Work to repair and mitigate against further damage to Port Arthur’s heritage buildings following last July’s major storm continues, with work underway to remove excess salt from the foundations of the Penitentiary.
The brickwork of the Penitentiary, like most stone and brick walls at Port Arthur, is very porous and prone to rising damp. Where walls such as these are built without a damp proof course, they draw up moisture from the ground in the same manner as a wick that is in contact with liquid (ie: capillary action). Groundwater contains small amounts of salt, and, when rising damp occurs, particles of salt are drawn up into the masonry.
Where the brickwork is exposed to wind and sunshine, the groundwater evaporates but the salt remains behind in the pores of the wall. Whenever this salt changes phase, from being in solution to being a crystal, its expansion causes the brickwork to break apart. This is evident as fretting or spalling of the surface. This erosive process can eventually cause structural failure.
It is possible to remove salt from a wall by applying a porous coating onto the salt-affected areas a technique known as “poulticing”. The poultice is a wet, sticky material composed of paper fibre, clay and water. It is applied either by trowelling or spraying. As the moisture that is in the wall travels to the drying surface, it carries the soluble salts and deposits them at the surface.
The poultice serves as a temporary surface in which the salts accumulate. After a period of weeks or months, this surface is peeled off and takes with it much of the salt that had been in the wall. In some cases so much salt can accumulate in the surface that almost half of the dry weight of the poultice will be salt.
Sometimes this treatment has to be repeated several times in order to make a noticeable difference.