The hearth was cold. The sulphates were still at work.
18th Century House
Alteration:
Heavy use of fossil fuels in fireplaces (assumed)
Deficiency:
Salt Contamination
Defect:
Surface salt accumulation and sub-surface accumulation causing masonry decay
Hygroscopic salts deposited by years of coal combustion do not leave when the hearth goes cold. They stay in the masonry, attract moisture, and make themselves visible long after anyone remembers what put them there.
The property was 18th Century semi-detached house. The chimney breast to the main reception room retained its original fireplace. The original open fireplace had been replaced for a wood burner with flue liner. The fireplace masonry showed heavy surface salt deposits and decay caused by the crystallisation of salt within the pore structure of the masonry. The extent of salt contamination was visible both within the fireplace and externally where the flue runs through the gable wall. This extent of salt accumulation is likely the result of a prolonged use of coal, producing considerably more sulphur dioxide than timber.
Efflorescence and surface deterioration to the chimney breast masonry were the visible result of hygroscopic salt contamination deposited by decades of coal burning
Original Specification
Modern Alteration
Findings
Solid masonry chimney breasts in traditional properties were built to manage the byproducts of combustion as well as heat. Pargeting would provide a protective and sacrificial component to traditional flues. Burning Comparatively clean fuels like timber tend to result in a layer of soot that prevents the lime pargeting from deteriorating.
Coal is a different material. It contains a high degree of sulphur compounds that, when burned, react with the calcium carbonate in the lime water and ultimately creates hygroscopic salts. The salt-laden masonry will cycle through wetting and drying phases driven by seasonal and daily environmental changes. Each cycle results in hygroscopic salts falling into solution, followed by crystallisation during the drying phase. The result was visible efflorescence, a white crystalline deposit on the face of the chimney breast, along with localised spalling where crystallisation pressure within the masonry had begun to break down the brick face or mortar joints.
The chimney breast showed efflorescence and surface spalling consistent with hygroscopic salt activity. The extent of contamination is likely a legacy of the fuel type. Replastering or applying a damp-proof treatment will not resolve it. Salt-contaminated masonry needs to be allowed to dry, with the source of elevated humidity addressed first. Surface salts can be brushed away and minimising the cycling of salts in and out of solution is key to managing stone decay. This is achieved through the creation of a consistent and stable internal environment.
Why this matters if you are buying an older property:
Salt contamination in chimney breast masonry is one of the more commonly misread defects in traditional properties. It looks like damp. It presents like damp. But treating it as damp, by injecting a chemical DPC, applying a waterproof render, or simply redecorating, will not work and may make it worse.
