<p>It appears that common model-making hydrocal (there are several products using &quot;hydrocal&quot; in their name) relies on the same chemical reaction as plaster of Paris, i.e. it's gypsum-based.</p>
<p>Gypsum plaster has to be <a href="https://en.wikipedia.org/wiki/Plaster#Gypsum_plaster_(plaster_of_Paris)" rel="nofollow noreferrer">calcined</a> (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.</p>
<p>However there are a couple of good reasons not to put much down the drain:</p>
<ul>
<li>Any heavy powder is likely to accumulate in the sink trap, leading to it slowly filling with sludge.</li>
<li>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.</li>
</ul>
<p>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.</p>

CC BY-SA 4.0
2024-06-26T14:39:00.783Z
2024-06-26T14:39:00.783Z
Chris H
<p>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...</p>
<p>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?</p>

<plaster>
Does hydrocal plaster work twice?
nuggethead
# Does hydrocal plaster work twice?

(Asked by: username_0 on 2024-06-26)

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?

**Tags:** plaster


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## 1 answers

### Answer by username_1 on 2024-06-26. Score: 2

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](https://en.wikipedia.org/wiki/Plaster#Gypsum_plaster_(plaster_of_Paris)) (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.


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