Posted 10/30/2010 11:47 am
It also comes from impurities in the alloy. The pre WWII cents are 95% copper, 3% zinc, and 2% tin. Most of the time the alloy is fine and even slight hot spots of tin and zinc won't cause an issue. However, if some ash (carbon or silicon) or slag (bits of metal and non-metal that don't alloy) get mixed in they can totally fracture the metalic structure of the intended alloy and cause laminations once the mix is rolled into cent thick sheets. So, in some sense it may be an over mixed alloy that is the issue at times, where something that should have been skimmed off the top or stayed on the bottom get mixed into the metal that gets poured into the ingots that later get rolled into sheets. It is more of an impurity issue than a mix issue really. Lamination issues can also arrise from overly aggressive rolling into sheets. If the step from one thickness to the next is too great, the metallic structure can be damaged and fractures can form in the sheet that later become weak spots in a planchet and untimately de-laminate.
