Cleaning Blocks?

[Neutronman]

I've always used 1/2 water and 1/2 vinegar ... if you have some light oxidation, an old toothbrush comes in handy.
There are some wiz-bang products out there, never used them.

Vinegar or ketchup.

Personally I use small amount of ketchup on rough cloth, then finish with vinegar and water and Artic Clean prep......

[asura]

Speeking as a jeweller sulphuric acid is the best method of removing copper oxide, but any acidic compound will work, just not as fast or througherly.