David in Normandy coughed up some electrons that declared:

Dave Liquorice wrote:
I'd not bother with a plastercine well and definately not oil of
anysort as a coolant/lubricant (it'll get into the grout...) little
bit of water from a sprayer or small squirty bottle will do.


I use a plant mister to occasionally spray the hole and drill bit. If
the bit gets too hot it will blunt. Very slow process though. I used
ordinary, brand new masonry bits without hammer action on some very hard
floor tiles and it took me around fifteen minutes per hole! After six
holes each bit was completely blunt. Someone suggested I should have
started with the smallest masonry bit available then work up sizes. I
live and learn.


Hmm. Perhaps a special bit then - or one of those Bosch ones....