In 1887, Wilson took out a patent for improvements in his process of
making compound plates. In this method of manufacture he takes a
wrought iron, fibrous plate, fifteen inches thick, built up from a
number of thin plates. While hot from the forging press, he places
this plate in an iron mould (see Fig. 7) about 28 inches deep, and
upon it runs "ingot iron" or very mild steel to a depth of thirteen
inches. In this form of mould the plate rests on brickwork, and is
held in place by two grooved side clamps or strips which are caused to
grip the plate by means of screws which extend through the sides of
the mould. After solidifying, the plate, which is twenty-eight inches
thick, is reheated and rolled down to eighteen inches. This is the
iron backing of the finished plate, and it is again put in the iron
mould and heated, when a layer of hard steel is run on the exposed
surface of the original wrought iron plate to a depth of eight inches.
This makes a plate about twenty-eight inches thick. It is taken from
the mould, reheated, rolled, hammered or pressed down to twenty
inches. After cooling, it is bent, planed, and fitted as desired, then
tempered and annealed to relieve internal strains.

[Illustration: Fig. 7.]

The method employed by Ellis in making compound plates is to take two
separate plates, one of good wrought iron and one of hard forged
steel, placing the forged steel plate on the wrought iron plate,
keeping them separate by a wedge frame or berm of steel around three
sides, and placing small blocks of steel at various points near the
middle of the plates (see Fig. 8). These blocks are called distance
blocks. After covering all the exposed steel surfaces with ganister,
the plates are put in a gas furnace and heated to a welding heat. They
are then lowered into a vertical iron pit with the open side