Here is a large brachiopod I found recently at Paulding, OH. Distinguishing Megastrophia from Stropheodonta titan can be difficult, but I think this one is S. titan due to its low profile. The side shown here is largely exposed, but the other side was covered in matrix. Unfortunately, this field photo is the only "before" photo I took.
I decided to leave this one on a pedestal of matrix. The matrix was really sticky. The bulk would pop off with the scribe, but a thin layer clinged to the shell. So rather than scratch up the shell with a pin vice or take hours blasting it, I prepped this one chemically with KOH flakes. About 80% of the "prep" was done chemically. The Silica Shale is rich in organics, and the KOH turns the shale into mud. I made several applications on the thick areas of matrix. Final procedure included reversal of the base by a quick dip in 5% glacial acetic acid and then a good soak in water.
Here's the result.