HOW TO MAKE HOLLOW PRODUCTS BY
SLUSH CASTING WITH PLASTER

The slush cast technique for hollow cast plaster lets you make lighter weight and hollow castings.

A slush cast plaster figurine	The base of the slush cast plaster figurine, showing the hollow interior

Slush casting is used for two reasons. One is if you need a hollow product - for instance, a piggy bank. Another is if you are making a large product, and the product would be too heavy if you cast it solid. An added benefit of slush casting is that you use less plaster, which saves money in materials.

The basic technique for slush casting plaster is on page 70 of the Castcraft Casting Guide. Pottery Plaster and Hydrocal are the best plasters to use for slush casting. Hydrostone will NOT give good results.

Most figurines found in craft stores and flea markets, and most imported items, are made from pottery plaster. Pottery plaster is the cheapest craft plaster you can buy, and makes good quality, but somewhat soft castings. Pottery plaster castings should be painted to seal the plaster and protect against chipping and scratching. Hydrocal makes a harder, more durable casting, and is sometimes used to give an imitation ceramic look and feel to products like decorative lamp bases. Hydrocal is more expensive than pottery plaster, and may be more difficult for you to obtain locally.

HOW TO SLUSH CAST

1. Slush casting is usually done in a rubber mold. If you are practicing the technique, you may want to get a small latex rubber mold from a local craft store. The technique is basically the same for a small casting or a large casting, so you won't waste a lot of plaster while you practice. Latex and Polyurethane molds are best. Silicone rubber is usually too water-repellant, and plaster is water based.
2. Mix enough plaster to fill about 1/4 of the mold. The plaster should be mixed fairly thick - almost like a syrup. It should not be watery, or it will not stick to the mold well.
3. Use a mold rinse in your rubber mold before you pour the plaster. The purpose of a mold rinse is to "wet-out" the mold so that plaster will flow over the mold surface easily. If you don't use a mold rinse, your finished casting may show a lot of tiny air pockets and air bubbles. You can make a simple and effective mold rinse from dish detergent. Just add three drops of dish detergent such as "Dawn" to a cup of water and stir gently so you don't create a lot of bubbles. Pour the rinse into your mold, tip and turn the mold so that the entire inner surface gets wet, and pour the rinse back out. You can save the rinse and use it again with another casting. Make your first slush cast while the mold is still wet.
4. Pour the thick plaster mix into your mold. Tip and turn the mold so that plaster coats the entire inside. (You can also shake, jiggle, and squeeze the mold if necessary to make the plaster flow over the entire surface and into all crevices). A coat of plaster will stick to the mold walls. If you are casting more than one mold at a time, you can pour the excess plaster into your next mold, and repeat the process. If you are only making one mold, pour off the excess as waste, or use it to fill some other small mold. Keep track of how much plaster and water you use, so that you don't use any more than is actually needed to make the slush casts.
5. Let the first coat of plaster set. When it sets, the plaster will get firm and appear to start to dry out. You can make your second slush cast as soon as the first coat is firm - usually after 45 minutes to an hour. The first coat of plaster does not need to set hard.
6. Mix a second batch of plaster. The second batch should be slightly more watery than the first batch, because the first coat of plaster will act like a sponge and draw water out of the second slush cast. If the second mix is thick to start with, when it loses water it will get so thick that you won't be able to slush it around the mold easily. Slush the second coat exactly like the first. The second coat will build an obvious layer on top of the first coat. Again, pour off the excess plaster - whatever doesn't stick to the first coat.
7. Let the second coat get firm, and slush a third coat over it, just like the second coat. This is all you need for a small product such as a piggy bank - the wall of the casting should be about 3/16 inch thick. If you are making a larger product like a column or pedestal, you might want to give one or two more slush coats, just like the third. For these large products, you can also add reinforcement such as chopped burlap to the plaster or lay in pieces of burlap to the wet plaster by hand.
8. If you wish, you can give your cast product a base, as described in the Casting Guide on page 70. Basically, for your last slush cast you use a little more plaster than necessary to coat the walls of the mold, then turn the mold over and put the base on a sheet of rubber, and let the excess plaster run down and make a solid base about 1/4 to 1/2 inch thick.
   If you are making something like a piggy bank and want to give people access to the hollow interior, you can put a rubber stopper on the sheet of rubber, then turn the mold over on top of the rubber stopper. When your casting sets, you will have a hole in the base exactly the size of the rubber stopper. Rubber and plastic stoppers are available at many ceramic supply houses.
9. Let the casting set for a few hours, and remove it from the mold.

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