Carbon, silicon, aluminum contaminations on contact

We are having contaminations on electro-mechanical relay contacts, mostly carbon and silicon, sometimes aluminum. We use AK225 as cleaning solvent. Also, we use ovens to bake our parts. We've been trying to identify the source of contaminations but it's not easy to pinpoint where and when these contaminations were introduced to the parts. Does anybody have any recommendations how to proceed on the troubleshooting of this particular problem.

It *could* be coming in from the outside air. Being less than 10 miles from the ocean there is some sand (Silicon Dioxide) in the air, and more in the soil. If you are near or downwind of a major street, truck route, or bus route, the exhaust is a significant Carbon source. Enough speculation, I'll let someone else try the Aluminium.