I gave this new method of mosaic a shot and now I'm totally in love.
You place the tiles on the bottom of the form, back-side-up, then pour the concrete on top. The following day, you pop them out of the form, use a stiff brush and water to clean off the "top" and here you go.
The paper shifted a bit on this one and they ended up crooked. Note to self to use something to keep them stuck in the future--double-sided tape maybe.
I used shelving paper, sticky side up, to adhere the tiles in place. It's a must that the tiles have one flat side that will adhere well to the sticky paper. Place the finished design in the bottom of the concrete form. I poured a "slurry" over the design, which is just some of the concrete mix with more water so it's a creamier consistency to get in between the tiles. Fill the rest of the form with concrete. I used a mortar mix for these, reinforced with a sheet of mesh.
The glass cubes came out BEAUTIFULLY! The effect is so cool. But I've got a ton of leftover tile--ceramic, porcelain, stone--so I'm going to make more using different materials. But I love these! They're so fun to make, and turning them over the next day and scrubbing them up is like opening a Christmas present. You don't know exactly what you're going to get.
Happy, Happy, Happy!!!