I hope this is the correct forum for this question. If no, please advise.

I would like to take some pictures of engraved lettering on a gun, that is all but invisible, so far, using my camera. The lettering is hard to see with the naked eye too.
What kind of substance is used to fill in the letters, then wiped off of the surface, leaving only the letters visible? I thought, maybe, crisco, or lard. Then, since this is an antique, I don't want to use something that might start to harm the finish.
If I keep writing it will probably only sound more and more confusing. I hope you get what I am after.
thanks,
LouM

This is what you need:

http://www.markal.com/lacquer-stik/