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Pottery Plaster

Pottery plaster is important both as an ingredient and as a working mold medium. From whiteware and sanitaryware to artware and statuary, from insulators and stoneware to special refractories, the versatility of gypsum is a valued resource. Without gypsum to shape and dewater the clay, slipcasting, jiggering and pressing operations would not be nearly as effective. To a lesser degree, the same could be said for our tooling plasters used in models, case molds and block molds – its secondary application.

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Pottery Plasters Description	Use Consistency (cc/100gms)	Vicat Set Time (minutes)	% Set Expansion	Dry Density (lbs./ft³)	Expected Compressive Strength (PSI)
K-55 Pottery Plaster This general-purpose product is ideally suited for producing large sanitaryware casting molds where high absorption is a top consideration. It is used in models, casting and jigger molds.	70 - 75	28 - 38	0.15	65 - 69	1500 - 2000
K-58 Pottery Plaster Formulated with special ingredients to minimize cracking, our K-58 pottery plaster provides extra protection against the effects of temperature extremes.	70	25 - 35	0.15	69 - 71	2000 - 2200
K-59 Pottery Plaster Designed for all types of potteries and sanitaryware. You cannot beat it for casts that are smooth and free of pinholes. Because K-59 contains Densite® plaster, the composition requires lower water to achieve high fluidity. The water:plaster ratio can be as low as 64:100 and the resulting cast is exceptionally strong and long-wearing.	68 - 70	25 - 35	0.22 - 0.27	69 - 71	2000 - 2600
K-60 Pottery Plaster Developed as an intermediate grade that provides improved absorption and longer mold life than K-59 plaster.	67	25 - 35	0.18	70 - 72	2100 - 2300
K-62 Pottery Plaster Our premium-grade pottery plaster, K-62 is carefully formulated with low fluidity - resulting in an exceptionally fine, smooth-working material having maximum strength and wear resistance. This product even contains additives to promote thermal shock resistance. Ideal for mechanical clay-forming applications, such as jigger molds, which are subject to the most wear.	66	20 - 30	0.2	71 - 73	2200 - 2400
K-63 Pottery Plaster Density, strength and absorption properties make K-63 plaster an excellent choice for use in dinnerware. Our customers rely on the plaster's formulation to satisfy complex requirements and maximize productivity.	55 - 57	24 - 28	0.23 - 0.26	78 - 81	3300 - 3700
Denscal® TL Plaster Medium-strength gypsum cement specially formulated to provide a long period of plasticity. Especially suited for loft template, splash cast patterns or models that require an extended working time. Denscal TL gypsum plaster provides the benefit of controlled setting expansion, and is particularly useful for tooling that involves close tolerances.	45	15 - 20	0.05 - 0.065	90	4000
Densite® K-12 Low Expansion Plaster With a density well suited for industrial tooling, Densite K-12 gypsum cement features a short plasticity period. You will appreciate its low-expansion and high-strength characteristics.	36 - 37	20 - 30	0.025 - 0.040	101	7500 - 8000
Densite® K-13 Plaster A high-strength, low-expansion gypsum cement with a medium range period of plasticity. Densite K-13 plaster is ideally suited for splash casting screeding applications and all types of tooling where a stronger material is needed.	36 - 38	15 - 20	0.18 - 0.23	99 - 102	6000 - 8000
Densite® K-25 Plaster High-strength, low expansion gypsum cement. Densite K-25 plaster has a long period of plasticity and is recommended for splash casting and screeding. Formulated to be non-corrosive when used in conjunction with aluminum templates.	40	20 - 30	0.04 - 0.05	96	6000
Densite® K-33 Plaster Regardless of application, material requirement or performance specifications, this product is suitable for all types of plaster tooling. According to many of our customers, you simply cannot go wrong with Densite K-33 gypsum cement.	36	15 - 20	0.025 - 0.04	102	8000
Densite® K-40 Plaster (Ram Plaster) Specifically developed for use with the automatic clay-forming press process, Densite K-40 plaster is the finest gypsum cement available for die press applications. The quality is exceptional for situations where unfailing performance is essential.	36 - 40	15 - 20	0.035 - 0.05	96 - 102	6000 - 8000

The best insurance against trouble is using sound practices at all levels of production. Good mold shop practices can be summarized as follows:

Plaster stock is rotated. The oldest is used first.
Plaster is stored in a dry and warm area.
Uniform mold density is assured by maintaining careful control of consistency used by the shop. Plaster and water are both weighed.
The soaking and mixing times are carefully controlled by time switches on the mixer.
Only water fit to drink is used and it is always used at the same temperature.
Molds are carefully dried at 125° F. with forced air before they are put into use.
Mechanical mixers with properly designed buckets and propellers are used or a continuous mixer of proper size and design is used.
Molds are treated as precision tools. They are handled with care and stored carefully.
The casting shops keep close checks on each mold's performance.

When trouble is experienced, the mold shop foreman should examine the problem to see which of the above conditions are being ignored. The best procedure is, of course, to see that the rules are not violated in the first place.

Mixing: The Most Important Step in Mold Making

Mold strength, hardness, absorption and density are developed to a maximum during the mixing process. At this time, gypsum particles are dispersed in water and all air is removed from the resulting mixture.

Two different methods can be used. One is continuous mixing, a process by which custom-engineered equipment automatically meters and mixes exact proportions of plaster and water. In continuous mixing, the ingredients are blended together at high velocity, instantly forming a slurry without the need for soaking. The resulting mixture is then pumped through a hose to the point of use.

The alternative method is called batch mixing, a process involving manual agitation of slurry produced in small amounts. Plaster is sifted into water and allowed to soak before it is hand mixed to the proper consistency. The slurry is then carried in buckets to the point of use. If batch mixing, follow these rules:

Avoid changing the mixing cycle.
Accurately weigh plaster and measure water.
Time the soaking period and mixing cycle (sift plaster into water, soak until all particles are wet).
Use proper mixer size, container shape and propeller clearances as dictated by the batch size and the physical properties of the plaster; mix the slurry to a creamy state to avoid settling.
Maintain a constant and proper temperature.
Keep all equipment clean.
When pouring, maintain a constant rate, keeping the lip of the bucket as close to the mold as possible.

Drying Rooms

So that a mold's physical properties can develop to the maximum level, the mold should be dried as quickly, thoroughly and safely as possible. Drying is the evaporation of "free" water from the mold which can be either a natural or forced air process. When the center of the piece reaches the temperature of the air surrounding it, drying is complete.

To prevent calcination (soft, powdery surfaces), drying temperatures generally should not exceed 120°F. Drying times can be reduced by increasing air speed over plaster molds. The following conditions are present in the best drying rooms or ovens:

Rapid, consistent air circulation with no 'dead spots'
A consistent temperature throughout the area
An air transfer system that replaces exhausted, moist air with dry air

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