Manufacturer and builder 4, 1871
The value of plumbago or graphite depends on its amount of carbon. In order to ascertain this, the latest, simplest, and best method is to dry the pulverized graphite at a temperature of some 350° Fahr., and then place it in a tube, four to five inches long and half an inch wide, made of hard glass, and closed at one end. To this is added about twenty times as much well-dried oxide of lead, and the whole is well mixed. The tube is then first weighed, and afterward heated before the blow-pipe till the contents are completely fused and no longer evolve gases. Adter this operation, which does no last longer than ten minutes, (if the quantity is not too large,) the tube is allowed to cool, and its weight is again ascertained. The loss in weight is carbonic acid, the oxygen of which has been taken fro mthe lead oxide; while the carbon is all that there was in the plumbago. For every twenty-eight parts f loss, there must have been twelve of carbon. In general, it is suddicient to take from one to two grammes of plumbago and from twenty to forty of oxide of lead.
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