Quote Originally Posted by Ian Grant View Post
Because the grits grind both surfaces, if the material's softer it'll get ground in preference to the glass if harder the ground particles from it will interfere with the grits ands it can scratch the glass being ground.

When grinding plastics glass tends to cause scratches very easily, so you grind with something similar.

No. If the material is softer, the abrasive will embed in it, forming a lap, grinding the harder material. It doesn't have to be a block of glass, a block of steel will work as well. One advantage of using a block of hard material is the fact that the abrasive will not embed, making it easier to attain a random pattern.