Protection of historic stained glass

The process of deterioration of a stained glass window, more precisely of its component glass, lead and paint pigments, has in recent decades been researched thoroughly and is now well understood. The need for the protection in particular of the potash window glasses of the middle ages therefore is widely accepted but not the means of how to achieve this aim. Early attempts such as coatings applied on the glass or the sandwich process, known as Jacobi–Process, have failed for a variety of reasons and with disastrous consequences. Short of removing the windows to safe storage, the only response currently available is the creation of a museum like condition on site in the form of the introduction of an isothermal glazing system. Despite its unquestioned merits, the isothermal glazing system is still in dispute mainly because of its interference with a given historic setting. A variety of designs for the isothermal glazing will be discussed, all aimed at minimising their physical and visual impact on the building, making a strong case in favour of this important protective measure.