When I first started mosaicing, I used titles and old ceramics, but these days I mostly use art glass. Glass is easier to cut and grout because all the pieces are the same thickness.  I mostly engage in mosaics when we are down at our holiday shack six hours from home ( my kiln is not transportable so I cannot do fused work when on vacation). I love thinking about colour and find it very meditative to do mosaics.