In spite of this difficulty very successful repairs have been made on cast-iron and it has been found that by mixing 2 per cent. of ferro-silicon and 1 per cent. pure manganese with the thermit for welding, a thermit steel is formed which is very soft and comes close to the properties of gray iron. By using this mixture important welds have been made on cast-iron flywheels, water wheels, and other cast-iron parts with great success. While industry is making progress with all these uses of thermit, science is experimenting all the time to add to the scope of the process. As was pointed out before, many other metals can be reduced to a high degree of purity with this process and in the laboratories they are always trying new ones and working out new formulas. Of the pure metals that can be reduced by the thermit process there are chromium, which is 98 to 99 per cent. pure; manganese, which is 96 per cent. pure; and molybdenum, which is 98 to 99 per cent. pure. These are used in the manufacture of very hard steel, such as armour plate, and "high speed steel." Among the alloys, or mixtures of metals, there are chromium-manganese, manganese titanium, ferro-titanium, ferro-vanadium, and ferro-boron, all of which have uses in industry and help us to travel faster and more safely by railroad, electric train, and steamship. It may have occurred to some bright boy that, since this heat is so intense and so handy, it might be a good way to make steam in locomotive boilers, or cook our meals, but it will be remembered that the heat is all over within a few minutes. In other words, where a terrific heat is required for a few seconds, thermit will fill the bill, but where a continuous heat for many hours is needed, electricity, gas, coal, coke, oil, or wood are better. The high cost of aluminum would probably prevent the thermit process coming into use in the manufacture of steel for our armour plate, ship plate, or structural steel, at least for a good many years. Earlier in this chapter I said that the slag, or aluminum oxide, from the thermit process was an almost useless product. This is not the precise scientific truth, for the slag becomes a black powder such as is used in making emery wheels, but the slag from thermit is never actually used for this. Another use for the slag from the thermit process in which chromium is used has been discovered. Potters use a material called corundum, which this slag resembles, except that it is superior to natural corundum in pottery manufacturing because of its freedom from metallic impurities. The slag can be mixed with clay and baked. It is especially useful in chemical apparatus that must withstand great extremes of temperature, because its experience has so tempered it that nothing less than a heat equal to that of the sun would give it much concern.