Don, Although the enamel fuses to the mica, the mica peels right
off, and has never caused a problem for me. The reason I switch from
the trivet to the mica, is that I avoid trivet marks on the enamel
where it touches at the edges, and also, by firing flat, I avoid any
problem of warpage.
Even, when I use my large kiln, I place my pieces on mica, then on a
firing rack for easy transportation in and out of the kiln.
You mention a mess when you fire on mica. Could it be that you are
overfiring your piece, and the counterenamel is flowing?? As I have
never had this problem I don't know what could be causing it, but
suspect it might be overfiring.