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Colour Investigation & Pigment Analysis
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By Richard Ireland

Headfort House -  Eating Parlour

‘Scrapes’ 

It is not possible to accurately establish the layer-chronology, colour and composition of historic paint schemes by the use of a scalpel blade and the naked eye. ‘Paint scrapes’ appear beguilingly credible.  

In practice, this technique is so thoroughly discredited that only the unaware and ill-informed would propose its use as a serious investigative tool for determining original paint schemes. Its value lies in revealing the extent and shape of any individual layer – such as the uncovering of a painted design, graining and marbling or signwriting, which clearly cannot be established by the vertical slice of a cross section through multiple layers. Where such horizontal developments are suspected, the cross section can identify the layer(s) to be investigated and their relationship with the other schemes. 

Colours and Pigments 

The need to describe and market colours to artists, decorators and the public, has resulted in a bewildering array of notional descriptions, marketing aspiration and whimsy. Some names have become enshrined in tradition like the eighteenth century common colours ‘pea green’, ‘straw’ and ‘drab’. Others draw from artists pigment names like ‘vermilion’, ‘cerulean’ and ‘burnt sienna’. Whilst other more fanciful modern descriptions include the likes of ‘aqua green’, ‘burgundy’ and ‘sunset’. 

Even artists pigments yield to marketing applying evocative traditional labels like Indian yellow in place of the modern synthesised chemical equivalent. Indian yellow was the inhumanely produced pigment derived from the crystallised urine of calves fed on a strict diet of mangos. 

Artists colours are usually single pigments and go by their common names such as yellow ochre, cadmium red and phthalocyanine blue, though others such as Payne’s grey, and Brunswick green describe combinations of at least two pure pigments. Colours like terre verte and emerald green on the other hand, are now obsolete and are synthesised from modern mixtures. 

The coloured ingredients that make up a paint are classified by internationally agreed standards. These describe every known pigment by a common name, unique code and chemical description. This makes it possible for the forensic and historic paint analyst to identify the unique natural or synthesised pigment mixtures of a coloured paint. 

Colourmen and pigment suppliers typically use the pigment common name, and usually label the back of their product with its ingredients and identifying codes. 

The combination of inferior pigments to mimic a more expensive colour has the suffix ‘hue’ added to the description such as ultramarine blue hue in place of genuine French ultramarine blue. The cheaper formulations for students artist colours and many printing inks generally fall into this category where additional extenders are combined with cheaper and inferior substitute pigments.

Dating 

The constant search for improved qualities like brightness, staining power, lightfastness and chemical resistance drives the invention and discovery of new formulations. This in turn helps establish the earliest date of possible pigment use. For instance blue verditer rapidly declined in popularity following the invention of the superior French ultramarine blue invented in 1828. 
Eating Parlour, Headfort House - East wall following plaster conservation Eating Parlour, Headfort House - East wall following paint investigation and reinstatement of 1775 Robert Adam scheme

The wide use of basic common iron oxide earth pigments like sienna’s, ochre’s, and umber’s, together with lead white and carbon black in common wall colours can frustrate attempts to date more subdued house painting schemes. Iron oxides have been used in art and decoration from cave dwellings to the present. 

Some pigments have fairly narrow periods of use in house paint, like orpiment also known as King’s yellow. This rare pigment, used mostly around the 1770’s, was rapidly eclipsed by the invention of chrome yellow in 1818 which was much safer and more pleasant to use than the miasmic arsenic based orpiment. 
Eating Parlour, Headfort House - Chimney wall following reinstatement.

Other pigments mark major shifts like the move from lead white to zinc white in the 1920’s. Whilst titanium dioxide white could not be made to perform satisfactorily in housepaint till made possible by the introduction of modern alkyd-resin binders ushered in from the 1950’s. 

Deterioration 

The combined effects of light, pollution and ageing dramatically alter colour. Understanding the nature of these effects and colour changes caused is critical to the appraisal of the original colouring. 

Reds are particularly sensitive to fading when exposed to light, especially organic pigments like red lakes. Whereas the high concentrations of pigment in a fine art painting afford greater resistance to U.V. degradation, the low dispersions used in housepaint are more readily degraded. 

The burning of fossil fuels like wood and coal in fireplaces deposit harmful acidic sulphur dioxide by-products on to the surfaces. Some pigments, like French ultramarine, are especially sensitive to fading when exposed to acid conditions. Conversely, lead white gradually darkens on exposure to acidic conditions. Lead white (lead carbonate) was the universal white pigment and base of oil paints used from the Renaissance to well into the twentieth century. 

Other pigments, like Prussian blue, fade and degrade when exposed to alkali conditions. This readily occurs when moisture migrates through the alkaline lime plaster of historic plaster walls and ceilings. 

Linseed oil, the medium or binder of oil paint, gradually darkens from a pale yellow to a dark toffee colour when concealed from light. Even modern alkyd-resin paints suffer from this, a phenomena readily observed by the yellowing of white paint in darker areas or, for instance, where shielded by static ornaments on a white painted shelf. Once painted over by successive obliterating schemes, the oil medium gradually darkens. 

Darkening linseed oil effectively adds yellow. A mixture of yellow and blue creates green. An exposed pale blue scheme can be identified as green in the absence of pigment analysis. 

Exposure of linseed oil to U.V. light will cause the medium to gradually bleach back to a paler colour. This technique is used to reduce the yellowing on an exposed and mounted paint chip to help evaluate the degree of change. Typically at least three months exposure is required with the sample mounted in direct light at a south facing window. 

Ageing of paint films also alters their appearance, generally causing a darkening, fading and yellowing as a result of all the above factors, combined with ingestion and deposition of dirt on the painted surface. 

To further compound the visual evidence, pigments react differentially to the agents of decay, both individually, as well as through the coating depth. This is particularly important as most housepaint colours are mixtures of pigments with differing levels of resistance and stability. A vibrant red lake will fade readily when exposed to light, whereas the duller iron oxide red ochre is especially stable, acid, alkali and light resistant, though it ages linseed oil much more rapidly. Each surface in a room receives differing levels of light and pollutants causing varying degrees of change. 
British Museum Entrance Hall ‘uncovering’ of 1847 scheme by Richard Ireland

The combined effects of light, ageing and pollution dramatically alter colour. Understanding the nature of these effects and the pigments and colour changes caused, is critical to an informed appraisal of the original colouring. See Figs 1-3 of the 1847 Coleman design executed at the British Museum Entrance Hall showing design exposure, colour reinstatement and completion. 

Cross Sections 

Microscopic cross sections of paint flake samples are used to identify the various paint layers applied to a substrate. It enables primers and combinations of undercoats and topcoats to be distinguished, and more importantly, determine the finish coat in any particular scheme. Equally revealing is the presence and thickness of dirt layers between schemes. 

Dirt layers help both in distinguishing new schemes as well as assisting in determining length of period between decorative schemes. This is important, for instance, when separating trials changes to a scheme from later redecoration. 

Development of schemes can be established from the first surviving treatment through to the latest decoration. Comparison with samples from differing sections and elements of the same ceiling, wall or room can identify broad sequences as well as highlighting minor detail changes. Likewise, comparison with other rooms in the same building can establish trends and connections that may not otherwise be apparent.
Resultant matching of colours by Dr Ian Bristow to original appearance

Sampling 

The calibre of a cross section rests to a large extent on the quality of the sampling. Samples lacking their substrate cannot be easily compared against a series of very similar schemes when an unknown number of layers are missing. 

Equally, a series of complete sections can show up instances where walls or particular surfaces and elements have been selectively decorated more often than others as is typically the case with walls and ceilings. 

Care and skill is required to ensure samples include a full set of coatings. This means selecting areas easily missed in preparation or stripping. Elements where paint accumulates are a far likelier source of complete information than flat open areas or the tops of relief.
 
Even where paint has been stripped of intermediary coats, careful selection of sample sites can usually include traces of the first coatings that can be identified under the microscope. 

This is not always the case though, and thorough chemical stripping, burning off, sanding or blasting can strip away much or all historic evidence. 

The normal preparation of washing down of water soluble distemper finishes from ceilings and walls also removes evidence. However, traces can usually be detected of the first application(s). It is highly likely though, that an unknown series of intermediary distemper schemes will have been removed and lost before application of a more permanent coating type. Equally, incompatibility between coating types and/or poor preparation between schemes can lead to unknown losses of subsequent layers from flaking and peeling. 

Convention analyses and annotates schemes consecutively starting at the bottom from the substrate and first scheme and working up to the last topmost scheme at the surface. Missing layers in comparative samples are skipped and omitted whereas additional schemes or layers are added in. This ensures that comparisons of sample sequences show gaps and additions whilst maintaining identical notation for coeval layers. 
Reinstatement of original 1847 Coleman design in upper hall

Thus all sequences start ‘1’ or ‘a’ next to the substrate at the bottom and may end with differently numbered top layers where surfaces like walls were painted more often than ceilings. This avoids the confusion that arises when attempting to equate related layers that have been labelled from surface to substrate that were painted on different occasions. 

Polarising Microscopy and Pigment Analysis 

Pigments in a specific layer are identified with chemical tests and observation with a polarising microscope. Paint particles from the relevant layer are dispersed on a slide. Microscopic examination through crossed polars reveals the crystallographic habit of the dispersed minerals comprising the paint layer. As each pigment has a unique ‘fingerprint’, it is thereby possible to identify the minerals and thus pigment identity contained in the layer being investigated. 

This is important, as it is impossible to determine the pigment used by unaided observation alone. The possible blue mixed with lead white to produce a blue paint might include, for instance, any of the following pigments: blue verditer, French ultramarine blue, cobalt blue, indigo a variety of different qualities of Prussian blue from green to violet toned, or phthalocyanine blue. 

Crucially, when mixed with white, each produces a very specific and characteristic range of blue. A strong staining modern pigment like phthalocyanine blue will have a totally different effect to the use of blue verditer. When establishing an historic decorative scheme, it can be seen that without such knowledge of the pigment(s), it is impossible to know the character of colour that could have been achieved. 

Conclusion 

As illustrated, there are many factors that distort the colour of paint and its appearance. This stresses the essential need for the benefits of analysis and the importance of a thorough understanding of both modern and historic pigment chemistry, paint technology and pigment use. In particular it emphasises the futility of ‘scrapes’ as a serious investigative tool for colour analysis. 

Further Reading 

Baty, Patrick R: “The Role of Paint analysis In The Historic Interior”  The Journal of Architectural Conservation.  (March 1995) pp 27-37.
 
Bristow, Ian C: Interior House-Painting Colours and Technology 1615-1840 Yale (1996). 

Bristow, Ian C: Architectural Colour In British Interiors 1615-1840 Yale (1996).







Richard Ireland works on a broad variety of buildings encompassing museums, churches, country houses and town houses undertaking consultancy, specification and physical work relating to decorative plaster and architectural decorative paint conservation.

View Richard Ireland's services in the Heritage Register 

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