just make a reaction... you can find out how on the wiki.
btw, glass is a HORRIBLE material for anything designed to kill.
This is only true most of the time. You see, Glass has some of its material values undefined. This means that each time you load up dwarf fortress, you spin the glass lottery wheel, and whatever value happens to be in the register at the time gets drafted as the MAX EDGE value for that session (and some others, I'm simplifying).
For this reason, it is not recommended to make danger rooms with glass weapons -- they work for seasons upon seasons, but one day, glass decides it wants to be three times sharper than obsidian, and the next thing you know you've got a military of quadraplegics.