This really has me curious, so I asked a couple of people with chemistry backgrounds to speculate on the cause.
- The water is likely just to hold the biomass (aka paper towel) in place and close to the metal.
- The laser could be forcing the salt in the biomass to result in a directed rust.
- The laser could be combining the biomass with the metal to form an oxidizing agent. The black line might even be magnetite (Fe304). (This sounded like the better explanation to me, but I’m not a chemist.)
Today or tomorrow, I’ll give it a try.
Assuming that it is salt, then salt water on a paper towel should work better.
If the water is only there to keep the biomass close to the metal, then kleenex might be better than a paper towel. (My thinking: thinner = closer.)
If it’s really just a need for biomass, then a thin layer of dried mustard (or Brother Mel’s BBQ sauce) will probably be the best option. By removing the water (waiting an hour for the mustard to dry), we might get a cleaner line since there isn’t water to do any refraction.
Then again, maybe dried Elmers glue will work. Dries fast, easy to remove, dries thin, and has biomass.
I can’t wait to play!