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Project Profile

 

2006 Award of Merit

Sir JJ School of Art Building

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Technical Brief

Conservation of Brick Work

One of the main technical problems to be resolved in the St. Andrew’s Church conservation project was how to repair damage from previous inappropriate maintenance work to the building’s brick exterior. The use of hard cement pointing had resulted in spalling of many bricks. The damage had been further compounded by cement patch repairs to the most badly damaged areas. The project sought to find an alternative to this destructive cycle of cement pointing and cement repairs to the brickwork.

The conservation work comprised the following key tasks:

• Removing all existing cement mortar pointing from the external walls

• Identifying and replacing defective bricks

• Identifying and replacing defective plasterwork

• Repointing all external walls using lime-based mortar

• Cleaning all stonework and brickwork on the external walls

• Repainting all plasterwork

As part of the tender submission, prospective contractors were first required to provide a method statement for the conservation works to demonstrate their understanding of conservation techniques. Small sample areas on the north and south transept walls were then allocated to the tenderers so they could demonstrate the effectiveness of their proposed techniques in situ.

The bricks in the sample areas were cleaned using water and nylon bristle brushes to remove accumulated mould, dust and grime. A water absorption test was carried out to ascertain the comparative porosity of the bricks and cement pointing. Due to excessive cement content, the pointing was found to be harder than the bricks and thus contributing to the accelerated deterioration of the bricks by inhibiting the normal flow of rainwater in and out of the joints.

The cement pointing was removed using hand tools and the recesses were raked clean to a depth of 15-20mm. Circular disc cutters were permitted to cut a centre line in the pointing (to facilitate removal by hand chisel) only where the cement pointing was difficult to remove.

The tenderers experimented with different mixes of mortars for repointing the bricks. The objective was to provide a softer and more porous lime-based mortar that was more suitable for the bricks. In addition, the relative merits of recessed versus protruding joints were considered. The protruding detail was adopted because sample areas using recessed pointing were unsightly due to the uneven profile of the bricks, which had been weathered and chipped by previous maintenance work. The protruding pointing was consistent with archive photographs and first-hand accounts from long-term members of the church. In addition, in the sample areas, the protruding pointing allowed a neater and straighter horizontal edge to be achieved that helped shed surface water more effectively. To achieve this detail and still produce a softer mortar, the sample area trials experimented with different proportions of cement, lime and sand before determining an optimum mix.

Once selected on the basis of the trials, the successful tenderer was required to make a comprehensive photographic record of the existing condition of the external walls before the conservation work commenced. Also, all windows were boarded up to prevent any damage to the existing stained glass.

Any defective bricks (i.e. those with spalled surfaces or significant chips and cracks) were identified in agreement with the consultant team, marked and counted, before being removed and checked. Bricks with intact reverse faces were rotated and reused, while bricks that were unsalvageable were replaced. Approximately 600 new bricks were required. Bricks of the same material, dimension and colour were sourced from the brick supplier, Blockleys, in England. The majority of the reused bricks were used in the external face of the church and the new bricks were used mainly in the inner wall cavities of the bell tower, to minimize the contrast between the new and old bricks. Repointing of the exterior walls was carried out using the optimum mortar mix described earlier.

Plaster mouldings were repaired in situ by an experienced plasterer using details from mouldings that were still intact for reference. Upon completion, all external plasterwork was painted using white masonry paint. 

Some areas of the granite stonework were noted to have small reddish-brown stains. Upon closer examination, with the benefit of the external scaffolding, it was determined that the stains were due to oxidizing of natural ferric compounds within the granite. Accordingly, it was determined that cleaning of the stonework would be limited to washing off superficial dirt and grime using clean water and nylon bristle brushes.

Adapted from “St Andrew’s Church” UNESCO Asia-Pacific Heritage Awards entry submission

Cleaning of Stone Work

A thorough mapping of all structural and architectural threats to the building revealed that, in addition to deterioration of soft limestone details, the main problem relating to the stone work was staining due to surface encrustation and biological growth.

Removing Surface Encrustation

On the carved stone capitals, decorative mouldings and sculpted panels where sulphate encrustation was severe, a poultice technique was adopted. The areas to be poulticed were identified by the consultants and prepared for treatment. Ammoniacal paper poultices were applied to the stone. Moisture was trapped inside by sealing the surface with a plastic film to prevent evaporation. The poulticed areas were kept wet by intermittent water spraying. After a period of two to three days, the poultices were gently scraped off using flat wooden spatulas. The pollutant, which had become soft and pliable by then, was removed with water sprays and nylon brushes. The run-off was immediately cleaned off to avoid other complications.

Removing Biological Growth

Peepul (ficus) plants with their root systems embedded into stone fissures were first manually uprooted. The remaining stumps were treated with a traditional application consisting of lime and hing (asafetida) to check further growth. In areas that were prone to algae and lichen growth, a light application of quaternary ammoniacal solution was applied to prevent future biological infestation after the surface was washed.

Removing Stains

The project was one of the first in the city of Mumbai to use only gentle water misting for the stone cleaning, in lieu of the normal methods: high pressure jet cleaning, sand blasting or use of chemical surfactants. A range of test patches were tried on the stone prior to beginning the work in full scale. Ultimately, the most sensitive and non-invasive technique was selected for treating the stains on the stone façade elements: the use of pure water alone. The technique involved first dry-brushing the stone using soft nylon brushes to remove superficial dirt. This was followed by mist cleaning, whereby a fine mist of water was sprayed at intermittent intervals. In the absence of expensive imported equipment, the team improvised with the invention of a water misting system mounted on a scaffold at a distance of three feet from the stone, with sprinklers that were calibrated to spray a fine mist for three seconds at three minute intervals. This allowed staining to be removed without over-wetting the stone. Each section of the stone masonry was wetted for a period of three days, after which it was gently cleaned again using soft brushes. The method yielded remarkable results: the golden colour of the basalt was once again revealed while avoiding damage to the stone from the use of solvents or harsh surfactants.

Adapted from “Sir JJ School of Art Building” UNESCO Asia-Pacific Heritage Awards entry submission