Does hydrocal plaster work twice?

Question

I know that disposing of wet plaster in a sink is unwise; the plaster can harden in drain pipes and cause blockages. But what about used plaster? Here is what I mean...

Say I have been using molds on a sheet of glass, which gets streaks and blobs of wet plaster all over it. To clean it off, I usually wetter it and scrape it clean with a razor blade. The plaster I'm scraping off has already been mixed and set fully once; I am wetting it a second time. Will these wet remnants set hard like plaster a second time, or have the active ingredients been used up?

Answer 1

It appears that common model-making hydrocal (there are several products using "hydrocal" in their name) relies on the same chemical reaction as plaster of Paris, i.e. it's gypsum-based.

Gypsum plaster has to be calcined (ground and baked) before it will undergo the hardening reaction, so your scrapings won't set like plaster unless you've cooked them in your oven.

However there are a couple of good reasons not to put much down the drain:

• Any heavy powder is likely to accumulate in the sink trap, leading to it slowly filling with sludge.
• While hydrated gypsum itself is a naturally occurring mineral, and not a pollutant, there are other ingredients in products that rely on it. Plaster mixes, especially lightweight ones can, in general, include microplastics.

So it's probably best overall to wash your tools in a bowl, allow the plaster to settle out, then pour off the water, disposing of the sediment in the bin.