Bear in mind that liquid thermit is exceedingly fluid - as much so as warm molasses - and as it is much heavier, it will search diligently for all openings in the mold. For this reason the mold must be tight at the entering of the iron pieces. The operator should have at hand a bucket of luting clay, made of equal mixtures of fire-clay and sand, made pasty with a little water.

If the molds are solid pieces, as in rail and locomotive-frame welding, he smears a thin layer over the surface of the molds where they come in contact with one another. This will make a fairly tight mold.

Also he must stuff luting clay around the mold where the iron pieces enter, otherwise the thermit may find its way along the iron and spurt out. The danger of an unexpected squirt of thermit need not be dwelt on.

When the mold is made of fire-clay tamped over a wax collar, there should be no leaks if the operator is careful. He must be sure that his mold is rigid and strong enough to hold the extra weight of the pour.

A possible overflow of thermit and slag must be provided for. Large pours of thermit are always made with this in mind. If the pour is made in the workshop, the floor should be of sand and the workman should remove his tools before tapping. After tapping the thermit he should remove himself as quickly as possible.