Analysis of poultice-based desalination of a nineteenth century brick and stone cathedral after the event; a longitudinal study of surface chloride readings

Abstract

St George’s Cathedral in Perth was consecrated in 1888 and constructed using hand-made bricks and limestone. After 120 years of exposure to salt-laden winds, rising salty water and leaky roofs significant deterioration due to salt efflorescence and water penetration necessitated an $18 million AUD restoration program in 2002. Before accepting the heritage architect’s proposal to replace all the worst damaged brick and stone elements, surface chloride activity mapping of the affected areas was conducted. The survey found that the worst decay was associated with high chloride ion concentrations, with a maximum value of 10,000 ppm on a brick buttress and 7,600 ppm on a stone colonnade on the west front. After four months of treatment, the Westox Cocoon papier-mâché pulp poultice had lowered the surface chloride readings to 10 ± 5 ppm on the bricks and stone. Ten years after the work, a series of tests were conducted on salt-damaged reference stone and bricks to find formulae for converting surface chloride readings into weight percent extractable chloride concentrations. This data was then retrospectively applied to the treated cathedral fabric to estimate how much chloride had been removed. Without the poultice treatment, large sections of the original fabric would have been lost.