Semi-hydrated calcium sulfate, or the powdered form of gypsum with half the water of crystallization driven off by heating. When water is added back into the plaster the calcium sulfate recrystallizes as it takes back the water it lost and could be said to have returned to its original gypsum state.
Plaster was known in early Greek and Roman times but it was not widely used in the ceramic industry until the early 18th century when potters brought the method of using plaster molds for production from Paris to England in an effort to improve their mold techniques. Even after the production of plaster started in England the term for the plaster brought from Paris stuck and today we refer to any pure plaster material as Plaster of Paris.
No. 1 MOULDING: An all purpose, “Plaster of Paris” type soft plaster. Used for waste molds or temporary patterns where surface hardness and strength are not important. Casts are very porous and must be sealed before decorating.
MIX: 35 lbs. water to 50 lbs. plaster. SET TIME: 27 to 37 min.* DRY TIME: 12 to 20+ hours. DRY STRENGTH: 2000 psi
WHITE ART: Similar in working properties to moulding plaster, White art plaster contains a surface hardening agent that minimizes paint absorption when dried cast is finished. Excellent reproduction of intricate detail. MIX: 35 lbs. water to 50 lbs. plaster. SET TIME: 27 to 37 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 2000 psi
No. 1 POTTERY: The best material available for sanitary ware and dinnerware casting. Formulated to produce strong molds with long life and reduced breakage. Highly uniform, noted for outstanding performance.
MIX: 35 lbs. water to 50 lbs. plaster. SET TIME: 27 to 37 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 2400 psi
PURITAN POTTERY: Unique, specially designed for use with mechanical clay forming machinery. Absolutely the hardest, most wear-resistant plaster made for molds. Ideal for jiggering applications. MIX: 33 lbs. water to 50 lbs. plaster. SET TIME: 27 to 37 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 2700 psi
No.1 CASTING: The industry standard for figurines, plaques and lamp bases. Special additives maintain smooth working qualities and upon drying results in a hard working surface with reduced paint absorption and a high degree of chip-resistance.
MIX: 32.5 lbs. water to 50 lbs. plaster. SET TIME: 27 to 37 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 2400 psi
LABORATORY DENTAL: A porous plaster that produces casts of nominal strength and hardness with accurate reproduction of most intricate detail. Fast set time for the dental industry. MIX: 35 lbs. water to 50 lbs. plaster. SET TIME: 6 to 9 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 2000 psi
HYDROCAL WHITE: A good multi-purpose gypsum cement that is very white in color, offers a gradual setting time and has a long period of plasticity. Ideal for both solid and hollow casting of lamp bases and figurines. Especially designed for thin sections which require high green strength to minimize breaking during removal from intricate latex molds. Moderate carvability. MIX: 22.5 lbs. water to 50 lbs. plaster. SET TIME: 25 to 35 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 5000 psi
HYDROCAL 105: A formulated hydrocal cement for dental stones with controlled expansion. Colored yellow for identification purposes. MIX: 20.5 lbs. water to 50 lbs. plaster. SET TIME: 7 to 10 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 6000 psi
ULTRACAL 30: A super strength gypsum cement recommended where extreme accuracy and greater surface hardness are required, as in mother molds. It has the lowest expansion of any fast set gypsum cement, a gradual set and long period of plasticity. Ideal for splash-casting molds and will accept tooling. MIX: 19 lbs. water to 50 lbs. plaster. SET TIME: 25 to 35 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 6000 psi
HYDROSTONE: A high strength gypsum cement with good resistance to wear. Used in high quality art novelty and statuary castings. Works well in most mold materials, especially flexible molds. Extremely fine detail duplication, self leveling, not carvable. Not designed for structural or outdoor applications. Best when mechanically mixed. MIX: 16 lbs. water to 50 lbs. plaster. SET TIME: 17 to 20 min.* DRY TIME: 12 to 20 + hours. DRY STRENGTH: 10,000 psi
* Set times may vary due to water quality, mixing, soaking style and temperature variations. 1 gallon of water typically weights 7 pounds.
PLASTER POINTERS
The quality of the water used to mix plaster will have some impact on the finished product. Generally speaking, if the water is drinkable it should be suitable for plaster, however high mineral content and the hardness of the water may be problematic in which case filtered water may be needed. Cold water will slow the set times and warm will speed them. The longer the plaster is mixed, the stronger it becomes, but be aware that it will also shorten the setting time.
WARNING: All plasters generate heat during setting that can cause SEVERE BURNS! Never encase body parts in gypsum plasters or cements.
STEPS FOR MIXING:
1. Sift or strew plaster into water slowly and evenly. Do not drop handfuls directly into the water. 2. After all the plaster has been added to the water allow it to soak undisturbed for 2 to 3 minutes (this is an important step as it allows the plaster to evenly hydrate without forming lumps) 3. Mix 2 to 5 minutes until a creamy consistency is obtained. 4. Tap the side of the mixing container with a wooden mallet or stick to release any air bubbles that might have formed during mixing.
Mechanical mixing is the best way to insure a consistent result, but hand mixing works well with small batches.
Plaster will adsorb moisture form the air thus giving it a shelf life of 6 to 10 months once the bag is opened.(depending on where you live) To increase the life span of your plaster try buying only what you think you will be using at the time and store any excess in an air tight container.
