from the April mosaicsmalti.com newsletter:

TIP OF THE MONTH

Strictly by accident, a mosaic friend discovered that plain ole coffee (and also Diet Coke!) can dissolve dried and cured thinset. While IÂm so sorry that this lesson was learned at the detriment of her mosaic, it was great news for those of us with thinset-encrusted tools that we thought were ruined. Just place them in a cup of this morningÂs coffee for an hour or so, and the thinset begins to crumble right off! It also dissolves thinset off of discarded pieces of smalti, chucks of thinset adhered to your studio table, etc. (Just be careful when drinking coffee around your completed

mosaicsÂdonÂt spill!!)

Here is a link that might be useful: Silva's blog