There are a variety of plasters and gypsum cements used in mould making, master models and finished art forms. These products originate from the highly versatile raw material, gypsum, commonly known as “plaster of Paris”.
Gypsum is a natural material found in the earth and was first mined commercially in France during the 1770’s near Paris, hence the name. It’s use predates this period by many thousands of years, for it appears in statuary made in ancient Egypt.
Before gypsum can be used it is quarried, crushed, screened, pulverised and then heated. This heating or calcining process drives off most of the plaster’s chemically combined water, leaving the plaster in a state of perpetual thirst, which explains its affinity for water. This is the basis of ceramic slip-casting work, the ability of plaster to absorb water from the pottery slip and produce greenware or unfired ceramic shapes ready for firing.
TYPES OF PLASTER
The plasters available cater for both the professional mould maker or statuary casting business and the amateur interested in producing plaster figurines in his home workshop.
Most creative people enjoy making objects with their hands, hence the popularity of plaster. Due to the even, homogenous nature of plaster, a material free from lumps or hard spots, it can be carved, sawn or turned on a lathe. A sculptors delight
Although there are a large number of plasters produced for specific purposes, such as dental plasters etc, the basic range used in Australia for craft and ceramic purposes consist of possibly three Australian and four American plasters.
The three local plasters, casting plaster, pottery and dental plaster cover most requirements with the four imported plasters covering specialty professional needs.
The plaster of paris sold in hardware outlets, is unsuitable for use in all model and mould making procedures, as it has not undergone final milling procedures to reduce the grain size.
CASTING or MOULDING PLASTER
This plaster is the choice for most of the plaster painting studios. It is economical and gives excellent detail when poured into latex or plastic moulds for the production of the figurines, plaques used for decorating.
Surface hardening agents are incorporated into this plaster to reduce chipping, which is essential for store retailing, and to give a superior surface when decorated.
When poured into plastic moulds it is capable of giving a superior finish to the cast plaster whiteware, due to its harder surface finish and finer, denser nature when compared to hardware store plaster of paris.
Pottery plaster is used by potters for making bats and wedging table tops, being a soft, highly absorbent plaster. Because this plaster has no surface hardening agents and is thus easily carved, it is used for making models, casting and waste moulds.
This is the most widely used local plaster for ceramic mould makers and potters. It is a high quality white plaster and is an excellent choice for slip casting moulds.
Normal set dental plaster for general purpose dental applications. Similar to moulding plaster.
FINISHED WARE PRODUCTS
A strong, hard gypsum cement especially suitable where high strength and resistance to water absorption are necessary. It is correspondingly more expensive than ordinary plasters. It is used in the production of high quality art castings. It works well in latex or RTV silicone or polyurethane cold moulding compounds.
Use 32% water by weight in mixing. Compressive dry strength 10,000psi.
WHITE HYDROCAL (USA)
This is a general purpose gypsum for the production of a wide range of finished pieces such as figurines, lamp bases etc. It is white in colour and can be carved or added to for additional ornamentation.
Use 45% water by weight when mixing for statuary production. Compressive dry strength 5,000psi.
L0W EXPANSI0N PLASTERS FOR MASTER MODEL MAKING
This special purpose plaster commonly called buff stone, is generally four to five times stronger and over three times the price of Australian plasters listed. Its use is limited to situations in which the plaster does not require carving or forming. It has high surface hardness.
It also has a very low setting expansion and is thus adaptable to the production of hard, strong, tough models of uniform and stable dimensional accuracy. It is used for the production of master models and prothetics.
Use 42% water by weight when mixing. Compressive dry strength 5,500psi.
Ultracal-30 remains the plaster of choice for making models and case moulds and is the widely used of the gypsum cements. It’s properties are the same as Hydrocal but it has higher surface hardness and compressive strength.
It also has the lowest expansion rate ensuring a higher degree of accuracy in model making. Despite this the workability and setting traits remain the same as Hydrocal.
Use 38% water by weight when mixing. Compressive dry strength 6,000psi.
ADDITIVES FOR PLASTERS
CRAFTCO PLASTER RETARDER
By adding a small amount of Craftco Retarder, the setting time of plaster can be delayed by 10 minutes to an hour. A quarter to two tablespoons is all that is needed per 50 Kg of plaster/gypsum. Dissolve in water before adding to the plaster.
This can be added to plaster mixes to reduce weight. It is an expanded lightweight material, volcanic in origin.
PLASTER OF PARIS BANDAGE
Used to make or reinforce back up moulds and for making lifecastingmoulds.
PLASTER REINFORCING SCRIM
Added to plaster castings to give added strength.
HOW TO HANDLE PLASTER
* Do not use plaster that has been stored for some time, without first mixing a test batch. Plaster may have absorbed moisture from the air if improperly sealed and will not harden properly.
* The rate at which liquid plaster thickens can be controlled by altering the water temperature or by adding salt or vinegar or adjusting the stirring time.Warm water speeds hardening. Small amounts of salt accelerate the hardening process, while adding vinegar retards it. By prolonging stirring, you make the plaster harden faster.
* You can simplify the cleaning up process by lightly greasing the mixing dish and stirring tools with a light film of paraffin or Vaseline before you start.
* Use a sheet of polyethylene film to cover your work surface. Plaster does not stick to this plastic film and clean up is expedited.
* After completing pouring, rinse your tools and bowls in a bucket of water before the plaster begins to harden on them Allow the plaster to settle in the bucket, pour off the water and dispose of the residue plaster separately. Never pour waste plaster into a drain, it clogs up your plumbing system.
* Store your plaster either in a plastic garbage tin or if in the original paper bag, enclose in a plastic garbage bag, in order to keep it air tight.
Stan Alderson P1286a