thirteen
sculptural materials
The word sculpture comes from the Latin sculptura, which means “to carve or cut stone.” These days, it encompasses far more materials and techniques. Carving stone is probably one of the last techniques you would ever use to make a prop.
While sculpting is frequently portrayed in the movies as a single artist finessing the details of a nose or ear on a large block of uncarved marble, real sculpting is far more methodical. You progress from an overall shape to fine details. First, you start with the basic outline, either by cutting away large chunks of material or by adding together big lumps of the material. You next shape all this material into a more refined form, progressing from aggressive tools to more precise tools. Only at the end, when everything is in place, do you begin adding in details, texture, and refining the surface.
The media used in sculpture can be divided into two groups. Rigid and mostly permanent materials like wood, metal, and foam are carved (or cut) directly, or beaten into shape. Impermanent and plastic materials, like clay and wax, are easily altered in shape by adding and subtracting material, or by “smooshing,” pulling, smearing, and stretching it around. (In this case, the word plastic refers to a material's ability to be formed; it does not mean clay is a plastic like acrylic or styrene.)
Figure 13-1: This mandrake root from Harry Potter and the Chamber of Secrets was created by sculpting.
Do You Actually Need to Sculpt?
When you are creating your prop, you often need to decide whether you are going to carve detail into the surface or whether applying a detail will work better. For example, your prop may have an area that is full of repetitive ridges. You can carve all those ridges in, or you can find a material that already has inherent ridges, such as a fabric, and apply it to the surface. You can build your sculpture out of any number of disparate materials, found objects, and appliqué.
In very low-budget theatrical situations (this may be scoffed at by the film and television crowd), three-dimensional details may sometimes be simply painted on. But if you remember the lessons from Chapter 2, as long as the needs of the prop are met using the resources available to you, this can be a viable solution.
Figure 13-2: The decorations on this cast iron bench are actually a mixture of applied resin pieces, upholstery fringe, Ethafoam, and yarn.
The first group is considered subtractive sculpture because you remove material to make your shape. The second is called additive sculpture because you combine material together to create a form. You can also call the first group “carving” materials, and the second “modeling” materials. Some materials overlap the two groups of carving and modeling. You can model a prop out of water-based clay, and then when it hardens, you can carve details into it.
This chapter will first look at carving materials, then at modeling materials. In props, the most popular carving material by far is foam. The most common modeling materials are clays. Though I will look at a few other materials, foam and clay are the ones I will focus on most in their respective sections.
Foam
The two most useful foams for sculpting props are polystyrene foam and polyurethane foam.
Both come in a range of densities, though polyurethane comes in much higher densities. A lower density is cheaper, lighter, and easier to cut and manipulate. A higher density allows you to carve much finer details and is more durable.
The density of foam is measured by how many pounds one cubic foot of that material weighs. The unit can be indicated by pound, pounds per cubic foot (pcf), or lb/ft3, all of which mean the same thing. Metric measures density in kilograms per cubic meter (kg/m3) or grams per liter (g/l), which are numerically equivalent.
Polystyrene comes in densities ranging from 1 lb to 3 lb, or 15 kg/m3 to 40 kg/m3. Many props people prefer the 2 lb (30 kg) density for the best compromise in affordability, ease of use, and durability.
Polyurethane can be found in higher densities, such as 2 lb through 12 lb and even all the way up to 50 lb. Common metric densities include 48 kg through 320 kg, and all the way up to 800 kg. The price increases linearly, so 12 lb foam is twice as expensive as 6 lb foam.
Anything below 4 lb (64 kg) is able to be carved by hand. Above that, you need to machine it or use power woodworking tools. Foam which is at least 8 lb (128 kg) is generally dense enough to vacuum form over without a coating or reinforcement.
Polystyrene foam can be cut with a hot wire; polyurethane should never be cut with hot wires or other high-temperature tools, as the fumes are very toxic when heated.
Polystyrene is dissolved by polyester resin, but not by epoxy. Polyurethane can be coated in either. This is important when considering what to coat your foam with (polyester resin is often used for fiberglass). Both foams will be dissolved by acetone and lacquer thinner.
Since foam is so expensive to ship compared to its actual cost, and there aren't any well-known chain stores I can point you to, you need to dig through your yellow pages to find local suppliers.
Gluing Foam
Foam comes in large blocks, sheets, or bars. You can make thicker pieces by laminating several pieces together. You can easily insert sharpened dowels or metal rods into pieces to give them extra support in holding them together.
Water-based contact cement (such as 3M Fastbond 30-NF) and spray adhesives specifically for foam are great for attaching large flat sheets of foam to each other and to wood. Epoxy works well for gluing smaller pieces together, particularly if you need some gap-filling abilities. Foaming polyurethane glues (such as Gorilla Glue) are especially good at sticking rougher pieces of foam together or to wood.
You will not be able to use a hot-wire cutter to cut through parts that have glue on them.
Most of these glues dry to a harder substance than the surrounding foam, or become a rubbery skin; this will affect your ability to easily carve or sand the foam. For gluing foam to itself without affecting its ability to be sculpted, some shops use two-part foaming polyurethane as an adhesive. The foam needs to be clamped together or weighted down because the polyurethane expands as it foams, and can push the pieces apart. When cured, it has a similar density as the surrounding foam, making it feel like one giant block of material as you carve.
Polystyrene Foam
Polystyrene foam can be either expanded or extruded. Expanded polystyrene foam (EPS) is a white, beady foam also known as “bead-board” or “white foam.” It is sold in sheets and blocks or scavenged from packing materials. The packing foam is usually not that dense, and it can even be somewhat spongy, but it is free. The beads make it difficult to achieve a smooth finish, but for large props that will receive thick coatings, it will suffice.
Figure 13-3: From left to right: white/bead foam (EPS), pink foam (XPS), blue foam/Styrofoam (XPS), dry floral foam (also Styrofoam), white craft foam (also Styrofoam). Not all polystyrene foam is called Styrofoam, and not all kinds of Styrofoam are the same; be careful about using “Styrofoam” as a generic term for foam.
Extruded polystyrene foam (XPS) is commonly known by its most popular brand name, Styrofoam. It can be found in home improvement stores in sheets up to 2″ (50 mm) thick, for insulating houses. These are often colored blue or pink, giving XPS the nicknames of “blue foam” or “pink foam.”
The brand or color you use is immaterial for sculpting purposes; what you should be more interested in is the density and rigidity of the XPS you can get.
XPS is easily dissolved in most solvents, and even heat as low as boiling water can begin to degrade it. This precludes many spray adhesives and paints, solvent-based contact cements, and hot melt adhesives from being used on it.
Urethane Foam
Another foam useful for carving is polyurethane (sometimes shortened to “urethane”) foam. It is not as economical or easy to find as XPS and EPS (taxidermy shops, strangely enough, often stock it), but fans like it for its greater density and ability to define more precise detail.
Polyurethane can also be found in some building supply stores as insulation sheets. Modern-day insulation is more likely to be polyisocyanurate foam, also called polyiso foam or PIR. Many places refer to polyiso foam as polyurethane or urethane. Some foam blocks are even a mix of the two. For prop-making purposes, they are interchangeable, just be aware that if you ask a supplier if they have polyurethane or urethane foam, and they say “no,” ask for polyisocyanurate foam, because they might have that and it is what you are looking for anyway.
The dust from polyurethane is pretty toxic, and you need to avoid breathing it and getting it on your skin. Wear protective clothing (like a bunny suit) and gloves when handling it, and a respirator when cutting, carving, and sanding it.
The two-part polyurethane foam you buy for casting can be used for carving. Some props artisans who cast objects in polyurethane foam will pour the excess liquid into a random block mold so that they have a block in stock for future carving projects.
Figure 13-4: Expanding spray foam can add material to a skeleton or framework that can be carved down. In this photograph, we sprayed foam onto a literal skeleton to construct a corpse. The top leg shows the fully cured foam already being carved. The bottom leg shows the foam after application; it will expand in size to match the top leg. The Bacchae, the Public Theater, 2009. Scenic design by John Conklin.
Spray foam (such as the brand Great Stuff) is a polyurethane foam that is automatically catalyzed as it is squirted out and cures as it is exposed to air. Because of its need to be exposed to air, you can only lay it down one layer at a time, otherwise it will take days to cure (the center may, in fact, never cure). It can be highly useful for selectively adding “carve-able bits” wherever you want, even around irregularly shaped supports and framework.
Many polyurethane foams off-gas isocyanates (among other toxic substances). These are highly reactive chemicals that can irritate the skin, eyes, and lungs, and lead to asthma or chemical sensitization when inhaled or absorbed through the skin. Eventually, you can find yourself having major reactions to even minor exposures (for isocyanates, this is typically an asthma attack) of a chemical. Sensitization can be permanent. When spraying, you need to take the same precautions as when pouring or casting other two-part polyurethane foams or resins; a local exhaust ventilation system (chemical hood) is the most appropriate precaution.
Floral Foam
Floral foam comes in a range of materials. It is divided into wet or dry. Wet floral foam is able to soak up water. Dry floral foam is meant to sit away from water. It usually comes in green, though brown and white are also found. Popular brands include Sahara (polyurethane), Floracraft (XPS) and Oasis (phenolic). Most floral foam allows flower stems to easily be inserted into them, which also makes them very easy to dent or break apart. With a strong-enough coating, they may be good enough, especially since they are one of the few types of foam you can buy at 3 A.M.
Figure 13-5: Floracraft is a crinkly type of Styrofoam made for the floral industry. It was cut, shaped, and painted to make these pieces of fake toast for Timon of Athens at the Public Theater. The glue helped seal the foam to make it easier to paint; yellow glue was used because it closely matched the color of the toast.
Carving Methods
Figure 13-6: A grid overlay is a useful method for enlarging your drawing onto your foam. See Chapter 6 for other methods.
To carve in the fastest and most efficient manner possible, you want to build your basic shape first. This will help you establish all the proportions first. You wouldn't want to start carving the intricate details of the ears on either side of a block of material only to find out later that they do not line up with each other.
Draw the furthest outline of your object on the three sides (front, side, and top) of your object. Cutting around this outline will make the removal of material go the fastest.
Figure 13-7 to 13-10: After you cut one face out of a block of material, you can reattach the offcut pieces with tape or toothpicks. Turn the piece on its side and cut out the profile. This gives you a flat surface to keep the material square as you cut the shape out of another face, which is especially helpful when cutting foam on a band saw.
Figure 13-11: When you are attaching several pieces of foam together for a larger sculpture, it can be helpful to rough-cut the individual pieces before gluing them together.
Figure 13-12: As you cut or carve away portions, it may be helpful to redraw guidelines as you go.
For large or complicated shapes, prop builders sometimes find it helpful to draw out various “slices” of the sculpture showing the profile at that point. You can cut these slices out of a sheet of material and attach them to a central framework. Fill the spaces in between the slices with your sculpting material. The slices serve as reference points so that you basically “connect the dots” while sculpting.
Figure 13-13: If your sculpture is meant to be symmetrical, you can use calipers or a divider to check the distances between two points on either side to make sure they are even. You can also check your sculpture against your reference image if it is printed in full scale.
Figure 13-14: For this three-foot tall garden gnome, my wife and I cut out the front and side profile from plywood, and then added slices indicating the top profiles. We placed foam between the plywood slices. Wherever possible, we precut the foam to a rough shape on the band saw before placing in the plywood frame to save time. Making the sculpture match the proportions and dimensions of our reference material was a simple matter of sculpting the foam flush with all the slices. Broke-ology, Lincoln Center Theatre, 2009. Scenic design by Donyale Werle.
Figure 13-15 to 13-17: This shield form used slices of MDF and pieces of plywood to establish the furthest extents of the shape. Spray foam was applied and carved down to fill in the gaps. This created a perfectly symmetrical shape with precise curves without the need for constant measuring.
Carving Tools
A band saw is one of the best tools to cut the initial shapes of any piece of foam that will fit in it. Larger chunks of material that will not fit on a stationary machine can be attacked with handheld saws, powered either electrically or with muscle.
Polystyrene foam can be cut with a hot-wire cutter, which is a long wire kept in tension and heated through electrical resistance; some can be long enough to require a person on either end to operate. Polyurethane foam cannot be cut with a hot-wire cutter.
Finer cutting can be done with an electric knife, handsaws, and snap-blade knives. Foam is easily shaped with saws, rasps, surforms, and sandpaper.
A wire wheel can quickly remove foam, albeit leaving a rough texture. Prop builders also improvise tools like a wooden disk with nails sticking out of the end that can be hooked into a drill.
Figure 13-18: This foam piece was cut in half using a large hot-wire cutter. The box served as a guide for the wire to slice the foam exactly down the center.
The Consortium of Local Education Authorities for the Provision of Science Services (CLEAPPS), at Brunel University in England, conducted tests with using hot-wire cutters on polystyrene foam and found that styrene fumes were released. Styrene is an eye irritant and can cause nerve damage; you begin to get dizzy after enough exposure (an indication that you do not have adequate ventilation). It is also a Group 2B carcinogen (possibly carcinogenic). CLEAPPS suggests that with adequate ventilation around handheld cutters, exposure can be kept to 10 percent of the MEL (maximum exposure limit). Large tabletop or two-person wires may require local exhaust ventilation, which suggests having a risk assessment done by a qualified professional. The Safety Data Sheet states that a respirator with an organic vapor cartridge will protect against styrene fumes.
Figure 13-19: Eileen Garcia carves a cast polyurethane foam bust with a surform (“surface forming”) tool, which resembles a cheese grater.
Rotary tools are useful for smaller or more detailed work. Props people often refer to all rotary tools as “Dremel” tools, though that is only one brand. More powerful tools such as die grinders are available for the same kind of work. You can choose from a vast array of bits designed for various tasks, such as grinding, sanding, shaping, cutting, and smoothing, and materials, such as wood, metal, and plastics.
An oscillating multifunction tool generates tens of thousands of oscillations per minute. It can accept a number of types of attachments, such as rasps, sanders, blades, grinders, and scrapers, making it a veritable chameleon of tools. It can cut a lot of time out of otherwise monotonous sculpting tasks.
Figure 13-20: A spiral curry comb (with steel blade) is a great tool for quickly but precisely removing large amounts of foam. Though originally designed for grooming horses, they can also be used for, well, carving horses.
Figure 13-21: A saw rasp removes material along a flat plane; its open body also keeps it from getting clogged.
Figure 13-22: Foam can also be turned on a lathe with standard wood turning tools.
Coating Foam
If you just want to paint the foam, you only need to seal the surface. Acrylic gesso, thinned PVA glue, or liquid latex are popular choices. However, if anyone will be handling your prop, you are going to want to coat it with some kind of protective shell. Even just transporting a foam prop can subject it to enough wear and tear to warrant a coating.
The cheapest method is to mix joint compound with PVA glue. This can be either the premixed joint compound or powdered drywall mud. The joint compound gives a sandable surface that offers some protection, while the glue keeps it from cracking or flaking off. Some prop builders add fillers to this mix, such as sawdust, which give it a bit more body.
Some ready-made products exist that offer a bit more protection and more consistency than a homemade solution. Rosco FoamCoat, Aqua-Resin, Steve Biggs' Original Foam Coat (similar to the now-discontinued Vanillacryl), and Aqua Form are popular products for giving a hard, durable, sandable finish. It helps to apply a thin layer on first to seal all the “pores”; when dry, you can add thicker layers until you build up enough of a body to protect your prop. These products may require a fairly thick coating to be durable, which obscures the details of your prop and add a lot of weight. You can mix in shredded fiberglass to maintain strength while minimizing the weight; some of these products come in versions with fiberglass already added.
Figure 13-23: Adding a hard coating to foam can also allow you to carve finer details into your prop.
Figure 13-24: This three-foot-tall foam sculpture has been coated in Rosco FoamCoat. Broke-ology, Lincoln Center Theatre, 2009. Scenic design by Donyale Werle.
A flexible coating will give your foam some protection against impact (though it's not good for thin protrusions that can break off). Sculpt or Coat, Rosco FlexCoat, and Rosco CrystalGel are all products that fit this bill. You can also use elastomeric roof coatings; a popular brand for theatrical use is Jaxsan. Note that these flexible coatings are not sandable; you need to give the surface its finished texture before it dries. These products can use cheesecloth or gauze for added reinforcement.
Figure 13-25: FlexCoat on a soft foam prop allows the whole thing to remain flexible.
Foam may also be covered in a shell that is laminated on. Papier-mâché and cloth-mâché will provide a strong but lightweight surface for painting. Applying cloth-mâché with cheesecloth rather than muslin will give a very thin coating that will not obscure as much detail as thicker coats.
For those foam props that need to be thrown into the back of a truck every night and last for years on a tour, fiberglass remains one of the best coatings. Standard fiberglass resin is usually a polyester resin, which will attack polystyrene foam. Use epoxy resin instead.
You can just brush on polyurethane resin as a coating too, though it is not the most economical choice. However, many of them cure very fast, and sometimes you have some sitting around that's about to go bad, so may as well use it. StyroSpray and Styroplast are a couple brand names of two-part polyurethane made specifically for use as a coating on foam. They are fairly pricey, and the equipment and safety gear needed to use them are typically not found in your standard prop shop.
Wood
For larger pieces, an angle grinder is a helpful ally in removing large amounts of material quickly (though less precisely). You can use a bench grinder or other tabletop tool as well, but sculptures often demand the greater control in positioning that a handheld tool provides. Chainsaw disks will remove material extremely quickly. Abrasive flap wheels, sanding disks, and wire brush wheels are good for finer shaping and finishing. Portable belt sanders will also enable you to remove materials quickly.
Figure 13-26 and 13-27: The process for carving wood is the same as the process for carving foam, but the tools used are more aggressive. The band saw can cut basic outlines, and rasps can rough out the shapes. Rotary rasps on a Dremel or drill are great for finer shaping.
Figure 13-28: A full-scale drawing is transferred directly to a piece of wood to carve it away. Photo courtesy of Natalie Taylor Hart.
Figure 13-29: An abrasive flap wheel on an angle grinder quickly shapes this large chunk of wood that will eventually become the back half of a dead deer. A Lie of the Mind, Ohio University, 2003. Photo courtesy of Natalie Taylor Hart.
Figure 13-30: Natalie Taylor Hart sands a basswood mask form, which she will use to mold a leather mask on top.
Basswood
Basswood is another good material to hone your carving skills on, or if you need a quick and easy sculpture made out of wood. It is pretty straightforward to graduate to fancier hardwoods from basswood. Soft woods tend to be less useful for carving, as their grain is larger and harder to control. Though similar to basswood, balsa wood is considered too mushy to be a satisfactory carving medium.
Sculpting
Now our chapter turns away from carving foam and other rigid materials and onto sculpting. Prop sculpting is mainly done with clay or some substance that acts like clay. What follows is a number of methods you can use to sculpt a prop.
It is easier to build up your basic shape bit by bit rather than starting with a giant lump or block of material. Add little balls of material to your shape until you have a rough form. Protrusions can be added a little bit at a time as well.
Figure 13-31 to 13-34: Start off by adding bits of material to create the overall shape. Build up the sculpture by hand to establish the proportions and the extents of the outline. Once you have this initial “sketch” of a sculpture, go in with more precise tools to flesh out the details.
Figure 13-35: A piece of cardstock with the profile along the center line cut out is used to check the progress of the sculpture. A second option is to print the picture out and mount it to a stiff board or stock; cut out and remove the picture itself so you are left with a hole the exact size and shape of your sculpture. You can pass this hole over your sculpture as you progress; when the sculpture fits through it with no gaps, you have successfully built your piece to its outer extremes.
A more three-dimensional sculpture may require some kind of underlying skeleton or armature to support the material. This is particularly true with human and animal figurines, where outstretched limbs can easily fall off due to the weight of the clay. The armature can also help define the shape and proportions.
Armature wire is a type of wire that is easily bent but holds its shape well. Any kind of wire or rod will work, but if you do a lot of sculpture, you will appreciate the reduced effort it takes to shape armature wire. You can build more rudimentary or sturdy skeletons out of wood, pipe, or rod. It need not be elegant.
When the shape is ready, some sculptors use epoxy putty to hold the wires together. For larger sculptures, the wires can be welded together as well. You can also lash the wires together, either with wire, tape, or string.
Figure 13-36: With a full-scale reference drawing, you can check the progress of your sculpture using calipers or a ruler. Photo courtesy of Natalie Taylor Hart.
Some materials will slide or rotate on a piece of wire; you may find it useful to wrap a thinner wire in a spiral around your thicker armature wire. This gives the wire some “grip” to better hold the material in place while you sculpt.
Another way to add an underlying shape to your sculpture is by wadding and crumpling up material. Aluminum foil is useful in this case, as it is cheap, readily available, and it holds its shape when crumpled. You can form a rough shape with a wad of aluminum foil and then proceed to add clay or whatever sculpting medium you choose on top. The same process can be used on wadded up paper. You can hold the shape together by wrapping either tape or wire around it. This also gives you some control over the shape you form. These methods work well for more free-form sculptures, for sculptures that need to remain lightweight, and for when time is of the essence.
Figure 13-37 to 13-39: Using armature wire with epoxy putty over the top.
If you work with a material that can be hardened in the oven, you can lay down the basic shape and then fire it so that as you add further details, you do not risk disturbing the underlying shape. If you work with a material that sets after a period of time, like epoxy putty or a thick resin, you can build your sculpture in stages; begin by roughing out the shape you need and let it harden. Add another layer of shape and detail and allow that to harden. Continue in this vein until you have the level of detail you need.
Working in stages like this helps keep you from ruining any work you have already done, either by handling the sculpture, or accidentally bumping or dropping it.
Some prop builders find it useful to make a waste mold of a piece in progress and then cast it into another material to continue working with a different technique. This allows you to “freeze” the progress of your sculpture and continue adding details without disturbing your basic shape and proportions. You may want to sculpt the basic shape in a soft and easy-to-manipulate material and then cast it in a hard material that allows you to carve sharp, precise details and textures. Similarly, casting in a hard material gives you the option of sanding and polishing your sculpture to a much smoother and glossier surface than you can get in materials such as clay.
Figure 13-40: For sculptures that are more geometric and precise in nature, you can build a base. For this sword hilt, the horizontal base was cut out of MDF, and then stiff paper was attached to establish the vertical lines. The clay was filled into the gaps to complete the curved surfaces.
Sculpting Tools
For the actual sculpting and carving of your object, the tools are as varied as any. Many artisans create and modify their own tools, so it should come as no surprise to see such a vast array of implements to choose from.
Ribbon tools have a loop of metal at one or both ends that scrape off material when pulled. The variety of shapes the ribbons can be found in are practically infinite, and the fact that they can be reshaped by hand means the choices are greater still. Other useful implements are wooden spatulas that come in a dizzying array of shapes and configurations. Some are rounded like spoons, and others are straight like knives.
Figure 13-41: Top left: ribbon tools. Top right: wooden spatulas and shapers. Bottom left: gouges. Bottom right: wire used for cutting blocks of soft clay.
Gouges are like curved or bent chisels that can scoop out material and make grooves.
For some materials, particularly water and oil clays, an easy way to quickly remove large chunks of material is by pulling a wire through it; to save your fingers, the ends of the wires are wrapped around sticks.
Choosing a Sculpting Medium
There is no single perfect sculpting medium. Some are good for large sculptures while others are more suited for smaller props with fine details. Some are great as final products, others are more useful as a piece that will be molded and cast. Other than a few practical considerations and cost, most of the choice comes down to personal preference. Whether you are just starting out or you have been sculpting for a while, it is always helpful to try out new materials that you come across.
Some materials are soft and pliable, making it easier and faster to create overall shapes. Harder and more rigid materials allow you to create sharper and finer details.
Different materials become permanent in different ways. Some harden through heat, either in an oven or a kiln. Some harden over time, either by curing or by air drying. Some become pliable when heated and rigid when at room temperature, allowing you to reshape them over and over again.
Some modeling materials are not meant to make a final product. They may be intended to construct a shape that will be molded and cast in another material. They may also be used to make a maquette, which is a small-scale model used as a three-dimensional reference for either a larger sculpture or for an animator making a film.
Oil Clay
Oil clay is a popular item in prop shops, sculpture studios, model shops, and animatronic studios. You may have heard it referred to by a number of names and spellings, and it's easy to see why there is confusion over its name. Plastilin, Plasteline (also known as “Chavant Clay”), Plasticine, Roma Plastilina, and Van Aken Plastalina are all different brand names of oil clays used by sculptors. Props artisans may use any one of these trademarked names as a generic term for all brands of oil clay. Other popular brands include Lewis Newplast and Monster Clay.
Mixing a clay with oil rather than water means it will never dry out, and so its ability to be modeled will remain consistent whether it takes you an hour or a week to finish. This also means that it cannot really be a final product; it is used almost exclusively to create a model for later molding and casting.
Oil clays can range in hardness from bubble gum to blocks that carve almost like wood. Some can be heated to become softer, and others can even be heated to the point of becoming pourable. Kneading the clay in your hands is often enough to get it soft. You can also keep your clay under a heat lamp as you work. Putting your sculpture in a fridge will help harden the clay and freeze the details.
Figure 13-42: A mask form sculpted out of a very firm oil-based clay.
Figure 13-43: A bit of mineral oil helps smooth out the surface of oil-based clay and remove fingerprints and tool marks.
Many oil clays use a sulfur base, which is chemically incompatible with silicone mold-making. If you will be molding your model in silicone, be sure to use a sulfur-free clay. This is usually pretty explicitly mentioned in the packaging or on the company's website, so if the clay does not specifically state it is sulfur-free, assume it contains sulfur. Many prop shops avoid sulfur clays altogether; they can easily get mixed in with sulfur-free clays, and there is no way of distinguishing between the two.
Water-Based Clay
Water-based clays are clays that are mixed with water; as the water evaporates, the clay dries and becomes hard. Ceramic clays, such as stoneware, earthenware, terracotta, and porcelain, are meant to be fired at high temperatures (typically between 1200°F and 2300°F or 650°C and 1250°C) in a kiln. We rarely use these to construct props because of the need for specialized firing equipment and other tools. They are, however, used on occasion to make breakable props, either by air drying or with a lower temperature firing.
You can also find water-based clays that air-dry (or “self-harden”). Though not as strong as their kiln-fired counterparts, they look similar, making them a viable alternative for a prop shop that needs to produce a real clay prop but lacks the ability to fire one in a kiln. Companies like AMACO and Sculpture House make a range of air-drying clays; other popular products include Activa La Doll, Polyform Model Air, Crayola Air-Dry, and DAS Modeling Clay.
WED Clay (Walter E. Disney) is a wet clay that feels like an oil-based clay and is very popular with special effects and creature sculptors. It is meant to be used for molding and casting; unlike other air-hardening clays, it will eventually crack and become brittle if left to dry for too long.
Figure 13-44: A lot of air-drying clays are found in the kids' aisle. A very popular brand is Crayola Model Magic. Though brittle, it is very lightweight and easily simulated the crust on this pie.
Polymer Clay
Polymer clay (or “hot set” clay) is a heat-fired clay that comes in a multitude of colors. Many harden when placed in a regular oven or toaster oven (rather than a high-temperature kiln). You may recognize them by brand names such as FIMO, Sculpey, Premo!, Cernit, Formello, Modello, Kato Polyclay, and Lewis Uro clay.
They have minimal shrinkage, so can be used on top of armatures or as embellishment. Pay careful attention to the temperatures and baking times of your specific brand of clay; if you heat it up too fast, the surface will harden before the inside gets a chance to. When in doubt, use a lower temperature and a longer baking time.
If your polymer clay is attached to a prop that cannot fit in an oven, you can use a hot air gun to heat and cure it.
Polymer clay is typically used for jewelry-sized pieces and embellishment. Anything thicker than 1¼″ to 1½″ (3 cm to 4 cm) is likely to crack while baking.
I must point out that an oven or toaster oven used to cure polymer clay should never be used to heat food again. The same is true for using an oven in any craft capacity (other than clays made entirely from flour and water or other food substances). Heating plastics releases fumes and chemicals into the oven. These can remain in there forever, and every time you heat food in there, the chemicals will propagate into your food.
Figure 13-45: These olives for fake martinis were sculpted with two different colors of polymer clay. Photo courtesy of Anna Warren.
Epoxy Clay
Epoxy clay is a two-part clay that hardens a few hours after mixing together. Epoxy will cure in the presence of water, so you can use water to smooth as you work. Shrinkage is very minimal, and it adheres to many other surfaces, making it ideal for applying sculpted details to an existing prop.
You can machine, sand, and file the dried clay. You can also push it into a press mold to shape it.
Popular brands include Apoxie Sculpt, Milliput, Magic Sculpt, and Smooth-On Free Form. You can also use the small sticks of plumber's putty or repair putty in a pinch.
Figure 13-46: Like most epoxy clays, Apoxie Sculpt has adhesive properties, so you can apply it directly to materials like wood to give sculpted details.
Baker's Clay
Baker's clay (also called baker's dough or salt dough) is a homemade material made of flour, salt, and water. It is a useful material if you absolutely need to avoid toxic materials; it is also handy for making fake food props that are intended to look like baked goods.
The basis of the recipe is flour (avoid the self-rising kind), salt (the finer the better), and water. Various recipes can be found online, but the most common involve 4 parts flour, 2 parts salt, and 1½ parts water. Food coloring or powdered sugarless drink mixes can be added for color; a bit of corn starch will help maintain the color while baking. Vegetable oil can be added to increase the working time.
The dough can be mixed cold or hot. Cold mixing involves mixing the flour and salt, and then gradually adding the water and kneading until smooth. Hot mixing can be done one of several ways. Some recipes involve mixing the salt in boiling water, cooling it, and then kneading the flour into it; others involve mixing all the ingredients together and stirring them over a low heat until they dry out and cease to be sticky.
The pieces can be air-dried (which may take two to three days) or baked in an oven. Thin pieces should bake at 150°F to 200°F (65°C to 95°C), while thicker pieces require temperatures of 250°F to 350°F (120°C to 175°C). It typically takes anywhere from one to three hours to bake fully; if the pieces start to brown, they are heating up too quickly.
The pieces can be sealed with shellac or any of the spray clear coats, or even just latex paint.
Polycaprolactone
Polycaprolactone (PCL) is a plastic that becomes soft and pliable at 140°F (60°C), making it possible to soften in boiling water and safely sculpt by hand without injury. You may recognize one of the many brand names, such as Friendly Plastic, InstaMorph, Shapelock, and Polymorph.
They often come in pellets or strips in a variety of colors. Their cost and workability make them more suited for jewelry and hand-sized objects, though pieces as large as masks are possible.
Drop the pellets or strips into a pot of boiling water, and wait for them to turn clear. Pull them out and begin shaping the piece with your hands. Organic-looking shapes are easy; more refined pieces are trickier. You can do minor machining to the final piece, such as drilling or sanding, but these processes can quickly heat the plastic back up to its softening point.
Joining pieces is difficult when they are cold. You can reheat the two pieces, but they will lose some of their shape in the process.
PCL can stick to many materials, making it useful to add sculpted details to a larger piece. It is especially good at making a strong but reversible bond with PVC.
You can also smash together scrap pieces of Worbla (or other thermoform sheets that don't have a fabric mesh) and use them as a similar sculpting medium.
Wax
Wax is a substance that is stiff around room temperature, but melts at a low temperature. Jewelry wax refers to any number of waxes, that can be easily carved, sanded, machined, and drilled. Their low melting temperature also allows you to smooth the surfaces with heat, or manipulate the wax like a liquid in a small area with a heated tool. It is preferred, unsurprisingly, by jewelers because it allows very precise and fine detail on a small object, which can be difficult to sculpt in clay.
Figure 13-47: The fillings of these sandwiches were made from paraffin wax tinted with small shavings of crayons. The lettuce on the outside is paper towels dipped in tinted paraffin wax that was scrunched up while still soft. Prop by Lisa Bledsoe.
Wax allows you to hold a small object from any side while working without distorting the other sides. This level of detail on such a small scale is usually unnecessary for theatrical prop making, but for small props in film and television that will be filmed up close, it is a great material to model in for later casting.
Sculptors who work in wax will often sculpt the basic figure in clay, make a waste mold, then cast the piece in wax, and work on the finer details.