discouraging" presumably because of the absorption of neutrons by some unknown impurity. So, in December 1940 Fermi and Szilard met with Herbert G. MacPherson and V. C. Hamister at National Carbon to discuss the possible existence of impurities in graphite. During this conversation it became clear that minute quantities of boron impurities were the source of the problem.

As a result of this meeting, over the next two years, MacPherson and Hamister developed thermal and gas extraction purification