Mark has addressed the issue exactly right.

What you may consider "grime" or a discolored overpaint may indeed an original coating that was intended as part of the creation of the plaster casts. And I really have to emphasize this here - attempting to clean such objects without a professional conservator's assessment of them can lead to unintended and irreversible damage. And I am not talking about hiring a conservator to do the whole enchilada - just getting them examined and getting informed guidance would be essential in knowing what is safe and not safe to do in this situation.

You can locate a conservator by using the Guide to Services of the AIC. It's FREE. Go to:  www/aic.stanford.edu and follow the link for public information on finding a conservator.

Cheers!
Dave

David Harvey
Objects & Sculpture Conservator
Los Angeles, California USA

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