flat glass

for windows, mirrors, etc.

See also: glazing materials.

Sheet glass and plate glass have now been almost entirely replaced by float glass, which is made by floating molten glass on a bed of molten tin, producing a very flat, high-quality surface, better than sheet glass, and almost as good as plate glass but without the expense of grinding. Float glass is made in five qualities, from best to worst:

Metric designation,
decimal inch
1 0.04 Micro-slide
1.5 0.06 Photo
2 0.08 Picture
(The sizes above are not used for glazing windows.)
2.5 0.09 Single-strength
2.7 0.11 lami
3.0 0.12 1/8 inch or double-strength
4.0 0.16 5/32 inch
5.0 0.19 3/16 inch
5.5 0.21 7/32 inch
6.0 0.23 ¼ inch
8.0 0.32 5/16 inch
10.0 0.39 3/8 inch
12.0 0.49 ½ inch
16.0 0.63 5/8 inch
19.0 0.75 ¾ inch
22.0 0.87 7/8 inch
25.0 1.00 1 inch
32.0 1.23 1¼ inches

The maximum size of pane that can be installed depends on the strength of the glass and on the load–mainly, how strong local winds are. The local building code may specify double-strength or even thicker glass.

Old “wavy” glass

Older types of glass are sometimes sought for aesthetic or restoration purposes. Crown glass, made since the 14th century by spinning discs of molten glass, has not been made commercially since the 19th century, but many artisans still produce it, though not in four foot discs! Ironically, the bullseye, the least desirable part of the disc in the past, is now the most desirable. See, for example, Sugar Hollow Glass.

What is called “sheet glass” in source 1, below, would now be called cylinder glass, is still in production in several countries, and can be had from such sources as the London Crown Glass Co. and in the United States Bendheim Restoration Glass.

historical sources


Glazing Windows.Crown glass is made in circular disks blown by hand; these disks are about 4 ft. diameter, and the glass averages about 1/15? [illegible-ed.] in. thick. Owing to the mode of manufacture there is a thick boss in the centre, and the glass is throughout more or less striated or channeled in concentric rings, frequently curved in surface, and thicker at the circumference of the disk. Consequently in cutting rectangular panes out of a disk there is a considerable loss, or at least variety in quality: one disk will yield about 10 sq. ft. of good window glass, and the largest pane that can be cut from an ordinary disk is about 34 × 22 in. The qualities are classified into seconds, thirds, and fourths.

Sheet glass is also blown by hand, but into hollow cylinders about 4 ft. long and 10 in. diameter, which are cut off and cut open longitudinally while hot, and therefore fall into flat sheets. A more perfect window glass can be made by this process, and thicker, and capable of yielding larger panes with less waste. Ordinary sheet glass will cut to a pane of 40 × 30 in., and some to 50 × 36 in. It can be made in thicknesses from 1/10 in. to ½ in.

Plate glass is cast on a flat table and rolled into a sheet of given size and thickness by a massive metal roller. In this form, when cool, it is rough plate.

Ribbed plate is made by using a roller with grooves on its surface. Rough and ribbed plate are frequently made of commoner and coarser materials than polished plate, being intended for use in factories and warehouses.

Polished plate is rough plate composed of good material and afterwards polished on both sides, which is done by rubbing two plates together with emery and other powders between them. Plate glass can be obtained of almost any thickness from 1/8 in. up to 1 in. thick, and of any size up to about 12 × 6 ft.

Ernest Spon.
Workshop Receipts, for the Use of Manufacturers, Mechanics and Scientific Amateurs.
London: E. and F. N. Spon, [no date, but 1873]
Page 51.


A description from 1832 of the making of crown glass is provided in a separate page.

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