The human relationship with plaster is ancient, and can be traced from 4000-year-old Egyptian walls to modern industrial applications. This fast-working material is easily manipulated from fluid slurry to the hardened stone, proving it to be an indispensable studio material.

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Defining the Terms 

• Calcination—Applying heat  to a substance so that it oxidizes, reduces, or loses water.
• Efflorescence—A whitish, powdery deposit on the surface of rocks or soil in dry regions usually consisting of gypsum, salt, or calcite. Powder is formed as mineral-rich water rises to the surface through capillary action and is exposed to air and evaporates.
• Exothermic—A chemical change that releases heat.
• Gypsum—(Calcium sulfate-CaSO4 2H2O), a white mineral found in sedimentary rock, few deposits in North America are large enough for commercial mining.
• Hygroscopic—To take up and retain moisture by all contact with water.
• Slake—Combining clay chemically with water in preparation for a solution.
• Slurry—A homogeneous and fluid, molecular mixture. With regards to plaster, it is the fluid mix of plaster and water anticipating a chemical reaction.

Good Plaster 

Plaster as we know it today comes from the mineral gypsum (chemically known as calcium sulfate). Gypsum crystals are mined all over the world and are processed for a variety of architectural, artistic, and construction uses. Gypsum is considered a sedimentary rock, and a water-soluble evaporate that crystallizes through the evaporation of trapped water. These crystals take on many forms and colors depending on where they are found.

From gypsum there are primarily two types of plaster, alpha and beta. The alpha plaster is gypsum that has been made into a watery mixture then heated in an autoclave under contained pressure. This plaster, upon setting, forms long, straight crystals that can become very hard. Beta plaster is processed by heating the powdered mineral in open containers, which cause it to dehydrate under atmospheric pressure. By either of these two methods, calcination of the gypsum occurs. Upon the reintroduction of water to the calcined plaster, re-crystallization happens, returning it to a rock like material. The size and shape of these crystals plays a large role in determining how the plaster performs. Of these two primary plasters, many other formulas have been engineered for specific applications and hardness.

Now that you know all about plaster, make some multiples!
Guy Michael Davis gives you all the information you need on his DVD Fundamentals of Mold Making and Slip Casting. 

An intriguing attribute of plaster is that it can be recycled indefinitely, by the same methods of the processing, but its availability and low cost outweigh the labor of reclaiming it.

Bad Plaster

Plaster’s hygroscopic nature makes it sensitive to environmental factors such as humidity. It should always be stored indoors, in a dry place, and used as soon as possible. Large amounts should be placed off the floor (i.e., on a pallet), as it will draw moisture from the ground and cement floors. Open and exposed bags can begin absorbing atmospheric moisture within weeks, eventually hardening and forming rocks. United States Gypsum Company (the primary producer of plasters in North America) estimates that there is an approximate six–month shelf life when stored under normal conditions (regions and seasons with high humidity may have a shorter shelf life) and bags are marked with the manufacturing date. Smaller amounts keep well in sealed plastic buckets and can last for well over a year.

The hygroscopic nature of plaster is also the reason we need to keep it from getting into our clay—it takes on water, expands, and then cracks our work. Once the plaster has been exposed to moisture it begins a chemical change. There is nothing more frustrating than opening a bag of plaster to find a bunch of hard chunks inside. Good plaster should be fine and soft, almost the consistency of flour, on the other hand old plaster will become coarse and have hard spots from the size of sand and rice up to rocks the size of baseballs and larger. Only in a pinch should aged plaster be used and it should be first pushed through a screen to separate any hardened areas. The difference in how it works can be noticed in all working stages from mixing and setting to the final result.

Water Considerations

Each type of plaster is mixed with water at specified ratios depending on its engineering and use—No. 1 Pottery Plaster is 100 parts plaster to 70 parts water (Hydrocal-White: 100 plaster to 45 water; Hydrostone: 100 plaster to 32 water; Ultracal 30: 100 plaster to 38 water). Water quality should be considered as it can affect the reaction. Simply, if you would drink the water, you can use it in mixing a satisfactory plaster batch. Large amounts of impurities and soluble salts in the water will affect the plaster slurry set time. These salts travel to the surface and the resulting efflorescence can cause hard spots on the plaster surface. Any inconsistencies in the plaster’s density can result in uneven water absorption from clay, resulting in inconsistent hardening and, in the case of slip casting, uneven thickness in the cast clay object.

Water temperature also impacts setting time. Room temperature water is ideal for mixing, but it is ultimately up to the user to standardize their process based on desired effects—the warmer the water, the faster the plaster will set up, while cooler water slows set time. It is important to know that plaster strength increases with the length of the time it takes to set up.

Mixing plaster slurry involves a period called soaking. During this time the plaster disperses evenly in the water, removing air and allowing the plaster to be absorbed by the water. The plaster should sink slowly into the water, allowing the particles to become completely saturated over the course of four minutes or less, depending on the temperature. Inaccurate soaking time adversely affects the mixing stages.

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Accurate Mixing 

• On a calibrated scale, weigh out the proper amounts of plaster and water in separate dry containers.
• Gently but swiftly sift all the dry plaster into the water, beginning the soaking period. Avoid dumping in large clumps at once, even plaster dispersal and water saturation is the goal. Always add the plaster to the water rather than the inverse. If the water is added to the plaster, a re-hydrated barrier will form leaving dry clumps stuck to the bottom of the mixing bucket.
• Soak 2–4 minutes. Nearly all of the visible plaster will become dissolved and the rising bubbles will slow.
• Mix for 2–5 minutes. Use a timer to make sure the soaking and mixing time are consistent with each batch. Batches under 5 pounds can be mixed by hand, but any larger amounts should be mechanically mixed using a drill and paint mixer. Hold the blade approximately 1–2 inches from the side and bottom of the bucket at a slight 15° angle, drill it downward fast but avoid whipping air into the mix. Mix aggressively for 1–2 minutes, then slow it down for the remainder of time to get air bubbles out. You should be working toward a creamy slurry. If you haven’t soaked and mixed long and aggressively enough, the particles settle and separate from the water and the finished cast is soft.
• Tap the outside of the bucket to release any trapped air.
• To gain a sense of timing the set of the slurry,  run your finger across the surface. A slight wake will notify you if it is time to pour. The more intense and permanent the line left behind, the less time you have before the mixture begins to harden; a deep, permanent groove may be too late while a slight record of the pass means it’s time to act.
• The chemical reaction that occurs when mixing plaster and water together is exothermic. High levels of heat are released upon the crystallization of the setting plaster.
• Be aware that there are safety and health concerns while working with plaster. At any point while handling plaster, use gloves to minimize contact. Burning and allergic reactions can occur, avoid exposing to skin, eyes, and lungs. Wear a dust mask; plaster contains silica that can lead to hardening of the lungs.
• Keep your buckets and tools as clean as possible. Too much cured plaster touching your mix can cause it to set up extremely fast. Keep a large drum of water in the studio where all tools and buckets can be immediately washed out. It is important to keep plaster out of the sink as it can build up and clog drainpipes over time.