Why the Pigment Oil to Ratio Matters to Artists
Every time you squeeze paint from a tube, you make a decision about the pigment to oil ratio. So does the manufacturer who filled the tube. So did the artisan who ground paint in seventeenth-century Antwerp. The pigment to oil ratio is the most basic variable in any oil paint, and yet most artists never think about it. We rarely ask how much pigment, by mass, is sitting inside.
A 2021 study by Eliise Tammekivi, Signe Vahur, Martin Vilbaste, and Ivo Leito at the University of Tartu makes the case that this variable matters — chemically, structurally, and over decades. The team used gas chromatography–mass spectrometry on artificially aged paints made from a single batch of linseed oil and seven different pigments. They show that pigment concentration shapes how oil binders age in ways the field has largely overlooked (Tammekivi et al., 2021, pp. 1–2). The findings also dismantle a quiet assumption resting under conservation science for sixty years: that fatty acid ratios in dried oil function as a reliable fingerprint for the type of oil used.
That assumption underwrites most of what we say when we talk about identifying linseed, walnut, and poppy oil in old paintings. Moreover, it underwrites a fair amount of what artists are told about which oils their tube paints contain. The Tammekivi data show the fingerprint dissolves the moment you add pigment. Furthermore, they show that the pigment to oil ratio matters as much as which pigment it is.
The Core Finding About Pigment to Oil Ratio in One Sentence
The same clarified linseed oil produced palmitic-to-stearic ratios ranging from 0.6 to 1.6 — a fourfold span — depending only on which pigment it was mixed with and on the pigment to oil ratio in the paint (Tammekivi et al., 2021, p. 6, Table 1).
That single sentence rearranges how artists and conservators should think about oil paint chemistry. The P/S ratio has been treated since Mills’s 1966 paper as approximately stable during drying and therefore useful for distinguishing drying oils (Tammekivi et al., 2021, p. 1). On clean, fresh oil, the ratio still works. On aged paint — any film more than a few months old — the ratio reflects the pigment as much as the oil.
For working artists, this matters in two ways. First, it complicates the technical claims you may hear about which oil is in a given tube. Second, and more importantly, it points to a deeper truth. The oil in a tube of zinc white at year thirty is chemically not the same oil as in a tube of Prussian blue at year thirty — even if both were ground in identical linseed oil on day one. The pigment is not a neutral carrier. It is an active chemical participant.
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