NSI Field trip:
Culzean Castle 2004

The NSI organised a visit to Culzean Castle in May 2004 to tour the masonry workshops where repairs to the fabric of Culzean are carried out. The group had a tour of the original quarries, stonework, repointing and repairs carried out on the Castle and observed preparation of slaked lime from a small-scale traditional kiln.

Culzean Castle, Ayrshire.

View of Culzean Castle through the ‘Roman’ viaduct and archway ruins built by Robert Adam.

Many alterations and repairs have been made during the Castle’s 400 year history – some better than others. Originally built as a tower house in the 1590s, Culzean Castle was remodelled and extended by Robert Adam in the 1780s-90s. The front of the Castle and the Clock Tower were re-faced. The Castle building was extended and Adam also built a ‘Roman’ ruin of a viaduct and arch.

The Clock Tower, originally harled, was re-faced by Robert Adam.

Robert Adam’s roof level alterations to the Clock Tower can be seen in the previous roof-line.

The Gas House. Recently restored as an exhibition area: coal gas was manufactured here to supply the needs of the castle and the home farm.

Location of an old quarry site on cliffs along the beach within a short distance of the Castle.

The original quarries for the sandstone used in the construction of Culzean Castle were on the cliffs above the beach, within a few hundred metres of the Castle. Remains from the quarries and the working of the stone can still be seen – quarried out sections along the cliffs, a boulder with plug and feather marks from masons’ tools and spoil heaps at the base of the cliffs.

This fallen boulder has plug and feather marks along the top right where masons have attempted to split the rock.

Waste from coastal quarry working has, in places, been piled up to form a working platform, presumably to access higher levels of stone from the quarry face.

This sandstone was of good quality, being resistant to weathering – a necessary quality as the Castle and its surrounding buildings are in a highly exposed location. The clay-rich sandstone later used by Robert Adam was also local (though its source is not precisely known), but was not so robust and many blocks have decayed very severely in the last 200 years.

The stone used by Adam during the 1780-90s has suffered badly from weathering in this coastal location.

Clay-rich blocks are especially prone to decay caused by expansion and contraction of clay minerals on wetting and drying - the surface of exposed sandstones are subjected to repeated size changes which do not occur in the unwetted interior, eventually resulting in detachment of the stone surface.

Honeycomb (also called alveolar) weathering, caused by the action of sea salts, is common on many blocks. This form of weathering is typical of sandstones (especially calcareous sandstones) in a coastal environment. Some extreme examples of this type of erosion can be seen at Culzean.

In some areas stone decay is extreme, especially (as here) where the sandstone contains a high proportion of clay minerals.

Expansion and contraction of clay mineral grains in the sandstone causes dimension changes on the exposed surface of the sandstone which may eventually cause cracking and spalling of the outer surface as seen here.

Honeycomb (also called alveolar) weathering of sandstone on the Clock Tower. This type of weathering is typical of sandstones in coastal environments, though not always to such an extreme.

Delamination of the sandstone block on the left is a result of it having been face-bedded – placed with its natural bedding planes parallel to the wall surface. A similarly decayed stone on the right has already been replaced.

This sandstone in a wall near the Gas House shows extremely thin contour scaling of the surface.

Blistered surface attacked by lichens.

In places, the stone is also vulnerable to attack by lichens. Severe pitting caused by lichen growth can be observed on a few areas. This lichen is Ochrolechia parella, also known as Crab’s Eye Lichen.

Lichens do not often cause this degree of damage. It is likely that the pitting shown here is caused by a combination of vulnerable stone, penetration of the stone by fungal filaments (hyphae) of the lichen, sea salt and attack by lichen acids.

This vertical view shows how the blistering occurs, as lichen growth clearly causes expansion of the outer layer of stone penetrated by the lichen’s hyphae, with detachment at about 5mm depth.

The worst (best?) lichen damage was observed in this gateway. This is where the images of lichen damage shown here were taken.

Pitting occurs in the central zone of lichen growth and may be as much as 10mm in depth.

The pitted surface may later be recolonised by the lichen, as can be seen here in the two larger pits to the lower right of the image.

This lichen appears to be enjoying itself!

Severe deterioration of the original stone has necessitated replacement work.

During the 1970s Springwell Sandstone was used for replacement and indenting. Since the 1990s Stanton Moor Sandstone has been used.

A variety of tooling was used on the castle and its surrounding buildings. Where stone is indented or replaced, the original tooling has been reproduced.

Front of Culzean Castle with Springwell sandstone indents. The colour of Springwell is close to that of the Culzean sandstone, but lacks the colour variation of the original stone.

The stone on the left is original and shows honeycomb weathering. The new stone on the right is Springwell. Tooling on the replacement stone matches the original.

Since the 1990s Stanton Moor sandstone has been used for replacement of decayed stone. The replacement blocks here (upper centre) are already weathering in to match the original stone.

A surprising variety of tooling may be seen. The rybats around this window in the stables show various horizontal and diagonal tooling styles.

In this example of original tooling, horizontal lines stand proud of the surface.

On this replacement stone (Stanton Moor sandstone), the original horizontal bands have been strongly reproduced.

On this replacement stone (also Stanton Moor sandstone) the horizontal tooling has been less strongly emphasised, producing a more ‘sympathetic’ result.

Replacement of decayed mortar and hard pointing from previous repairs has been necessary. Lime mortar was made using sand from the beach as this is thought likely to have been the original source of aggregate at the Castle. The presence of sea salt does not interfere with setting of the lime mortar.

Pointing dating from some decades earlier often used hard cement mortar which has accelerated decay of the sandstone by trapping moisture in the wall. Pointing of this type is being replaced with lime-based mortar.

Lime mortar could not be used for pointing this retaining wall below the driveway as it is constantly saturated with water. Instead, hydraulic lime was used as this can set when wet.

Where possible, lime mortar was used for repointing. Aggregate was obtained from the beach since this is likely to have been the original source of aggregate.

At the end of our tour, the NSI group observed slaking of lime from a small, wood fired kiln. The limestone had been burnt on the previous day.

This sequence shows the wood-fired kiln used for burning limestone, the lime in the top part of the kiln and slaking of the lime. In the centre picture, steam rises from the barrow as the slaking reaction produces intense heat. Within half an hour the lime has been converted to hydrated lime putty.

Wood-fired lime kiln.

Lime chunks in the top of the kiln after firing.

Lime pieces removed from the kiln prior to slaking.

Slaking lime produces intense heat – steam can be seen rising from the barrow.

Within a few minutes the lime chunks have broken down to form a hot slaked lime putty.