Material.

Density.

Weight per cubic foot in lbs.

Dark-grey foundry-iron......

6,80

425

Grey foundry-iron.........

7,20

450

Mottled foundry-iron......

7,35

458

White iron...............

7,50

474

For wrought-iron Unwin gives this analysis :

Carbon 0,02 to 0,25 per cent; Manganese 0,0 to 0,3 per cent; Silicon 0,0 to 0,2 per cent; Sulphur 0,0 to 0,015 per cent; Phosphorus the same, and Pure Iron 99 to 99,5 per cent.

For steel, of course, the proportions vary greatly with the amount of carbon it contains.

From pig-iron is produced cast-iron, steel and wrought-iron. Castings are generally made from pig-iron. In some rare cases cheap castings are made the same as the pig-iron, from the "first melting," that is, directly from the molten ore. Most castings, however, are made from the second melting, that is, the pig-iron is re-melted in a cupola or reverbatory furnace with more flux to take up any remaining impurities, and the molten mass run out into the moulds. The second melting makes of course very much better work. The moulds generally consist of a wooden flask or box, made in parts which are secured together by clamps, the parts generally being halves, top (cope) and bottom. This flask is filled all around the inside with a lining of a special dampened black or green (fascia) sand ; into this sand the mould or pattern of the outside of the casting is pressed and, after ramming the sand, removed, leaving, of course, the impression of the outside of casting. The core is then secured into position in the flask. The core is the reverse of the mould being sometimes a sand pattern covered on the outside with a similar rammed sand made to fit the hollow spaces on the inside of the castings, and stiffened with an iron piece called the "arbor," but usually made of a baked mixture of sand and clay or flour, which forms a friable, infusible mass, which can be easily broken up and removed after the casting has cooled. The top flask is then secured over the whole, and the molten metal is run in through convenient holes left through the flask, until all the space between the core and mould is filled with metal. The gases generated escape through holes left in the cope, and from the core by putting iron rods along the arbor which are withdrawn before casting. The whole is then covered with sand and allowed to cool slowly if of varying thickness; if the casting is long and thin it is "stripped" of the moulding sand to prevent warping. The inside of the mould is frequently painted with coal dust or charred oak, or dusted with flour of plumbago, which generates hot gases that prevent, the too quick chilling of the external surfaces of the casting by the dampened sand and makes a clearer surface finish. When cold the top flask is lifted off. the arbor withdrawn and the finished casting removed from the bottom flask. The casting is then chipped off and finished. Small and cheap castings, such as separators, are revolved in a rumble (a sort of barrel) for the finishing process. It will be seen, therefore, that the most economical castings will be those which are so designed that the pattern can be easily withdrawn and the flask readily made in simple parts; in which case it is only necessary to line the flask with a fresh layer of sand for each casting. Whenever possible, castings should be made in an upright position (the metal being, of course, run in from the top), as the long core, if horizontal, being unsupported except at its ends is apt to sag in the centre, or float with the metal, making uneven thicknesses in the casting above and below the core. To avoid this the core should be supported by small iron "chaplets," which mix and combine with the casting. There is also danger of slag and other impurities that may be carried into the mould floating to one side. In upright castings or where there are large vents they will float off on top.1 To discover whether the casting is or is not uneven, the architect should have every column or long hollow casting "tapped " at about midway of its length. Tapping consists of drilling a hole, about three-eighths of an inch diameter through the shell. A small wire with the end bent at right angles is inserted in the hole ; the bent part is drawn closely to the inside of shell and the outside of shell marked on the wire, which on being withdrawn gives the exact thickness of shell. In case of rectangular or round castings there should be four such holes bored, two opposite each other and the other two the same, but at right angles to the line of the first two. All of course in the same plane, and about midway of the length of casting.

Manufacture of Cast-iron.

Moulds, flask and core.

The amount of metal in the shell is then readily ascertained, also whether the casting is of even thickness.

For a good strong casting it is very essential that all parts should be designed (and cast) of even thickness. Castings of uneven thickness vary greatly in strength for two reasons. For some reason not thoroughly understood, though generally attributed to the influence of the gases produced in the flask, the outside layers or "skin" of castings is supposed to be very much stronger than any other part. The metal on the inside seems to diminish in strength the farther it lies from the outside. (Experiments as to whether the skin does or does not add strength, however, are contradictory, the latest seeming to disprove the theory of its extra strength.)

Economical

Castings.

Inspection by Tapping.

Even thickness best.

1 On the muzzle end of cannons are cast heavy "sink heads," which contain the greater part of all impurities, leaving the good solid material for the guns proper. In heavy castings a flow or pathway is provided, more metal is poured than needed, and as it flows off it carries with it the dirt and impurities.

It is found, however, by actual experiment that castings from the same metal, made at the same time, vary according to their thickness in strength, the thinnest being the strongest in proportion to their metal, and the thickest the. weakest.