What Is a Lake Pigment? Turning a Dye Into Something a Painter Can Use

Pigments are insoluble solids suspended in a medium, and their colour comes from particles scattering and absorbing light while remaining physically separate from the binder. Dyes dissolve and bond chemically to what they colour, which suits textiles and does not suit paint, since a dissolved colourant in a binder produces a stain rather than a workable paint film. The conversion precipitates the dye onto a colourless inorganic substrate, typically an aluminium compound, producing solid coloured particles that behave as a pigment while retaining the colour of the dye. That process was used to make paint from the enormous range of colourants available from plants and insects, which was otherwise inaccessible to painters, and the resulting colours are characteristically transparent and intense rather than opaque and covering.

Several such pigments were among the most valuable materials in a painter's stock:

• •Carmine, from cochineal insects imported from the Americas, which was extraordinarily expensive and gave an intense crimson
• •Madder lake, from a plant root, giving a range from pink to deep red and used since antiquity
• •Kermes, from a different insect and used in Europe before cochineal displaced it
• •Weld and buckthorn yellows, from plants, which gave the yellow lakes
• •Brazilwood, from a timber that gave a colour so associated with it that a country was named after the tree
• •Indigo, which was used both as a dye and, prepared appropriately, as a pigment

These colours share a serious weakness, which is that organic colourants fade on exposure to light and the conversion into a pigment does not fix that. Fading varies with the specific colourant, with the substrate, with the binder and with how much light the work receives, and some are severely fugitive while others are moderately stable. The consequence is visible across the history of painting, since works containing them have lost their reds and pinks progressively, and a painting that now shows pale or absent passages was frequently intense when made. Documented cases include portraits whose flesh tones have gone, landscapes whose greens have shifted blue as the yellow component faded, and works where an entire compositional element has effectively disappeared. Reconstructing original appearance is a substantial area of technical art history and depends on identifying what was used.

Galleries holding works made with fugitive colourants face a decision that has no good answer. Light causes the damage and is required to see the work, so any display is a trade between access now and preservation later, and the relationship is cumulative rather than having a safe threshold. Standard practice limits illumination to low levels for works on paper and for paintings known to contain sensitive materials, rotates such works out of display for recovery periods that do not actually reverse the fading, and monitors total exposure over time. Reproductions and digital reconstructions show visitors what the work looked like, which is informative and is not the work. Some institutions display a faded original alongside a reconstruction, which is honest and disconcerting. The underlying fact, that a painting is a physical object degrading at a rate viewing accelerates, is rarely stated to visitors plainly.

Detecting these materials in a painting is harder than detecting mineral pigments and the methods are correspondingly specialised. Mineral pigments contain characteristic elements and are identified by element mapping, while organic colourants contain only the common elements of life and are invisible to those techniques. Chromatographic methods separate the colourant from a tiny sample and identify it by comparison with references, which works and requires removing material. Fluorescence under ultraviolet light distinguishes some. Spectroscopic methods applied without sampling have improved considerably. Identifying which colourant was used matters beyond curiosity, since it establishes trade connections, dates a work by when a material became available, and tells conservators how much light exposure the work can safely receive, which is why galleries light such paintings at levels that visitors sometimes find too dim.

Precipitating a soluble dye onto an inert powder produces solid coloured particles that behave as a pigment, which made plant and insect colourants available to painters as transparent intense reds and yellows. Organic colourants fade in light and the conversion does not prevent it, so many paintings have lost their reds entirely. Identifying them requires different methods from mineral pigments.