29-31 George Street, The Rocks (the Victorian Pair)

Desalination of Stone Façade

Client: RM Watson

Location/Site: 29-31 George Street, The Rocks

Project: Desalination of stone façade

Scope of Works: Let’s Clean was asked to perform salt extraction on parts of the stone façade at heritage-listed 29-31 George Street, The Rocks (also known as the Victorian Pair) in order to maintain less salt in the masonry. Desalination took place in designated lower, upper and rear areas of the historic terraces. 

Prior to desalination, the stones of the façade were all quite clean, given that we had already stripped them of paint and washed the surfaces. (Read about the paint stripping works here.)

29-31 George Street, The Rocks.

Lower area 29 George Street.

Upper area 29 George Street.

Lower area 31 George Street.

Upper area 31 George Street.

Rear area 29-31 George Street.

Method: Using the Captive Head Washing technique via our BlueVac System, Let’s Clean performed salt extraction on all applicable stone surfaces of the façade. We completed three passes, where each pass means passing the Captive Head over a given area once.

The Captive Head uses low pressure water to dissolve surface salts and simultaneously removes these dissolved salts by vacuum to a capture tank. 

Water samples were obtained from the capture tank and had their salt level measured by testing for conductivity. Where possible, up to one-square-metre (1 m 2 ) sections of the lower and upper areas of each terrace and the rear area were designated as sampling areas, allowing us to measure the change in surface salts after each pass.

In order to obtain accurate samples, the apparatus (BlueVac system) was thoroughly rinsed prior to each sample taken in order to avoid cross-contamination.

Electrical conductivity (ppm): Electrical conductivity is used as a measure of impurities in water. We use it here to infer the presence of various salts such as nitrates, phosphates, and chlorides.

Note: Local Sydney water measures approximately 128 parts per million (ppm).

Interpreting the Results: A general downward trend in the concentration of salts is desirable. I.e. All other things being equal, we want to see a lower reading with each subsequent pass.

Salt sits within masonry. The waiting period between passes is to allow salts to migrate towards the surface, where each pass effects a reduction of salts contained within the masonry. An uptick in the salt concentration from one pass to the next may mean that:

  • Enough time has passed to allow more salts to migrate nearer to the surface.

  • A longer waiting period has passed (when compared with the one before it).

  • Some factor other than time has influenced the salt reading from samples taken.

At 29-31 George Street, salt extraction (and hence sampling) did not take place until a significant amount of paint had been removed from the rear of the terraces. At the front, much of the desalination areas were already bare of paint, however some of the front areas were also washed with the BlueVac system. This relatively intense amount of activity would affect the readings gained from the surface. In our case, it is most beneficial to look at the final results.

Local Sydney water reads at approximately 128 ppm. Any reading that approaches this, e.g. 512 and lower, is exceptionally welcome. All front sampling areas showed results within this parameter. Depending on the site and source of salts, even higher concentrations can be deemed acceptable.

The rear area shows an anomalous, steady increase in the results, although the final reading is still quite low. In addition, since the rear surfaces were completely painted, and hence received thorough washing after paint stripping, one can reason that this washing assisted in removal of surface salts.

Outcome: We were encouraged by the overall reduction in salt readings obtained during desalination at 29-31 George Street, and also by the generally low final readings. Importantly, these readings approach the base salinity of local Sydney water.

Given the increase in electrical conductivity results at the rear of the properties, it is recommended to choose several locations at the rear for monitoring. It may be prudent to return within twelve months for another course of desalination at the rear elevations.