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Message:: [QUOTE="-jeffB, post: 2487684, member: 27832"]So, if you give a coin an acetone bath, and then "dry" it by rubbing it vigorously with a paper towel, it gets hairlines -- and that's because of the acetone bath? <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie11" alt=":rolleyes:" unselectable="on" unselectable="on" /> If you "let contaminated acetone dry on the coin", first, you're doing it wrong -- you use two or three separate baths, as I understand it. Each time, the drop or two of contaminated acetone from the previous stage is diluted by the entire bath full of clean acetone. This is not going to leave a film, at least not one detectable by the human eye (or TPG instruments). Suppose you've got a Morgan dollar that's caked with gunk, a full gram (and at that point we're talking "wad of chewing gum" amounts). Dip it in 50ml of acetone, about a shot-glass worth. Assume that 100% of that gunk dissolves in the acetone. Pull the coin out. Let the acetone, with its very low surface tension, drip off. You'll have maybe a tenth of a ml of acetone sticking to the coin's surface, and that bit's share of the gunk amounts to about two milligrams. Now do another bath. Those two milligrams dissolve into another 50ml of clean acetone. Pull the coin out, let it drip. Again, maybe 1/10ml of acetone doesn't drip off -- but that bit is carrying four micrograms of gunk. Now, do the third bath. Those four micrograms dissolve into another 50ml of clean acetone. Pull the coin, let it drip, and the acetone staying on it until it evaporates leaves behind eight nanograms of the original gunk. And a full gram of gunk is a big stretch to begin with.[/QUOTE]

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