The description of the art of the ironfounder does not come within the range of these Notes.
The few remarks which follow are intended only to give such a general idea of the process of ironfounding as will enable the student to understand the points to be observed in examining and testing castings of different kinds.
Castings, such as are used in building and engineering works, are generally made by pouring molten iron into sand, in which an impression of the article required has been formed by means of a wooden pattern.
The sand is of a fine loamy character, free from oxides, and is filled into iron frames or boxes, without tops or bottoms, called "flasks," made in two similar parts, one of which fits over the other.
The "pattern" having been accurately formed in wood (a little larger than the required casting, so as to allow for contraction in cooling, see p. 355), is placed in the lower flask, and the space round it is tightly tilled with damp sand, the surface of the pattern having been dusted with dry "parting sand."
The upper flask is then placed upon the lower one, and in its turn filled with damp sand rammed round the pattern.
The box is then opened, the pattern taken out, and the halves carefully put together again without disarranging the sand, an orifice being left for the fluid metal, which is poured through it, into the space, in the sand, previously occupied by the pattern.
In order to prevent the metal from being chilled (see page 266) by contact with the sand, the inside of the mould is painted with a blacking made of charred oak, which evolves gases under the action of the hot iron, and prevents too close a contact between the metal and sand.
1 Application of Iron to Building Purposes, p. 85.
The sand is also pierced with holes to allow of the escape of the air, and of gases evolved when the metal is poured in. If these are allowed to force their way through the metal, they will cause it to be unsound and full of flaws.
The passages through which the molten iron is poured into the mould should be so arranged that the metal runs together from different parts at the same time. If one portion gets partially cool before the adjacent metal flows against it, there will be a clear division when they meet, the iron will not be run into one mass, but will form what is called a cold shut.
The above is the simplest form of the process.
When a casting is to be hollow, a pattern of its inner surface, called a "core" is formed in sand, or other material, so that the metal may flow round it.
The core for a pipe consists of a hollow metal tube, having its surface full of holes. This is wound round with straw bands, and the whole is covered with loam turned and smoothed to the form of the inside of the pipe.
The strength of a casting is increased if it be run with a head or superincumbent column of metal, which by its weight compresses the metal below, making it more compact and free from bubbles, scoriae, etc. These rise into the head, which is afterwards cut off.
For the same reason pipes and columns are generally specified to be cast vertically, that is when the mould is standing on end. This position has another advantage, which is that the metal is more likely to be of uniform density and thickness all round, than if the pipe or column is run in a horizontal position.
In the latter case the core is very apt to be a little out of the centre, so as to cause the tube to be of unequal thickness.
In casting a large number of pipes of the same size iron patterns are used, as they are more durable than wooden ones, and draw cleaner from the sand. Socket pipes should be cast with their sockets downwards, the spigot end being made longer than required for the finished pipe, so that the scoriae, bubbles, etc., rising into it may be cut off. Pipes of very small diameters are generally cast in an inclined position.1