Glossary of general stone decay forms: (Compiled by Jonathan Kemp)
Alveolar erosion – circular/nodular erosion of areas forming deep cavities or alveoli.
Back weathering - differential weathering resulting in large volume loss of material in a stone.
Bioinfestation - evidence of animal and/or biological infestation.
Blistering - swelling and superficial lifting of the thin outer surface of the stone to create hollows.
Calcite leaching - formation of hard, brittle crusts of calcite through percolation of calcite rich water leading to increased stone porosity and loss of integrity.
Clay weathering - breakdown of clay minerals.
Contour scaling / Spalling - where cleavage, unlike delamination/scaling is along the plane of the architectural profile, rather than the bedding plane of the substrate.: The plane of detachment of the scales is located near the stone surface (a fraction of milliimetres to several centimetres).
Delamination - see Scaling.
Differential weathering or preferential erosion - selective weathering which involves the loss of less consolidated material exposing the most resistant fragments.
Disaggregation - see Granular Disaggregation.
Dissolution - detachment of weakly bonded ionic components of minerals through the attraction of water molecules.
Dry jointing - mortar loss from masonry joints.
Granular Disaggregation - loss of material by the progressive detachment of single grains.
Encrustation- development of protruding black, grey or white deposits with irregular rough surfaces showing dendrites, folds and pores.
Exfoliation - scaling or delamination.
Fault lines - planar rock fractures.
Feldspar weathering - transformation of feldspar into clay minerals.
Flaking - detachment of thin fragments of stone with irregular edges and varying sizes.
Fracturing - occurrence of breaking, usually mechanical rupture induced by stress, sometimes along vent-lines.
Freeze-thaw cycle - repeated freezing followed by thawing of a material often resulting in granular disaggregation and spalling.
Friable - where a surface can be reduced or made particulate by the use of little force (eg. rubbing a finger across the surface) resulting in disaggregated material.
Gypsum deposition - see Sulphation.
Induration - the redeposition near the surface of a soluble component (eg. silica) to form a hardened and less porous surface crust prone to delamination.
Micro-fracturing - small-scale fracturing.
Salt efflorescence - deposits of salt crystals on the stone surface. Possible sub-fluorescent salts.
Scaling - or delamination - lifting and eventual detachment of large, planar elements from the stone surface according to the bedding plane or, in the case of metamorphic stone, influenced by the isotropism of its minerals.
Scialbatura - a patina of calcium oxalate, found on calcareous building stones, mortars and also on natural outcrops of limestone and marble. The patina consists mainly of calcium oxalate which exists in two forms: monohydrated whewellite and dehydrated weddelite. Both are chemically pure, although impurities are bound up in the crust during their formation. Calcium oxalate is colourless but when organic fragments, mainly lichen, and inorganic fragments of quartz, feldspar and other minerals are present they give the patina a yellowish brown colour.
Formations of calcium oxalate can penetrate deeply into intergranular spaces or along microfractures in calcareous stone, including marble. Calcite crystals become isolated creating a mixed metasomatic oxalate/calcite band with no clear definition between the carbonates and oxalates, making it difficult to detect in cross-section. This layer can be less than 100 µm thick, but also may exist as a heavy encrustation. Calcium carbonate is significantly more soluble than calcium oxalate at pH 7. The solubility of the carbonate increases until pH 5, beyond which irreversible decomposition occurs. In the absence of pollution, rain has a pH of 5 and dilute sulphuric acid can be expected to have a pH of 3 or less. Thus the oxalate is far more acid resistant and not as susceptible to the degradation caused by acid gases.
There appear to have been purposeful attempts to encourage the formation of the patina in antiquity. An examination of Trajan's column found the patina evenly distributed over the entire surface, which may suggest an intentional application.
Oxalic acid reacts with calcite to form, over centuries, a thin calcium oxalate membrane. Lichens are one source of oxalic acid, but the patina may not be caused exclusively by lichen. Oxalic acid can also be produced through the transformation of hydrocarbons from industrial areas, thus it is possible the patina could also be the result of air pollution. There is a lot of debate about how the patina is formed, with theories ranging from the activity of lichen and other micro-organisms to electrical storms as the source of calcium oxalate.
Soiling - accumulation of dirt on the stone surface.
Staining - strong change of the natural stone colouration. Black, pink and other colours can be produced by microorganisms. Iron staining as brown colouration induced by iron compounds.
Sulphation – the reaction of sulphate minerals or sulphur ions with minerals eg. CaCO3 with its reaction product CaSO4 gypsum. The process is accelerated in polluted environments where sulphur dioxide and nitrogen oxide in the atmosphere combine with water to form sulphuric and nitric acids and reacting with the calcium carbonate to form water soluble calcium bicarbonate. If marble is in an exposed position and regularly water washed then the reaction products will be washed away and the surface will recede; if, however, the stone is sheltered then the reaction products will accumulate and form gypsum crusts on the surface, often incorporating atmospheric particulates including carbon, giving the crusts their characteristic blackness.
Surface dissolution - occurrence of rough or irregular surfaces with material loss (see dissolution).
Vandalism - intentional damage e.g. deliberate breaking; graffiti.
Vent – planar slit/openings often along fault lines or mineral planes.
Glossary of polychromy decay forms: (Compiled by Lisa Wagner)
Blistering - detachment of parts of polychromy or within layers thereof in hollow, convex shape.
Crack, hairline crack, fissure, drying crack - types of cracks of various causes within layers of polychromy.
Craquelure - net of cracks within polychromy of distinctive pattern.
Cupping - detachment of parts of polychromy or within layers thereof in concave, cup-like shape.
Delaminating - separation of polychromy or parts thereof from underground in multiple layers.
Fading, blanching, leaching, darkening, change of opacity - forms of discolouration of polychromy or layers thereof.
Flaking- detachment of thin fragments of polychromy or within layers thereof with irregular edges and varying sizes.
Loss - of polychromy down to the stone or within layers of polychromy.
Migration - of binding media, salts, or other particles within polychromy or from the underground to the surface. Can lead to forms of discolouration and/or forms of detachment.
Powdering - loss of binder causing powdering of pigment.
Protrusion - result of a special type of migration.
Tenting - detachment of parts of polychromy or within layers thereof in forms of small tents.