To make the mould, the brass model with its movable type set as required is placed upon a flat table or plate, face upward, and surrounded by the tube, as shown in partial section in the cut, [page 114]. The tube is a strip of sheet iron, which is bent around the flange and is secured in place by a wire twisted around it. The melted alloy (type, metal and lead) is poured into the space thus formed until it rises a quarter of an inch above the face of the model. In a few minutes it sets and is removed and allowed to cool. This gives a cup with the inscription and design depressed or in intaglio upon its inside base. This is shown in the cut, [page 115], partly in section; it will of course be understood that the mould forms a complete cup.
To make the stamp the interior surface of the mould is oiled with a stiff brush. It is not material what oil is used. The composition melted by heat is then poured into the cup, and is allowed to solidify. Owing to the conical shape of the mould it is readily removed. The mould must be hot but not too much so.
In the Post Office stamps the date requires to be changed frequently. Some of the figures do duty for two or three days each month. Thus the figure 8 is in the designation of three days, the eighth, eighteenth, and twenty-eighth. There are three changes involved therefore in connection with this day numeral. When a stamp mould or matrix is cast the place of numerals that are to be changed is filled with a blank space in the part where the type would otherwise come. A number is stamped in this space when needed, by means of an ordinary steel number-punch.
When the number is to be changed the old character is scraped or cut out, leaving a small irregular hollow. A very small piece of soft lead, about one-sixteenth of an inch on each side, is dropped into the hollow. With a flat faced punch it is flattened out, and on it the new number is impressed by a steel punch. This operation is repeated a great many times before the matrix is worn out.
OPEN SHUT
Composition Stamp Handle.
In the cut, [page 115], one number is shown as stamped into the soft lead, and at the other end of the stamp is a blank space ready for a number.
The casting of a stamp is so extremely simple that no attempt is made to use movable type, as in permanent rubber dating stamps.
While it is obvious that these composition stamps could be attached directly to wooden handles, a special style of handle, shown in the cuts, is employed by the Post Office. A wooden handle carries at its end a brass base, to which is pivotted a swinging piece that is perforated by a conical oval aperture a little larger than the small end of the stamp. The edges of this aperture are slightly rounded.