Once lead is exposed to the elements it starts ‘taking on’ it’s environment and reacting to what’s in it. The surface of the lead interacts with oxygen and water in the air. The change of state only happens on the surface of the lead. Firstly, lead oxide is produced and the surface turns to a matt. Water in the atmosphere will spot the lead and may produce a white powder, this is ‘basic lead carbonate’. This can run down onto the glass but can be wiped off without staining. (the Science bit) Basic lead carbonate turns to ‘normal lead sulphite’ then ‘normal lead sulphate’. This is when you get to the grey patina state, familiar on old churches and Victorian terrace houses where the glass such an age.

The patina on the lead protects the lead underneath. Think about a lead unit that was made up 150 years ago, the lead looks pretty much the same as the lead on an 85 year old 1930s leaded light because the state of the lead hasn’t changed for decades, the main body of the lead is protected now and will be unaffected.

This is why leaded windows cannot be entrusted to just anyone as when the surface is scrubbed shiny it will leak as the protective layer has gone

To clean windows on historic houses, you have to use gentle, non-abrasive techniques especially  purified water, soft cloths, and minimal pH-neutral, non-scratch detergents to avoid damaging delicate glass and frames. Avoiding harsh chemicals and high-pressure washing, which can cause permanent damage. Gentle methods, such as using a soft-bristle brush with purified water and carefully drying with a soft cloth or squeegee, preserve the integrity of the historic windows