Deterioration of Stained Glass


Copyright © 1996-2012  Geoffrey Wallace Stained Glass


Stage 1: is where the leadlight cement has dried out to the point where it is no longer performing its function of binding the glass/lead structure together in a rigid and water tight manner. Cracks and powdering are observed in the leadlight cement indicating that the linseed oil component of the cement mix has dried out and is decomposing.  Rain water on the outside is now being actively siphoned to the inside through capillary action created by the voids left behind by the decomposing linseed oil.

Stage 2: is indicated by break down of exterior leadlight cement to the point where it is beginning to crack and dislodge, increased bulging or buckling perhaps to the point where tie wires are breaking away from the saddle bars and cracks or breaks near solder joints begin to appear on exterior lead came. Generally this won't occur for some considerable time after leakage first becomes apparent and is a result of continued advancement from stage 1.

As the leadlight cement deteriorates the fragile lead structure is required to bear more and more of the weight and pressures of the window until it fatigues to breakage at the points of stress. At this time a stained glass conservator should be contacted to evaluate the condition and provide written recommendations. lntervention early at this stage does not necessarily require total re-leading of the window and repairs are usually carried out on site by our highly skilled craftsmen.  It may be necessary to remove some badly buckled or fatigued panels for straightening or partial re-leading of badly decayed sections but as a rule this work is carried out in situ  and when performed correctly should return the window structure to an "as new" condition. It is important and cost effective to intervene early at this stage to avoid progression to Stage 3.

Stage 3: Progression to this stage is at an accelerated rate over Stage I to Stage 2 and is the result of loss of reinforcement. Lead failure occurs due to a lack of earlier maintenance resulting in lead fatigue caused by gravity, wind pressure and the continual thermal expansion and contraction of the glass. When the lead came has lost its maleability it begins to crystallise and crack and eventually break at the points of greatest flex, usually close to solder joints. If lead failure becomes widespread it can result in glass dislodging or, occasionally, even total collapse of the window. Indicated by glass breakage, unhousing of raw edges of glass from the lead, daylight visible through gaps in the window and widespread external loss of leadlight cement. Windows will usually have to be very carefully excavated to under go partial or more extensive lead calme replacement and glass conservation work to save them.

The basic indications of stained glass deterioration occur in three primary stages:

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. Intervention at this stage is preventative and is aimed at prolonging the life of the window and halting the progression to stage 2.  A regular 25 year maintenance program at stage 1 will prolong the life of a window for several centuries

Dry and cracking leadlight cement

Fatigue crack in lead calme

Daylight visible through splits in window