The Plaster FAQ—Working With Plaster

Plaster is made from the mineral gypsum, which is called Calcium Sulphate by chemists (Gypsum in the ground is calcium sulphate dihydrate, after calcining, it becomes calcium sulphate hemihydrate, also known as hydrous calcium sulphate.) It is often called plaster of Paris, because of the large deposits of pure gypsum underlying the French capitol, which were utilized early on by local artisans. When heated, the mineral loses some water which is chemically bound into it and gives it its hardness. After it is cooked (“calcined”), it is easily reduced to a powder, but the addition of water will reconstitute its original hardness, once the material sets.

This remarkable material is stored as a powder, and is mixed with ordinary water into a liquid that gets gradually thicker and thicker until it becomes plastic, then a paste, then a cheese-bodied mass; finally turning rock hard in about an hour. Unlike practically any other compound, when plaster turns from liquid to solid it does not shrink, rather, it expands ever so slightly as it forms crystals. It can be carved, sanded, drilled, cut, textured, added to, reinforced and remain incredibly strong throughout. Because it expands, plaster castings do not lose any detail, and a mold with a glass smooth surface will result in a glass smooth plaster casting.

Plaster absorbs water and can therefore be used to make molds for casting porcelain and ceramic slipware as well as latex forms such as those aliens and monsters used in the Star Wars movies. Plaster is used extensively in the manufacture of pottery, and plaster is used to cover the walls in quality homes. Plaster can be “screeded” with a template to form straight and curved patterns with any cross-sectional contour. At one time, virtually all decorative moldings in houses were of plaster and, of course, plaster is the principal material of pattern making and foundry processes. In moldmaking, plaster is used to make mold casings (mother molds), waste molds, casting and retouching masters.

While plaster can be cast into plaster molds if they are sealed and well-soaped, it works best in flexible molds. These were originally made from animal gelatin, which was superseded by vinyl hot-pour, which in its turn was made obsolete by the modern synthetic rubber molding materials based on silicone and urethane. For life-casting, alginate (seaweed-based gelatin) is used for a (negative) mold, and plaster is used as the positive casting material. Plaster molds can also be used to cast wax, if the mold is well-soaked beforehand.

Nearly all the plaster available in the United States is manufactured by U.S. Gypsum, (USG). There are quite a few different kinds. Pottery Plaster, Number One Casting Plaster, Art Plaster, Hydrostone, Ultracal 30, Ultracal 60, Moulding Plaster, and so on. Each of these products have different characteristics that make them well suited to particular tasks. Some have surface hardening agents that make for a more chip-resistant casting, others have fillers and binders that lower their cost, while still others are formulated to maximize their absorbency for slip casting ceramic products or are specialized to minimize their coefficient of expansion. All in all, the best all-round, multi-purpose plaster I have ever found is USG’s Puritan (Number 1) Pottery Plaster. If you will be running a one plaster bin shop, fill your bin with Puritan.

Plaster can also be alloyed with plastic resins, such as melamine, to form “Modified Gypsum” cements, which combine plaster’s ease of use with plastic’s durability and water resistance. Forton MG is one of these alloys, and there are others as well. It can also be mixed with Portland cement, to produce a product with qualities intermediate between the two materials. Mixed with glue and an inert filler, it becomes “composition”, which found early use as a material for picture frames and other small decorative items. Mixed with sand or another refractory aggregate, it is used as “investment” for lost-wax casting, and will hang together just enough, after being brought to more than 1000 degrees F, to hold molten bronze and impart shape and fine detail, while still being friable enough to remove easily from the casting. (Premixed investment plaster is still generally used by the jewelry industry, while its use in the art bronze-casting industry has been largely superseded by ceramic shell).

Buy plaster from ceramics suppliers in 100 pound bags. It costs between 7 and 18 cents per pound. The plaster sold in hardware and paint stores, as well as being much more expensive by the pound, is often stale, and is generally formulated for patching walls, not casting. Because it is hygroscopic (absorbant of moisture), the powder will draw in water from humid air and this will cause it to “perish” ( get too old and gritty to work with) So, plaster should be stored in an airtight container—a 16 gallon Rubbermaid container will hold one full bag.

On every bag of plaster are big letters that say Warning! May Cause Severe Burns. This is because as plaster sets it generates heat. The exothermic reaction, as its called, doesn’t really set in until after the mixed plaster is pretty hard. Why I’m telling you this is the same reason USG prints the warning on the bag: many people think they can make an easy mold of a hand or a face or some other body part by sticking it in a bucket of plaster or slopping plaster all over it and waiting for it to set. Just after the plaster gets too hard for you to pull your self out it begins to get hot—then it gets hotter. People have been badly burned before being extracted. While it isn’t considered a toxic material, it can dry out and irritate eyes and skin. It also isn’t a good idea to inhale a lot of it, so a good dust mask should be used when mixing it.

Be aware that many bubbles in plaster castings from rubber molds are caused by water beading on the surface of the mold. The surface tension of the water prevents the plaster from mixing with the water bead. Once the plaster has set—it sucks the water up, leaving what looks like an air bubble. Combat this by rinsing your mold in a surfactant, something that causes water to sheet instead of bead. Permaflex Mold Co. of Columbus Ohio make a product called mold dressing for just this problem.

Other holes in the casting’s surface are actually caused by air bubbles, either entrained in the mixing process or during pouring. Professional shops often use vacuum chambers to “boil” the wet plaster at room temperature, displacing any entrained bubbles, or pressure vessels which force them to the surface. Lacking this equipment, most amateurs endeavor to avoid trapping air by mixing and pouring as gently as possible, avoiding vortex and splash. If you do decide to set up a vacuum de-airing chamber, use a piston-type vacuum pump, (Kinney is one brand) which won’t corrode internally from the water vapor entrained in the pump oil. The vane-type pumps (Welch is one brand) have a problem with this. (If all you have is a vane-type pump, use an inline desiccator to dry the air coming into the pump, and change the pump oil and desiccant frequently.) You will need a pump capable of boiling water at room temperature, which happens at about 28 hg. (inches of mercury). Since the plaster rises considerably in the bucket under vacuum, don’t fill it more than about 1/3 full before vacuuming, at least until you’ve determined by experimentation that a given size of mix won’t overflow.

Consistency is very important when dealing with plaster, so when you find a technique that suits your application, make a note of it, writing down how much of what you mixed together, any variable factors like how long you mixed it, warmer or colder water, old vs. new bucket, how the plaster felt—smooth or lumpy—things like that—and how things worked out. That way, if a mix turned out well, you can duplicate your results pretty closely. It’s important to get a sense of material’s setting curve. Plaster has a certain short period of plasticity, when it can be worked by hand and will stay where it is put, even vertically or upside down. But this setting curve is different for each type of plaster. Learn what you can do with it at each of its constantly changing stages. It’s pourable, stackable, toolable, and eventually immovable. After it has set, but before it hardens fully, it can be manipulated with steel tools, some of which, like sets of curved toothed riffler/scrapers, are specialized for the job. If you plan to carve the plaster directly when semi-soft, try a hard plaster like Hydrocal White, which is recommended for its toolability. It is best to execute any of these refinements soon after setting occurs. After a few days it will dry out. Carve and scrape when it’s wet; saw, drill and sand it when it’s dry.

You will need buckets to mix your plaster in. My favorites are those high density polyethylene (HDPE) 2 and 5 gallon buckets you can buy at the hardware store (or scrounge from a food-service company or hotel laundry (they get detergent in them). Be sure to cut the reinforced and ribbed rim off the buckets. This makes the bucket more flexible for breaking out set plaster dregs, and less likely to crack. I have buckets like this I have used for 5 years. Some people rub a light coating of petroleum jelly on the inside of the bucket every so often to facilitate removal of the old plaster, but I never grease my buckets—I find that, by avoiding scratching up the inner surface of polyethylene buckets, the set plaster just falls out. Restaurant supply houses sell a container that holds about 40 quarts. They’re great for mixes of twenty pounds and up. Rubber balls, cut in half, can be used for small batches. Set plaster pops out by turning the ball inside out.

My advice here is mostly based on artists’ needs, who seldom need industrial quantities mixed. Make a mixer by bending the end of a ½″ diameter round-section steel rod into a rectangular shape with radiused corners. (This will work on rubber too.) Fitted into a heavy-duty electric drill, this will scrape the sides and bottom of your bucket without scratching as much as a propeller-style or Jiffy mixer, the commercially-available stainless steel cylindrical mixers.

Most professional shops casting production quantities of gypsum products (that I have seen or consulted with) use an industrial screw mixer that mixes the material without introducing significant air. The mixing head is a tube with a wormy kind of crankshaft down the center. As long as operation is continuous, everything is fine—the instant you stop mixing, you flush the head with water and then break it down and spend about 5 minutes cleaning the screw and cylinder. At one company I set up a mold shop for, they had an overhead conveyor feeding rubber molds to the mixing station. They used a mixture of sand, hydrostone and cement and poured 3,000 figures a day. The flush water has to go thru a set of settling tanks and a silk filter before going down the drain.

USG offers tables that will show you precisely what weight of plaster to water will get you the best compressive strength. But most experienced plaster workers do it by eye. Mixing plaster is a pretty straightforward proposition; first get out one of those plastic buckets I told you to buy and fill that bucket to a little more than half full with water. This will give you a full bucket of mixed plaster once you’ve added the plaster. In fact, however much plaster you think you might need you will have to start with somewhat more than half that amount of water, as the plaster you add will almost double the volume of the water.

Using your hand, start adding the plaster to the water by sifting it through your fingers handful by handful. You want the plaster to sprinkle onto the water’s surface to ensure that it gets thoroughly saturated. This helps to minimize air bubbles in your mixture and gives you a better-bodied plaster. (We casting masters use a flour sifter to ensure perfect saturation.) As you add the plaster to the water, do not stir it!

When the level of plaster remains just a little higher than the water’s surface, hold up and let the mix settle a bit, adding more plaster when there is clear water visible. Ever seen a mud flat when the water starts to dry up and cracks begin to appear? You want the small island of plaster that won’t settle below the water to look like that; then you’ll know you have enough plaster in your water.When you want to minimize air bubbles in your plaster, say for casting a master, you can let the plaster sit for five minutes before mixing it. The use of a vibrator motor on the outside of the bucket can help bring trapped air to the top, or failing that, one can simply kick the bucket gently as one waits.

The rate at which plaster sets is directly related to how long it is mixed. The longer you mix your plaster, the faster and more homogeneously it sets. This kind of mix is very desirable when you’re casting a pattern or a master; it makes for easy, controllable retouching and carving. However, when making a plaster casing or other part that you will be laying up by hand, then stir the plaster very briefly. By mixing your plaster lightly you will be given up to fifteen extra minutes of precious time in which to work. This is the central fact that you must work hardest to overcome: once the plaster is mixed, it is going to set. And the plaster doesn’t care if you’re done or not. The oil in your skin, the dirt in the bucket, the minerals or salt in the water, chips of set plaster—even looking too hard at it—will accelerate the setting of mixed plaster, and cut into your working time, whereas lemon juice, vinegar, and anything else acidic will retard the setting of your plaster. A solution of sodium citrate is often used (sparingly) as a retarding agent, as are certain proprietary powders. Be careful—a little too much acid and it won’t set at all. In general, do not try to buy time by putting lemon in your plaster. It will yield weaker plaster that can be sheer hell to work with.

Plaster mold casings ( the “mother mold” that holds the rubber) can be made stronger by reinforcing the plaster with “Sisal”. Sisal or “Manila hemp” is the raw fiber from which manila rope is made. Look for it at construction materials supply houses. They may call it hemp, or casting fiber or by some other term. A bale of Sisal will set you back about $150 but will make100 or so average sized molds. Others like to use fiberglass-either loose chopped strands or loosely woven scrim cloth—or 7 oz. burlap (the more common 10 oz. burlap is too tightly woven to be useful). In general, the woven materials are best for larger or simpler molds, the non-woven ones conform better to the concavities of a small or complex one. Larger molds may benefit from more rigid reinforcement as well as the use of harder plaster, like Ultracal 30 to allow thinner molds. (This is more difficult to work with, as the setting curve is more abrupt.) You can use steel for reinforcement if it is galvanized. Uncoated steel will rust and crack the plaster as it expands, wood absorbs water and acts similarly. Copper tubing, though relatively expensive, is easy to conform to a desired shape.

Large sculptures can be made directly in plaster by constructing an armature—wood and chicken-wire are commonly used—and applying plaster-soaked cloth or sisal. These are not suitable for outdoor exposure, however, since the action of rain will quickly erode the plaster. They may, however, be refined and used as models for molding and casting in other materials. The most common sculptural use of plaster is for making models, small castings, ceramic molds, and casings for rubber molds.

To make a mold casing with plaster, you will need to do it in layers. Start with a putty or “face” coat of pure plaster, topped by 1 or 2 layers of plaster reinforced with sisal, and finish with an optional “gloss” coat over that to smooth it and make it pretty. You do not want to use separate mixes of plaster to make the separate layers of the casing. Because plaster expands when it sets, the second layer would continue to expand after the first layer has set and cause the mold section to curl and warp. Instead, you have to use a single mix of plaster to do all the different layers. You can do this by “fractioning” your plaster.

Here’s how: after you sift the plaster into the water, don’t mix it. Instead, pour off a section into a separate bucket and stir this up well, leaving the remainder in the original bucket unstirred, then apply this first section to the mold, flopping it over the surface by hand, creating a nice clear putty coat. (Properly done, the action of flicking the wet plaster onto the surface of your model will force the material into all the crevices and cavities, excluding pockets of air.) This will take some patience, you will have to stand and wait for the plaster to thicken a little so that you can build up the putty coat to at least a quarter inch thick.

As soon as it is somewhat firm, go back to the original bucket and pour off another section into another clean bucket. Since the action of stirring accelerates the setting of plaster, the unstirred plaster will still be liquid. Don’t even bother to stir this second section, since the act of dunking sisal in it will stir it up enough. Use this section to apply a mat-like layer of plaster-soaked sisal over the entire putty coat to reinforce it. Because the first section has thickened up, you will get no sisal coming through the putty coat. Once you have covered the putty coat, apply a thick, ropelike layer of plaster-soaked sisal just around the edges of the mold section. If the mold will be cast upside-down, use the thickening plaster and sisal to form feet for it to stand upside down on. You will have to move pretty quick to get all this done defore the plaster gets too thick to work.

Keep in mind that plaster sets faster on your hands—keep a rinse-bucket full of water nearby and rinse the plaster off your hands and tools between each layer. It has a special affinity for the hair on ones arms, (and ones head, if it can get there) and can be painful to remove if it gets a chance to set hard. Do not rinse plaster off in a sink. It will set in the pipe and that will mean digging it up to repair it.

As the second section begins to get too thick to work, go back to the original bucket—hey! Still liquid! Use it to do any last minute sisal work, to finish the casing and level the feet. Square up the thicker edges of the mold; in a multipart casing, make them square so that “C”clamps can hold the mold together. This technique can double or triple the working time of plaster. Because the entire body of the plaster is one mix, all three sections achieve ultimate cure at very nearly the same time and all have almost identical coefficents of expansion. This minimizes or eliminates the warping that occurs whenever you laminate two separate mixes of plaster.

Other considerations in working plaster: Avoid acute (sharp) angles—try to bisect every angle and separation line. A square edge is stronger and less likely to chip or crack than a narrow pointy one. Try to make the edges of a mold uniformly 1 ½ to 2 inches in thickness, keep the field ½ to ¾ inch thick. This helps reduce warpage and distortion and makes for a far stronger shell.

If you are making a “waste mold”, that is, a mold made entirely of unreinforced plaster that you will chisel off of the casting (which can also be of plaster or of another material) then add a mortar dye pigment (sold as concrete coloring—red or black iron oxide from a ceramic supply is also adequate) to the putty coat as you stir it. When you are chiseling off the mold later, the color coat will warn you when you are getting close to the surface of the casting.

To separate set plaster from a new section of plaster, as along the separation line of a mold, you must apply a separation agent. For plaster the ideal separation agent is mold soap, also called green or potter’s soap. You can get Purelube mold soap from most ceramic suppliers. Soap is ideal for ceramicists because it will not transfer to the cast part—that is, plaster poured or cast against a soaped surface will not pick up or absorb any trace of the soap. This retains the plaster’s ability to absorb water. Soap can be a pain though because after you lather up your separation line with soap you must wait for nearly half an hour for it to dry before proceeding, if the mold is fresh. For this reason lots of people use petroleum jelly to separate plaster from plaster. Slather it on—then use a paper towel to get as much back off as you can, leaving a thin sheen of grease. You can then immediately cast plaster against this surface. Do not wait an hour—the plaster will absorb the Vaseline and you will no longer have a good separation.

I always recommend sealing the plaster first by painting the separation line with orange Shellac (the real stuff with beetle wing bits in it—make it up fresh each time from shellac flakes and denatured alcohol, since it goes bad quickly once mixed, and then remains sticky). This stops the plaster from absorbing anything, and it gives you a visible orange line to help you gauge how thick the edge of your plaster is as you lay it up. However, be sure you don’t forget to Vaseline or soap the shellacked suface, because, if you don’t, shellac will give you the most perfect bond between two pieces of plaster. Indeed—if you ever want to add plaster to an existing plaster casting, you have to seal the area you want to add to with shellac first. This will keep the old plaster from sucking the water out of the fresh plaster—allowing it to properly crystalize. If the casting is new and still fairly damp, you can patch or add plaster to it by soaking the area in water first, again, so that the old plaster ( pre-saturated with water) will not suck water from the fresh, which weakens it dramatically.

Shellac is not entirely waterproof, however. Wood finished with shellac can continue to absorb or release moisture in response to changing environments—it just does so over weeks instead of hours—making the wood more stable. This is why violins and other instruments are finished with shellac. Also—shellac is soluble in alcohol, and can be removed from wood even decades later without harming the wood. In plaster-working, when adding fresh plaster to set plaster, you do not want the set plaster to absorb the water in the fresh plaster, but you don’t necessarily want it to be vapor proof. Shellac inhibits the flow of water across the join so that the fresh plaster can properly crystalize. However—you do want both bodies of plaster to be able to breathe so they will dry completely, so don’t put shellac anywhere except on that area where you will be adding fresh.

To patch bubbles and small flaws in plaster, open them with a fine-pointed steel tool. With a small amount of mixed plaster (mixed with a slight excess of water) a spatula, and a bowl of water and a sumi-e brush, wet the hole with water, spatula in some plaster, then use the wet brush to brush off the excess and shape the plaster. If you’re making major additions to a plaster part, avoid adding plaster in a fashion that results in the added plaster getting thinner and thinner to a feather edge. You will not be able to sand or carve across such an edge smoothly; the feather edge is sensitive to the slightest difference in hardness between the new and old plaster. Rather, determine where the edge of the patch will be and engrave deeply and roughly into the plaster, about ¼ inch all along this transition area, so that when filled with plaster, the plane of the joint at the edges will be perpendicular to the surface. Then shellac the area to be patched. This adds what is called a good ‘mechanical’ attachment and results in a transition between new and old that can be finished like glass. Be aware that plaster expands when it sets and the addition of more plaster to the surface of a large area, or a thin piece, will cause some warpage of the work.

Following a good plaster-casting technique called “slosh” (or slush) casting, the rubber mold, rinsed in a surfactant, is filled with plaster, then it is inverted and most of the plaster is poured out. The mold is then partially filled with plaster and “sloshed”, that is, picked up and moved violently in a circular motion in a plane parallel to the ground. (It helps if you’ve got a guy named “Godzilla” in the shop to do this). This action causes air to move toward the center of the mold and away from the surface—like the pulp when you stir orange juice. The mold is then topped off, making sure you pour carefully to avoid turbulence. Professional shops use rotational casting machines, which hold the (sealed) molds in gimbaled arms that mechanically slosh them around until the material inside sets, thus achieving a hollow casting. But most artists work on a shoestring.

I have slosh-cast life sized ponies by myself, rigging a simple hoist and sling tied to the mold just a wee bit above its center of gravity (when half filled.) I could rotate it fairly freely in this way. I once created a plaster casing around a piece of greased electrical conduit set in off to one side and tied into the plaster casing like a hinge pin. This allowed me to cast an heroic figure singlehandedly in plaster by laying the mold open and casting both halves of the figure with a putty coat reinforced by sisal. I then shellacked the separation line, slapped some thick plaster on it, and hoisted it up and let it swing shut on its hinge. Clamp it, Slosh it around. Perfect. I have also designed production mold systems with casings incorporating circular flanges in the plaster that allow a mold to be ‘rolled’ on the table without picking it up. Some sculptural configurations won’t pour correctly from the top, or need vents as part of the mold, but these can often be averted by using rotational techniques. Moldmakers need to be creative in solving problems and anything that works is gold.

Plaster castings and casings can be “tempered” by wetting them thoroughly within a few hours of setting. Like with concrete, this helps cool the plaster and extends crystallization, resulting in a denser final part. Plaster castings need to be completely dry before painting them. This takes fifteen to twenty days in a normal environment—longer in a humid one. You can force-dry plaster by putting it a room with a constant stream of warm dry air—like from a gas furnace. Do not let the room exceed 125 degrees F. This will dry plaster overnight. If you are planning to paint it, seal the surface by applying a coat of shellac diluted with alcohol and letting it sink in, following with heavier coats until it lies on the surface, then let it dry before applying paint.

The very best plaster-working tools in the world are made by Caselli in Italy. While the surviving Caselli brother has retired, the last time I was in Bagni di Lucca his apprentices were still cranking out the finest steel tools anywhere. Mold knives, spatulettas, carving tools, rifflers hooked and toothed, mixing paddles large and small—they make it all, even including stone chisels and a damn fine Parmesan (cheese) knife. Anyone who has worked with these tools is spoiled for life. They are hard to come by in the states. An art store called Flax in Los Angeles will order them for you. (none of the other Flax stores will; and the catalog of that name doesn’t mention Caselli’s) The second best thing to a Caselli tool is Milani; also Italian. You will find that the temper is not so perfect as a Caselli and they often come without a finished edge—but they’re better than the knockoff tools Sculpture House sells. Montoya Sculpture Studios in Florida imports Milani tools.

Plaster does dry out the skin terribly, and on a cold day, in an unheated shop using cold water, moldmaking can be a freezing hell. My woman bought me the greatest gift a moldmaker could get. It’s called a “Therabath” and it’s a big electrically heated tub full of (barely) molten paraffin wax with some aromatic oils added. After a full day of moldmaking—which always makes you feel like someone has been beating you with a pipe, there is nothing so nice as dipping my hands into the hot wax, over and over, building up a nice warm thick layer of oily wax. Ten minutes of this and my hands feel great and are as soft as a baby’s bum.

One more thing—when you carve and sand plaster, you will, sooner or later, be confronted with a lot of plaster dust and chips obscuring your view of some recess. You will naturally blow the debris away. this is the first lesson any apprentice learns—Before you blow, close your eyes. (You will thank me for this tip.)

by Christopher Pardell

With contributions from Andrew Werby and Tom Battersby