Molten pitch on a 1.5 inch thick ply base.
Pressing the pitch lap mat to form the facets.
Looking good.
Please don't stick.
Messy business!
Good contact between lap and mirror is essential.
Smear soapy water on the mirror surface and press on the lap, a channel allows air to escape. Repeat until good contact is obtained. Add extra pitch if needed. Place a lap forming mat over the still warm pitch lap and press with the mirror. Repeat until the grooves between the facets are as deep as possible. Remove the forming mat, add a mix of Cerium Oxide and water to the lap, re-press. Leave it to cool overnight
As seen through the mirror the lap should look like this:
During cold-pressing pitch laps have been known to stick to mirrors. This can be a problem. My solution is to cold-press with a layer of baking parchment paper between lap and mirror. It works a treat.
After cold pressing the channels between the facets will probably have closed up, this can be remedied with a hand saw and a little care.
Here is the finished pitch lap after one hours work with the 19 inch mirror;