The wax is weighed before
and after filling _the entire space that the thermit will occupy_.
This does not mean only the wax collar, but the space of the mould with all
gates filled with wax. The number of pounds of wax required for this
filling multiplied by 25 will give the number of pounds of thermit to be
used. To this quantity of thermit should be added I per cent of pure
manganese, 1 per cent nickel thermit and 15 per cent of steel punchings.
It is necessary, when more than 10 pounds of thermit will be used, to mix
steel punchings not exceeding 3/8 inch diameter by 1/8 inch thick with the
powder in order to sufficiently retard the intensity of the reaction.
Half a teaspoonful of ignition powder is placed on top of the thermit
charge and ignited with a storm match or piece of red hot iron. The cover
should be immediately closed on the top of the crucible and the operator
should get away to a safe distance because of the metal that may be thrown
out of the crucible.
After allowing about 30 seconds to a minute for the reaction to take place
and the slag to rise to the top of the crucible, the tapping pin is struck
from below and the molten metal allowed to run into the mould. The mould
should be allowed to remain in place as long as possible, preferably over
night, so as to anneal the steel in the weld, but in no case should it be
disturbed for several hours after pouring. After removing the mould, drill
through the metal left in the riser and gates and knock these sections off.
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