Minor undulations can enhance the visual appeal of an old window – in fact, with ‘Bombay glass’ the unevenness is deliberate. Slightly distorted but firm windows may, therefore, be left undisturbed.

More serious deflection, however, admits rainwater, fractures cames and cracks glass. Panels are sometimes straightened in situ (by reattaching broken ties, for example) or temporarily removed and laid flat under an even load. Copper patches can be adhered to fractured cames. The most extreme cases could justify some releading or perhaps, with very important glass, a protective glazing system. Additional supporting bars may be required. Severe distortion may indicate a structural problem.

Releading entails removing a leaded light to replace cames. This should only take place when there is severe deterioration (such as corrosion) that cannot be addressed in other ways and not because a window is simply a certain age.

When releading, retain and reuse old cames wherever possible, along with components such as pierced lead ventilation grilles. Where replacement is unavoidable, the existing leadwork should be examined and recorded, noting, for instance, any milling marks on the hearts (cross-bars), including names or dates. New cames should usually match the type, size and profile of the existing ones.

Any metal bars and ferramenta are integral to a leaded light and should not be overlooked during maintenance or other work. As with metal window frames, their treatment generally involves derusting, priming and top-coating using a suitable rust-inhibiting paint.

Rust stains can irreversibly damage glass. Where the expansion of heavily corroded iron bars causes the masonry to fracture (ie ‘rust-jacking’), the ends of bars may be cut off and substituted (‘tipped’) with non-ferrous metal. Consider replacing low quality mild steel bars with stainless steel or bronze.