U.S. Gypsum WHITE HYDROCAL -REGULAR 50 lbs. Bag
WHITE HYDROCAL is used for making plaster objects, a neutral gypsum cement having a normal set of 25 minutes. When Gypsum Cement is formed under a template it is slightly on the “short side” and has a tendency to tear. The setting expansion is somewhat greater than ULTRACAL 30.
HYDROCAL White Gypsum Cement
A neutral gypsum cement having a normal set of 25
minutes. When formed under a template it is slightly
on “short side” and has a tendency to tear. The
setting expansion is somewhat greater than with A-11
or B-11 cements, but higher wet and dry strength is
Sealers and Parting Compounds
Parting compounds (also called “separators” or “releasers”) are needed to prevent adhesion of plaster or gypsum cement to patterns or models. They also permit removal of plaster or gypsum cement from the surface of the pattern or model upon which it “sets up.” Porous model materials require sealing prior to application of the parting compound.
All porous models, patterns and molds must be sealed before the parting agent is applied. Sealing is particularly essential for wood patterns to prevent dampness from causing the wood to swell, resulting in release difficulty. Gypsum cement patterns also require sealing to prevent them from absorbing water from the plaster mix and causing the mix to adhere to the gypsum
cement pattern. Patterns of metal, glass, plastic and other impervious materials normally do not require sealing.
A quick-drying lacquer applied by brush or spray gun is an excellent sealer for wood, plaster and gypsum-cement patterns and master models. Two coats of lacquer provide more resistance to moisture than shellac. Correct procedure is to reduce lacquer with
lacquer thinner so that the model or pattern readily absorbs the first
coat. Apply second coat after first coat has thoroughly
dried.Where a plaster or gypsum cement pattern is used, apply lacquer immediately after set.
Mahogany patterns or models require special treatment because gypsum cement casts may adhere to undercuts caused by multiple
pores or grain of the wood. A paste wood filler can be used to fill the
grain of the wood, but should be reduced to a satisfactory
brushing consistency with the recommended thinner. Apply a heavy coat and let it dull down; then wipe off excess by
across the grain. After wood filler has set overnight, apply a coat of
a clear acrylic sealer and allow to dry. Sand the pattern
or model and apply several coats of a clear lacquer.
Satisfactory separation agents must provide the following functions:
1. Prevent adhesion to the cast.
2. Protect and lubricate pattern surface.
3. Spread easily and uniformly in a thin, continuous insoluble film.
4. Do not react destructively with the plaster or gypsum cement surface, or with the pattern.
Some of the most common and widely used separators are as follows:
Soaps—A number of special “potter’s” soaps are available for use as parting compounds. “Green soap” (a liquid) is an effective parting compound when cut 10 to 1 with distilled water. Apply parting soap with a sponge and work into plaster surface. Several coats may be required on a new mold. Remove excess soap with a clean wet sponge before pouring mold.
Stearic Acid and Kerosene (Stearine)—Formula: 1/4 lb. stearic acid shaved to flakes; 1 pt. kerosene.
Melt stearic acid by warming:
remove from heat source and add kerosene with constant stirring until a uniform mixture is obtained. Apply to pattern with
a soft brush, preferably camel’s hair or sable hair. If brush marks show, thin mixture with more kerosene or by warming. This
separator is one of the most widely used of all parting compounds and is particularly recommended for use with glue molds.
Jelly (Vaseline)—Formula: 1 part petroleum jelly; 2 parts kerosene. Use
petroleum jelly if it is cut back with approximately two parts kerosene
to one of jelly. Blend mixture by carefully heating and thoroughly
stirring (Caution—Fire Hazard).
Apply the casting cement as a thin coat and brush out thoroughly.
For special purposes, separators other than the above may be advantageous:
Light Lubricating Oil—Use sparingly.
Light Mineral Oil—Particularly effective as a fine fog spray on metal patterns. Must be thinned to be effective.
Olive Oil—May be used if applied carefully while warm. Crisco and other similar hardened vegetable oils used as frying fats are particularly recommended for use on lacquered wood.
Powered Mica—Has been found useful when finely ground and added to most parting compounds. Satisfactory results have been obtained by adding 1/4 to 1/2 cup of the powdered mica to 1 qt. of parting compound solution.
EPOXICAL® Mold Sealer Separator—Works well as a release agent of gypsum cements.
EPOXICAL Mold Sealer Separator as it comes in the can, using white
gasoline or kerosene at a ratio of 1-to-1 by volume. Apply a generous
coat to the pattern: allow to dry 2 to 3 minutes and then wipe off
excess with a soft cloth.