The mold in which type is cast consists of two essential parts--the steel box in which the body is formed, and the matrix which contains a sunken image of the character. The matrix covers the opening at one end of the mold, and on the opposite end (which is the foot of the type) is an opening through which the melted metal is injected. A mold is made for a single body-size of type but it is adjustable sideways to correspond to the various widths of the letters in an alphabet. One mold may be used to cast, in succession, all the letters of a font of type, or it may be used to cast any number of fonts of different faces that may be made on the same body, by merely changing the matrices that form the face. The mold is made in two sections, which are fitted together so as to close up to the required width of the letter, and, after the cast is made, to open slightly in order to release the type.
_The Matrix._ There are three different methods of making type matrices. By the oldest method the first step is to cut the character on the end of a small bar of soft steel, called a punch; when this is done, the steel is hardened and it is used to stamp an impression in a bar of copper. This copper bar is a matrix in the rough, and its sides are next trimmed and squared so that it will fit the mold.
[Illustration: Type-mold complete, in two parts, but without the matrix (which is shown separately above). Position of the matrix when cast is made is indicated by the letter H on the end of the type within the mold. The upper and lower halves of the mold slide horizontally, to make the interior casting-box wide or narrow to conform to the required widths of various letters.]
The second method of making a matrix is by electrotype process. The original pattern of the letter may be engraved by hand or by other operation, or it may be a perfect type-face of a previous casting. This is fitted into a small frame of brass and then held in the vat of an electro-galvanic battery, which deposits a thick film of copper around the pattern and fills the opening in the brass frame. When this electro-plating process is completed, the pattern letter is extracted and the brass frame, with its copper impression of the letter, becomes the essential feature of the matrix. It is then reinforced by riveting another plate on the back, and is trimmed and fitted to place on the mold.