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Project Profile


2007 Award of Distinctionion

The Convocation Hall

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Technical Brief

Stained Glass Conservation

Every attempt was made to keep as much as possible of the original material – glass, lead, metal and surrounding masonry – and four basic conservation principles were adopted in the restoration of the historic stained glass.

Minimum intervention: the conservators kept in mind that a dramatic transformation of the existing glass was not the objective and undertook a minimalist approach.

Documentation: the conservators made extensive records of the original fabric status of the windows. First, they took photographs of the stained glass panels before removal of the windows, and during the bench-repair and re-fixing. Second, they made rubbings or impressions on tracing paper of the lead-matrix of every independent panel that was removed for repair or restoration. The second set of documentation records the steps taken in the repair process and is therefore a significant reference for future conservators.

Reversibility of technique: the conservators were told to use repair techniques that could be reversed in the future. Certain harmful processes carried out in the past (previous interventions) were eliminated. Viewing the stained glass in its entirety: the conservators understood and were sensitive towards the materials surrounding the stained glass windows such as original fixing methods, together with the timber framework, steel armatures, lime mortar, stone rebates and iron dowels. All repair approaches were discussed with the architect, consultant and civil contractor when these composite materials had to be addressed alongside interventions to the stained glass.

Numbering of the Panels

The Corpus Vitrearum system of numbering was adopted. Though all the panels were fixed from the outside, all numbering was from the inside. The lancets were assigned Arabic numerals, from left to right, the wheel window was divided into radial sections in capital letters and the traceries and the tympanums were assigned capital letters.

Removal of the Panels

All the independent panels had to be removed from the lancet windows, tracery windows and tympanums. This was because the surrounding framework holding up these panels had been found to be in need of extensive repairs. In the case of the lancet windows, the steel armature was in poor condition and had resulted in heavy buckling of the panels. With the traceries, the stone cusps were damaged and needed plastic repairs, and the circular steel rings that held the octofoils had to be replaced on account of corrosion. As for the tympanums, the fixing method had to be improved as the outer rebates that held the panels had been too narrow, and the panels had been reinforced internally with only one tie-bar each. In fact, the “accordion effect” had already been seen in all the 24 panels, with at least two horizontal rows of quarries collapsing. In the wheel window, only the damaged panels were removed and in situ repairs were undertaken in the rest.

The panels were carefully removed after observation of the original fixing method, non-abrasive forms of cleaning, the consolidation and stablization of the components and their protection against further damage. In the case of panels located in stone rebates, they were first freed of the mortar before untying the copper-ties from the inside. In the case of panels fitted into steel armatures, one panel was removed at a time, beginning with the removal of the metal wedges, then untying the copperties and finally releasing the panel from the mastic-putty from the outside. In the case of panels joined with interlocking division leads, care was taken to start removal from the top, moving downward, and by loosening the copper-ties to flex the panel.

Cleaning of the Panels

Mechanical methods of cleaning were used rather than chemical methods. The more tenacious dirt on painted areas (if stable) was cleaned with glass fibre brushes. In the case of calcium or lime deposits on the glass, soft-bristled brushes were used with a non-alkaline liquid soap. This was done only when the paint was found to be stable. In the case of unstable paint, only dry cleaning was done, using soft cloth or cotton swabs. The cleaning process revealed the exact status of the paint and improved the translucency of the glass, making the colours brighter.

Types of Repair

The panels were inspected closely by the conservators to determine the type of repair required. In most cases, all single, neat cracks in the glass with a stable and firm matrix around them were either “strap-leaded” or sealed with an epoxy resin in-situ. Glass with multiple cracks was removed from the leads and mended using either the copper-foil method or by edgebonding with silicone. The repaired piece was then reassembled in the panel with a two millimetre glass back-plate introduced inside the same leads. As for the lead matrix, care was taken not to dismantle it unless it showed over 50 percent lead-fatigue or lead-joint fractures. Usually only the damaged lead was replaced. In exceptional cases where the panels were found in a buckled state, compounded by the above-mentioned damage, complete re-leading was undertaken.

As for the painted glass, the approach was damage specific, as repainting is an irreversible intervention and therefore unacceptable as it destroys the historic value of the glass. This argument is more rigid in the case of medieval glass.

For the glass, the following was done:

- In cases of more than 75 percent paint loss, the glass was replaced and painted using the existing original as a reference guide. Care was taken to match glass and paint oxides with the original.

- In cases of less than 75 percent paint loss, the “leave as found” principle was adopted, especially if the glass was in a fairly stable panel.

- In cases of missing glass, the area was recovered by glass newly cut and painted using the surrounding glass as a reference guide.

The following procedures were undertaken in the studio:

- Making digital recordings and rubbings of the panels to record surface dirt, glass condition, structural stability of the lead and unstable or lost paint.

- Determining the repair approaches to each panel based on its condition and recording the “line-of-treatment” on the rubbings and in notes, after discussion with the consultant.

- Making one team member responsible for the treatment of a given panel, who would execute tasks from cleaning and stoprepairs to replacing broken or missing glass and re-leading.

- Cementing and finishing the repaired panels by a team of workers.

Cementing of the Panels

Every single panel, whether in situ or on the bench, was cemented on both sides to render them waterproof. In case of in situ cementing, the cement-putty was introduced into the lead by hand instead of using brushes. This had to be done carefully, without applying pressure on the glass.

Re-installation of the Panels

All the panels were re-installed using the same method as in the original. The external framework around the stained glass was repaired or replaced with new as required.

Adapted from “Convocation Hall” UNESCO Asia-Pacific Heritage