What is the composition of the discoloration build up on long buried clad coins?

The composition depends on what metal the coin is made of , but a buried coin is exposed to acids and minerals in the soil , the reaction of the metal coming in contact with the substance and slight electrical charges ( natural , in ground electrolysis ) is what causes the surface of the coin to oxidize or break down. A stronger acid can strip off the outside layer of oxidation or discoloration , and Im not sure about the exact reaction of a base like ammonia used to clean coins only that it softens the material you want to clean off a coin and has the ability to penetrate so that it comes off more easily. Either one can also ruin or discolor certain metals if not done correctly but generally , cleaning with a base is less destructive than with an acid.

When tumbling with acid , the acid does most of the work.....when tumbling with a base the media does most of the work. I used to use vinegar ( acid ) for tumbling and it was quick to work but usually caused more damage to the coins. I now use ammonia ( base ) because even though it seems to take longer it does less damage to the coin but still gives a good result. Just be sure to tumble pennies separate ( and shorter time ) with either one.