Most malleable iron castings are made in sand molds, and, as stated, the iron poured must be of such composition and temperature that the castings so made will be white of fracture. It is possible to get a quick indication of the condition of the iron for pouring by making test pieces, every one in the same way, which, after cooling and breaking, will show by fracture the approximate composition of the metal. According to these test pieces, called “sprues,” which, at times, may be cast as often as every five or ten minutes, the mixture is regulated to produce a uniform product. [Illustration: FIRING AN AIR FURNACE ] To illustrate: The fracture of a round sprue, or test piece, always ⅞ inch in diameter, when poured in the sand, cooled there to low red heat, quenched in water and then broken, should be white with only a few flecks of dark constituent. A gray iron fracture indicates too high silicon content and such iron is usually termed “low” iron. Castings of medium or heavy section, which, therefore, cool slowly in the sand, if poured of too high silicon, i.e., “low” iron, might precipitate a little graphite during cooling, even though thinner-sectioned castings which cool so much more rapidly would come white from the same iron. While iron giving nearly white sprues is necessary for particularly large castings, to make sure that the usual run of malleable castings will come white in the sand requires very slightly mottled test sprues. Test blocks also, with one side cast against an iron “chill” are poured to determine the depth of chilling, and test bars of various shapes are regularly made, to test after annealing, for tensile strength, torsion and other physical properties. “Air furnaces” are much like longer puddling furnaces. They vary in capacity from ten to forty-five tons while occasionally small ones of as little as three or five tons capacity are met with. [Illustration: TAKING OFF THE SLAG ] The usual fuel is soft coal. The long flame passes from the grate at one end over the bridge wall and is deflected by the roof down upon the bath beneath. A chimney at the outgoing end furnishes draft. The furnace bed is usually of brick upon which is fritted (slightly fused) a mixture of sand with a little lime. In order to facilitate charging of the materials to be melted the roof is usually removable in parts, called “bungs.” These have frame work of iron which hold in place the fire bricks that come in contact with the flame. During charging these bungs are lifted off one at a time, and the iron materials are dumped through the openings. Small doors in the sides just above the bath allow “rabbling” or mixing of the charge and skimming of the slag which forms, and one or more spouts lined with fire bricks and clay provide for tapping out the metal when it is ready to pour. [Illustration: TAPPING ]