I use Elmers all the time for mosaics and it works fine. You can mosaic
over lots of surfaces-like clay tiles, cement stepping stones (about $1 each at
Walmart), flower pots. I have been using some 1/2 inch fore core board that
was donated but I also have been taping 2 thinner pieces together. I get
this free from frame shops. The main thing is to get students to put the pieces
really close together. Use the sanded grout from Lowes or Home Depot and mix
it up really thick like putty. If you buy white you can add a squirt of
acrylic or tempera for color. Meryl