N_Cook Inscribed thus:

1.8mm slabs of porcelain under TO220 devices.

I would check that ! Its more likely that they are Beryllium_oxide !
http://en.wikipedia.org/wiki/Beryllium_oxide

Tried grinding a clearance notch on the side of one
pad but got nowhere as ceramic.
Not only that but I had no trouble holding the slab in fingers while
trying to grind a slot, it was barely getting warm - good thermal
insulator or small grinding wheel not generating heat as not cutting
into the material ?

The dust from grinding or abrading Beryllium_oxide is highly toxic !

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Best Regards:
Baron.