Everything is now ready for the thermit. There has been some elaborate preparation of the thermit too. The coarse powder or grains of iron oxide and aluminum previously have been prepared according to the job to be done. In very large welds, or welds where very hard steel is required, certain additions, to be explained later, are made to the thermit. The amount of thermit to be used is an important factor, of course, as there must be plenty to fill the mould, and yet not so much that it will overflow the riser. To decide on the amount takes a careful calculation because in large operations there are certain additions to the thermit which have to be considered. In general, however, the engineer must remember that he must have just twice as much molten thermit steel as he needs to fill the space left by the melting of the wax pattern. The surplus flows up into the riser, heating aperture, and gate, effectually closing all of them. The calculation, then, is that it takes four and a half ounces of steel to fill a cubic inch. It takes nine ounces of thermit to produce four and a half ounces of steel, so the engineer directing the weld must figure on eighteen ounces of thermit to each cubic inch in the wax pattern, including the space between the parts to be welded. After seeing that the proper amount of thermit is measured out the engineer must see that the crucible in which the reaction is to take place is ready to contain the strenuous battle that is to be fought in it. As before mentioned there are very few products that can withstand the heat of the fire produced by thermit. Ordinary fire brick and mortar would melt or be burned to powder in a few seconds. Metal would go the same way that the metal in the crucible goes. Science, however, has established that magnesia tar is not affected by the thermit fire, so the crucible in which the thermit is reduced is heavily lined with magnesia tar. The crucible itself is shaped like a cone with the point downward. At the bottom is a magnesia stone, which has a conical-shaped hole for the "thimble." This "thimble" also is made of magnesia stone, and has a hole through it for the molten thermit steel to run through after the reaction has taken place. Before filling the crucible with the thermit, however, the pouring hole is very carefully plugged up by a special process, with a little steel pin protected by fire sand and fire clay. This pin extends below the lowest point of the crucible a couple of inches, and by knocking it upward the molten metal is allowed to flow out. The upper end of this little plug that otherwise would be melted instantaneously by contact with the burning thermit, as indicated above, has to be protected by a layer of fire sand. The hole through which the metal flows is never more than half an inch in diameter.