Any metal forming process creates heat, in some amount. Rolling
produces a lot of heat in a very small area at one time (see
attached). As the pinch increases the local heat increases. With a
very heavy pinch, the metal becomes plastic, meaning there is enough
heat to reduce the metals strength making it easier to form. If too
much heat is introduced the rapid heating and cooling will force the
crystal structure to grow and imbrittle the metal causing the ingot
to fracture. When working with production rolling mills the ingots
are routinely very hot to the touch, with gloves required. When
pushing the process the ingot can be hot enough to require tongs. If
several ingot are worked rotationally the rolls themselves can be
come so hot that the heat treatment is damaged, so we monitor them
as well.
I have melted hundreds of Kruger coins in my life. Back in the '70’s
and 80’s in South Africa, gold was very difficult to buy legally. So
KG’s proved an easy alternative, and we used it to buy gold that was
not on the police records. Only copper is used to bring it down to
22kt.in the minting process and it contains 2.8 grams of copper when
minted. Hence that ugly red look. After alloying with 6 grams of fine
silver, it produced about 40 grams of 18.5kt yellow gold. Nice yellow
alloy. Never, ever had a problem casting, rolling and drawing it… I
doubt that micro impurities affected your fabrication. I suspect
that your machining methods need revisiting, rather than the KG
alloy. To say that the impurity of the copper used in the coin causes
fabrication difficulties is completely untrue.
There have been more than 50 million Kruger coins sold and believe
me, if impure metals were used, the banshee howl would be heard world
wide.
And to another historically challenged poster, the Kruger coin is
not named after an “apartheid politician”. Paul Kruger (1825-1904)
was an earlier politician who had a particular dislike of the
British, as it were. Apartheid was started in 1948. It was the only
idea the Nationalist party had in some 40 years of rule and it
proved to be a very bad one.
Not suggesting the Krugerrand is an impure coin, but, for metals
production that requires maximum and reliable working properties
from the alloy, the Krugerrand is useless as a source material.