I agree with Dave that it's likely to be an IMC.  What are you tinning?  Back in my first job in printed circuits in the through-hole days and when we used gold-plated boards, I was engineer for a drag solder machine (note that I did not pick this equipment, and do not advise it).  We had to lower the temperature in the pot, let it sit for a while, then dredge out the gold-tin intermetallic from the bottom of the pot.  You should definitely have this stuff analyzed if you've been pre-tinning gold parts.  It could have some commercial value.