Bristol Cathedral: Abbey Gatehouse: Report on cleaning and conservation works
Winter 2003/4. Nimbus Conservation Ltd.
It is not the purpose of this report to discuss in detail the architectural significance, build history or geological history of the stonework of the Abbey gatehouse. These will be dealt with elsewhere, by others in more detail. It is sufficient to say here that the Abbey gatehouse as it stands at present is the result of several periods of dismantling and rebuilding, that it retains significant elements from each rebuild and because of this provenance contains more than one stone type and varies in condition depending on the age and type of stone used. Significant numbers of Jacobean/Elizabethan elements of carved stonework(Tudor roses/Niche canopies and niches) remain, rebuilt into the major refurbishment which took place at the beginning of the Twentieth century. The lower main arch is of Norman origin and is in generally remarkable condition.
The original object of the contract was to ascertain suitable cleaning methods for the Norman Arch, alongside an emergency program of repair of the two lower niches which were in poor condition and threatened passers by with falling masonry. Funding allowed this program to be extended to include an overall general clean, repair and sheltercoating of all older sculptural elements, cleaning and repair of the major arches and lower niche canopies.
Methodology throughout was of minimum intervention. Cleaning methods were chosen to cause least disturbance in shortest time. The object of the general clean was not to reveal bare stone, but to remove damaging sulphation from sheltered areas and loose surface dirt from the face of the stonework overall. Plastic repair where undertaken was used to fill voids and prevent water/frost traps. Where ornamental areas were subject to plastic repair no attempt was made to repair back to the original line, where necessary the line was suggested to give the repair coherence.
During cleaning great care was taken to avoid the removal of tool marks, masons marks, alignment marks and polychromy.
2.1. Cleaning Trials .
Sections of the Left section of the Norman arch were chosen and subjected to the following cleaning methods.
2.1.1. Nebular spray: A timed nebular spray was used with a 10 second on period in every five minutes. This was sufficient given the temperature and wind conditions to keep the stonework moist and detach the sulphation staining over a three day period. It must be remembered that in was not possible to leave sprays unattended at night. The method cleaned the stonework successfully.
2.1.2. Microabrasive. A microabrasive pencil was used using aluminium oxide as the abrasive powder. This also removed the sulphation, but very slowly and left a minor amount of burnishing.
2.1.3. Ammonium carbonate poultice. This consisted of a 7.5% solution of ammonium carbonate in water mixed with technofil paper pulp and sepiolite clay to crate a malleable poultice which could be kept in contact with the stone surface for up to 24 hours without excessive drying out and cracking. The method was very successful, removing sulphation in one application. The residue was scrubbed using a variety of tools, - toothbrushes, stainless steel brushes, etc. The poultice removed the sulphation without affecting areas of polychromy.
2.1.4. Waterwashing and scrubbing using nonionic detergent. This had little impact and was not pursued.
Final choice of cleaning method was between nebular spraying and ammonium carbonate poultice.. The poultice was thought to be suitable and the most cost effective and was therefore used for the final clean of the arch and other sulphated elements of the elevation. Nebular spraying although equally effective created problems of unattended equipment and water runoff over a passageway used by the public.
Examples of sulphation on carved areas of stonework
Shows poultice applied to archway and left to dwell for 24 hours.
Shows partially cleaned arch stonework.
2.1.2. General Cleaning.
Dirt was removed from the entire surface of the elevation using a domestic power washer with a lance set to low volume and low pressure. This removed loose surface dirt but did not clean down to the bare stone surface which was thought to be undesirable. Mosses and algal growth were mechanically removed.
2.1.3. Sulphation in sheltered areas.
Where sulphation growth was quite thick, the bulk was removed mechanically. This process was followed by several (where necessary) applications of the ammonium carbonate poultice described above.
2.1.5.Norman arch and smaller arch.
Cleaned throughout using ammonium carbonate poultice.
3.1 Pointing to lead chases at high level.
This was done after taping the joints to prevent staining to the lead with a 3:1 mix of Chardstock pit sand and hydraulic lime. Pointing kept damp for some days after application.
3.2. Pointing to high level chimneys and cupola.
After careful raking out of cementitious and failed mortars, the open joints were repointed using a mix of 1 part fine yellow sand, two parts of sharp sand and 1 part of hydraulic lime. Hydraulic lime was used to give the mortar strength in the face of extreme weathering conditions.
3.3. Plastic repair at high level.
Instituted into water traps and hollows on the chimneys and cupolas using various mixes of the three sands (Chardstock, fine yellow and sharp pit sand) in proportion of 3:1 with hydraulic lime. Sands used in different combinations depending on the depth and size of the hollow to be filled.
3.4. Pointing to north elevation.
Narrow joints pointed with 3:1 aggregate:lime putty. Aggregate a mix of fine yellow sand and sharp pit sand.
Wider joint pointed with a 3:1 mix of Chardstock sand and lime putty.
3.5. Plastic repair to elements of north elevation.
All executed in 3:1 aggregate to lime putty mix. Aggregates a mix (depending of stone type and texture of Guiting stone dust, fine yellow sand, brown sharp pit sand and Chardstock sand.
I all cases of pointing and plastic repair the joint or hollow was prepared for pointing/repair by removal of extraneous matter and thorough soaking prior to institution of pointing/repair. All repairs kept damp for some days to prevent premature drying and assist carbonation.
4.Lower Niche Canopies.
Photos showing original condition of the lower niche canopies.
4.1. Condition of lower niches.
These appear to date from an earlier build that the present early 20th century. These two niche canopies presented the biggest problem on the faŤade. Built it would appear from face bedded stone there was much evidence of deep fissures and delamination combined with disintegration and weakening of the stone surface due to acid rains frost and other forms of weathering. The condition was bad enough to cause worry of major falls of large pieces of stonework. The problems were therefore in the main twofold- to fix the remaining stone soundly to avert fears of collapse and to consolidate the remaining stone surface to make it resistant to further weathering. The methods of repair have been described in an earlier method statement and were successful.
Lower Niche canopies after repair.
Condition prior to cleaning and repair
After cleaning, repair and sheltercoat.
The older carved elements of the faŤade were treated similarly to the niche canopies. This meant cleaning of sulphation staining using ammonium carbonate poultices followed by multiple applications of Limewater. Friable areas were treated with a slurry of stonedust, lime putty and casein, applied very dilute in order to facilitate penetration into the pores of the stonework. After this foundation work was undertake, necessary repairs consisting of fine yellow sand, Guiting stonedust and sharp brown pit sand were applied (3:1 Aggregate to putty). Subsequent to this sheltercoat was applied to provide weathering protection.
The archway was cleaned using applications of Ammonium Carbonate poultice as described above. Cleaning revealed location marks and red oxide polychromy in sheltered areas esp on the interior face of the arch. Cleaning also revealed the poor and cracked condition of the stonework in some places.
Removal of hard cement Large sections of hard grey cement repair were removed using light tapping action from hammer and chisel, shallow cuts with angle grinder and penetration with drill.
5.2. Damage to archway and repairs.
As has been stated above cleaning revealed damage to the stonework hitherto invisible. This ranged from broken stone to cracks and fissures, hard skin blisters with zones of stone depletion beneath the hard skin. Projecting elements of the topmost hoodmould were lost and large sections of the topmost decorative band were eroded esp to the lower section to the east and west.
Topmost decorative band to lower west, shown post cleaning post cement removal with repair in progress. Repairs made only to prevent water traps and to suggest original line.
The picture below show typical problems found once the stonework had been cleaned: Blistered and friable stone, loss of mortar to joints, cracks and fissures.
Cracks and fissures Loss of jointing mortar Blisters and stone loss
Cracks and fissures
Loss of jointing mortar
Blisters and stone loss
5.3. Repair methods and mixes.
The arch was limewatered throughout using multiple coats. Subsequent to this repairs and repointing was undertaken to fill cracks and fissures and cap blistered and friable stone. Throughout the repair mix consisted of Lime putty, Guiting stonedust (sieved 2mm down), fine yellow sand and were a wider gap was filled either sharp pit sand or chardstock sand. Pointing was undertaken using the same combination of lime putty and aggregates.
The older carved elements, niches and niche canopies and some of the Norman arch (topmost mouldings, columns and bases) were all sheltercoated. The sheltercoat consisted of casein, Guiting stonedust sieved 1mm down, fine yellow sand and lime putty. Because a strong colour was required, yellow ochre, burnt sienna and crushed charcoal were used in small quantities (all Cornelison earth naturally occurring pigments)
The sheltercoat was applied to the wet stone using a paint brush with the mixture being constantly stirred to keep the aggregates in suspension. After application the excess was rubbed off the surface using a hessian cloth.
Rene f. Rice for Nimbus Conservation Ltd. February 2004