Crown Glass

Crown glass was one of the two most commonly used types of glass for windows up until the 19th century, the other being blown plate glass. The process of making crown glass was first perfected by French glassmakers in the 1320s. Crown glass is made without lead, chiefly by fusing fixed alkali with silica sand, to which is added some black oxide of manganese – which gives the glass a tinge of purple.

For the best crown glass, the ingredients must be prepared in the same manner as for mirrors, and mixed in the following proportions: 60 lbs. of white sand, 30 lbs. of pearlash, and 15 lbs. of nitre, 1 lb. of borax, and 1/2 lb. of arsenic.

Crown Glass Making Process

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. Spinning the semi-molten ball then causes it to flatten and increase in size, but only up to a certain diameter.

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.

The finished “table” of glass was thin, lustrous, highly polished (by “fire-polish”), and had concentric ripple lines, the result of spinning; crown glass was slightly convex, and in the centre of the crown was the bull’s eye - a thickened part where the pontil was attached. This was often cut out as a defect, but later it came to be prized as evidence of antiquity. Nevertheless, and despite the availability of cheaper cylinder glass (cast and rolled glass had been invented in the 17th century), crown glass was particularly popular for its superior quality and clarity.

This process allows the colour range to be limitless; crown glass is used ecclesiastically, commercially, domestically and for restoration purposes.