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It’s easy to find references to the decomposition of calcium sulfate to calcium oxide and sulfur dioxide - under the right conditions (Wikipedia mentions this was once how sulfuric acid was produced industrially). But of course this assumes two things: one is that blasting plaster in air with a CO2 laser is the right conditions to get the sulfur out. And the other one is that the sulfur ends up forming sulfuric acid and not just blowing out the exhaust as SO2. I’m curious to hear what the chemists have to say.
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I wondered similar things about sulfur dioxide. Eventually sulfur dioxide will combine with water vapor and make acid rain and all, but I’m not sure how aggressive that reaction is, it seems like it would be a fairly slow process, as it requires the interaction of sulfur dioxide and water vapor and the concentrations may be pretty low.
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I can’t thank you all enough for doing the science to find out what is happening at the chemical level. Please let me know if your daughter, her boyfriend, you or their chemistry professor has a favorite saying/phrase/quote that they’d like to see on a retro-modern plaster plaque! Message me your mailing address and I’ll make it & pop it into the post as my gift to you all for the work you’re doing. Thanks so very much! <3
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Sulphate Ions are not Chlorine ions but they sre not far away. We have seen that lasering marble actually makes a hole expelling CO2 and leaving CaO that became lime by sucking water from the air as a pile of fluff. If not all the CaSO2 is brokeninto ions at the laser temps, then at least some of it will be and of course there will be some watervapor in the air for the ion to sieze on. Perhaps also the various parts of the glowforge might be less vulnerable to Sulfuric Acid than Hydrochloric Acid but I still would not wish my machane at least to test it.
There is probably a table somewhere that ranks strong acids like would AgSO4 and NaCl give you AgCL and Na2SO4? I think it would but a certain percentage would remain as Ions.
That is my reasoning. We will see what the universe thinks.
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This rocks. It’s a fascinating discussion overall, and the fact that you offered to thank them this way is perfect for for what I expect from this forum. Friendly, generous people 
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Thank your friends for all of us! This is so coooool!
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Swell… now I have something else to try…
Seriously though, great write up.
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Found this:
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FWIW this is the brand of plaster of paris I’ve been using. I don’t know that there are lots of brands or if the brands have widely varying ingredients (I imagine that plaster of paris is basically plaster of paris from brand to brand…) but in any case, DAP is what I’ve been buying in the bulk 25 pound bags.
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Thanks. They’re en route to Boulder now. I’ll let them know tomorrow after they’ve gone back to work.
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Simple chemistry with some amount of filler (sand) The amount of sand might differ.
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I came for the retro plaques, but stayed for the incredible science brain talk 
You all inspire me so!
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So the word from UC Boulder is:
Plaster of Paris is actually a calcium sulfate hydrate, as some amount of water must’ve been involved to make it a paste or pliable material
The laser itself is, yes, introducing energy. And yes, it probably is causing enough energy to push forward some collisions and some reactions. An insignificant amount of sulfuric acid is momentarily forming. But the water in the air and in hydrated calcium sulfate is so abundant that the sulfuric acid will wind up reacting to reconnect with water molecules and make more water molecules, and leave you with HSO4(2-). At that point, that acid is both low in abundance and also a weak acid, as opposed to the strong acid of H2SO4(-), meaning that you won’t be seeing health effects or laser housing corrosion.
My daughter said it was a big hit in the lab to have something of a real world application vs much of their more canned work they do when they’re teaching.
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I’m jelly that you have a chemist on call!
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She says so much of what they do is repetitious in the university setting - they’re always cycling new students in and teaching the same curriculum. So it’s only when they have new research projects or questions like ours that they get interesting questions to chase.
I asked her about the plaster being dried still containing water and she explained it a bit more.
Yes exactly. But even dried in the mold or shape the actual chemical compound remains hydrated to some extent. More like a trihydrate versus an aqueous solution, but three water molecules per calcium sulfate molecule is enough to just drive that weak acid formation over the strong acid formation, and to pretty much swamp out the acid and make it pretty nonsignificant.
It does explain why we don’t see a lot of lasers eaten up by sulphuric acid.
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I’m thrilled to hear this as I have a great many ideas I want to see through to fruition with this process! Big thanks to you, to your daugher and to the whole class at UC Boulder for doing the work of analyzing what is going on at a chemical level! <3
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Big thank you to you and your daughter/university for checking all of this out for us! My mind is now filling with ideas on what to make so I can laser it! I’ve had a box of Plaster of Paris sitting in my craftroom for a couple years now, so long that I forgot what I originally intended to use it for! But now I have something else I can use it for! Awesome!
And I’m glad it turned out to be such a fun research for your daughter and others involved!
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Thanks to the Colorado taxpayers who help fund the University 
Of course there are students who may end up with new lab exercises now that we’ve given the staff new ideas 
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I’m glad to see that some of my money is, in a roundabout way, funding something that I was interested to learn about! 
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