I use any carbon based glue, including mucus (yes, nose drippings)
cow gum, school glue - in fact almost anything including superglue.
The purpose of the glue is twofold. First to hold the granules in
place and second to combine with the oxides of copper to leave a
pure copper micro-coating on the granules and the object.
The copper salts I use are copper sulphate, copper nitrate or even
"blue" acid from the pickle pot.
There is a series of reactions.
The copper sulfate is reduced by the heat to copper oxide, the
"glue" is reduced by the heat to carbon, the copper oxide is reduced
by the carbon from the glue to copper.
Then the copper combines with the metal to form molecule thin coat
of a lower melting point metal (solder.) This alloy attaches the
granules to the base and then diffuses through the two - so much so
that it almost “disappears.”
I’ll repeat that you can use almost any copper salt or oxide and any
carbon based glue.