Empirically. I got my hands on a piece and measured the density myself. It's roughly 430 kg/m^3.
Still have a box full of the damn stuff, actually, just in case someone can figure out any half-sensible way to translate the other properties into something DF uses.
Hmm, I think I remember coming across the material properties thread a long time ago.
Wow that was some data gathering. It's kinda strange to think that along with DF you can get a dataset with the properties of various real world materials.
Part of it was crowdsourced to my fellow forumites, but a lot of that thread was just me, Google & Wolfram Alpha. You could probably still find it buried somewhere on the suggestions forum with that giant table of materials, material properties & sources.
Here is the thread where the initial !!Science!! on whether bolt weight made an impact on their lethality - [Dwarven Research: The Effect of Bolt Weight on Crossbow Performance](http://www.bay12forums.com/smf/index.php?topic=115683.0)
Still have a box full of the damn stuff, actually, just in case someone can figure out any half-sensible way to translate the other properties into something DF uses.