FMF - Friends of Minerals Forum, discussion and message board

[bennefie]

I am new to mineral collecting but have always admired the subject. I recently purchased a (I believe Brazilian) amethyst cathedral. It is a beautiful specimen! I purchased through craigslist at a very discounted rate because of what I believe to be a few minor issues. The issue I can't get rid of is what I believe is a calcium deposit. It is a thin, hard, white layer over about a 6"x2" area of the amethyst. I have tried soaking the entire cathedral in a warm water and ammonia bath. I also tried dawn dish detergent. For both of these attempts, I soaked in a bathtub for about half an hour and then scrubbed the inside of the cathedral with a toothbrush. While this made the amethyst shine and sparkle and removed years of built up dust, the deposit didn't budge. I have done a lot of research and can't find anything that refers to the kind of deposit I'm seeing. I'm thinking of trying a oxalic acid and water soak. Does anyone think that would remove it? I'm worried of how the acid would affect the paint on the back side of the cathedral, but I don't mind repainting if I need to. Has anyone seen a deposit like this and had any luck removing it? If you think the oxalic acid is a good idea, how long do you recommend soaking? What ratio of acid to water should I use? Should I soak the entire cathedral or just the bottom where the deposit is? Thanks for reading and for any help provided.

JJ

[alfredo]

If the white deposit is calcium carbonate (calcite) then using oxalic acid would be a very bad idea! Under certain circumstances it can create an insoluble crust of calcium oxalate (what most kidney stones are made of) that is even more difficult to remove. Best use vinegar (5% acetic acid). Gallon jugs of vinegar are cheap, non-toxic, and with patience are quite efficient at removing carbonate crusts. It might take a few days, but is much less risky that using more aggressive acids like HCl (pool acid).