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Plasters and their harder brethren, gypsum cements, are manufactured from naturally occurring, high-purity gypsum deposits. The gypsum is finely ground and the chemically-bound water is then removed through one of two calcination processes depending upon the type of plaster desired. The calcination process is one of heating the finely ground material which removes the chemically-bound water which then results in a different independent crystallline structure. The manufacturer controls the process to achieve different size and shape crystals which then results in different final characteristics. The term "gypsum cement" refers to harder and higher strength plasters manufactured by using heat and pressure. Industrial plasters require 65-160 lbs. of water to 100 lbs. of plaster to make a workable slurry whereas gypsum cements require 22-45 lbs. water to gain workability. With the reintroduction of water to any plaster, a new interlocking structure of needle-like crystals forms. This is a very complex process with many variables affecting the outcome from the grinding and calcination process to the reintroduction of water. Each step is critical to achieving the desired end product.

The general characteristics of plasters are:

• Ease and variety of uses

• Mixed as a fluid slurry, it can be cast or sprayed
• Worked in a plastic state by screeding, template forming or paddling
• As a semi-moist powder pressed between dies
• As a solid carved or machined

• Homogeneity with no grain, hard or soft spots
• Expansion from close to zero to 3/16 inch per foot
• Setting time from 3 minutes to 20 hours
• Strength and hardness from the weakest chalk to four times greater than concrete
• Absorption from that of a rigid sponge to an impervious surface that sheds water
• Density from that of popcorn to stone
• Compatible with other chemicals, aggregates, colorants
• Fire resistant and electrically non-conductive
• Non-toxic, non-allergenic, odorless & non-irritating to the skin with few exceptions
• Dimensionally stable under normal changes in temperature and humidity
• Extreme accuracy in reproduction

In addition, new plaster adheres to old plaster, especially if the old plaster is moistened and roughened. The bonded surface will be harder, however, which may be undesirable (on a mold, for instance)

Plaster goes through several setting stages after it is mixed with water. Different plasters and mixing procedures affect the time of each stage:

• 1-Liquid stage
• 2-Plastic stage. This is the "workable" stage when the plaster can be manipulated as desired. It can be hand modeled, paddled or carved to desired shapes and forms.
• 3-Final setup. This is when final crystallization takes place and heat is generated

Plaster types and Uses

It is very important to use the proper plaster or gypsum cement for the specific application and it is important that it be fresh and mixed properly. "Plaster of Paris" from your home supply store is not suitable for most ceramic applications as it may lack the strength, hardness, absorption and/or structural integrity needed. The shelf-life of quality plasters is approximately 6 months. It must be kept dry, preferably in a sealed container off the floor (it will start to "set" just by picking up moisture from the air). Also, it is a powerful flux at high temperatures, so be very careful to avoid any contamination of clay with even a small amount of plaster. Plaster pieces are generally not suitable for applications exposed to water such as fountains or outdoors. All are very fine plasters and pick up detail very well.

To assure freshness, we regularly stock the following plasters but can special order any of the dozens of others available.

• 20 Minute Casting Plaster – An economical general purpose industrial plaster suitable where expansion control, hardness, and strength are not of major importance. Working properties are similar to Hydrocal-White.

• Hydrocal-White – This gypsum cement has a gradual setting time and long period of plasticity and is recommended for both solid and hollow castings of figurines and lamp bases. Especially suitable for thin sections, it has a high green strength and can be carved and built up. High expansion.

• Hydrocal-FGR95 – Designed for use with the addition of fiber for strong, resilient products. Adaptable to deep patterns. Accepts most coatings. Expansion and strength influenced by fiber used.

• Hydrostone – Very strong and resistant to wear. Extremely high compressive strength makes it the ideal gypsum cement for ram press molds. Also used for high-quality art and novelty castings, molds, figurines, anchoring materials, density, fill applications, lamp bases, and case molds.

• Pottery Plaster No. 1 – The industry standard for slip molds, jigger molds, and wedging tables combines strength, surface integrity and absorbability.

• Ultracal 30 – Very low expansion and absorption for critical case molds and close tolerance tooling. Provides excellent hardness, accuracy and freedom from efflorescence.

• Ceramical – A low consistency form hydrocal compounded to produce a dense, smooth-wearing die material for mechanically pressing clay ware. It is characterized by low absorption, high strength and ability to purge easily, and resistance to abrasion and wear.

Specifications and Mixing

  parts water to 100 parts plaster by weight pounds plaster to 1 gallon water

Settimg Time

% Expansion

20 minute casting 70 11.4 30 .200 2000
Hydrocal - White 45 18.5 25-35 .390 5000
Hydrocal- FGR95 30 27.8 50-70 n/a n/a
Hydrostone 32 26.0 17-20 .240 10000
Pottery Plaster #1 70 11.4 27-37 .210 2000
Ultracal 30 38 21.9 25-35 .080 6000
Ceramical 40 20.8 18-23 .165 6500

Volume and Plaster Mixing Ratios

Created (cu.in.)

Plaster -.70 ratio

40 0.5 1.40
80 1 2.85
160 2 5.70
240 3 8.55
320 4 11.40
400 5 14.25
480 6 17.10
560 7 19.95
640 8 22.80
720 9 25.65
800 10 28.50


Mixing and Pouring Instructions and Tips

The basic objective is to uniformly and simultaneously wet each particle as best as practical. Proper weighing and mixing is critical to performance. A chemical reaction is occurring which produces heat. Protect your hands with gloves. Set-up time is affected by water-to-plaster ratio (less water=faster set-up but decreased strength). Temperature of water (warm=faster set-up), and speed and length of mixing (long/rough agitation=faster set-up). Set-up should be at room temperature without fans or drafts. To facilitate clean up, the mixing container may be treated with WD-40 or other release agents.

If water rises to the surface after the pour, the plaster has not mixed sufficiently or the water to plaster ratio is incorrect. Mix longer, check your calculations or review the ratio.

By Weight

1. Weigh proper amount of room temperature water in clean container

2. Weigh proper amount of plaster/gypsum.

3. Sprinkle plaster/gypsum into water until smooth mounds of plaster peak above the surface of water. Allow the mixture to sit undisturbed while the plaster soaks up the water, about 3 or 4 minutes.

4. Manual Mixing (not generally suitable for batches over 10 pounds) – using a spoon or wood trowel, mix from the bottom using a side to side motion. Circular motion will cause bubbles. Mix to the consistency of heavy cream.

Electric Mixer – Use an electric drill with a Jiffy Mixer attachment. "Stir" at no more than 1750 RPM while holding the Jiffy Mixer at 15 degrees off vertical.

5. The mixing container should be solidly tapped or even dropped on the floor several times to help air bubbles rise to the top.

6. The plaster is ready to pour when mixing is complete. " Marking" is a way to determine when it is ready. It is the moment to pour when a finger is trailed over the surface of the plaster and a slight wake is left in the plaster.

7. Pouring should be done in a comfortably quick, smooth, steady manner directed towards the center of the base of the mold. Avoid allowing the slip to run down the side of the mold as "hard spots" may occur.

By Volume and Weight

1.Determine how much "mixed" plaster is needed (Volume)

2.Weigh half that volume of water (adding the water to plaster does not substantially increase the volume as the water is absorbed by the plaster and the air in the plaster is displaced).

3.Divide that water weight by the water to plaster ratio which in the case of No. 1 Pottery Plaster is .70

4. The result of 3 above will be the weight of the plaster needed

Note: as a check, the volume of plaster and water should be close to the same

5. Follow steps 3 through 7 under "By Weight" above

By Eye

1.Those with sufficient experience sometimes us a method called "Peaking" or "Island Method" whereby the plaster is sifted into the water until it forms a mountain of plaster in the water with the "peak" rising above the water surface.

2. Follow steps 4 through 7 under "By Weight" above

Coloring Plasters

Color can be added to plaster but it is difficult to achieve uniformity. As the excess water migrates from the plaster cast, areas of light and dark color concentrations inevitably appear. Adding color does not replace surface decoration such as paints.

If attempted, the color should be mixed with the water and should be tested as it may accelerate or retard setting time, cause excessive pinholing, affect any later surface decorative coating or weaken the finished product unsatisfactorily. (Additions of 10% dry pigment may reduce dry compressive strength by up to 30%, and dry impact strength up to 50%)

Surface Texturing

Sand, aggregates or other materials can be added to plaster but the same precautions as noted above in Coloring Plasters apply even more critically, particularly the loss of strength. Such additions are not recommended. It would be best to consider adding the texture to the finish surface decoration only.

Painting a Finished Plaster Item

The thoroughly dried plaster should be sealed with thinned shellac, lacquer or acrylic first (see below). Once sealed, any paint or wax can be used.

Sealers/Releases/Separators for Models

All porous models, patterns, molds or dies must be sealed before a parting agent is applied. Thinned shellac, lacquer or acrylic can be used for a first coat, then a second coat applied un-thinned. If it is a plaster model, apply immediately after set. If it is wood, the grain must be filled so the plaster doesn’t adhere to the micro-undercuts of the grain which may require multiple coats and light sanding between coats.

Satisfactory agents for separating plasters from models are:

• Soaps (not detergents)- cut 10 to 1 with distilled water, sponged on. work several coats into a plastic model and remove excess with a damp sponge.
• Stearic Acid & Kerosene (Stearine)-apply with soft brush
• Kerosene & Petroleum Jelly-1 part jelly to 2 parts kerosene heated and brushed on
• Light Mineral Oil
• Olive Oil-apply warm
• Vegetable Oils-including PAM (non-stick aerosol for cooking)
• WD-40








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