Short answer
Colour on a wall or a statue cannot be taken out of its environment: the plaster is alkaline, water moves through the masonry carrying salts, and the surface faces fire, smoke and weather. Lead pigments turn brown or black on plaster, azurite goes green with chloride or black with heat, cinnabar greys, and yellow ochre reddens in fire. On stone sculpture and classical buildings most of the paint has simply gone, and its former presence is established from microscopic traces — which show that there was colour, and where, more reliably than what the whole once looked like.
A panel or canvas is chemically quiet beside a lime wall. Lead white, the standard white of easel painting, caused great problems when artists tried it in murals: it transformed into brownish lead dioxide. Red lead behaves similarly, darkening to lead dioxide or whitening to sulfate and carbonate, and a review of the literature finds the process still debated, with light, humidity, pollution and microbes all implicated; murals in Bezeklik Cave 51 on the Silk Road show red lead with lead dioxide as its alteration product. Azurite blackens to copper oxide under alkaline conditions. Colours added a secco, on top of the set plaster, are vulnerable in a different way: on the Last Judgment much of the lapis lazuli applied a secco has been lost, leaving only the preparatory colour.
What comes through and onto the wall matters as much as what it is made of. Where chloride is available, azurite converts to green paratacamite; at Quaracchi near Florence this was identified only after a consolidation treatment turned a green patch dark blue and the blue then changed again within two years. Red cinnabar walls from the Pompeii area are grey where mercury has combined with chlorine and black where sulfation has coated the surface with gypsum. Rainwater leaking through the Sistine Chapel vault deposited white salt efflorescence; earlier restorers masked it with animal glue, and the glue, with centuries of candle smoke, became the dark veil removed in the 1980s. In each case the visible colour is a layered record of pigment, alteration product, deposit and past repair.
Heat alters mineral colours permanently. The best-known case is the conversion of yellow goethite to red hematite: in the towns buried in AD 79, extensive yellow background panels turned red where hot ash, gas or pyroclastic flow reached them. The same reaction is the craft process for making red ochre from yellow, which poses researchers the question of which reds were painted red. They answer it with measurable properties — the mix of minerals present and the width of hematite's diffraction and Raman peaks, which differ between a natural red earth and one made by sudden heating. The consequence for anyone reading colour in an archaeological site is that 'Pompeian red' is partly a product of the eruption, and that the proportion of red to yellow in a surviving room may not be the proportion its painters chose.
Marble sculpture and architecture of the Greek and Roman world were painted, and almost none of that paint is visible now. A 2023 study of the Parthenon Sculptures in the British Museum found traces of Egyptian blue, used extensively for figurative designs on carved drapery, and of a purple colourant, and showed that the sculptors had finished surfaces differently for skin, wool and linen before colour was applied. The Liebieghaus in Frankfurt has built more than thirty full-size painted reconstructions from such traces. Where paint does survive on excavated objects it may be barely attached: the polychromy of the Terracotta Army sits on an organic ground that deteriorated during two millennia in waterlogged soil, and keeping it in place was described by the chemists involved as a problem in its own right.
For painted façades and interiors the equivalent evidence is stratigraphy. Architectural paint research takes small samples through every layer down to the substrate, mounts them in cross-section and reads the sequence of schemes under the microscope, identifying pigments in each layer; it replaced the older practice of scraping back and matching by eye, which misleads, not least because exposed old paint has itself yellowed or altered. A cross-section shows what was applied and in what order. It does not by itself give the colour the room appeared, since that depended on the fresh binder, the gloss and the light. Limewash, renewed repeatedly and weathering between coats, leaves a different kind of record; the pages on limewashed settlements and on heritage colour rules cover how that evidence is used.
| Colourant | What changed | Driver | Evidence and caveat |
|---|---|---|---|
| Cinnabar (mercury sulfide) on lime mortar | Deep red turns grey or black. | Chlorides and salts; Gaseous pollutants | Strong. Samples from one site and one mortar type. |
| Lead white on plaster | White passages turn brownish and dark. | Acidity or alkalinity; Chlorides and salts | Moderate. The drivers come from one study of fragments from one church; the 1990 abstract names none. |
| Red lead (minium) on plaster | Orange-red turns brown-black, or whitens. | Light and UV; Humidity and water; Gaseous pollutants; Acidity or alkalinity | Moderate. Several competing pathways; which operates depends on site conditions. |
| Azurite (basic copper carbonate) on plaster | Blue turns green. | Chlorides and salts; Humidity and water | Strong. One painting; laboratory tests on plaster samples. |
| Azurite (basic copper carbonate) on plaster | Blue turns black. | Heat; Acidity or alkalinity | Moderate. Laboratory samples on plaster. |
| Yellow ochre (goethite) | Yellow panels turn red. | Heat | Moderate. Cited from a conference abstract; how much of the red is heat-made is not quantified there. |
| Fresco surface under soot, dust and restorers' animal glue | Colours appear dark and brownish, 'as if through a smoked glass'. | Gaseous pollutants; Adjacent materials (binder, support, varnish) | Moderate. The detailed account is by the restorer responsible; the critics' case is cited from a reference work. |
| Fresco surface with soluble salts | White efflorescence over the painted surface. | Humidity and water; Chlorides and salts | Moderate. The salts are not identified in the passage read. |
| Paint layers over an organic (lacquer) ground on terracotta | Painted colour is at risk of being lost with its ground. | Humidity and water | Moderate. The abstract does not quantify losses. |
| Manganese-bearing medieval window glass | Brown spots and patches darken the glass. | Oxygen | Moderate. Four glass samples; the environmental cause was not tested. |
| Object | What changed in appearance | Method | Rests on | What is disputed or uncertain |
|---|---|---|---|---|
| The Parthenon Sculptures (British Museum, London) | Marble long treated as white is shown to carry traces of Egyptian blue, used for figurative designs on carved drapery, and of a purple colourant. | Close examination and archaeometric imaging of the surfaces. | Surviving pigment traces and tool-finished surface textures. | Traces show that colour was applied and where; they do not give the full original scheme, and the authors call for reconsideration rather than offering a reconstruction. |
| Greek and Roman marble sculpture (more than 30 painted reconstructions) (Liebieghaus Skulpturensammlung, Frankfurt) | Full-size painted copies stand beside the bare originals, showing patterned garments in strong colour. | Physical replicas painted on the basis of pigment traces found on the originals. | Surviving traces of polychromy and ancient literary sources. | The museum page presents the reconstructions as research results and does not discuss their uncertainty. A replica must commit to a colour everywhere, including where no trace survives. |
| Painted figures of the Terracotta Army (Lintong, Shaanxi) | Polychromy that would otherwise be lost with its ground is kept in place. | Consolidation of the paint layers by polymerising a monomer mixture within them. | Chemical study of the deteriorated organic ground layer. | Introducing a polymer into the paint is not something that can be taken out again, which sits uneasily with the reversibility that conservation codes ask for; the authors present it as the answer to an otherwise unsolved problem. |
| Wall painting of San Antonio Abate, church of San Pietro at Quaracchi (Near Florence) | A green-altered area of azurite turned dark blue after consolidation, then changed colour again within two years. | A standard two-stage consolidation treatment used on wall paintings. | Infrared spectroscopy and X-ray diffraction of the altered and treated areas. | The study's own conclusion is that the procedure is questionable where copper chlorides are present: the 'recovered' blue was unstable copper hydroxide, not azurite. |
| Michelangelo, Sistine Chapel ceiling and Last Judgment (Vatican Museums) | Frescoes seen 'as if through a smoked glass' emerged with bright, strongly contrasted colour. | Cleaning of soot, dust and earlier restorers' glue coatings by the Vatican's own restorers. | The restorers' technical examination, which attributed the dark layers to smoke and to glue applied in past restorations. | Critics, notably James Beck, argued that Michelangelo finished shadows a secco in carbon black and glue, and that the cleaning removed his work along with the dirt. The restorers held that the layers were later. The removal cannot be undone, so the dispute cannot be settled by looking again. |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Lead white used in mural painting transforms into brownish lead dioxide; a 1990 study summarised research on the cause and reported a procedure that reconverted the altered pigment.
Caveat: From the abstract; the conditions that trigger the change are not stated there.
FactStrong evidence
The green alteration product of azurite on the wall painting at San Pietro a Quaracchi was paratacamite, a basic copper chloride, not malachite; after a consolidation treatment an area turned dark blue copper hydroxide and changed again within two years.
Caveat: One painting, with supporting tests on plaster samples.
FactStrong evidence
In darkened cinnabar wall paintings from the Pompeii area, grey regions contain mercury–chlorine compounds and black coatings are gypsum about 5 µm thick over intact cinnabar; metacinnabar was never detected.
FactStrong evidence
Examination of the Parthenon Sculptures in the British Museum found traces of Egyptian blue, used extensively for figurative designs on carved textiles, and of a purple colourant.
Caveat: Surviving traces; the full original scheme is not recoverable from them.
Source: The goddess' new clothes: the carving and polychromy of the Parthenon Sculptures (Antiquity, 2023)
ConventionModerate evidence
Architectural paint research reads paint layers in mounted cross-sections under the microscope, and is regarded as more reliable than scraping back and matching by eye.
Caveat: Cited from a reference work summarising the practice.
Colourwise interpretationModerate evidence
Trace evidence establishes that colour was applied and where far more securely than it establishes the overall original appearance.
Based on: Colourwise's reading of the difference between the Parthenon study, which reports located traces and calls for reconsideration, and painted reconstructions, which must assign a colour to every surface.
Source: The goddess' new clothes: the carving and polychromy of the Parthenon Sculptures (Antiquity, 2023); Gods in Color (exhibition)
Reviewed 6 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.