilities of molding and casting materials narrowed down, the rest will depend on practical constraints, such as budget, time, availability of materials, and what your shop is best set up to deal with. There is no one “right” material to mold and cast with, but most prop builders stick with a small group of materials because they satisfy all the needs and considerations of what the prop must do.

Often, the manufacturers or distributors of molding and casting products will be able to guide you to the best choice, particularly when you have adequately described what exactly your mold and your cast need to do.

When you are first getting started with molding and casting, it is better to learn on cheap and simple materials. There are many concepts and techniques to understand in molding and casting, and you can learn these on materials that act consistently and reliably and, most importantly, are inexpensive. Such materials include plaster, latex, alginate, and papier-mâché.

Materials such as resins and silicone rubbers, though preferred by the professionals, have their own special considerations. They are more temperamental, require more safety precautions, and their chemistry can cause them to fail miserably if you do not know what you are doing. You can make ten to fifteen plaster molds for the same price as one silicone rubber mold. If you cannot make a plaster or alginate mold work correctly, then you will not be able to make a silicone mold or a polyurethane resin casting work correctly, and you would have wasted more money in the process. Once you can make successful molds and casts with the cheap and easy materials, then you can start trying to figure out molding and casting with the more advanced materials that require precise measurements and a knowledge of what may inhibit their curing or cause the casts to come out imperfectly.

No two molding and casting projects will ever be exactly the same, so the materials and techniques one person uses may not necessarily be the same ones you want to use.

Plaster

Plaster of Paris remains a highly economical, intuitive, and less-toxic material for mold making in prop shops. It is also one of the few mold materials used for absorption casting of neoprene, slip clay, and latex. For one-piece and simple two-piece molds with no undercuts, even beginners can grasp the concepts needed to make molds out of plaster.

The largest manufacturer of plaster in North America by far is United States Gypsum Company (USG), and prop builders often refer to their brand names when discussing different types of plaster. In the chart, you can see a few of their plaster products and how radically different they can be. Success in using plaster involves determining exactly what you need and choosing a plaster that will fulfill those needs.

Plaster of Paris is also known as “gypsum plaster” and is the type used by prop builders for molding and casting. Other types of plaster you may hear of include lime plaster and cement plaster.

plaster type	density (dry)	dry compressive strength	water : plaster ratio (by weight)	set time (machine mix)
USG White Art Plaster	69 lb/ft3	2000 psi	70:100	27–37 min
USG Lab Dental Plaster	69 lb/ft3	2000 psi	70:100	6–9 min
USG No. 1 Moulding Plaster	69 lb/ft3	2000 psi	70:100	27–37 min
USG No. 1 Pottery Plaster	69 lb/ft3	2400 psi	70:100	14–24 min
USG No. 1 Casting Plaster	72.5 lb/ft3	2400 psi	65:100	27–37 min
USG Puritan Pottery Plaster	72 lb/ft3	2700 psi	66:100	14–24 min
USG Duramold	75 lb/ft3	2900 psi	62:100	14–24 min
USG Tuf Cal	85.6 lb/ft3	4300 psi	50:100	27–37 min
USG Hydrocal White	90 lb/ft3	5000 psi	45:100	25 min
USG Hydrocal Statuary	96 lb/ft3	6500 psi	40:100	25–35 min
USG Ultracal 60	97.5 lb/ft3	5000 psi	39:100	75–90 min
USG Ultracal 30	99 lb/ft3	6000 psi	38:100	25–35 min
USG Hydro-Stone	108.7 lb/ft3	10000 psi	32:100	17–20 min
USG Tuf Stone	112 lb/ft3	10000 psi	32:100	25–30 min
USG Dry Stone*	128 lb/ft3	10000 psi	20:100	5–10 min

Note
* Dry Stone is a mix of plaster and EVA polymers, making it fully dry and paintable within two hours.

Soft and porous plasters (less dense) are useful for waste and temporary molds. Regular molds should be made out of light but strong plasters (the ones in the middle of the chart). Molds for vacuum forming (bucks) do not need any absorptive properties, but do need incredible strength to withstand the vacuum pressures. Harder and less absorptive plasters are also good for castings.

Plaster is used in a liquid form (with a consistency close to cream) for molding and casting. It can also be mixed a little thicker for slush and rotational casting. As it sets, it reaches a point where it is more like a plastic material, with a consistency of butter or soft clay. During this short period, it can be manipulated, troweled, and moved by hand or with tools, and it will stay put on vertical or even upside-down surfaces. Some plasters are manufactured to go from a liquid to a solid rather quickly, while others can remain in this plastic state for quite a while. If you are casting in a slush mold or making a brush-up mold, you want a plaster that maintains a plastic state for a long time.

To mix plaster, first make sure your plaster is dry and well-sifted. Old plaster or plaster that has been stored in a damp environment will form lumps, and may be unusable because it has already absorbed moisture. Always use breathing protection and adequate ventilation when working with powdered plaster.

The ratio of water to plaster you use varies depending on what plaster you are using; most manufacturers give the ratio in terms of weight. More water leads to a more brittle piece of plaster. Less water leads to a denser piece of plaster, which may be nice for castings, but for molds, it will be less able to absorb water when doing absorption casting. The water should be between 70°F and 100°F (21°C and 37°C). Colder water makes the plaster set up more slowly, while warmer water will make it set up quicker; very cold water will keep it from setting altogether, while water over a certain temperature will start increasing the set time again. You do not want the plaster to set up so quickly that you do not have time to stir it and pour it before it begins hardening.

Figure 14-45: Larger plaster molds may need reinforcement; you can mix sisal or hemp fiber in with the wet plaster. Copper tubing or galvanized steel can also be embedded in the plaster while it is wet. In this mold from Costume Armour, the bars also serve as handles for easy maneuverability.

It helps to mix plaster in a plastic bucket so that the dried plaster can just be broken off by flexing the bucket. Always add the plaster to the water; never pour water onto plaster. Do not stir the water as you sift the plaster in.

Let your plaster soak for one to two minutes before mixing it. Tap the sides of the bucket to loosen some air bubbles, and then begin mixing. For batches under 5 lb (2.27 kg), you can mix by hand (wearing gloves); this helps you feel for lumps that you can break apart with your fingers. An egg beater or wooden spoon is also useful. Larger batches can be mixed with an electric drill and a propeller or squirrel cage. Stir vigorously, but do not whip it or create a vortex, otherwise air bubbles will be created in the plaster that may show up in the final piece. Make sure to break up all the lumps of dried plaster while stirring.

The longer you mix, the stronger the plaster will be (and the shorter the setting time will be), which is good for casts, but reduces the absorptivity of molds. Generally, mixing time is two to five minutes (after soaking for one minute). When fully mixed, the plaster will have a creamy consistency with no lumps. Give the bucket another few quick taps, and skim the layer of air bubbles off the top. You are now ready to pour.

You can achieve some success with brush-up molding in plaster. Rather than pouring it, let it sit for a few seconds or minutes. At a certain point, the consistency changes from a liquid cream to more of a thick paste. You can scoop this up and smear it on the sides of whatever you are molding. It will run down, but continue shoveling it on the top until it hardens enough to stay in place.

With the “islands” or “sea level” method of mixing plaster, you do not weigh your materials. You simply start sifting plaster into the water. It will land on the water for a second, and then sink down. At a certain point, the plaster will no longer sink down, and will remain above the level of the water (forming the above-mentioned “islands”). This indicates that you have added enough plaster, and you can start mixing.

This is not a consistent method of ensuring you have the proper ratio, and you have less control over achieving the desired hardness, softness, or absorption of plaster you may need. For simple castings where consistency is not necessary, this method can save time; just be aware that mixing plaster this way may not give you the properties that your plaster promises.

Let the excess plaster harden in the bottom of the bucket. Never ever pour wet plaster down the drain, because it will harden inside the drain and create a blockage that no drain cleaner can break. You cannot dilute it once you have poured it down a drain either because it is a chemical reaction, not a physical one. It can be helpful to have a waste bucket where you dump all excess liquid plaster and allow it to set.

As plaster sets, it heats up. In a large enough mass, it will generate enough heat to burn the skin. It should set in about fifteen to thirty minutes. When it is cool to the touch, it can be demolded.

At this point, though it is “set,” it is still damp. Fully drying out a piece of plaster can take anywhere from twenty-four hours to a week. You can tell the difference between damp plaster and dried plaster, because a piece of damp plaster feels cool, while dried plaster feels room temperature. Additionally, a piece of dried plaster weighs much less than a piece of damp plaster.

You need dry plaster if you want to paint it or glue something to it. Dry plaster is also vital if you are using it for absorption casting. You can dry plaster in a cool oven (no more than 100°F or 37°C) or under lamps; you can also set up fans to increase air movement, but do not blow them directly on the plaster pieces, as this will make them dry unevenly.

Plaster also comes impregnated on rolls of gauze. These are meant for making medical casts, but they are extremely versatile for the prop builder. Cut the bandages (as they are sometimes called) into strips. Submerge the full length of a strip into a tub of water. Remove it from the water, and lay it over the side of tub for a few minutes. A “slurry” will begin to form as the plaster becomes hydrated. Lay the plaster bandage on the surface, and rub your finger over it in a circular motion so that the plaster can fully impregnate both the fabric and the material underneath. Cover the surface with a complete layer of the bandages, slightly overlapping each one with the next. On the second layer, run the strips perpendicular to the first layer.

Newer products combine plaster with polymers. Their use is the same as straight plaster, but they have almost no drying time.

Silicone Rubber

Silicone rubber is the preferred mold material for prop builders to make long-lasting and flexible molds. They are more expensive than many other molding materials, but if cost were not an issue, I imagine some prop builders would never make a mold out of anything else.

The silicone rubbers used for prop mold making are a two-component system known as “Room Temperature Vulcanizing,” or RTV, silicone. They can be poured, brushed, or sprayed on, depending on their viscosity. They have the best release properties out of all the mold rubbers, and most materials do not require any mold release. This makes them the ideal choice for production casting of any plastic resins. They have a high temperature resistance (higher than 400°F or 200°C, depending on the specific product), making them the only mold rubber suitable for casting low-melt metal alloys, such as tin, pewter, and lead, and with some types even capable of handling aluminum.

RTV silicones use either a tin or platinum-based catalyst. Tin-catalyzed rubber is cheaper and not as sensitive to cure inhibition from contamination. Platinum-catalyzed rubber does not shrink as much as tin-based rubber, is more durable, and has a longer life.

They are among the most flexible and tear-resistant molding materials, making it possible to cast shapes with severe undercuts without resorting to needing a dozen separate mold pieces. This stretchiness also makes it necessary in most cases to construct a mother mold around the silicone mold so it can hold its shape; making the silicone mold thick enough to hold its own shape is usually very expensive, and often takes away the flexibility that you wanted in the first place.

Some silicone rubbers are mixed in equal proportions by volume, whereas others can have a ratio up to 10:1, requiring an accurate scale. Always check the instructions, as even a slight variation from the required proportion can keep it from fully curing. Silicones intended for mold making are tinted to make mixing easier.

When mixing any “two-part” molding or casting compound, you should measure each part in a separate container, and then pour them both into a third container. This ensures a better mix, since a small amount of each material will stick to the sides of their original containers. If the two parts are mixed by weight, remember to account for the weight of the container itself.

Some substances, particularly sulfur and latex, will inhibit the cure of silicone rubbers (especially platinum-based silicone) on contact. The clay must be specifically labeled “sulfur free,” or you have to assume it contains sulfur. Though more expensive, it is worth it to only buy sulfur-free clay, otherwise, you do not know if that clay you have sitting around has sulfur or not. Many petroleum-based waxes will also contain sulfur. Latex is such a strong inhibitor that even wearing latex gloves while mixing it can prevent it from curing (yet another reason to just avoid latex gloves altogether).

Many silicone rubbers are too viscous for air bubbles to float out naturally. They need to be vacuum degassed. After mixing the two parts, the liquid is placed in a vacuum chamber, which is a rigid enclosure that can suck all the air out of the inside with a vacuum pump. By letting the mixture sit inside the vacuum chamber for a few seconds or minutes (as per its instructions), the air bubbles are also sucked out. Some newer silicone rubbers are formulated to avoid the need for vacuum degassing.

As a casting material, RTV silicone replicates skin and flesh very well, and creates much more durable prosthetic body parts than latex. It can also be simply painted or poured onto surfaces. Thickening agents exist so it can hold onto a vertical surface without dripping while it cures (making brush-on molds possible).

Any of the two-part molding and casting compounds can be harmful and toxic in a number of ways. Polyurethane resin, polyester resin, epoxy, and even silicone rubber all need particular precautions taken if you want to work with them.

The liquids themselves are harmful, and will irritate the skin on contact, possibly causing an allergic reaction. Resin allergies can be nasty things, causing rashes, open and oozing sores, and even the closing up of your throat. This can happen after one exposure or after decades of use. Some people have worked with resin for over twenty years with no ill effects before suddenly developing an allergic reaction. Once developed, it is irreversible, and you will have a reaction with even the slightest exposure. The allergy is actually to the chemicals in the catalyst, which are similar in most of these resins; developing an allergy while working with polyurethane could mean you are now allergic to epoxy and polyester resins as well. Wear the proper gloves (not latex or cotton gloves), as well as chemically resistant sleeves, splash goggles, and even a face shield in some cases.

While in a liquid form and curing, they release irritating and toxic fumes which can lead to the same sensitization. You need proper ventilation not just when working with them, but while waiting for them to cure. Even though some of them become solid in minutes, they can be off-gassing at least twenty-four hours after mixing, and up to several years if not mixed correctly. Cover the molds with aluminum foil or plastic film, or keep them in an airtight container while they are curing to capture and contain the fumes. Alternatively, you can sequester the molds to a space with its own exhaust (such as a spray booth or chemical hood) until they are cured.

Polyester resin fumes can be explosive. Many epoxies will require a combination organic vapor/ammonia cartridge.

Polyurethane resin requires its own word of warning. They do not give off any odor while curing, so many prop builders believe they are safe. But the presence of a smell does not mean something is dangerous, and the lack of a smell does not mean something is safe. Because they have no recognizable scent, you cannot use a respirator with them, because if the respirator is not sealed right, you would not be able to tell that you are breathing toxic air. You need to work with a lot of ventilation, such as a spray booth, chemical hood, or glove box. You can also use a supplied-air respirator. This is no joke; people have died from inhaling polyurethane fumes.

Remember too that the containers and mixers you use to mix them will have liquid residue left in them that will off-gas indefinitely. These should be sealed in a trash bag and removed from the room as soon as possible. The implements you use to stir and measure the polyurethane should be disposable, because these will also off-gas toxins for quite a while, particularly because the stuff left in cups is not fully mixed.

Take the same precautions when sanding, machining, or cutting cured resins to keep the particulates off of you and out of your lungs. All cured resin will contain uncured particles within that are liberated as you turn it to dust. Wear a disposable “bunny suit” (Tyvek overalls) or keep a second set of clothes at the shop that you change out of at the end of the day to avoid bringing any dust home with you.

Shops that use two-part molding and casting compounds on a regular basis also have the safety infrastructure in place to minimize the risks. Many less-toxic alternatives exist, and more are introduced into the market every year. Always ask yourself whether you really need the specific properties that only these products can give, or whether a safer material will satisfy the needs of your prop.

Figure 14-46: This hand was also cast in Dragon Skin, a colorless silicone rubber. It was painted with special silicone pigments, which are one of the only types of paint that will actually stick to it. Titus Andronicus, the Public Theater, 2011.

Companies that produce a wide variety of silicone rubbers include Smooth-On, Silicones, Inc., EnvironMolds, and Castin' Craft. Companies such as Smooth-On manufacture a vast array of silicone rubbers for all sorts of uses, such as budget mold-making rubbers, high-end mold-making rubbers, and stretchy casting rubbers for prosthetics and animatronic skins.

A mold release is rarely required, but one may be used to extend the life of the mold. The major exception is when you are pouring silicone rubber onto a silicone rubber mold, whether you are making a two-part mold or casting a silicone rubber piece inside a silicone rubber mold. Waxes or special mold releases made by molding and casting manufacturers are most useful.

Silicone rubber also comes in a two-part putty; it may be handy for quick and dirty molds, but it loses a lot of the advantages of silicone rubber without saving you money like other materials.

Though silicone rubber is less toxic than other two-part molding and casting materials, the curing agents still create vapors, and the uncured liquids are irritants that can injure the skin and eyes upon contact.

Resins

A resin is the liquid form of a plastic that serves as a precursor to the solid forms. Resins for molding and casting come in a two-part system. One of the liquids is the actual resin, while the other is the hardener or catalyst which hardens the liquid into a solid when mixed together. The three that prove most common and useful to the prop builder are polyurethane resin, polyester resin, and epoxy resin.

With some products, you can alter the amount of catalyst used to speed up or slow down the hardening time, while with other products, you must mix the two components in exact proportions.

Casting resins have endless variations. Some resins turn into a rigid plastic, while some become flexible. Some turn to foam, some to rubber, and some to foam rubber. Some are clear, some translucent, and others opaque. Various working properties can be different as well; some set up very quickly, whereas others remain liquid for a long time to allow you to do extensive work with it, such as brushing it over a large surface.

Since these types of resins are also used as adhesives, it should come as no shock that they require plenty of mold release to keep from sticking to everything. Make sure that if the resin overflows, leaks, or spills out of your mold when pouring or curing that it doesn't run onto anything nice.

Avoiding Moisture

Some resins, especially polyester and polyurethane, have their curing inhibited by exposure to moisture. Make sure your mold is completely dry before using it, as well as your mixing containers and stirring sticks. A wooden stirring stick can contain enough moisture to mess with some resins, so use plastic stirring sticks instead (polyethylene sticks, such as HDPE or UHMW, have the added advantage that cured resin will not stick as much to them).

The air can have enough moisture to keep any exposed resin slightly tacky. This is fine if the only exposed area is a sprue that will be cut off. But if the top is the final surface, that's a problem. You can top your mold with a piece of glass or thick plastic (covered in mold release) to cut off the air; this will also give you a smooth flat surface, though it may also trap air bubbles. Alternatively, seal off the top with a piece of shrink wrap or cling wrap that is not touching the surface of the resin.

If you cannot physically seal the air off from your resin, you can buy surface curing agents that float to the top of your resin and form a barrier against the air; in clear resins, these will cloud the surface, so they may not be desirable. You can also buy aerosols that you spray on after the resin has mostly cured.

Polyurethane Resin

Polyurethane resins have been around for a while, but it was not until the 1980s when efficiency breakthroughs in their manufacture made them more economical to use in prop making. Polyurethanes have dominated the art and hobby market since then, resulting in a dizzying array of products; some companies can have nearly forty different kinds of casting polyurethanes. Brand names of polyurethane plastic include Smooth-On's Smooth-Cast and TASK, Alumilite Super Plastic Casting Resin, Por-a-Mold and Por-a-Kast, Model-Pro casting resin, TAP Quik-Cast, and Polycraft casting resin.

They come in a wide variety of strengths and hardnesses. Their forms include hard plastics, rubbers, and foams. They can be poured, brushed, or sprayed on. They are very easy to use. Polyurethane can reproduce extremely fine detail and cure incredibly quickly (compared to other resins).

One of their main advantages for prop builders are their fast cure times. Many resins will cure in under an hour, with some curing as quickly as ten minutes. This allows you to cast several pieces in a single day. Polyester and epoxy resins can take up to twenty-four hours to cure.

Their disadvantages are that they will stick to just about everything (except polyethylene). Mold releases are vital when using them, and it is necessary to work them into every nook and cranny. They are moisture-sensitive, and may remain tacky or bubble if the mold is a bit damp or it is a humid day. They have a very limited shelf life, and the components only really last five to six months after opening them. Finally, they release toxic fumes as they cure and the dust released when sanding them is harmful to the skin.

Always use the least toxic alternative that will get the job done. While polyurethanes seem to be a miracle solution to every problem, there are often cases where the strength they give is simply overkill for a prop. Also, if you are not set up to work with them safely, you just need to avoid them altogether.

Polyurethane resins come in a range of white, off-white, brown, and black colors. You can get clear polyurethane resins, though these are exceptionally toxic (even more so than regular polyurethane resin). Special colorants let you tint the liquid resin so that you can cast it in whatever color you wish. Always mix your colorants, as well as any fillers or thickening agents, into the resin first before mixing in the catalyst.

Polyurethane is best cast in silicone molds. Plaster, latex, polyethylene, polyurethane rubber, and fiberglass molds will require a mold release. Plaster will also need to be sealed before applying the mold release. Dusting a layer of corn starch or baking soda over the mold release can help break the surface tension of the polyurethane resin and assist its flow into all the cracks and crevices of the mold cavity.

Figure 14-47: A model made of wood, PVC, and clay, and the polyurethane resin casting made of it.

Figure 14-48: Polyurethane rubber can range in hardness and rigidity from floppy rubbers softer than a rubber band, to rigid and hyper-firm rubbers harder than a shoe heel. They can be used for both molding and casting.

You can also brush a thin layer of liquid latex over the mold's surface and allow it to dry for about eight hours, though this will cause some minor obscuring of intricate details.

Polyurethane Foam

Besides rubber and plastic, polyurethane resin can also be formulated to create foam. This is sometimes called A-B foam because it is made by mixing two parts (an “A” part and a “B” part) together. The foam will expand while it cures, with some varieties foaming only slightly and others expanding up to twenty times the volume of the liquid. The hardness and rigidity of different foam products can range from soft foam rubbers to rigid foams denser than polystyrene foam.

Spray foam (such as Great Stuff) is usually bad for casting as it needs exposure to air to cure, and anything thicker than a single layer may remain wet on the inside nearly indefinitely.

After mixing, the polyurethane will foam and rise. Make sure it has somewhere to go and that it will not spill out over anything important; remember that it sticks to everything and is difficult to clean up. It sets in around three to five minutes though the actual curing will take another thirty minutes to several hours. Most foams will create their own skin, though a few need a protective skin added on top once cured.

Do not touch the foam as it is curing; you will break the surface tension of the bubbles and cause it to fail in some way.

A lightweight yet strong casting can be had by slush casting or roto-casting a high-impact polyurethane to make a shell, and then filling it with an expanding foam polyurethane. This gives you an impact-resistant and smoothly detailed exterior with a lightweight but rigid interior.

Figure 14-49: Unpainted polyurethane foam castings.

Unless you buy polyurethane in a fire-retardant form, it will be very flammable and release many toxic gases as it burns.

Polyester Resin

Polyester resins are the cheapest and easiest of the resins to use. While they can range between rigid and semiflexible, they do not come in nearly as many forms as polyurethanes. They also shrink a lot more than epoxies. Still, they remain one of the most popular resins for beginners and for casting clear and translucent objects. Fillers can also be added to keep shrinkage down, though for transparent pieces, these can affect the clarity. Popular brand names include Castin' Craft Clear Polyester Casting Resin and Aristocrat Casting Resin.

Polyester resins are one of the most popular resins for laminating fiberglass; in fact, many types of polyester resin are labeled “fiberglass resin” (which is misleading, as you can use other types of resin on fiberglass, as well as various polymer emulsions). See Chapter 15 for more information on working with fiberglass

Do not use laminating resin for casting thick or solid parts. Laminating resins are catalyzed hot so they can cure when spread in thin layers. If you concentrate a large amount in a mold cavity, it may generate so much heat that it degrades and release toxic fumes or even catches fire. Likewise, if you use a casting resin to laminate fiberglass, it may not get hot enough to fully cure.

Mix any fillers, thixotropic fillers (chemicals that thicken the liquid resin), and colors in the resin before mixing the catalyst. Common thixotropic fillers include silicas, a popular brand being CAB-O-SIL. Polyester resins can be tinted with vat dyes, dry pigment powder from art stores, or any number of specialty products. A small amount of pigment can go a long way in coloring your resin, so start off by adding small amounts. Water-based tints and colors may inhibit the cure of your polyester.

Pay attention to the manufacturer's recommendations for how much catalyst to mix in. More catalyst will lead to a hotter mix that cures faster, but too much heat can cause it to shrink too rapidly and crack, break, and warp (not to mention the potential fire danger; buildings have burned down from fires started with resins that catalyzed too hot). The large masses of resin in thick castings will generate a lot of heat, and in some cases, you will have to use less catalyst than recommended to keep it from cracking. Alternatively, when you have thin parts, the recommended amount of catalyst might not be enough to generate the heat needed to cure. For especially thin parts (less than ⅛″ thick at the thickest part), you may wish to use a laminating resin.

Polyester resin will not fully cure when exposed to the air and retains some “tack.” Casting resins have a small amount of wax in them that floats to the surface and forms a coat against the air to allow it to cure. You can also purchase separate surface curing agents to mix in to achieve the same effect. There are also “resin sprays” you can buy to spray on top of a tacky surface and seal it. If you are pouring the resin into a sprue hole, of course, the tacky part will be on the end of the excess sprue material, which will be removed anyway.

Polyester resins, unless poured in a mold made of vinyl or silicone, should have a release of PVA, silicone, or wax.

Because some auto body fillers use the same catalyst as polyester resin, they can be mixed together when you want a material with an “in between” consistency. This material is often referred to as “Rondo,” which is a combination of “resin” and “Bondo,” a registered mark.

Water-extended polyester is a specific type of polyester resin which you add water to as a filler (you cannot just add water to regular polyester). It dries to a hard, white “bone-like” material similar to a hard art plaster such as USG's Hydrocal. The advantage of WEP is that it is far cheaper than other resins because it is up to 60 percent water. The disadvantage is that it tends to warp badly over time. For the life span of most props, that is not an issue, though if you mean for your prop to hold up for several years, you may wish to avoid it.

Epoxy Resin

Epoxy can be found as a casting resin as well, though it tends to be more expensive by volume than either polyester or polyurethane resin. It may also have much longer curing times (usually around twenty-four hours).

Epoxy is very sensitive to mass. As with polyester resin, do not use lamination epoxies for casting thick parts. Even with casting resins, many will give you a maximum thickness or volume you can cast; read the instructions carefully to avoid casting too large a piece. The heat generated will release toxic fumes, destroy your cast, or even start a fire.

Some brands include EasyCast Clear Casting Epoxy, Smooth-On EpoxAcast, and Polytek PolyPoxy.

The toxicity of epoxies varies greatly, though all warrant protection of some kind. Check the Safety Data Sheets for the ingredients in the specific epoxy you are using. Amines are the safest curing agents, though they are still sensitizing and toxic.

Latex

Latex, or natural rubber, is extracted from rubber trees from Southeast Asia. It is processed with ammonia and water to form a milky white or gray viscous liquid. As the liquids evaporate, it becomes a highly elastic solid called “rubber.”

Latex has long been a staple of prop shops, costume crafters, and special effects makeup artists because it is an inexpensive single-component system (no need to measure and mix multiple parts). A certain percentage of people are allergic to latex, though, and others can develop allergies with repeated exposure. This must be considered not just for the artisans working with it, but for the actors and crew that will handle the finished prop.

Latex gives off ammonia (and sometimes formaldehyde) fumes while drying, so work with plenty of ventilation. If using a respirator, be sure your cartridges are designed to filter ammonia, which standard organic vapor cartridges are not.

Latex can also be slow to apply. Many latex formulations need to be brushed on. It can take up to twenty coats of latex to form a skin of sufficient thickness, and each layer needs to dry up to four hours in between coats. One final disadvantage is its tendency to shrink anywhere from 10 percent to 20 percent.

Latex comes in a variety of formulations for different tasks. Many standard latex formulations can only be brushed or sprayed on. More specific formulas can be cast (through absorption casting) or slush cast. Some latex-based rug backing products or latex glues can even be used in a pinch for glove and brush-on molds. Latex varies in the amount of “solids” it contains. The higher the solids content, the greater its ability to withstand tearing.

As with other molding and casting materials, latex can be thickened with various agents.

Latex is good for making molds to cast plaster, cement, wax, low-temperature metals, and, to a limited extent, plastic resins. Because latex shrinks around the model, it captures the finest of details. Latex is often brushed over the outside of a master to make a glove mold. Remember to let each layer fully dry between applications. You can tell it is dry when the milky white areas have completely disappeared. The dry time cannot be accelerated through heat. Like a glove, it can be peeled off the model by rolling it inside out.

To cast with latex, you want to use a “casting latex.” The easiest method is with absorption casting in a plaster mold as described earlier in this chapter.

You can also make reinforced castings with latex. Again, use casting latex, not molding latex. Brush one layer over the inside of a plaster mold. Repeat until you have four layers. When these are dry, brush a layer of latex, lay in a piece of cheesecloth or similar gauze (use several pieces if the shape is complicated; overlap each piece slightly, but try to keep the cheesecloth as flat as possible), and then brush another layer of latex immediately over the top of this. Let this dry, then add at least five more layers, letting each layer dry before adding the next. If you want it thicker, you can add another layer of gauze after the ninth layer, and then coat that with around three more layers. The cheesecloth has a grain and a bias; you can work with the bias if you want your piece to be able to stretch, or apply with the grain for greater strength.

Figure 14-50: Latex “horns” for a piece of KISS armor are cast in a plaster mold. Molds and castings by Costume Armour, Inc.

Finished latex pieces can be dusted with baby powder or corn starch to keep the latex from sticking to itself.

You can vulcanize your latex for additional strength. When all the layers are completely dry, simmer your piece in hot water for around fifteen to twenty minutes without boiling it. Dab the water off and let it dry while fitted back in the mold. Some latex can be vulcanized by heating it to 120°F (48°C) for four hours, and others you can just let sit for three days.

Certain forms of latex, particularly casting latex, come prevulcanized; always check the instructions of the specific latex you purchase. Prevulcanized latex gives you a final piece with the same strength and durability of vulcanized rubber without the need for a separate heating process.

Latex is great for casting plaster because it can withstand the heat created during the curing process, and it captures excellent detail. No mold release is needed. It is also good for other hot melts, such as wax, vinyl, and hot melt glue. It often requires a rigid mother mold around it to hold its shape when casting heavier items.

Plastic resins generate too much heat and have chemical reactions with the latex to cause it to degrade with every pour. Still, with a properly vulcanized latex, you can get twenty to fifty pieces out, which is often good enough for props.

Latex has a limited shelf life of a year if kept in a cool dry place. It will turn a cottage cheese consistency or develop a putrid odor when it spoils, and spoilage is irreversible.

Molding the Human Form

Props often require molding parts of the human body. Severed limbs find their way into numerous shows. Many productions also require either a severed head, a bust, or a full statue of a specific actor, and molding their form is the most accurate way, and often the quickest and cheapest way, to get this done. Making a mold on a person has special considerations though, both in the choice of materials used, and the methods you go about doing it.

Figure 14-51: An alginate waste mold is a common way to cast body parts. Most boxes of alginate sold at hobby and art stores will indicate on the box how many hands can be molded with the quantity inside. You need a container that will fit your actor's hand plus at least a half an inch around all sides. Mix the alginate with the specified amount of water, and pour it into this container. Have the actor pose their hand, and slide it inside. They should be seated comfortably as they will need to keep their hand frozen in place for about ten minutes. Titus Andronicus, the Public Theater, 2011.

Figure 14-52: They do not need any mold release on their hand. The alginate will not heat up either; it will just slowly get stiffer until it solidifies. It is helpful to use warm water when mixing your alginate just so that your actor does not have to plunge his or her hand into a cold and slimy bucket. When the alginate has solidified, have the actor slightly wiggle their fingers to break the suction, then slowly draw their hand out. The alginate should have enough flexibility that the hand can be drawn straight out even if they were making a fist. Thank the actor, and let them know they can clean up with regular soap and water. Titus Andronicus, the Public Theater, 2011.

Figure 14-53: The alginate will begin drying and shrinking right away; you only have about two hours to cast your material before it becomes visibly deformed. Plaster is a good material to make a quick cast with, since most plastic resins are sensitive to the moisture in the alginate. Titus Andronicus, the Public Theater, 2011.

Figure 14-54: When the plaster has set, you can simply break and peel away the alginate. Plaster poured into alginate does not need a mold release; the alginate will fall away easily and cleanly. You can now take the time to make a more permanent mold from this plaster hand without needing an actor to wait around. Titus Andronicus, the Public Theater, 2011.

Alginate is the preferred material for casting hands, feet, faces, and ears. It is nontoxic, easy to use, and cleans up with water. It is fast and comfortable for the actor, and reproduces very fine detail, such as the texture of the skin.

Flexwax is a brand of wax that melts at a low enough temperature to safely be used on the skin. First, cool the model's skin with cold water. The Flexwax can be applied by brushing or by dipping. When dipping, plunge the model's limb into a container of Flexwax and immediately remove it; let this layer solidify before dipping again. Build up the mold's thickness until it can hold its shape (usually around ¼″ or 6.35 mm thick). Flexwax molds can be remelted and reused.

Figure 14-55: This severed head was cast from the actor's face. In the middle is the plaster cast made from the alginate mold taken from his face; on the right is the silicone mold made from the plaster cast. On the left is the final piece, cast in silicone rubber and mounted on a fake skull. The Bacchae, The Public Theater, 2009.

For larger portions of the body, a skin-safe silicone is often a better choice. Even with a “skin-safe” silicone, you should always do a small test on the model's wrist to check for possible allergic reactions. These can be brushed or poured on. As with alginate, a mold release is not necessary.

If you need to cast the entire body, it may be easier to do it in pieces, cast them separately, and then attach them together.

You may not need to do the entire body. Generic mannequin pieces can be bought to fill in the torsos and limbs, and you only need to cast the distinctive parts, such as the face and hands. If the fake body will be wearing clothes, you only need to cast the parts that are uncovered; the rest can simply be rods and stuffing.

Some artisans mold body parts directly with plaster. I do not recommend this if you are new to mold and casting. Plaster heats up while it sets, and if you put it on the body too thick, it can burn the model's skin. It can also trap body parts with undercuts inside.

Storing and Caring for Your Molds

Moisture, dust, exposure to sunlight, and even exposure to air will degrade your molds over time. If you wish to keep a “mold archive” of molds that you may wish to use in the future, store them in plastic storage tubs with sealed tops. Try not to stack the individual molds on top of each other in the tubs, as the flexible mold materials may deform over time and the rigid mold materials may chip as they run into each other.

Figure 14-56: A portion of the mold storage area at Costume Armour, Inc., which maintains a mold library for most of the shows they have built since the 1980s.