Solution for dissolving plaster wanted

[Korto]

As some of you know, I do a bit of lost wax casting, using plaster. Now, when it comes to cleaning off the plaster, it's a bit of a bugger. There are all kind of nooks and crannies that the plaster can hide.

I recently experimented using white vinegar, but the results were uninspiring. After perhaps two months, definetly more than one (I lose track of time easily), there was still plaster hiding there.

What I'm looking for is a solution that can dissolve the plaster in a reasonable time-frame (a couple of months is actually fine, as long as it works). The plaster is common casting plaster (high temperature fired and water free), and the metal is bronze (copper-tin)

Ideally, the substance will have the following properties.
1) Cheap
2) Easily available
3) As safe as vinegar
4) For preferance, leave the metal shiny clean
5) Obviously, it can't harm the metal.

And yes, I do get the pun.

[Broomstick]

Assuming you're using some form of plaster of Paris (gypsum) or lime plaster, that's going to be a problem because, during casting/hardening either one under goes a chemical reaction. Plaster of Paris goes back to being gypsum, and lime plaster essentially becomes limestone. You're looking to dissolve rock.

Now, water and/or vinegar will, eventually dissolve either but you're looking at something more like geologic time scales for that. Well, OK, a thin film of plaster can probably be dissolved in a few days or even weeks, but large chunks? Lots of time.

Citric acid and vinegar are both recommended as plaster solvents, but always with the caution it will take time and you should break up/ break off as much plaster as possible before using the acid. I've also seen hydrochloric acid suggested BUT - I can't caution you strongly enough on this - hydrochloric acid requires serious safety equipment for hand and eye protection, and don't breathe any fumes. Chemical burns are absolutely no fun whatsoever. Hydrochloric should clean it up better and faster, but I have no idea what the effect would be on the metal you're using.

[Korto]

Gypsum, I believe. Plaster of paris.

Yeah, not a particular fan of using serious acids (it's just stomach acid! How dangerous could it be? ) I run enough chance of getting severely burnt in normal life, without adding corroded.

[someone_else]

Not sure I understood what you want, but maybe I can help anyway.

If you want to remove plaster from an object that can stay in water/oil for a few hours/days without ruining, you can place it in a ultrasonic cleaner.

In theory it should detach all non-metallic stuff from the metal thing you put into it, and it is pretty common in jewellery (where I got the idea from).

[Broomstick]

Some dental picks might work for getting it out of the nooks and crannies - really, you want to remove as much as possible by mechanical means before using the acid to get rid of the final bits of residue.

I'd forgotten about ultrasonic cleaners - that might be an option if you can find one large enough for your casting.

[Korto]

I hadn't thought of ultrasonic cleaners, either. Someone mentioned them to me once, last year some time. I'd forgotten all about it. They would seem to be quite suited. Thankyou.
Having a look on ebay at the moment. Main problem is the shallowness of the tank, more a tray than a tank on the ones in my price bracket, although there is something (secondhand) that may be suitable. If only I hadn't paid my mortgage today.

And I don't think there's too much worry about bronze being damage by immersion in water for a few days. Or centuries, for that matter.

[madd0ct0r]

but presumably bronze is damaged by acid yes?

Seeing as you do this a lot...

http://deiwhy.blogspot.com/2008/12/diy- ... eaner.html

[Korto]

but presumably bronze is damaged by acid yes?

Seeing as you do this a lot...

Been a long time since I did chemistry, and even then I never made a particular study of acids (didn't particularly study period. May explain why I flunked 2nd year BaSc ), but I believe that different acids will affect or not affect different materials. Acid strength may have an effect there too.
I personally wouldn't expect vinegar to have any noticable effect on bronze within my lifetime. Unfortunately, the same seems to apply to the plaster as well.

Everything I did before these figures was open and simple to clean by hand, so this is a new thing. Never had to think about acids before, except for the idea of doing some etching one day.

[Gil Hamilton]

Acids are great at solvating sulfate compounds, but they are also rough on the metal.

However, I have a thought. Normally if you want to dissolve something, you put it in solution and heat it, but gypsum (by my reading) is the opposite. It dissolved better in COLD water.

So I have a thought. Get a bucket of water, ice, and rock salt. Add to it a bit of vinegar to add make the solution a bit acidic (hydrochloric acid would be even better). Once it is good and cold, wearing gloves if you used HCl, submerge your statue in that for a bit.

The water won't hurt bronze and the acid effect on bronze will be limited by temperature. Meanwhile, it's cold and ionic and that should dissolve some of that plaster of paris off. After a bit, take it out and see that doesn't loosen the plaster.

[someone_else]

There is also the point that metal tends to shrink when cold.
Maybe a good freezer is enough, but I think liquid nitrogen will be better.

My granpa used liquid nitrogen to shrink the internal ring of a ball bearing to assemble the bearing, in the days when he was a factory worker.

I still think ultrasonic cleaners are orders of magnitude better, but if they fail... you can try this plan B.

[Gil Hamilton]

You don't want it too cold because you'll risk cracking the metal with uneven compression (what your grampa did isn't easy to do right, which is a criteria Korto has).

Ultrasonic cleaners are great through.

[Korto]

However, I have a thought. Normally if you want to dissolve something, you put it in solution and heat it, but gypsum (by my reading) is the opposite. It dissolved better in COLD water.

So I have a thought. Get a bucket of water, ice, and rock salt. Add to it a bit of vinegar to add make the solution a bit acidic (hydrochloric acid would be even better). Once it is good and cold, wearing gloves if you used HCl, submerge your statue in that for a bit.

This is definately counter-intuitive for me, but also cheap and easy enough to test, so I'll give it a go.

As for using liquid nitrogen, that seems even more dangerous than strong acid, doesn't store, and would quickly cost more than the $220 an ultrasonic cleaner looks like it will cost me (although I may try the DIY plans from madd0ct0r first). Other than that, great!

[Chardok]

Hey I had a thought yesterday. Since plaster is mostly calcium, why not try CLR? it's a commercially available sleaner with lactic acid, gluconic acid, glycolic acid, sulfamic acid, citric acid, and surfactants (Per wiki) I used it yesterday on some hard water staining and POOF! it was awesome.

[someone_else]

yeah, whatever you use to clean your bathroom from water stains and scale may be able to do it.
Although some (the majority?) of the commercial products tend to be overzealous and may damage or stain the metal. (some indeed damage the taps and fittings)

Would be better to try on a piece of your spare metal to see if it damages/stains it before risking your artwork, imho.

Maybe you can discover that the stains add something to your art.

[Broomstick]

That's not "stains". It's "patina". And I meant to do that.