You _might_ find a commercial two-pack filler for sale for doing this.
They used to be around, but they're pretty rare now.

The other alternative is a good epoxy (non-yellowing with age and heat)
and a mineral dust filler. I'd use West System's. When you put it in
place, tape a piece of Mylar (polyester) tightly across the surface and
this will give a glossy surface without polishing.

White tiles are fairly easy, terracotta not too bad, but '30s off-pink
are a bit of a nightmare.