Stone Capitals at St Pauls Cathedral.

16 Oct 2014

The re-carving of a section of a Corinthian capital at St Paul’s Cathedral presented many problems,
partly due to the sensitivity of the structure, which, as a Grade I listed building, demanded the highest standards of
carving as well as the retention of as much of the original carvers’ work as possible.
The very public location and height of the scaffolding also presented some interesting challenges.

Work began with the cutting open of the original capital to discard badly eroded stone. Up close it was possible to see the original geometrical setting out lines, still scribed on the stone. In order to save as much historic fabric as possible, only a wedge of badly weathered material was removed, instead of the capital being sliced in half. A replacement piece of stone was then roughed out to the approximate shape of the capital to reduce its weight.
This was placed on a bogey and moved by a team of masons up the tower of the cathedral via scaffolding and a hoist. The most difficult part of the route was then to push the stone up and over the v-shaped rooftop before it could be lifted up into the south west bell tower. The route required careful thought and preparation, since any protrusions and traps in the supporting scaffolding could have halted progress and caused a serious safety hazard.

The fixing of this capital had many complications. In particular the replacement section was top heavy and inclined to tilt forward. However, by accurately cutting the stone and leaving only narrow gaps for mortar beds (in this case 5mm) tilting could be prevented as the stones above it and below would trap it in position once it was in place.

The dowels were secured by drilling three dowel holes, two at the top and one at the bottom in the back face of the existing capital and the new section of stone, so that the holes could be accurately aligned when the two sections were joined together. After clearing the dowel holes of dust, they were filled with an epoxy resin. The stainless steel threaded dowels were then quickly inserted into the dowel holes of the existing capital. Time is of the essence as the resin can set in as little as five to ten minutes in hot weather.

The stone then had to be ‘keyed up’, a process which gave it extra grip. This involved making cuts with an angle grinder (traditionally a hammer and chisel would have been used to create pits and punch marks) on the adjoining joints into which the mortar engages itself. A bed height of 5mm was prepared using a ‘St Paul’s mix’, a proprietary mortar which sets hydraulically – that is to say, without the need for air – enabling it to set deep within the structure where there is insufficient air for lime to carbonate. (Ordinary non-hydraulic lime mortars need atmospheric carbon dioxide to set.) It also increases the strength of the mortar, enabling it to take the weight of the stone in compression without any large aggregate particles.

Once this process was completed, three masons lifted the stone with the aid of a block and tackle and straps, and rolled it into location on 5mm stainless steel dowels, thus ensuring sufficient space for the mortar bed. The stone was then grouted.

Grout is essentially a fluid mortar designed to fill voids and bed joints. It is absorbed by the new and old stone, keeping them apart. When set it remains softer than the stone and retains some plasticity to allow for some structural movement in the building. The grout used at St Paul’s was a hydraulic mixture containing bentonite, a clay mineral which made the grout more fluid.

An indent repair at the Royal Pavilion, Brighton, showing the hole in the underside of one of the palm leafed capitals after cutting out the damaged section (top) and the repair (middle). The detail (bottom) of the work before pointing shows more clearly that the lines of the new work copy the lines of the original carving, and are slightly proud of the weathered surface above

Before grouting was carried out, the new stone was secured with wooden wedges which had been soaked in water to swell the wood, and the joints were pointed with mortar. Once this had set, the grout was poured into a hole left at the top of one of the joints though a funnel (actually half a plastic cup fixed temporarily to the wall using mortar), keeping one eye on the joints below in case a leak occurred. Pouring continued steadily until the grout filled to the top of the hole. Later, after the wedges had dried out, it was possible to remove them easily, without disturbing the stone.