Water Proofing

 

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A stained glass window gains it’s inherent strength, rigidity and waterproofing from the leadlight cement that is worked under the flanges and around the heart of the lead calmes, forming a tight bond between glass and lead.  Leadlight cement,  like all oil based putties will eventually dry out and progressively lose bonding strength.

Cracks and powdering will be observed in the cement and rain water from the outside will be siphoned to the inside through capillary action created by the voids left behind by the decomposing linseed oil. The siphoned water is usually observed internally as a damp band around each piece of glass and when dry, often leaves a white calcium deposit on the inside surface of the glass.

To combat this a normal 25 year cyclic maintenance program includes re-cementing of the lead calmes.  When this does not occur, not only will the window begin to leak, but the weight of the window will need to be supported more and more by the fragile lead structure alone.


It is the combined cement/lead structure  that supports the weight of the window with the lead providing the malleability to cope with expansion and contraction and the leadlight cement providing rigidity.

Water is also the major accelerant of paint loss on 19th century windows and leaking water can have disastrous effects on old windows unless properly maintained.

Bleached, dry and cracked leadlight cement.

The same lead section after re-cementing.

Calcium deposits left behind by water leeching through degraded leadlight cement

Recementing is carried out on site and should be repeated about every 25 years on old windows.