Glass contributes a lot to the character of a building, the distortion from imperfections reflect light that modern glass does not equal. Traditionally glass was held in place by lead or a series of nails and linseed oil putty, with old glass panes usually holding air bubbles.

Seed visable and telltale sign of traditional glazing

By the late 17th century sash Windows were becoming popular throughout Britain leading to a rise in demand for glass

  • Cylinder Glass – also known as broad sheet and popular until the first half of the 18th century, was made by blowing a cylinder of molten glass then cutting it along it’s side and flattening it out in the furnace. This gives it a slightly rippled surface and can be recognised by elongated air bubbles, the ‘seed’, in straight parallel lines.
  • Crown Glass – Increasingly popular from the mid 18th century, crown glass is made by blowing and spinning a large thin disk known as a table which is then cut into smaller panes. Thinner than cylinder glass it is also shinier and brighter as it never came in contact with a hard surface while molten. The seed lies in distinctive semi-circular lines with the glass being slightly curved.
  • Plate Glass – using cylinder or glass made from a cast, the glass was ground and polished until smooth.  It was expensive to produce so usually reserved for high status buildings and mirrors. Patent Plate Glass was invented in 1839 and used a thinner initial sheet of glass resulting in more glass being produced from the same amount of raw material.
  • Drawn Flat Sheet – With mechanisation glass production was able to develop methods to draw continuous sheets of molten glass out of a furnace. This was then passed through rollers, cooled, ground and polished.
  • Modern Float Glass – developed in 1959 it is the standard type of glass used for glazing today, made by floating molten glass on a bed of molten tin to produce perfectly flat glass.
Building character with distortion form traditional glazing

Maintaining traditional glass is usually limited to cleaning with a soft cloth and water as abrasive cleaning agents may cause damage. Where putty is needed to be replaced, linseed oil putty has not changed in character or composition and is still widely available and easy to apply. Ensure when painting, the paint overlaps slightly from the putty onto the glass.
Removing putty can be difficult and an infrared heat lamp is invaluable for this procedure. Used to soften linseed putty and ease its removal, this reduces the risk of damage to glazing.

Most types of glass are no longer produced so it is preferable to retain original glazing whenever possible. Small cracks in the corner of panes can be left in-situ unless they allow air or water penetration, larger cracks in very valuable glass can be repaired using epoxy techniques.

Cracked corner of old window glass pane

Crown and plate glass is no longer produced in the UK, reproduction cylinder  glass is along with modern reproduction ‘antique’ glass which may provide a better match to original glazing. But either will not replicate the appearance of older hand made glass.

by Matthew Evans of Welsh Heritage Decor

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