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2007 Award of Merit

The Bonython Hall

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Technical Brief

Stone Repair and Conservation Techniques

The following techniques were used in cleaning and repairing the stone elements of Bonython Hall.

Indent repairs

Damaged and decayed limestone was repaired using stone indenting (also known as piecing in). The limestone was prepared by cutting and chiselling out the damaged parts, and slightly undercutting the edges to provide a mortar wedge. Indents were made using Murray Bridge limestone to match the original stone and were installed with hairline epoxy bonded joints.

The indents were held in place with stainless steel all-threaded dowels of minimum 8 millimetre diameter, ensuring that at least two dowels in each indent were longer than 200 millimetres. After ensuring the holes were clean, dry and free of dust, dowels were epoxy bonded into the indent and existing limestone. 

Stabilizing

Fine fractures in the stone were stabilized by drilling a series of small holes along the complete length of the fracture and injecting epoxy with high penetrating properties. This consolidated the friable substrate and will prevent any future water penetration.

Mortar repair

Mortar repair (or plastic repair) was used as an alternative to indenting. This involved the careful removal of decayed stone, cleaning of the cavity and filling the cavity with a mortar mix. The limestone was prepared for repair by cutting and redressing to a minimum depth of 10 millimetres. For deeper cuts, reinforcing pins were anchored into the existing limestone with epoxy resin. The mortar mix for the repair was 3 parts lime putty to 1 part white cement to 10 parts well-graded sand and limestone dust. After application of the mortar, the repair was covered with damp cotton waste and was allowed to cure slowly.

Cleaning of stone

The cleaning of the combination of Murray Bridge Limestone and cast cement elements used to construct Bonython Hall required a careful approach. It was necessary to choose a cleaning system that would be effective and non-damaging for both materials. This was achieved by using the JOS system, which is used across Europe and had been trialled successfully on many projects in South Australia. The JOS system is a patented chemical free, low pressure, rotating vortex process. This cleaning technology operates with an extremely low and variable air pressure (between 0.1 and 21 psi), which can be regulated with minimal amount of water and an extremely fine granulate. The combination of special fine blast grain and the rotating vortex made it possible to achieve good cleaning results efficiently and without harming the material of the building.

Adapted from “Bonython Hall” UNESCO Asia-Pacific Heritage Awards entry submission