Crown Glass is made as follows: - A blowpipe is dipped into melted glass, which is then blown into the form of a large globular bottle. A rod tipped with a blob of hot glass is so placed that the blob or "punty" sticks to the centre of the bottom of the blown globe. The globe is then detached from the blowpipe, heated, and rotated vigorously until it whirls out by centrifugal force into a flat disc or "table" having a blob or "bullion" of glass in the centre.
It will be seen that this process of manufacture tends to make the disc gradually thicker from the circumference to centre. In cutting the disc into panes the boss in the centre must be avoided, so that there is a good deal of waste.
The area of panes that can be produced from a table varies according to circumstance.
Of course the centre bullion must be cut out in a small pane. This pane varies in size from 5 to 10 inches square, and is often used for stables or very common cottages. Lately, however, such panes have been in demand for use in better houses built in the Queen Anne style of architecture.
If the remainder of the table be cut into panes of the most advantageous size to produce a maximum quantity, it may yield about 13 feet superficial. But if the panes are cut as required, they will amount to only 10 or 11 square feet The largest "squares" produced are about 33 x 25 inches.
The portion containing the bullion cannot he flattened; the smaller halves of the disc (which do not include the boss or "bullion") may be flattened, if desired, at an extra cost, so as to correct the slight convexity that exists in the tables.
Crown glass is sold in crates of tables, i.e. half discs; crates of slabs, flattened or unflattened; and in squares, i.e. rectangular pieces cut to various dimensions.
There are two thicknesses -
The usual, about 1/20th inch thick, and weighing some 10 oz. per square foot; and the extra, about 1/15th inch thick, and weighing some 16 oz. per square foot.
The Quantity in Crates varies according to the thickness and kind of glass, and is shown in the following Table : -
Crate of Tables
18 Tables, averaging 53 inches diameter.
12 Tables, averaging 52 inches diameter.
Crate of Slabs
36 Slabs, averaging in extreme width
24 Slabs, averaging in extreme width
22 3/4 „
22 3/4 „
21 1/2 „
21 1/2 „
The extreme widths of slabs given 211/2, 223/4, 24, etc., refer to the distance from the line where the disc is cut in two to the farthest point of the circumference. Extra sized slabs, flattened and unflattened, are made in 12 sizes, each increasing 1/2 inch in width from 241/2 to 30 inches.
The maximum area of the squares kept in stock is 5 superficial feet.
There are four qualities classed as mentioned below, which may be used where comparatively small panes or squares are required.
A These may be used for pictures, or for the very best window.
Picture qualities.. B glazing.
Glazing qualities. Best. For the best class of dwelling-houses.
Seconds. „ second „ „
Thirds. „ third „ „
Fourths, or coarse, for agricultural cottages.
Crown glass is said to be more free from colour than Bheet glass, and it has a finer surface, as it does not come into contact with any other substance during the process of manufacture; but it is being rapidly superseded by the latter, in consequence of the demand for large sizes, and Borne of the principal manufactories have ceased to make crown glass altogether.