The classic contact pattern indicator is white lead (lead carbonate) and linseed oil. I've got a lifetime supply of it in a 5-lb. can of Dutch Boy white lead pigment I bought on Ebay a couple of years ago. It's dried up, but grinding a little of it at a time back into a powder with a mortar and pestle and adding a few drops of linseed makes it usable again.

If you're a tree hugger and you go into heart palpitations and froth at the mouth at the mention of "lead" anything, try Prussian Blue as an indicator.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!