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repairing leaky leading in situ

 
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davidpalmer



Joined: 01 Jan 1970
Posts: 3

PostPosted: Fri Aug 31, 2007 3:28 pm    Post subject: repairing leaky leading in situ Reply with quote

I have a customer who wishes to carry out his own repairs in situ to leaking leads in a window facing directly into prevailing rain-bearing winds.
Although I have not been able to look closely at this particular window to see the state of the leads, the other lights in his converted chapel date from about 1947/8 and are in their original leads which are reasonably well preserved. The window in question is clearly a part of the same glazing scheme.

Apart from lifting the lead leaves and cleaning out loose cement and re-cementing internally and externally, has any colleague any other tips, miracle solutions or products to suggest for an in-situ repair? Question
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geoffreywallace



Joined: 24 Sep 2006
Posts: 7
Location: Melbourne Australia

PostPosted: Sat Sep 01, 2007 12:57 am    Post subject: Reply with quote

G’day David,

Yes. I have a miracle solution for your problem. Here is an extract from an article I wrote for the SGAA’s Stained Glass Quarterly, Summer 2005.


"Like all linseed oil based glazing compounds and paints, the effective life of leadlight cement is usually somewhat less than fifty years. Except, unlike these other products, traditional leadlight cement is able to be regenerated.

As the leadlight cement ages, the linseed oil component continues to oxidise and becomes drier and drier until, being organic, it starts to decompose. As the linseed oil decomposes, it becomes water soluble and is washed away during rain The oxidation process continues to progress until, usually by about fifty years, secondary problems arise as a result of this gradual decomposition of the linseed oil.

The first of these is ingress of water. This occurs because the degraded, now partly porous, leadlight cement begins actively sucking water, through capillary action, from the outside of the building to the interior. The effect is the same as dipping the corner of a dry sponge in liquid and watching it rise up through the sponge in defiance of gravity. This action is what is meant by the term “hygroscopic” that pops up so often in condemnation of plaster of paris as an ingredient of leadlight cement. The siphoned water is usually observed internally as a damp band around each piece of glass and, when dry, often leaves behind a white calcium deposit that had been dissolved in the water as it made its way through the now degraded leadlight cement.

As the leadlight cement continues to lose its major bonding component, linseed oil, it becomes more brittle and cracks appear in the cement and small sections of cement begin to dislodge from under the flanges of the lead came. By this time, the fragile lead matrix is under extreme stress as it is required to bear the whole weight of the window and absorb the various forces acting on it without the rigid, bonding strength once provided by the leadlight cement. As we all know, lead cames cannot carry even their own weight for very long and as a result metal fatigue will occur at points of major stress, the lead matrix will easily stretch and eventually buckle, bulge or sag often with dire consequences for the raison d’etre of the window structure, the glass component.

Regeneration: New from old.

The most wonderful thing about traditional leadlight cement is that it can be regenerated. The results of its decay are also the means for the gentlest and most effective method of window conservation, recementing. The fact that gypsum is hygroscopic or porous not only means that it is able to draw in water but it can alternatively draw in new linseed oil and reseal itself, once again becoming structurally rigid and impervious to water. With leadlight cement you can actually replace the decomposed organic matter which was linseed oil in the first place .

Thankfully, for the windows, this procedure can be performed in situ with no need for invasive excavation and potential damage to the fabric of both window and building. Furthermore, the original lead cames can be preserved indefinitely if maintenance is carried out on a regular basis to prevent excessive stress and weight from being transferred to the fragile lead matrix alone.

Recementing is carried out using the traditional mixture but it is now made up to a very thin, soupy consistency. After thorough cleaning of the window, the mixture is liberally applied to the exterior surface using a paint brush and left to be absorbed by the old cement. Quite quickly you will observe the mixture dry out around the lead cames as the oil is sucked in.

At this point more thin cement mixture is applied and repeated 2 or more times until the old cement is thoroughly saturated with boiled linseed oil. The excess cement is then cleaned off the window and during this process the solids in the mix will fill any cracks or dislodged sections of the old cement.

Once the new mix has cured, usually sufficient for handling after about 48 hours, the window is just as solid and resilient as it was when new. A perfect bond between old and new cement has been achieved through saturation and capillary action or, as plasterers and stone masons have always called it, “suction”.

The same principles of saturation and capillary action can be used to straighten bulged or buckled panels after they have been excavated. The affected panel is placed on a work bench, painted side down, and water saturated towels are laid over the top. After a time the old cement becomes saturated with water and at this time a flat board can be placed on top of the panel and weighted accordingly. Depending on the age of the window, the water will either soften the old cement or at the very least provide total lubrication for the straightening process. I usually leave the panel over night and in the morning it will be flat again, more stubborn panels may take longer. If the panel has no other problems it can be left to totally dry out and then be recemented to once again securely cast the glass and lead into a straight and strong stained glass window. "

Cheers

Geoff

web.mac.com/gwsg



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davidpalmer



Joined: 01 Jan 1970
Posts: 3

PostPosted: Sat Sep 01, 2007 8:50 am    Post subject: repairing leaky leads in situ Reply with quote

Hi Geoffrey

Thanks for your prompt and detailed reply. Indeed I had read with interest your series of articles in the SGAA quarterly. I mentioned this to my client and gave him the gist of the treatment as I remembered it. Unfortunately I didn't have your article to hand, nor had I ever tried it myself. He did do some trial cementing using a thin mixture of linseed oil and whiting which he applied to the leads with a small paintbrush. He claims that this appeared to work for awhile until some extremely heavy rain leaked in again.

What I don't know is how "liberally" he applied the cement,or whether he cleaned the glass prior to application. I'm pretty sure he only applied the cement once.

From what you say it seems it would be worth him persisting with this treatment. How much time is normally required between each application of the cement? Any recommendations for cleaning - both before and after the applications? Would you advise treating the leads internally as well as externally given that this is unpainted glass?

Thanks again

David
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geoffreywallace



Joined: 24 Sep 2006
Posts: 7
Location: Melbourne Australia

PostPosted: Mon Sep 03, 2007 3:52 am    Post subject: Reply with quote

David,

The mixture should be reasonably thick, paint like, and needs to be stirred constantly as the solids will settle to the bottom. Masking the frame and sill before hand is advised and of course any section of the lead structure that is missed will continue to leak. Suction should be apparent in 5-10 minutes and the cement is then reapplied, checked in a further 5-10 mins, reapplied until suction is no longer evident.

When suction is complete the window is cleaned with hessian or cotton cloths which will remove the oil and push the solids up and under the cames. It is then left to cure a little before picking as with normal cementing. The operation must be completed in one go because if left overnight the cement will set up hard and be a pain to remove. If the window is in full sun the cement can set up in a couple of hours so it needs to be cleaned off as quickly as possible.

The mixture is made with BOILED linseed oil thinned 50/50 with mineral turps to make it thin enough to allow good absorption. Raw linseed oil will not cure. For in situ work we only treat the exterior of the glass.

Good luck

Geoff
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