Leo Montagnese with Omega Modern Mouldings in Woodbridge, Ontario makes faux stone mantels that look like real carved stone. When you look at one and compare it to a plaster-molded mantel, you notice that the colour is different, but mostly that there are interesting and random holes in the surface that make it look like stone. So I set out to find out how they made the holes.
Let's work back from the final product. The thick bread-dough-like plaster is hand placed, not poured, into the mold. That allows the air holes in the mix to stay. It is carefully patted into place to fill the whole mold, but without pushing out the air holes. So that is what keeps the holes in the finished product.
But where do the air holes come from? When they mix the product, they do it with a drill-powered beater, giving them a good "feel" for the mix. That too is part of getting just enough, but not too much, air into the mix.
But let's go back to the ingredients that go into the mix. Oops, Leo is not talking. He said it took years and lots of garbage mantels to get the right things into the mix for the colour, texture --- and air bubbles. I thought I was hot on the track to figuring this out -- but some things in life are just classified secrets. We can, however, appreciate how the absence of a little material (the air holes) makes something look so good.