I teach sculpture to high schoolers, and we use a lot of plaster of Paris. Most of it turns into sculptures, but there is a substantial amount of waste. I keep scraps and chunks of old plaster around to use as filler when casting, but I'm hoping to find some way to use the spent plaster short of putting it back in a kiln for reprocessing. The waste I'm dealing with is everything from quart container sized chunks, to thumbnail-sized flakes. I'm in Dallas, Texas, and our soils are either Blackland prairie (clay silt, vertisol, alkaline) or chalk prairie, so soil amendment is probably out of the question, at least locally. Any ideas?
Put out a post on free cycle for your area. Not sure, but totally guessing this gypsum/calcium sulfate dihydrate might turn soil more acid if incorporated?
Dane, if you put the chunks in water to soak will they breakdown? If so, could that be easier to use in a garden to add gypsum/calcium sulfate to the soil?
Anne and Michael, thank you both for taking time to respond. Plaster won't break down in water (water actually catalyzes the crystallization process), but it's so fragile it can easily be powdered or broken down into small pieces with a hammer once it's dry.
As I've been researching more, it turns out gypsum seems to be the best option for increasing available calcium in soil without raising the PH. Our soils are already alkaline, so we never add lime. Gypsum seem like a good option.
Dane. the temperature required to "reactivate" plaster is quite low. I've done it at about 150C on top of the wood fire. All you need to do is drive off the water, and this works for stale plaster that has been sitting in an open bag in humid conditions as well.
However, it is a great soil amendment for increasing Ca without raising pH. Go nuts with it.
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