The method of welding steel pipes is an exact reversal of the principle of welding together solid pieces of steel or iron. After the pipes are cut off clean, the mould, which is made of cast iron, is placed around them with specially constructed clamps to force the two ends closer together after the thermit has been poured in. The thermit is then set off in a flat-bottomed crucible like a long-handled ladle, and poured into the mould by hand as if from a ladle. As the slag rises to the top it goes into the mould first and coats the pipes. The thermit steel does not touch the pipes, but merely supplies the heat to weld them perfectly, so that they are as strong as the piping itself. Just after the pour has been made, the clamps are tightened up and the white-hot pipe ends forced together. They are thus held until cold, when the mould is broken away. The slag coats the outside of the pipes and this is chipped away, leaving a perfect weld. Another interesting use of thermit is in the great foundries where cauldrons of metal have to be kept at a very high temperature. To help keep the mass in a liquid state thermit can be introduced in it either by throwing it into the cauldrons in bags, with a little ignition powder so fixed that it will be touched off by the heat of the boiling metal, or by putting it in especially designed cans affixed to the ends of long rods. By these rods the thermit can be plunged to the bottom of the cauldron before it "burns." The reaction of the thermit, with the intense heat caused by it, helps to keep the mass at the proper temperature. Also thermit is used in the same way with a small amount of titanium oxide, to purify iron and steel. The metal becomes much more liquid, and a commotion like boiling is started. This is the result of the titanium driving out the impure gases and driving other impurities such as metallic oxides and sulphur contents to the top. Chemically what happens when the titanium is introduced by the thermit process is that the titanium combines with the nitrogen in the molten iron, giving it a much finer grain, and making it a much lighter colour, more like steel, than previously. One of the things thermit is not extensively used for is the repairing of gray iron castings. The first reason is that gray iron is cheaper than steel, and a new casting often can be turned out by the mills quickly. Another and a more interesting reason is that gray iron melts in a much lower temperature than does thermit steel and consequently has a lower shrinkage. Therefore when the molten thermit, with its terrific heat, cools there is a large shrinkage. Thermit steel being much stronger than gray iron, its shrinking sometimes strains and cracks the iron casting.