Old Sessions House, Clerkenwell Green
London Stone Conservation carried out a full programme of external cleaning and repair to the Grade II* listed Portland stone building. This included the formulation of bespoke non-aqueous poultice treatments to target and remove different types of staining from the historic stonework.
The exterior stonework at Old Sessions House had suffered severe staining due to the build up of atmospheric pollutants, moisture damage from historic water ingress and biological growth. Initial site trials using the DOFF superheated water system had proved successful but were found to mobilise the natural mineral content in the Portland stone, bringing the iron to the surface and causing heavy orange and brown staining. The iron content, present in limestone, is water-soluble and becomes mobile during cleaning or heavy saturation of the stonework. As the stone dries out the iron then deposits on the surface and oxidises, causing discolouration.
The use of poultice cleaning allowed for large areas of the façade to be chemically cleaned in an effective, targeted and controlled manner. Following further site trials, multiple poultice formulas were prepared depending on the type, location and severity of the staining to be treated. Different reagents were blended and suspended in natural clay or paper pulp medium to form a stiff paste. This was then applied over an intervention layer to aid the removal of the poultice after cleaning. The contact time was dependent on the type of soiling to be treated and ranged from an hour to more than a day in some areas. The increased dwell time of poultice application allows for much lower concentration of reagents than would be necessary for other forms of chemical cleaning. The poultice was then covered with plastic film to slow the rate on evaporation. As the moisture disperses from the poultice, the active agents loosen the dirt and draw it into the absorbent support. The poultice was then mechanically removed with wooden tools and nylon bristle brushes, and the surface neutralised to remove any remaining contaminants. The use of multiple non-aqueous cleaning methods allowed for a localised and controlled program of cleaning, which was sensitive to the historic patina of the listed stonework.