@jbv, I thought about including it, but then when I hit reply and shutdown my computer I realized I had forgot! The product I used was black (more like charcoal…dark grey/gray 
). It’s flat black when applied, and if I remember right, turns a little more deeper black when “baked” on. After thinking about this more, and how some might tackle etching glass, I’m thinking if I were to do this again, I would apply a mask to the metal, cut out on the laser the area I would want marked, take it out of the laser and remove the mask where I want to apply the dry moly, spray on the dry moly, let it dry, pull off the rest of the mask, and take a blow torch to piece of metal to “bake” on the dry moly. This seems a lot more practical than wasting your laser tube life on something that could be done at a higher temperature in less than half the time (conservatively speaking)!
Nice. So not a sienna/brown like I was thinking could happen. Good to hear. I would still be interested in seeing your pictures if you happen to still have them.
I agree. And then you would not have as much risk of rubbing off the marking as you would not have to scrub off the excess moly afterwards.
Why not just use Cermark/Thermark? It “just works” and the marks are permanent. Aluminum can be tricky though, never could get the marks to stick to 6061. Might sound pricey but a can goes a LONG way, and your time is probably worth more than the $20 you’d save testing moly lube four hours if not days.
Do you have photos of the results of your trial?
We recently built a custom laser cutter for a group of artists from Mexico City called Astrovandalistas that they used to engrave a bunch of rocks and spread them around as sort of retro-future artifacts. You can see some pics of the cool rock engraving they did on their FB page.
The custom cutter we built had to solve a similar issue as the GF. Because wild rocks are never perfectly flat we needed to keep the focal distance over the entire surface. Instead of cameras + motors we just 3D printed a spring-loaded lens holder and put some wheels on the bottom so it could skate across the surface. Worked like a charm!
The machine could also be bolted to rock walls to do futuristic tech-cave drawings.
p.s. One reason the moly lube might not be sticking permanently is cleanliness of the material. Make sure your metal passes the Water Break Test before applying any coating.
At the time I was a student so it was cheaper to go this route and at that point my time was cheaper! Plus, it was something to experiment and see. There’s nothing more satisfying than finding a hack that works just as good and is load cheaper. In this case, I think I might just buy the Cermark (especially the pearl color!), but not after I try the blow-torch idea on the dry moly lube!
I looked this morning and couldn’t find it 
I can see it in my head but that’s not useful. Now that I think about it, I might still have it at home in a box. Sorry!
Yeah, #picsoritdidnthappen.
This was done with Thermark on Titanuim alloy knives with a 35 wt laser. I’ve also marked stainless steel, brass and regular steel alloys.
you also use Thermark on the brass?
Yes, here are the settings for the laser…
http://www.thermark.com/TM_Downloads/Settings/LMM14_Laser_Settings.pdf
This is what I am going to be most interested in. I already have an order for 50 Cal BMG shell casings with text engraved (marked) on them for custom pens. I am going to be doing a rather large lot and will be looking into the different marking sprays as well as dry molly lube as that might be a suitable replacement to cermark and thermark seeing as those two are rather pricey!
Just to clarify, having operated several different industrial production grade CO2 CNC lasers (Mitsubishi & Bystronic) I can speak on which metals are the hardest to cut/engrave with a CO2 laser.
Worst to easiest:
Copper
Brass
These materials were rarely cut in the shop, their reflective pretty much prevent them from being cut with a laser. We tried many things, from various focus depths, assist gas pressures, modifying the surface by fogging/misting specialized surface treatments, even mild sanding. The best results were achieved with combining an 400grit orbital sand AND application of a surface treatment mist. Even then the second shift operator cause machine damage when the beam reflected inside the laser and distorted some of the cutting chamber “windows”.
Aluminum
Stainless
Steel
These materials were and are cut all the time, but not with 40 watts More like 4000 watts, and 100->280psi assist gas.
not to mention an issue i’ve read about with copper is how reflective it is when it melts, so your laser needs special shutters; even when you manage to cut it, as you indicate, you’re always going to have the reflectivity issues…
Don’t forget that copper can also be smelted with arsenic