Here are two simple tests for determining if leather is either 100% vegetable tanned (safe to laser), or chrome tanned (definitely not safe for you or your machine). I’ve seen this info buried in other threads, but I thought a demo and freestanding post might be appreciated. Also, keep in mind that there are many other kinds of tannage (alum tawed, oil tanned, etc.) that may or may not be either safe or laserable, but the only one you can trust is 100% vegetable tanned, and you should never laser chrome tanned.
Siegal Effect Test
First, here’s the Siegal Effect test, created by Steven Siegel, who inherited his family leather business and went on to get a Master’s degree in physical chemistry. He serves as an expert witness in leather-related cases (and his company, Steve Siegal Leather sells some REALLY nice bookbinding leathers). To test, you just drop a thin strip of your leather into boiling water. If it immediately curls up into a spiral, it’s 100% veg tan. If it doesn’t, there’s some presence of stuff you don’t want to laser.
In the ash test, go to a well-ventilated area and light a small sample of your leather on fire. If it is hard to light and goes out as soon as you remove the match, it’s veg-tan. Its ash is also an indicator–smear the leather ash on a white piece of paper and the ash will show up as black. If the leather smolders and holds an ember, it’s a no-go. If its ash has a greenish color, it’s definitely chrome tanned.
For more info on these two kinds of leather, go here: http://rainierleather.com/quick-guide-vegetable-tanned-leather-care/