When conserving stained glass, the need is to preserve the glass in is original state as far as possible. This may require the window to be removed from it setting and re-lead glazed, or it might require the glass to be protected from external elements, by isothermal glazing. This involves making a clear screen of glass with some ventilation, to allow air movement, but not allowing the ingress of bugs and insects. The clear screen is divided up with lead to replicate some of the main features and shapes of the stained glass, giving a pleasing appearance from outside.
Below can be seen isothermal glazing on the oldest glass in Devon where the glass had been damaged by grass strimming.