When you’re searching the hardware store for materials, you’ve probably wondered: What’s the difference between all these plaster varieties, anyway? Here are my empirical observations:
Patching Plaster: Sold in all hardware stores, this can be used in a pinch but sets up very quickly as it has been designed for patching interior sheetrock and lath-and-plaster walls and ceilings (it’s too expensive, anyway, in the quantities needed for sculpture).
Plaster of Paris: A soft , coarse plaster which is OK for hobby molds and readily available in most hardware stores. It is about half the price if you buy in bulk from a building supply place.
Molding Plaster: A fine, hard plaster for architectural work and good for most sculpture and molds. Usually available only in bulk (100 lb bags) from building supply yards (see my list of materials suppliers for one in Boston).
Hydrocal: A plaster formula developed for hobby, sculpture, and molding use. It is extremely hard when fully cured and also has good “green” hardness before the cure is complete. The cured surface is very white, dense, and fine, and accepts faux finishes and paint very cleanly. Available at some art supply shops, or online from a number of bulk distributors.
Temperature: It is true that cold water can make your plaster mix set up more slowly, and hot water makes it set up more quickly. This is all relative, however, and factors like ambient temperature and humidity are also important.
Tinting: Once you are familiar with plaster varieties and mixing, you can go on to do crazy things with plaster, like tint it. Use a good quality water-based pigment (I use gouache) thoroughly mixed in the water before the plaster is sifted in. Use a lot, or the resulting tint will be very, very pale after the white plaster has been added. Or, pour plaster into a mold and brush swirls of pigment across the wet surface for a marbled effect (used to create those fabulous plastic-flamingo-and-seashell-encrusted Florida souvenirs of my childhood).
Surface decoration: Plaster readily accepts all kinds of surface decoration. Brush water-based pigments directly on damp plaster and you have–voila!–a fresco. Use waxes or linseed oil mixed with metallic powders for a faux-metal finish on plaster that is thoroughly dry. Or paint on it with whatever is your preferred medium. I have pieces painted with oils, gouache, watercolor, acrylics, and encaustic-like waxes that look great after 20 years. Butcher’s wax (in an orange and white can from the hardware store) is the best I’ve found for waxing the final piece, necessary to preserve your beautiful surface from dust and dirt. Make sure the surface is absolutely dry and the wax is warm and spreadable when you use it, so it doesn’t leave cold white lumps behind.