Skibo Castle - Scotland

Westox Cocoon Skibo Castle

The first record of Skibo Castle is a charter from 1211. From its early history, the castle was a residence of the Bishops of Caithness. Skibo Castle remained the residence of subsequent bishops until 1545, when the estate was, as a tactical measure by the church, given to John Gray in order to reinforce its alliance with a powerful family as the threat of a Protestant uprising spread towards the north.

It is currently operating as The Carnegie Club, a members-only residential club offering members and their guest’s accommodation in the castle or estate lodges, a private links golf course and a range of activities including clay pigeon shooting, tennis and horse riding.

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The Problem

Skibo castle was built on a foundation of sandstone. Over the centuries the castles sand stone foundation has been exposed to the elements with wetting & drying cycles from lateral & rising moisture, resulting in exfoliation of the sandstone.

The caretaker of the castle Mr. Gary Gruber has been in contact with several heritage organizations for example BGS (British Geological Survey) SLCT (Scottish Lime Centre Trust) and HES (Historic Environment Scotland – formerly Historic Scotland) to solve the problem before he got in touch with Westox and Mr. Kevin Bannerman.

After a short survey on the site it was felt that the problem was caused by harmful soluble salts, which was soon confirmed after an analysis of samples taken from the stone, a trial was then arranged to verify if the harmful salts could be removed with a poultice treatment on one of the sandstone blocks.

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The Test

The product to be used named Westox Cocoon originated in Sydney Australia, an invention by Barrie Cooper, the managing director of Westox building products. Three samples were taken at three different depths to be sent to a laboratory to be analyzed by Ion chromatography.

The analysis results show high readings of Nitrates and Sulphates a bit over the preferred 0.2 % “safe level” required to prevent further deterioration. The type of salts found is explained later in this report. The trial commenced with Mr. Bannerman from WCRP applying the Westox Cocoon on the selected area immediately over the area where the initial sampling was carried out.

The cocoon was left on the trial area covered with a temporary shelter to prevent washing off by rain, once dry the temporary covers can be removed. Each application was left for a dwell time of 3 weeks with samples being taken before removal of the first application and again following removal of the second application. All samples were taken within a 50mm radius of the initial drill site. The samples taken were clearly labeled in small plastic bags and sent to the laboratory for analysis.

The Result

The analysis shows that the chloride and nitrate contents have been lowered to well below the “safe level” of 0.2% of total an-ion salt by weight, The Sulphates however have only reduced by approximately 30%.

It is probable that the sulphates are an integral part of the stone or from the cement joints and as Calcium Sulphate (Gypsum) is only sparingly soluble in water the solubility increases with temperature up to 40 degrees C and then decreases again. Although there has been unusally high temperatures in Scotland during the year it is possible the temperatures did not reach 36 to 40 degrees C which would assist in the removal of sulphates.

There has however been a significant transfer of sulphates into the cocoon so the surface of the wall temperatures must have been close to the ideal. It appears there is a slight increase in the samples from test hole 3 compared to test hole 2, although this is not unusual if rain has occured during the test period, we would expect the results from 2 and 3 to be the other way round in stable weather.

The total salts in both test holes 2 and 3 are now considered safe, the only depth where the 0.2% of total an-ions in exceeded is at 0 to 10mm, as the content is predomonately sulphate we would expect the deterioration of the stone surface to cease which should be monitored over the next few months to confirm our expectation.