In the 70's Plaster of paris skim coated with thinned epoxy paint was the basis for my molds. Aluminum and "home molds" were totally unknown. The worms if left curing directly on an aluminum cookie sheet overnight would discolor. I don't know if it was the plastic, or the dyes, or the aluminum. All three have made great strides since 1975. Even aluminum has better alloys. I used window glass back then.