Historic Glass Processes
Introduction
There have been many different ways used to manufacture flat glass, but here you can find the processes used by Pilkington UK since our establishment in 1826.
Crown Process
The first glass manufacturing process we used was the crown process. This is where a gob of glass was blown into a sphere, which was cut at the end to create an opening.
The sphere was then reheated to keep the glass softened, and spun so that the sphere would stretch out and flatten to make a crown or disc. This would create a slightly distorted but clear pane of glass. The distortion was mostly because the middle of the pane was thicker than the edges which had been stretched out.
The finished product from this process was circular. Windows, however, are usually square and making square panels out of a disc of glass led to considerable wastage.
This method also resulted in a bullion being formed in the centre of the disc where the pipe had been attached. Although these bullions created a warped, distorted view they are still created to satisfy a fashion in domestic glazing. Examples of these bullions can still be see in pub windows.
When Pilkington UK first started production in 1826, it was named the ‘St. Helens Crown Glass Company’ after this process which we used to produce glass.
Information sourced from The Glassmakers by T. C. Barker and Float: Pilkingtons' Glass Revolution by D. J. Bricknell and in-house.
Cylinder Process
The cylinder process had two main versions in which the cylinder was created.
In the earlier version of this process, a blowing technique was used. First, a ball of glass was blown at the end of a blower’s pipe. Once the desired circumference was reached the blower would begin to create the cylinder. This was done by reheating the ball in a furnace and swinging the glass ball in a trench in a pendulum motion to stretch the ball into a cylinder shape. This took great craftsman skill and was an energy consuming process for the worker.
The later version (pictured) was mechanised and could create cylinders 13 metres high. This was done by dipping a round metal bait into a bath of molten glass which was then raised to create a long cylinder of clear glass. The advantage of this version of the process was that bigger panes could be made, and less human energy went into it.
In both versions, after the cylinder had been made, the cylinder was scored and split down one side and laid flat on the opposite side. The cylinder was then moved into a furnace where it would be softened again and would be flattened with the split edges peeling outwards. This would create a flat, straight edged piece of glass.
The advantages to this process was that in the latter version a lot less effort had to go into making the glass meaning less man-power was needed. Also, with the edges of the product being straight, panes of glass could be cut out of it, creating minimal wastage.
The disadvantage was that the surfaces could get damaged when the cylinder was split and flattened.
Information sourced from The Glassmakers by T. C. Barker and Float: Pilkingtons' Glass Revolution by D. J. Bricknell and in-house.
Flat Drawn Process
Flat drawn or Fourcault process involved drawing the molten glass upwards out of a bath and allowing it to cool as it is lifted into the air. This would ensure a transparent but distorted sheet of glass.
This was done using a straight metal bait which would be lowered into a bath of molten glass, which would then draw a continuous sheet of glass. To create large sheets of glass a tower was built to allow the metal bait to continue rising uninterrupted. The glass was gradually cooled and hardened as it rose through the tower.
To prevent the edges of this sheet of glass ‘necking in’ rollers were used to keep the sheet as flat as possible. The rollers would also help cool the glass and stiffen it into shape.
After the full height of the tower was used, the glass was cut into sheets at the top of the tower and the sheets were lifted off at the top.
Information sourced from The Glassmakers by T. C. Barker and Float: Pilkingtons' Glass Revolution by D. J. Bricknell and in-house.