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Author: Subject: Cleaning Marble
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[*] posted on 12-5-2019 at 10:00
Cleaning Marble


Hello everyone, long time no see, hope everyone has been ok.

Anyone, I have a question, I have a piece of polished marble that is rather old and has become stained with a large yellow stain that I assume to likely be Iron from local water, I need to remove the stain, hopefully without destroying the finish but I assume I could likely repolish it if possible.

I have tried Oxalic acid in the form of Bar keepers friend however that did not really work. Is there by any chance a way that I could mobilize the stain, or even better, just convert it into a clear substance to make it vanish even though the substance is still there. Something along those lines would likely be one of the best ways as it would not destroy the finish, I would just have to soak the stone for a while.

It's rather important and any help would be appreciated.





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[*] posted on 12-5-2019 at 11:12


Quote: Originally posted by Frostboi  
Did you think of putting it in some dilute phosphoric acid solution? I’m quite sure it wouldn’t damage the rock a lot.

Phosphoric acid is sold for the express purpose of damaging calcium carbonate.
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[*] posted on 12-5-2019 at 12:09


Not sure thats what I am curious about. I mean, one of the test for marble out in the wild is to place Acetic acid on it and look for bubbles if I am not mistake, that means an acid even as weak as vinegar will cause corrosion so I would think Phosphoric would as well if not faster. What I was thinking was something I might be able to soak it in that overtime will convert the presumably Iron Oxides embedded in it into something colorless.





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[*] posted on 12-5-2019 at 12:09


Quote: Originally posted by Frostboi  
[ Anyways, maybe phenol could work? It won’t certainly rip off CO2 from a carbonate but will make a salt with iron. If it doesn’t work quickly you might need to add some hydrogen peroxide in order to oxide any leftover iron that isn’t in the form of Fe2O3.

OK, so that's two ways to make things worse.
You can make the iron/ phenol complex or Fe2O3 .
Those are both very dark colours...
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[*] posted on 12-5-2019 at 12:35


Ferric fluoride complexes are colorless. If your marble is exposed to HF, then you may have reaction with CaCO3 as well (albeit slowly, HF is only a weak acid, comparable to acetic acid). The reaction with CaCO3, however, forms CaF2, which is totally insoluble in water and also in acids. So, corrosion of CaCO3 does not proceed.

With H3PO4 there is no such protection. FePO4 is colorless, but the CaCO3 dissolves. You get Ca(H2PO4)2, which is soluble in water.


I would try with 3% HF on your marble. Be careful with that. Although it is a weak acid, it is VERY dangerous for your skin, even at such low concentrations. In some countries you can buy a cleaning product, which contains severals percents of HF. If you can't get HF, you can get NH4HF2 on eBay.




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[*] posted on 13-5-2019 at 00:28


Hmm, Ty Woelen, That sounds very close to what I am looking for. At the very least gives me a jump off point.

The idea that it forms CaF2 that is insoluble seems like a great thing to be honest because as long as it strips the color I do not care if it is in there. If the formed salt is mobile in water there runs the risk of over time it converting back to iron oxide and being back to square one. I like the idea of it being insoluble to be honest.

Whats CaF2 stability in higher temperatures, nothing extreme, just a slow 100degree bake in the oven to remove water before sealing it.





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[*] posted on 13-5-2019 at 03:34


What about EDTA? It might remove the iron



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[*] posted on 13-5-2019 at 04:50


You can use sodium thioglycolate. It dissolves iron deposits at neutral pH and it is sold as rust stain remover (especially for clothes). Once it dissolves the iron, it forms an intensely colored violet complex.
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[*] posted on 13-5-2019 at 04:52


We have a rust-removal product here that I found quite effective and is safe to use on marble.
https://nl.hg.eu/producten/klus/hg-roestvlekken-verwijderaar

I don't know if something similar, perhaps even the same brand is available where you live, but I recomend it if you can get it.
I have repeatedly used it on a marble shelve in our shower, which becomes stained with iron from the water over time.

It contains (2-hydroxyethyl) ammonium thioacetate, which chelates iron, forming a soluble, purple iron complex which you can then rinse off. Sometimes I have to repeat it a few times to completely remove the discoloration.

[Edited on 13-5-2019 by phlogiston]




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[*] posted on 13-5-2019 at 07:42


Doesn’t oxalic acid also form iron complexes which are water soluble? Article here

Edit: Oops didn't see you'd tried that. How come it didn't work?

[Edited on 13-5-2019 by Keras]

[Edited on 13-5-2019 by Keras]
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