Towards the end of 2024 I performed some research on the chemical composition of a number of watercolour paints. In those experiments the mass fraction of the pigment was determined using UV-vis spectroscopy and thermogravimetric analysis in a number of paints with a quinacridone pigment.
During that research I correlated some properties of the paint, such as sheen, to this mass fraction and the composition of the binder, which is gum Arabic. This in turn led me to make some predictions about their behaviour in water, which is the ultimate application of the paint.
In order to test these predictions, I devised a simple experiment where a small amount of dissolved watercolour paint was introduced with a brush to a small channel of water. Then it was merely a matter of observing the behaviour of the paint in the water.
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| Six watercolour paints with PV 19 pigment in water after 0, 15 and 60 minutes. |
In the above image you can see the dissolution of six paints in water over time. From left to right they are arranged from low to high pigmentation. This also corresponds to a transition from large to small particles of gum Arabic.
As you can see, the smaller particles of gum Arabic, with higher pigmentation, generally dissolve faster and show more movement in the paint. It is however notable that the paint with the highest pigmentation and lowest particle size dissolves less fast than the other paints with high pigmentation. An explanation for this counter-intuitive observation is that the more tightly packed particles have less room for the water to enter and thus dissolve the paint.
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| Five Schminke Horadam watercolours in water after 0, 15 and 60 minutes. |
I then repeated this experiment with five paints of the same brand, that I assume to posses a similar composition of paint. Their behaviour was indeed similar, while some small differences were still present. These can be explained by the differences in chemical composition of the various pigments, which will give different properties during the mixing and milling of the paint.
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| Winsor & Newton Professional (l) and Cotman (r) watercolour in water after 0, 15 and 60 minutes. |
As a final experiment I tested if there was some validity to the observation that paints with a more matte appearance have larger gum Arabic particles and therefore dissolve more slowly. In the above image two paints from the same brand are tested. They are of different qualities, with the Cotman branded paint having less sheen than the Professional branded one. As it is clear to see, the professional branded paint, with higher sheen, did dissolve much more readily than the Cotman branded paint, which is an indication that the hypothesis might be correct.
With these positive results, I then repeated these tests with a more common version of paper chromatography. The liquid phase in this instance is of course water.
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| From left to right; VG, RT, SCH, SCH', DS, DS', DS'' and KP |
The first of these chromatography tests immediately produced some interesting results. The same paints are presented in the same order as in the first image. For the first three paints, we also see the same behaviour, where increased pigmentation and smaller particle size likewise give a further travel in the paper. It is then for the two highest pigmented paints that we see almost no travel at all. This was surprising so the experiment was repeated with the SCH and DS paints, to ensure that no kind of error was made while performing the experiment.
As this wasn't the case I can only assume that there is some kind of limit, whereafter the solubility of the paint in water is actually hindered by the paint particles being small enough to get 'stuck' in the fibres of the paper. So while smaller particles are generally are more easily brought into solution, there is probably some point where the paint particles are small enough to penetrate more deeply into the paper and then aren't removed as easily.
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| Winsor & Newton Cotman (l) and Professional (r) Permanent Rose |
This is somewhat confirmed by the above experiment. Winsor & Newton paint was the slowest to dissolve in the first test and consequently they showed no travel at all in the paper chromatography experiment. This result is in line with the reasoning that larger particles of gum Arabic dissolve less quickly in water.
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| Schminke Horadam Mars Black, Mars Brown, Ochre, Lamp Black and Quinacridone Rose |
A further test with the most soluble paint, Schminke Horadam, shows a near identical result as in the earlier tests. The explanations for this result are of course also similar to what we have already seen.
With the knowledge gained from these experiments in can therefore be said that paints with large particles of gum Arabic have generally lower pigmentation and dissolve less quickly in water. There is however a turning point when the paint is applied on paper, where smaller particles of gum Arabic attach more readily to the paper fibres and therefore adhere more strongly to the paper, hindering dissolution.
