Just like anything else leaded windows need cleaning. But why do they get in the state they do? Why does the lead age like it does? How do you clean leaded glass windows?
Oxidation of leaded windows
Firstly, let’s understand why lead goes blotchy and eventually a dark grey matt patina.
Remembering that the lead in leaded windows is probably 120 years old we might have a little empathy with them. Everything ages and lead is no exception.
What’s actually going on is oxidation of the lead.
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 Science of lead oxidation
Lead gives off electrons to oxygen, it’s state changes.
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.
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.
So, the grey matt oxidised patina on lead is normal, inevitable and actually protects the lead. Don’t fight it, let it happen and enjoy it’s authentic qualities.
Speeding up the process of aging lead
When we make traditional stained glass for leaded glass windows we solder all the joints. The lead cames and soldered joints are all cleaned up and wiped down. We then treat the cames and solder with an acid to blacken the lead, take the shine out and speed the process of oxidation up.
The units look better than they would do shiny.
Stripping paint from glass & lead
For years now we’ve used Home Strip Paint & Varnish Remover to strip paint from glass and lead caming. It’s water based and has no toxins or fumes. Therefore, you can use it indoors without ventilation unlike Nitromors which will have you high as a kite.
Just brush a little on the bits of paint on the glass or lead and rub in with a toothbrush, especially good for textured glass and around the soldered joints on the stained glass.
Home Strip paint stripper (you can get some from Toolstation) will have the paint soft and removed within an hour when you can just wipe it off. Be careful not to let it drip down onto the door. When we’re refurbishing the glass we’ll take it out of the door or we’ll be stripping the door anyway so we don’t have this problem.
Don’t use an abrasive like wire wool or sand paper as you may scratch the soft old hand made stained glass.
On flat glass you can use a sharp Stanley knife blade for removing paint. On textured glass you can use the point of the blade but be careful not to damage the glass.
Cleaning leaded glass windows
This chemistry lesson is intended to help you with the cleaning process. Hopefully you will now know that taking the patina off the lead will only expose ‘fresh’ lead which will inevitably oxidise.
Leaded lights, including the glass should simply be carefully cleaned with warm soapy water and dried off with a paper towel, lint free cloth or microfibre cloth. Don’t use an abrasive material like wire wool because you may scratch or dull old hand made stained glass.
If you have any questions please don’t hesitate to contact us or comment below and feel free to share this article.