Question: Are there other "unsafe" materials besides green-flame chlorine, what about reflective metals?

In my time at TechShop we learned that we were never to do any “uncoated” metals due to the possibility of a reflection. Another one to avoid is Polycarbonate materials as they will burn severely and that is not good for the machine, not sure what the combustion products are but I would not want to breath even the tiniest amount.

So, I did install my backstop. Then I taped a paper to it. Told it to do full power with a .5" focus for an engrave.

Ready for the Go button! So go!

I stopped it early with a hole burned through the paper and marking the tile beneath. I’m pretty sure the laser does NOT defocus enough to avoid reflection for potential damage. It’s hella bright.


So the manual and support are quite adamant about this, and I’m certainly not going to ignore them.


WOW, I’d put that in the “Avoid at all costs” category. That’s bright even on my screen!

It was VERY bright, brighter than hitting slate with full power. That’s hitting a tile (laser resistant) around .3" from the bottom. I’d hate to think what it could do hitting the reflective bottom and bouncing back.



Good point, could have used a lot better word for my intent.

Using my harbor freight stainless steel ruler. I coated with CRC Dry Molybdenum Lube (spray). Lasered it at 100power and 100 speed… then wiped off the moly.

It seems to have etched the metal… rather than marked… Seems to have pits in the metal now. You can see depending on the light angle, it’s dark or reflective - I think because of the raw unpolished metal underneath…

To test this idea, above the ® you see a “k” rotated 90deg… That’s been lasered without any moly at all…

Maybe harbor freight uses a special stainless that is meant for laser marking… Or maybe coats these in a very thin stainless layer to save costs, and my laser obliterates that. Is all I can think of.

My laser is still good. Done many projects since this test.


Here is my experiment with Aluminum marking, using CRC Dry Moly Lube (spray).
Lasered at 100power and 100 speed, 3 passes.

As you can see, the result is faint. And the laser needed the extra passes to get this dark. The metal appears to be very slightly raised where the moly was (the metal wasn’t pitted like the stainless was.).

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With dry moly lube, I’m always doing multiple coats with dry time between, then multiple full power passes. I’ve been able to get some ok marking, though fades a bit when seen at an angle.

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Yeah, I didn’t mention that that was with 2 coats and drying time.

It may/may not be overkill, but I’m usually using about 4-5 coats.


So really I want to know what metals will damage my laser.

Or if none will.

Or if any do, how can I tell, or is there a list?

Or my tests above, did I somehow degrade my mirrors?

Not seeing definitive answers like for “the green flame test”. Everyone unanimous on the no chlorine advice, that’s obvious. So this question is now about metals

Here’s the thing (well as far as I can tell anyway). Many metals are reflective of infrared light including aluminum, nickel (in stainless steel), especially copper, but…

I think the risky bit is if you get a specular reflection off of it. Straight back seems like it could melt the plastic of the head around the lower window and perhaps stuff inside of the window around the lens. This is because the focusing lens made a cone out of it whose apex is at the focal point and then it makes an hourglass shape after that and if it hits a mirror will turn around but keep expanding outward. If it expands enough it’ll no longer damage anything just warm it like a 40W light bulb would.

Seems like there is potential for damage here though since someone on the forum did an experiment to see if there was still a tight enough focus to do damage at the bottom of the unit. Plus support said, don’t let the laser hit the bottom of the unit with the crumb tray removed.

However, you don’t get a specular reflection off of any old hunk of reflective material. For visible light, you need to polish it by making the surface smooth by grinding finer and finer grooves into it until the surface features are very fine. It seems to me like longer wavelength light wouldn’t need as smooth of a surface as visible light to reflect specularly (any physicists around?)

So, I think to be safe, you’d stick with anodized aluminum (anodized has a somewhat diffused reflection) and only other metals that are coated with something. The reason I said particularly copper above is that if I understand it correctly, sometimes copper is actually used as a mirror in some infrared laser applications.

But, in spite of this, several folks on the forums have experimented with etching and cutting steel directly to no apparent ill effect on the laser.

I think this is why there isn’t a definitive answer like “green flame” because you’d need a test for specular reflection of infrared light and the definitive test for that is to shoot a laser at it an angle and then measure the power density of the photons that bounce off of it. And since a material’s behavior with respect to infrared light is different than with visible light (glass, for example, is transparent to visible spectrum, but opaque to infrared), you can’t go by what you see.

(By the way, another material to avoid is anything containing fluorine, when it meets atmospheric water it makes a dangerous, corrosive gas similar to what happens with chlorine, only worse.)


To add on a bit to mark’s comments, one must avoid the top three Halogens: fluorine, chlorine, and bromine. Probably best to avoid all halogens, but I’ve never seen Iodine used in a plastic, and I’ve never run into Astatine into my life (except for jigsaw puzzles), probably because it is the rarest naturally occurring element on earth.

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That’s correct. It’s sometimes used for homemade mirrors in the D-I-Y laser community :slight_smile: (Or those modding out a K40 type laser.)


I don’t like squirrels as they attack our fruit and veg so HCN is too good for them.

Think of your fruit and veg?

I have used HCN when working with silver and gold and never got dizzy from it. a big advantage is that unlike mercury it does not build up in your system, and reacts with everything just about so did not hang around even in solution.

Nothing about Aspen on this list. Do we think that means it’s safe?

There are some relatively cheap 1/2" x 6" x 4’ Aspen boards available at Menards that I was thinking about picking up.


I think Aspen is a close relative of Poplar. It’s likely not any more dangerous than other stuff we cut in the Forge.

Nothing we cut or burn or cook is completely “safe”, but at reasonably small doses with good ventilation the risks seem acceptable to most people.

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Dunno… So I looked.
Found info on Aspen on indexing extras (very bottom of page).
Interesting read and I can see why they did not break it down into individual species since it would have really extended the listing.

Way I would approach this is to call it Popular, which seems to be all inclusive to popular, cottowood, and aspen.
Seems safe but that Low Flammability probably means tweaking the pews a little. (A is for Adventure).

Added: Info seen during this quest below:

For identification purposes, Populus species can be divided into two main groups: cottonwood (including poplars), and aspen.

And from Wiki:

Aspen wood is white and soft, but fairly strong, and has low flammability. It has a number of uses, notably for making matches and paper where its low flammability makes it safer to use than most other woods. Shredded aspen wood is used for packing and stuffing, sometimes called excelsior (wood wool). Aspen flakes are the most common species of wood used to make oriented strand boards. It is also a popular animal bedding, since it lacks the phenols associated with pine and juniper, which are thought to cause respiratory system ailments in some animals. Heat-treated aspen is a popular material for the interiors of a sauna. While standing trees sometimes tend to rot from the heart outward, the dry timber weathers very well, becoming silvery-grey and resistant to rotting and warping, and has traditionally been used for rural construction in the northwestern regions of Russia.

Happy Trails and Keep On Burnin’