Patent Plate Glass, or Blown Plate, is made by polishing sheet glass 011 both sides.
Patent plate may be distinguished from British plate by the bubbles in the glass. In the former case these are elongated and irregular, in consequence of the glass having been blown after the bubbles were formed. In British plate the bubbles are circular. The surface of the patent plate is also more wavy than that of British plate.1
Patent plate is made in the three qualities which are respectively used for the purposes mentioned below.
Best. B, For engravings or very good glazing.
Second. C, For good glazing.
Messrs. Chance of Birmingham make each of the qualities above mentioned in two colours - the Usual (or Crystal), and the Extra white. The usual is the better for glazing, as it is harder, more lustrous, and less liable to be scratched in cleaning. The extra white is better for engravings and water-colour drawings, etc.
Average weight per foot superficial.
Extra white is made in Nos. 1 and 2 thicknesses only.
1/9 to 1/8 in.
The squares kept in stock do not exceed 10 or 12 feet in area, the length being not greater than 50 inches, or the width than 36 inches.
"Flattened sheet glass and patent plate should be cut with the convex side of the air bubbles downwards, or it will be liable to crack starwise, and it should be glazed with the convex face outwards, or it will present the appearance of being hammered on the face." 1
The surface is either left rough, or polished, or indented by a pattern cut upon the surface of the table. The several varieties of plate glass differ from one another according to the nature of the surface thus formed, and are named Rough-cast Plate, Rolled Plate, or Polished Plate, accordingly.
All plate glass has the advantage of being strong. If of sufficient thickness it keeps out the cold, and, moreover, is a "preventive to robbery, as it will not yield to the diamond and allow of being noiselessly removed." Other advantages are possessed by the different descriptions according to the nature of their surface.