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The main ways to damage your machine with a material are through fire, melting, or chemical reaction.
Fire:
Layered materials like cardboard and stacked papers can be fire hazards. Some foams are prone to fire. Acrylic is prone to fire. There are good reasons why Glowforge insists that you monitor the machine when it’s cutting, fire is at the top of the list. Search the forum any time you’re trying a new material, chances are someone’s tried it or will have advice.
Melting:
Lots of plastics aren’t great candidates for lasering, they melt rather than cut. HDPE (think milk jugs or soda bottles) is a prime example. It’s possible to laser it, but it’s quite melty and you could end up with a mess.
Chemical:
Cutting PVC–aka vinyl or polyvinyl chloride–forms acids inside your Glowforge and will kill your machine, possibly the very first time you cut it. The bottom line is that you should never use it. There is a lot of discussion on the forum:
https://community.glowforge.com/search?q=PVC%20damage
Some resins can also pose the same chemical risk as PVC. Here’s a great tip:
Other:
Some (not many) materials reflect laser light, copper is a well known example. Cutting or engraving reflective materials could theoretically cause your laser to bounce back at your Glowforge and damage it. I’ve never seen this actually happen to anyone, but it’s a good idea to just avoid copper altogether. If you’re not sure if a material might reflect CO2 laser light, search the forum and the internet at large.
The main way to damage yourself is via toxicity. Almost everything you laser will produce toxic or harmful fumes, that’s the whole reason for good ventilation and/or filters. There’s an old saying, “the poison is in the dose”, so the entire idea is to get rid of all the smoke or gasses or dust that your laser might kick up and minimize your exposure.
In general when it comes to toxicity and otherwise dangerous byproducts of lasering a specific material, the final word is on a document called a material safety data sheet, commonly referred to as an MSDS. If you want to be completely sure, try to get that document from the manufacturer. In particular look at the “combustion” or “thermal decomposition” sections. MSDSes can be a little tricky to read, but they’re the authoritative source for this info.
A nice roundup of things to look out for:
And here’s another list of "don’t"s in plain english:
And another:
As with anything, this is a guideline and you should use your judgement on any and all info in this post and anything linked here.