The qualities of ordinary moulding sand have already been given as infusibility and adhesiveness. Such sand, when used in its natural condition is called green sand. Green sand moulding is most extensively practiced. In many foundries it is used exclusively.
Loam is usually made in the foundry where it is used. It consists of sharp sand, moulding sand, manure and clay wash. The best way of mixing the loam is by the use of a mill. Loam mixing mills are practically the same as flour mills. Each has its upper and lower stones. Grindstones are, however, used instead of burr stones. When the loam is mixed by hand the mixture is beaten with an iron rod as it lays upon an iron or wooden bench. The mill is the better means of mixing.
The ingredients of loam vary with the purpose for which it is intended and also with the method of mixing. When ground in a mill a good proportion is: sharp sand 7 parts, manure 2 parts, moulding sand 2 parts, mixed with a thick clay wash. Clay wash is made by stirring ordinary blue clay in water. A thick wash is obtained by using a surplus of clay, and stirring frequently. When the loam is mixed by hand the proportions vary somewhat. They should be: sharp sand 5 parts, moulding sand 2 parts and manure 1½ part, mixed with a thick clay wash. The sharp sand is used as a regulator for the moulding sand. The more clay there is in the moulding sand the greater the quantity of sharp sand that must be used. Experience alone can tell the exact ratios. A skilled workman can tell by the feeling as to whether the loam is properly mixed or not. A practical test is to take a well-dried lump and drop it in molten iron. If the iron boils after the first bubble, the mixture is too dense and contains too much clay. In this case more sharp sand must be added.
Dry sand is the name applied to a class of moulding where the moulds are baked in an oven before being used. The most common example of dry sand moulding is that of cores. The great majority of all cores used are made of dry sand. Loam cores are sometimes used in green sand moulds. Dry sand cores and moulds are made of sharp sand with a binder to cement the grains together. Good sharp sand should be free from clay. The grains should stand out separately and distinct from each other. It should consist, as nearly as possible, of pure silicon. The best of sharp sand may be obtained from any of three sources; the beach where it has been washed by the waves of the ocean; river beds, where it has been cleansed by the scouring action of the stream and the artificially washed sand taken from banks.
The binders used as cement for cores are beer, flour, molasses, resin, clay and glue. Those in most common use are flour and molasses. As in loam moulding, the proportions of the ingredients used in cores vary with the work to be done. A good common mixture is the following: 1 part flour, 7 parts clean sharp sand and 3 parts fine moulding sand, mixed with enough water to dampen. A tougher core can be made of 12 pounds of sharp sand, 5 pounds of moulding sand, 2 pounds of very finely ground resin and ¼ pound of flour. This should also be mixed with water in which molasses has been stirred in the proportion of 1 gill to the quart. The ingredients should be carefully mixed so thateach portion is evenly distributed throughout the mass.
Cores are made in core boxes. The construction of these boxes is fully described in "Pattern Making." After the core mixture is made it is rammed into the core boxes, turned out on the bench and then carefully and thoroughly dried in a core oven. As already stated in "Pattern Making" cores are frequently made in halves and pasted together. The paste is made of a cheap grade of wheat flour. After the core or dry-sand mould has been dried it should be blackened so as to present a smooth surface to the iron. The blacking may also be applied before the core is baked. Preparation of the blacking calls for the same skill as that of the core and loam mixing. A good blacking may be made of 1 pound of black lead, 1 pound of charcoal blacking, 4 pounds of Lehigh blacking, mixed with beer or molars s water and a little fire clay. In mixing molasses water for this purpose, use about one-half a pint of molasses in a pail of water.
Core ovens are used for drying cores and moulds. There are many styles of ovens. Where large and small work is done there should be two ovens for the two classes of work. It is uneconomical to heat a large oven in order to bake small cores. The oven and fireplace are usually in the same enclosure. Thus the products of combustion come in contact with the cores without injury to them. A convenient form of oven for baking small cores is shown in Fig. 21. It consists of a number of circular shelves mounted on a central shaft which is held by a step at the bottom and a bearing at the top. By turning the shelves any parts of .them may be brought opposite the door for loading or removing the cores.
The shelves should be in the form of gratings so that the hot gases may pass through and around the cores. The furnace and stack should be arranged so that the hot gases and products of combustion are made to pass across the oven.
Large ovens are made with a traveling carriage. The cores are loaded when the carriage is outside the oven. The carriage is then run in, the doors closed and the fire lighted. Such an oven is shown in Fig. 22. The furnace is located beneath the floor of the oven. There are racks upon each side upon which cores may be dried as well as upon the carriage.
The best fuel for firing a core oven is coke. Wood is very frequently used, as is also hard and soft coal. It is desirable to have a steady fire. Wood and soft coal do not make steady fires and they require constant attention. Hard coal makes a good, hot fire, but it is expensive. The time required to dry a core varies from one to fourteen or more hours, according to its size. Where large cores are to be dried it is customary to bake them all night. In such cases the dampers of the furnace must be arranged so that the fire will burn steadily for a long time without attention.
Dry sand moulds are baked in the same way as cores, except that, in some instances they are dried in position. Loam moulds are also dried in position.