The Romans made beautiful glass that was clear and thin glass before the Roman Empire collapsed. When it collapsed the water in the astrolabes dried up and people had to move away and find nice clean water. Since they had to survive they had no time to make glass and improve their skills. People forgot how to make glass and other skills and had to start farming in order to provide for their families. When they had a steady income and could maintain a steady supply of food they started doing things like making glass again. However, it was not as nice. The glass they made was thick and the quality was not as good as before because it was dirty, not as pure. It still worked well and looked nice and they still could sell glassware and make money.