Depending upon the type of stone, the climate, exposure, orientation, building use, provision for moisture management, and type of construction, the causes and manifestations of stone deterioration can be diverse.
As moisture within the wall evaporates from the exterior face, it leaves behind waterborne salts, which remain on the stone surface. (Fig. a) This characteristic white stain, or efflorescence, is a result of rising damp, which occurs when groundwater is drawn up into the base of the wall. To the naked eye, efflorescences often look like deposits. However, their constituents come from the stone itself whereas deposits come from outside.
As trapped moisture migrates through stone, it may leave behind a potentially harmful accumulation of crystallized salts along veins or internal cracks, which can damage the stone’s internal structure. (Fig. b) In some cases, these salts may accelerate corrosion of naturally occurring ferrous compounds in the stone, leading to characteristic rust-colored stains.
Khair-ul Manazil, New Delhi
Delamination takes place when the outer surface of the stone splits into thin layers, which then peel off the face. (Fig. c) Sedimentary stones are naturally prone to delamination, which occurs along the bedding planes when the rock is set perpendicular to the direction in which it was originally formed.
Like delamination, exfoliation is a type of surface disintegration in which the stone sloughs off in very fine layers. (Fig. d) Although the two terms are often used interchangeably, delamination tends to refer to fracture along natural bedding planes, whereas exfoliation is usually due to thermal stress, impeded moisture movement, or other environmental factors.
Khair-ul Manazil, New Delhi
Where a crack in the masonry unit leads to a clean break, the fracture is referred to as detachment. (Fig. e) Detachment may be due to failure of an original construction joint, or it may be the result of a weakened plane within the stone.
Cracking in historic stone masonry can originate from a variety of reasons, ranging from structural settlement to a repointing mortar that is incompatible with the stone. (Fig. f) If cracks are narrow and short or confined to within a single stone unit, the issue may be relatively minor; cracks that are wider and longer or those that extend over large areas may be indicative of systemic problems.
Sher Mandal, Purana Qila, New Delhi
Patina is the chromatic modification of the material, generally resulting from natural or artificial ageing and not involving, in most cases, visible surface deterioration. (Fig. g) It is caused by oxalate or iron deposits that result from direct reaction with the carbonate substrates or from transformations occurring in the overlying coatings with no direct interaction with the substrate.
As the building ages, the natural disintegration and erosion of stone due to wind and rain leads to weathering, which results in worn and rounded surfaces. (Fig. h) Exposure to acid rain tends to drastically increase the rate of weathering and produce a noticeable softening or loss of detail.
Sher Mandal, Purana Qila, New Delhi
2.7 CLEANING OF STONE
Masonry façade cleaning removes pollutants and soiling that have the potential to damage and erode the substrate Soiling and contaminant build up can harm the fabric of a building façade. Cleaning can reduce soil build up and contaminant deposits, therefore negating the potential damage they may cause.
Selection and Approach
- Each building should be assessed individually and in relation to its construction material.
- Initial investigations, sample cleans and testing should be carried out in appropriate positions on the façade, which may include samples on different elevations to ascertain the degree and type of decay.
- Following testing and sample cleans, the specialist team can ascertain the appropriate cleaning technique to use.
- It is essential that cleaning is sympathetically undertaken to avoid serious, long-term damage to the building.
Common Cleaning Methods
i. NEBULOUS SPRAYS or NEBULIZED WATER: a very fine water spray, or mist.
Removing dust and relatively soft layers of soot, rich in water-soluble materials.
The solvent action of water (with respect to water-soluble materials) is enhanced by the high surface area of the droplets. Cleaning is thus accomplished without mechanical action.
Using a nozzle which creates a fine mist, spray water in such a way that it wets the subject stone surfaces but is not aimed directly at them.
When the stone is so heavily damaged by the presence of water-soluble salts that the hydration process (triggered by the penetration of water into the stone) could produce spalling and cracking.
ii. ATTAPULGITE: A hydrated aluminum-magnesium silicate, having the typical structure of a clay mineral; the chief ingredient of fuller's earth.
a) Extracting soluble salts.
b) Removing stains caused by organic materials such as oils. In the latter cases, it is necessary to use an organic solvent (carbon tetrachloride, dichloromethane, white spirit, etc.) instead of water.
Paper pulp, Attapulgite, "AB 57", plastic wrap, air-abrasive equipment, water nebulizer.
Because of its crystalline structure, this clay has a high surface area and absorption potential. Salt solutions can thus be absorbed by the clay and extracted from the stone.
Mix clay and distilled water (1:1) into a paste. Apply packs of wet clay to the stone and cover with plastic film. After a few hours, remove the plastic wrap to permit evaporation of water from the clay. (It is possible to add some paper pulp to the clay to prevent cracking of the compress during drying). When the pack has dried, carefully remove it from the subject area. Repeat if necessary.
When the stone surface is in bad condition. Application of the clay compresses could lead to detachment of loose fragments.
iii. "AB 57": A solution of slightly basic salts with chelating agents added to surfactants, fungicides and thixotropic substances to form a paste. The ratio of the water to thixotropic agents always remains the same, while the quantity of the other components can be varied according to necessity. A typical mixture is obtained by mixing the ingredients in the following proportions:
- Water 1000 mL
- Ammonium bicarbonate 30 g
- Sodium bicarbonate 50 g
- Disodium salt of EDTA 25 g
- Desogen (10% solution) 10 mL
- Carboxymethyl cellulose 60 g
When the stone surface is covered with a layer of soot rich in gypsum and other salts.
The ammonium and sodium bicarbonates and the EDTA (ethylene diamine tetra acetic acid, a chelating agent) facilitate the dissolution of calcium salts (especially gypsum); Desogen (Ciba-Geigy) is a surfactant (reduces surface tension) with a disinfecting action. The carboxymethyl cellulose gives the paste the needed bulk (to prevent flow on vertical surfaces) and prevents the solution from penetrating too deeply into the stone.
(It is possible to use micronized silica instead of carboxymethyl cellulose, thus creating a paste that can be more easily removed at the end of the treatment. It should be noted, however, that the adhesive strength of micronized silica is lower than that of carboxymethyl cellulose, so its cleaning action on surface crusts is slower and often less effective.)
Make a solution of the bicarbonates and the EDTA in water. Add the Desogen and then, slowly, the cellulose. Mix constantly to form a homogeneous paste. Apply packs of the paste, covered with plastic wrap, to the surface to be cleaned. Leave in place for 1 to 12 hours (depending on the condition of the stone), then remove. Removal should be accomplished by delicate scraping (using scalpels) of the subject area followed by careful washing with water to eliminate sodium and ammonium salts. As a final phase, damp, absorbent compresses can be applied to completely eliminate the salt residues.
Inadvisable: When the stone surface is in bad condition and fragments might be detached.
iv. AIR ABRASIVE CLEANING: Micro-sandblasting using glass microspheres (diameter 0.001 mm) or a fine powder of aluminum oxide.
Cleaning thick crusts from sound stone or to clean consolidated stone.
Cleaning is accomplished by the mechanical action of a compressed air jet containing an abrasive. Because of the small size of the spray and the low hardness of the powders used, the cleaning action is slow and fairly easy to control.
Turn the air-abrasive equipment to the lowest pressure setting, aim the pencil at the subject surface and begin cleaning. Adjust the pressure of flow and the distance between the pencil and surface based on the response of the material to the abrasion.
When the Stone surface is in bad condition or the operator unskilled in the use of the air-abrasive apparatus.
- Ashurst, J. Cleaning stone and brick. (Technical Pamphlet N°4). London: Society for the Protection of Ancient Buildings, 1977.
- ICCROM, Scientific Principles of Conservation Course. Course exercises, 1977.
- Mora, P. and Mora-Sbordoni, L. Metodo per la rimozione di incrostazione su pietre calcaree e dipinti murali. In: Problemi di Conservazione, Bologna: Editrice Compositori, 1975, pp. 339-344.
- ICCROM, ARC laboratory manual for Architectural Conservators, Jeanne Marie Teutonico, Rome, 1988
- Ashurst, Nicola. (2015) Cleaning Historic Buildings: v. 2: Cleaning Materials and Processes, Volume 2, Routledge
- C Andrew, Stone Cleaning: A Guide for Practitioners, Historic Scotland & The Robert Gordon University, Edinburgh, 1994
- Jonathan Ashley-Smith, Science for Conservators, Book 2 - Cleaning, Conservation Science Teaching Series, The Conservation Unit, 1983
- John Ashurst and Francis G Dimes, Conservation of Building and Decorative Stones, Butterworth-Heinemann, London, 1990
- Robert C Mack and A Grimmer, Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings, Preservation Briefs 1, HPS, National Park Service, Technical Preservation Services
- F Matero et al, 'An approach to the evaluation of cleaning methods for unglazed architectural terracotta in the USA', Architectural Ceramics: Their History, Manufacture and Conservation, A joint symposium of English Heritage and the United Kingdom Institute for Conservation, 22–25 September 1994, James & James, 1996
- RGM Webster, Stone Cleaning and the Nature, Soiling and Decay Mechanisms of Stone, Donhead, London, 1992