(Contributed by T. H. BISHOP, A.R.I.B.A.)

In a variable climate such as ours it is necessary that the openings to admit light be filled with a substance which will keep out the wind, rain, etc., and let in as much light as possible. The material used in this country is glass, and the operation of fitting in the glass is called Glazing.

The majority of bye-laws in relation to building state that the window area, exclusive of the frames, must be equal to one-tenth the floor area, but no mention is made of the loss of light in passing through the glass.

The following table shows the proportion of light intercepted in passing through the different qualities of glass: -

British polished plate, 1/4 inch thick .

13 per cent.

Rough cast plate, 1/4 inch thick . .

30 ,,

Rough rolled, 4 flakes to 1 inch . .

53 ,,

2-oz. sheet glass . . . .

22 ,,

Many makes of glass, such as brilliant, muffled, and Venetian, make up for the loss by diffusion and diffraction.

The different qualities of sheet glass are distinguished by their weight in ounces per foot super.; those of rolled glass, by their thickness. A sheet of glass 1/10 inch in thickness weighs about 21 oz. per foot super.

It may be noted that one-seventh of the glass used in this country is British, the rest is mainly Belgian. The latter, although attaining a high standard of excellence, is inferior to the British in clearness.

The following are the various qualities of glass principally used.

Crown Glass is finer in surface and whiter in quality than sheet glass, and is slightly convex. In fixing, the convex side should be placed outside, as each disc of glass has a bullion in the centre. The largest panes that can be obtained are about 18 inches square. It is obtainable in one thickness only, equal to about 13 oz., but the demand for it is very small.

Sheet Glass is made in various qualities (Specification):-

A, for pictures (best).

B, for pictures (ordinary). Best, for first-class work. Seconds, for good glazing. Thirds, for ordinary glazing. Fourths, for coarse glazing (little used).

Sheet glass is described in Specifications according to weight, and sheets can be obtained up to 54 by 36 inches.

Weight.

Thickness.

15 oz.

1/15 inch.

21 ,,

1 ,,

26 ,,

1 ,,

32 ,,

1/7 ,,

Fluted Sheet Glass is corrugated to increase its strength, and is used where privacy is required without undue obstruction of light. It is obtainable in 15, 21, 26, and 32 oz. sheets.

Patent or Blown Plate is sheet glass polished on both sides. In patent plate the bubbles are oval or irregular, but in the British plate they are spherical. Patent plate is particularly thin, and is used for airtight show cases on account of its lightness.

Patent Plate Glass is obtainable in two "colours," the usual, crystal; and the extra, white. Sheets can be obtained up to 50 inches long and 39 inches wide, or 13 feet in area.

Plate Glass is made by casting on a table, and afterwards rolled. The rolling consolidates and toughens the glass.

It is manufactured in three qualities: rough plate, rolled plate, and polished plate.

Rough Plate, or Rough Cast Plate, is the commonest quality of glass cut into sheets. One side appears polished and the other dull, but both sides are wavy. It is used where great strength and little light are required, and is obtainable 1/4, 3/8, 1/2, 3/4, and 1 inch in thickness, 60 feet in area up to 3/4 inch thick, and 40 feet in area for 1 inch.

Patent Diamond Rolled Rough Plate, and Patent Quarry Rolled Rough Plate have each an oblique lozenge pattern on one side only (the pattern should be placed outside). The lozenge is larger in the latter. The quarries measure 6 by 4 1/2 inches and 3 by 2 1/16 inches. The stock thicknesses for both are 1/8, 3/16 and 1/4 inch; and these can be obtained in sheets 100 feet long by 34 inches wide.

British Polished Plate is first cast on a large iron table, and then rolled by a heavy iron roller into sheets from 3/8 to 1/4 inch thick. It is afterwards ground down to an even surface and uniform thickness, and polished by special machinery.

Rolled, Rough, Figured, and Pattern Plate Glasses are cast and then rolled to impress the pattern in the back surface of the glass.

Muranese, Hartley's, etc., are made in a similar manner.

The plain and fluted can be obtained 1/8, 3/10, 1/4, and | inch.

The extreme sizes of plain rolled glass that can be obtained are:-

Length.

Width.

1/8 inch .

120 inches.

42 inches.

3/16, 1/4 and 3/8 inch .

120 ,,

44 ,,

Fluted or Ribbed Glass is especially useful where a large amount of diffused light is required. The horizontal ribs give more light in the middle and less at the sides, whilst when placed vertically the reverse holds good. Horizontal ribs give more brilliant light than do vertical ones.

Ground or Obscured Glass (sheet or plate) has one side rendered opaque either by grinding the surface or by coating it with a vitrifiable matt flux. It is supplied in 15, 21, 26, and 32 oz. sheets. In fixing, care should be taken to size the edges.

Enamelled Glass is similar to ground glass, except that parts only are obscured to form a pattern. Figured rolled is rapidly superseding enamelled glass, owing to its better appearance and strength.

Another glass used largely in hotel work is Embossed Glass. It is made in two ways, the "sand blast" and the "acid process" (ordinary or single). The "sand blast" is the cheaper, but the "acid process" is the better. The embossing is done after the glass has been cut to shape, and any required degree of transparency or opacity can be obtained.

Coloured Glass can be obtained in sheet, muffled, rolled, or antique. It is usually 1/8 inch thick, rolled or muffled. The glass is either coloured throughout, and is then known as cathedral tinted, or coloured only on the face, when it is called "flashed" glass. The term "cathedral" is also applied to unbleached or un-coloured glass.

Perforated Glass may be either "sheet" or "plate." The perforations for ventilation are cut in the glass after manufacture.

Wired Glass is made by embedding wire netting in the centre of the thickness of sheets of plate glass. It is used for glazing roof skylights and doors subject to jar or vibration.