At the suggestion of friends I decided to set up an Etsy Store, Concrete Thinking, where I would offer for sale some of my concrete casts.
Since the vast majority of my casts are created using one-off paper molds I set about designing a few things suitable for use with rubber casting material from Polytek that I've used previously.
Coasters seemed straightforward enough.
They're 4" square and .75" thick, thicker than ordinary coasters, but I wanted to be sure they wouldn't crack too easily, and I generally like concrete when it gives the appearance of heaviness.
The positive and negative casts shown are made of Polytek 7420.
An imperfection showed up along the top edges of the mold very early on and were subsequently cast into each of the coasters. I'll correct this in subsequent molds.