Each time the player successfully finishes working a sheet of glass, one of three things will happen:
Sheet Glass Fabrication ranges from unlearned (effectively 0) to 7. A lower skill means greater chance of breakage but also a greater chance of skill gain when a break occurs. At 7, the glass never breaks, and you gain the option to make mirrors by fusing silver powder to a piece of sheet glass.
Note that crafting any glass item can always fail if the bench's temperature gets too high or too low. That is an entirely separate mechanic from breakage, and this skill doesn't affect it in the least. Also, pieces lost due to bad temperature do not help raise this skill.
Note: Making Sheet Glass is not possible using Soda Glass
I was working ryhtmic and sheet glass at the same time one (in game) night. The region was nearly empty. I bounced back and forth between two spots picking grass next to the glazier's bench I was using. Both skills rose very fast, usually alternating. I went from level 2 to 6 in sheet, and from level 2 to 5 in rythmic in approx. 30 minutes. Noobiehotep