You don't want to make the pitch too hot to begin with so is won't bother the plate glass at all. I generally use a microwave to heat the pitch and tool at the same time. I get the pitch warm enough that it's slightly painful to maintain contact with it and it's generally quite plastic. like butter at about 80F. This will allow the pitch to adhere to the tool and yet flow readily whereever it's needed. I also use a mold to form the pitch squares on the tool. The latest is to use the plastic fencing for flower beds which has a grid of about 3/8" or so. Place that over the pitch and under the mirror and press until most of the squares are in contact with the mirror. Don't be afraid to redo the mold contact again if necessary. The grooves don't need to be very deep at all to work properly. Also, get some of that netting that the wife uses for things like old time hats and use that for micronetting to make smaller blocks of contact. Do insure that you don't get the stuff buried into the pitch (which happens when the pitch is too hot) as that will merely mean cracked pitch pieces all over the place as that stuff makes extra strength Ex-Lax very mild as laxatives are concerned.