Getting the formula right for graphite ‘lead’ has involved a surprising amount of fiddling, historically; and the results are dreadful if you don’t get it right.
Possibly even worse with mechanical pencils, where bad lead invariably manages to be hard enough to tear at the paper without leaving any other mark; while brittle enough to snap all the time. Wooden jacketed pencil lead can also be abundantly awful; but the support provided by the wood masks some of the brittleness.
