Crown Glass

Crown glass: The earliest style of glass window

The earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and cut into small sheets.  

This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process.

At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain. They are known as bull's eyes and are feature of late 19th century domestic lead lighting and are sometimes used with cathedral glass or quarry glass in church windows of that date. Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diamond pane windows was in part due to the fact that three regular diaper shaped panes could be conveniently cut from a piece of crown glass, with minimum waste and with minimum distortion.

This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.

Types of Glass

Glass Types by manufacturing method

There are several ways of categorising glass and this overview of glass types looks at the way the glass is manufactured.

Crown Glass

Crown glass is the oldest method of producing sheet glass and continued to be used until the 19^th century.  This method consisted of blowing a very large bubble of glass.  It was then spun rapidly over a pit until the bubble collapsed into a disc that ranged from 1500mm to 1800mm diameter.  

This gave the thinnest and least marked glass at the outer portion of the disc.  The centre was the thickest and became known as the bullseye.  The glass was cut to provide the best use of the disc.  This limited the size of panes to what could be cut from the disc.  Diamond shapes were often cut from the remainder and the central bullseye was used in less expensive glazing.

Corning Museum of Glass

Cylinder Glass

Cylinder Glass is a handmade process that includes broad sheet glass. It was widely used from the 17^th to the 19^th century, and now is limited to a few manufacturers.  

"Among the Glass Workers" Harry Fenn, 1871

An elongated bubble was blown.  The top and bottom of the bubble are broken off and annealed.  Later the cylinder is placed in the lehr for reheating.  It is scored and when it breaks open along the score, the glass is flattened. Characteristically, it has a gradation of thickness with thicker edges where the top and bottom of the cylinder were cut off.

From IdoStuff

Flashed Glass

A development in cylinder glass was to make the bubble of two colours, with the dark colour gathered first and then encased in clear (or sometimes other pale colours) and blown into a cylinder.  This made dense colours more transparent and enabled more detail through abrading and etching.

Drawn Glass

Industrialisation of glass production began with the development of drawn glass.  This method of mass production of window glass was invented and developed by Emile Fourcault in Belgium. Full scale production began in the early 1900’s.  

The glass is drawn upwards from a vat of molten glass until it cools enough to be cut into sheets at the top of the tower.  The process is subject to slight variations in thickness due to uneven cooling and gravity. It enabled much larger panes of glass without the astragals that are common in Georgian and later houses.  It was the most common method of producing window glass until the 1950’s.

Table Glass
Table glass is the process of putting molten glass onto a flat surface (the table) and rolling the glass flat.  This has been used from the latter part of the 19^th century to the present.  It enables textures to be pressed into the glass from the rolling cylinder.  It is easier to produce streaky and wispy glass by combining different colours on the table. 

Kokomo Glass Co.

This can be done as single sheets or further mechanised to roll out long ribbons of glass.  This is now mostly referred to as machine or hand rolled glass depending on the amount of mechanisation.

Float Glass

The glass that we now rely on for large clear windows began with the development of experiments by Alastair Pilkington and the company named after him.  This consisted of floating near molten glass on molten tin, hence the name, float glass.  This has been the standard method of glass for windows since the 1950’s.