Chimney

Since glasswood is the only kind of lumber that has been positively identified by the people of the Deeganverse as such, they are forced to use it for the purpose of warming their houses. Burning glasswood produces a smoke so thin, so volatile that it can pass through solid bricks, allowing the Fifth-Casters to not build hollow chimneys in their fireplaces. Instead, they build solid brick columns. However, while glasswood smoke can pass through bricks, it doesn't pass through cloth. For this reason, people wanting to look all badass and cool often prowl atop these chimneys, so that their capes will billow out dramatically, lifted by the fumes.

A side-effect of glasswood fumes is terminal stupidity.

Possibly the source of the Deegandrug.