Having prepared one or more molds, the next operation is that of melting and pouring. An ordinary cast-iron glue-pot makes a good crucible and can be easily handled by a pair of tongs, made out of steel rod, as shown in the sketch. In order to hold the tongs together a small link can be slipped on over the handle, thus holding the crucible securely.
A second piece of steel rod bent in the form of a hook at the end is very useful for supporting the weight of the crucible and prevents spilling the molten metal should the tongs slip off the crucible. The hook is also useful for removing the crucible from the fire, which should be done soon after the metal is entirely melted, in order to prevent overheating. The metal should be poured into the mold in a small stream, to give the air a chance to escape, and should not be poured directly into the center of the opening, as the metal will then strike the bottom hard enough to loosen the sand, thus making a dirty casting.
Fig. 4 -Pouring the Metal
If, after being poured, the mold sputters and emits large volumes of steam, it shows that the sand is too wet, and the castings in such cases will probably be imperfect and full of holes.
A mold made in the manner previously described may be poured with any desired metal, but a metal which is easily melted will give the least trouble. One of the easiest metals to melt and one which makes very attractive castings is pure tin. Tin melts at a temperature slightly above the melting point of solder, and, although somewhat expensive, the permanent brightness and silver-like appearance of the castings is very desirable. A good "white metal" may be made by mixing 75% tin, 15% lead, 5% zinc and 5% antimony. The object of adding antimony to an alloy is to prevent shrinkage when cooling.
A very economical alloy is made by melting up all the old type-metal, babbitt, battery zincs, white metal and other scrap available, and adding a little antimony if the metal shrinks too much in cooling. If a good furnace is available, aluminum can be melted without any difficulty, although this metal melts at a higher temperature than any of the metals previously mentioned.
In casting zincs for batteries a separate crucible, used only for zinc, is very desirable, as the presence of a very small amount of lead or other impurity will cause the batteries to polarize. A very good way to make the binding posts is to remove the binding posts from worn-out dry batteries and place in the molds in such a way that the melted zinc will flow around them.
The time required for a casting to solidify varies with the size and shape of the casting, but unless the pattern is a very large one about five minutes will be ample time for it to set. The casting is then dumped out of the mold and the sand brushed off. The gate can be removed with either a cold chisel or a hacksaw, and the casting is then ready for finishing.