Traditional Victorian glaziers always specified putty for the soft component of the window as it was the cheapest part of the whole system. The designers had in mind that the putty would fail, slowly, but be an easy job for a craftsman to mend, before any other part of the window, such as the glass or the timber rebate, was put under stress. All other parts of the window were designed to last for many years, perhaps decades or even centuries.
Historic England’s guidance on traditional glazing materials includes a full description of the behaviour of linseed putty within a timber window frame, as well as a clear explanation of the material science behind its use as opposed to harder more modern materials used for modern glazing. The linseed oil in the putty is slowly oxidized by the air as it dries. This process causes the outer skin of the putty to become increasingly hard while the core remains soft and flexible for long periods of time. The putty is thus able to move with the timber throughout the seasons, preventing the glass or the rebate joints from being put under undue stress.
Putty does two more important jobs as well as passing the weather to the glass. It seals capillary water – water that gets into hairline gaps between objects by virtue of its surface tension – and it holds the glass in the window frame in compression, rather than adhering to it. This means the glass is free to float in a soft cradle of putty. Even when the putty eventually chalks and cracks away from the surface of the joint, water cannot get behind it because the putty has a body, and that body is still doing its job. That is the principle of a sacrificial material – the outside of the material will slowly fail while the inside of the material will remain sound for years to come. For Sash Window Repair contact www.sashwindowpreservation.co.uk/services/sash-window-repair.
Most modern polymer seals have a longer surface life than traditional putty. However, once they have failed, they tend to ‘lock in’ moisture, unlike traditional putty which would allow moisture to escape when the surface skin failed. There is a useful background explanation of Historic England’s guidance on traditional glazing materials.
Cracked or missing putty does not indicate poor maintenance of old sash windows, rather the materials are performing as intended. Repairs of older timber sash windows are increasingly viable alternatives to replacing entirely.
