Molten glass is used in the Crafting and Construction skills, as well as some quests, to form glass items. It is used in construction for bedroom hairdressers, and skill rooms.
Molten glass can be created through the Crafting skill by using a bucket of sand and soda ash (both can be bought at a charter) on a furnace, granting 20 Crafting experience. Alternatively, you can use the lunar spell Superglass Make, which converts all buckets of sand and all seaweed or soda ash into molten glass, granting 10 Crafting experience per pair of items.
In order to make many of the items, you have to use a glassblowing pipe.
Molten glass is used in some quests such as Observatory Quest, Horror from the Deep, and Another Slice of H.A.M.