I have had some experience with Stained or lead light glass. I used to make Lead light, light fittings and windows as a hobby.
In a stained glass window the sections of glass are held together in the window in a LEAD section called CAME. It is in effect an extruded LEAD section that is “H” shaped. The glass sections fit in between the arms and legs of the “H” and the cross bar on the “H” is like a web between each piece of glass. Once the sections of the window are all set in place and the ends of the CAME are soldered in place the glass is glued into place with window putty which is made from linseed oil and WHITING. ( whit·ing 1 (hwī′tĭng, wī′-) n. A pure white grade of chalk that has been ground and washed for use in paints, ink, and putty. [Middle English whityng, from ...)
What happens is the linseed oil evaporates out of the whiting and it hardens to lock the glass into the CAME and it waterproofs the joint.
My caution to you would to be sure you do not apply pressure, be it water or brush, to the stain glass windows. If they are old you could run the risk of literally pushing the glass out of the frame.
Stain glass windows are really quite soft and will damage very easily. Huge amounts of respect required when working around them.
The Lead it’s self is in the metal form as opposed to a powder form as in paint. It would be fair to say it poses no health risks if that is a concern for you.
With regard to the algae on the windows I would suggest you use a spray and walk away type product on them and be sure that the formula in the product DOESN’T contain Sodium Hypochlorite in any way shape or form.
Lead probably will corrode like hell if there is any residual Hypo left on it or in the joints between the whiting, glass and the lead came. The end result probably would be that the windows would start to leak in time.
If the windows were made in the 1920 they are nearly 100 years old and would be very delicate.
Any damage or leaking would cost a huge amount of money to have fixed or replace.
I hope this has shed some light on stained glass windows for you.
