On Mon, 18 Apr 2011 17:36:42 +0100, Baron wrote:

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 !

BeO is very expensive. I would be very surprised to find any in such
application.