I also tried using some dry moly lube as an engraving agent. This also works well. The laser can't touch metal, the metal acts like a mirror unless much more powerful than I have.
I did a test with a quarter of the tin at different number of passes. Top is 4, bottom is 1 pass. There is a cut across just to check the pass count.
I burned the tin to darken it. I also used moly lube to label it. I accidently labeled the wrong side.