Notice how the walls seem to breathe as you crouch down to view an old sash window from the inside. A straight edge appears to bend as it crosses the pane. The light appears to pool in the corner of the glass and then seems to move as you alter your view. This is not an illusion – it’s the glass.
Cylinder or broad glass blowing was the method by which Victorian window panes were made. The process started with a glassmaker gathering a large amount of molten glass onto an iron blowpipe. Once sufficient glass had been gathered the blower would begin to blow it out into a long cylindrical shape. Once the cylinder had reached the required size it was cut lengthways down the centre while still hot. The two halves of the cylinder were then placed in the annealing oven where they would slowly cool over a period of days. During this time the glass would flatten and spread out in the oven. It was unlikely to be completely flat, there may be faint ridges on the surface of the glass. Tiny bubbles that had risen to the surface of the glass during the blowing process would also be caught in mid flight. There were also likely to be variations in the thickness of the glass from edge to edge.
For over a century the glass making industry attempted to remove all of these imperfections from glass, finally succeeding in the 1950’s with the invention of ‘float glass’ where molten glass is floated across a bath of molten tin where it flattens out and cools evenly. If you are looking for Sash Window Repair, see www.sashwindowpreservation.co.uk/services/sash-window-repair/.
It is now commonly accepted that much of the beauty of Victorian glass is lost in modern cylinder glass. Firstly the glass is marginally thicker in some areas than others. This means that the glass behaves as a very gentle lens. As the sun moves around in the sky, the angle of refraction changes, and thus the position of the light falling on a floor or wall changes. In modern float glass this does not occur. Also the same piece of glass will appear to be a different size when viewed from inside a room than it is when viewed from outside on the street. There is a useful background explanation of Historic England’s guidance on historic windows.
Historic England has a clear policy on historic windows which highlights the distinct optical characteristic of the earliest form of cylinder glass and strongly advises its retention in historic windows wherever possible. When a single pane fails it is the intention to retain the original glass, even if it has cracked, as the very best replacement unit, however well made, cannot replicate the slow and subtle flicker of light through a slightly thicker area of glass in the afternoon sun.