dave
The tiles are set with a flexible adhesive made for ceramic tile. The backer is made up of 2 layers of 1/2" 7 ply plywwd. alternate direction glued and screwed together. The thickness was partly because I needed this thickness to get face of tiles where I needed them in floater frame. I painted edges of ply with black flat paint as well as joint locations before setting tiles. I set tiles yesterday and let cure overnight before finishing this morning. Decor floater frame.
Also there is a black spacer that fits under tiles to support them against the floater frame. This does not show against the black frame. The whole thing only weighs 16 lbs. I used 4 screw strap hangers screwed directly to plywood backer with 100# braided wire. I did not want to let the frame carry the weight and this works well because the backer is recessed about 1/2" from back of frame. The finished size is 18.25x24.25 (6" tiles).
Randy