I believe it is TARN-X. At least that is what I use. The only problem I have noticed with it is after you use it, the blocks tarnish much quicker next time around.
The first time I had read about people using it was on the Prometheus build. But he was spraying some kind of clear (I think) on the bare copper directly after. It does work really well though. Does anyone remember those commercials on T.V. when the guy dips the silver tarnished spoon in it and it comes out clean.
I don't know......Just a goofy childhood memory. The stuff has been around for at least 40 years though.