They were popular not only in Mesopotamia but also in Egypt and the Aegean. So that glass from Egyptian or Roman times is more ornate and complex than early medieval glasswares. Up until the latter part of the 20th century, glass making was a closely guarded skill. To a certain extent, it still is. As we no longer have secret guilds to apprentice into, trial and error still seems to be the mainstay of learning to work with fusible glass.
Modern students learning to use fusible glass can read books and take classes.
See the Bookstore link and selected titles at the bottom of this page. The larger the piece is, the longer the time required for this process. In general, it takes about 20 hours before the fused piece is ready to be removed from my kiln. The COE means that the glass heats and cools at the same predictable rate. Without this feature, different pieces of glass could not be fused together, they would shatter.