Stan,

The coin analogy may be instructive. You know that coins are dirty and
greasy after they have circulated for a while so why do you think the grease
is a function of the caulk?
So, what are you actually caulking and why do you believe that it is
clean? I suspect that you are seeing a residue of crud from your coin that
has reacted with the acid of the caulk in some way.

Dave M.