I will laugh if my use of a wet paper towel ends up being the mind blowing find here! lol Of course I now need to try it…
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If this seems to be possible, there will follow a temptation to try other metals.
The general thought seems to be that the possible reflection of the beam bak onto the exit lens is going to do a lot of harm.
Is there a general approach that might prevent this ?
I can only think immediately of hitting a convex urface for testing, so that the beam is scattered(de-focussed) .
I tried the mustard and vinegar on aluminum and it did NOTHING. 
Try an alkaline solution - soda ?
The alkalis will attack ally anyway, but hitting it with a laser will up the ante considerably.
Le Chatelie’s principle !
I wouldn’t expect it to. Aluminum naturally develops a thin layer of aluminum oxide on its surface when exposed to air. Aluminum oxide is transparent so it’s not visible. (Sapphires and rubies are made of aluminum oxide; the colors are due to impurities. A pure crystal of aluminum oxide would look like colorless glass.) Aluminum oxide is quite tough and pretty inert under normal conditions; it’s not likely to react with anything and is why aluminum doesn’t corrode like iron.
If the steel is turning black due to the formation of oxides (which seems likely, given that plain water is working) then it clearly wouldn’t work on aluminum given that aluminum oxide is, well, clear. (Not to mention the fact that every piece of aluminum you look at is already covered in aluminum oxide.)
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OK, so what would interact with Aluminum? I know cooking tomatoes in aluminum cookware, or having foil make contact with a tomato-based sauce, leads to discoloration, and transfer of something unpleasant from the metal into the meal.
You can’t wash aluminum in the dishwasher or it loses any finish, and I use the combination of dishwashing machine fluid and high heat to clean things that would otherwise require physical contact. Hmm…
if your arm rubs along the side of an aluminum canoe while paddling it turns black. Your arm, not the canoe.
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Very acidic substances weaken aluminum oxide and tend to find their way into small fissures to reach the bare aluminum inside. Tomatoes have a pH of around 4.5, which is pretty acidic.
If you have aluminum foil on a pan made from another type of metal and the tomato comes into contact with both then you end up with a battery. You’ll end up electroplating aluminum onto the pan (or vice-versa). Fun! (You may see holes develop in the foil as it’s eaten away.)
Aluminum is often colored or otherwise modified using an anodization process.
While the layer of aluminum oxide that is deposited during the anodizing process is much thicker (5–15 nm) than the 2–3 nm layer of aluminum oxide that aluminum develops naturally, you tend to get more and larger fissures in the anodized surface. (This can be a good thing: the rougher texture allows for coatings to stick better. It also provides places for dye molecules to live.) So anodized aluminum can be more susceptible to damage from acids.
Because the natural aluminum oxide layer that re-forms afterwards is thinner, marks made by layering off the anodized aluminum oxide layer often remain visible. The thinner, smoother aluminum oxide layer is shinier than the thicker, rougher anodized layer. (And of course if there were dyes in the anodized layer they won’t be in the newly-formed aluminum oxide layer, so there will be a color difference.)
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See my earlier post.
Perhaps you people still refer to it as ‘lye’ ?
Try dropping cooking foil into hot washing soda - standard method for kids to produce hydrogen.
Please feel free to school me on this, as I know I’m showing off my ignorance here. I know just about zero regarding metalic properties or chemistry (just ask one of my exes. ::rimshot::).
Doesn’t that suggest that hitting a piece of aluminum coated with soda with a laser would release free hydrogen inside the Glowforge? Unless I’m completely off base, and I probably am, I’m thinking it wouldn’t be enough hydrogen to make a Hindenforge, but…
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Yes, you’re right !
I would expect the hydrogen to be instantly consumed by all the oxygen present and converted into water, so no big deal.
The amounts involved are definitely minute, and not a problem for the fan assist and vent to cope with.
Sorry, I am new to this thread and have not checked out any of the referenced videos. This is an intriguing possibility for the Glowforge that I had not considered. Has anyone seen a faux Damascus pattern done in this regard? I can see some great possibilities here
LOL! Having just said “maybe not” after seeing the $80 price tag on a can of ceremark, you’re now totally my favorite!
I have some experimenting to do 
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Not water alone, water plus paper towel.
Sorry for resurrecting an old thread, but nobody ever pointed this out, and I’m wondering if the act of burning the paper towel created enough of some kind of chemical reaction to help with the marking. Wood ash is alkaline, right? Isn’t that where lye comes from? (It’s been a long time since organic chemistry, so I’m not even trying to pretend I know anything here…!)
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Unless you choose acid-free paper towels, most are going to be acidic to start with. But what then happens when you burn it, like you said, the products might be alkaline. But again, the steel is going to be attacked either way, I suspect, just so long as the pH is away from neutral, ie plain water.
John 
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Aren’t you afraid of the fumes? Mustard gas maybe?, Dr Evil laugh
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Stop, stop you are killing me! LOL - mustard gas
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Too funny 
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