How did they make the old mosaics in the past?

Answer 1

Very similarly to the way we do them now. Cement was discovered, sort of invented, by the early Romans. Many mosaics were set in cement. We glue them down with glues that weren't available then, but they also had glues, especially rabbit glue which is still very popular in art. There are huge mosaics on the floors of Renaissance cathedrals. We've just added acrylics to our glues and to some of our mortars.