Karin Groen article about Rembrandt’s paint and binder, particle sizes

Karin Groen article: Investigation of the Use of the Binding Medium by Rembrandt :

Big conclusion: For thick impastos, especially in late Rembrandt, a pure white layer was commonly used, composed of a fine grind of lead white in unpolymerized linseed oil, combined almost universally (!) with a protein emulsifier, probably egg. Polymerized oils were used more with slower drying pigments, not with lead white.  Some glazes may have been emulsions employing cherry gum, with signs of encapsulated water droplets.

<0.1 micron (.0001 mm) is a fine grind; this is found in lead white impasto samples (along with a lot of chalk in some cases: 15% up to 45% by volume).  The fine particle size seems to give elasticity to the impasto, though in their tests with unpolymerized linseed the resulting impasto was short, not long. Polymerization of the oil was needed in their tests to make a longer rheology, so there is still a mystery here.  Egg is not likely to make long rheology in my experience, though it will hold brushmarks. This implies an unknown quality of the lead white, or as Essman suggests some combination of oils that is not currently detectable.  Or they may be overlooking the character of the added chalk, which could be contributing a lot to the rheology.

Some details and quotes:

In the Jewish bride, there is a pure white impasto layer, glazed with yellow and brown. In the Draper’s carpet, there is a pure white layer, then a thin brown (drawing?) layer, then a yellowish white impasto layer.  In one case, ” It appears that lead white and chalk have been set beside each other in a single stroke of the brush,” perhaps chalk added to the brush and dragged through the paint. She suggests that late Rembrandt impasto was more often pure lead white, with only a little chalk added on the spot.  There is an odd thread-like microscopic structure seen in lead white sample from the Jewish Bride.  Several samples of white impasto indicate ” in all cases an oil that has not been pre-treated (not subjected to pre-polymerisation, accelerated oxidation and thickening.”  Modern tests to  simulate paint indicate that paint

“made with un-treated (uncooked), cold pressed oil, the paint is a rather’short’, buttery paste whereas with a heat treated oil a paint results with stringy threads. The stringy character was not reduced by the addition of more pigment. So although the oil is thicker as a result of the pre-polymerization (oxidation) due to the heat treatment, the paint made with such an oil is actually found to be less solid. Tests conducted by Hayo de Boer demonstrate that the higher the temperature used, and the longer the heat treatment, the more stringy the paint becomes. Pre-polymerizing o1l by heat treatment excluding air is a later invention. In the seventeenth century in The Netherlands oil was sometimes thickened while having it bleached by placing it in the sun. Painters saw a disadvantage in the use of. a sun-bleached oil because such oil suffered from aiso being thicker. A higher value was attached to a thin, good drying oil than to a colourless oil, since after painting the oil darkened in any case, whether it had been sun-bleached or not. Lead oil would thus have been used for colours other than lead white. while lead white would have been made with untreated oil and without siccative.”

All white pastoso paint was found to include protein, most likely egg.:

Given that egg appears in the paints under investigation together with oil, it would seem obvious that we are dealing with an emulsion paint whose major constituent is oil. This would then be an oil emulsion that could be thinned with a volatile solvent. In the sources, both egg yolk and egg white -as well as whole egg – are mentioned. From the experimental research of Hayo de Boer it appears that only egg yolk or whole egg are capable of being emulsified with oil. This is the natural conclusion, since egg yolk is the essential component in emulsification.

Rheologists describe the thick impasto

The rheologists’ response to micrographs of Rembrandt’s paintings was that these clearly showed that very little flow had occurred in the thick paint, which sometimes remains pulled out into threads. After application the paint has remained, as it were, standing. It can be said that the paints used still possessed a considerable yield value. (Materials under tension immediately start to flow once the tension exerted on them exceeds their yield value…

…. If a distinction is made between the fine powder that occurs in the lead white samples (diameter < 0.1 pm) and the coarse powder (diameter > 0.1 pm, elongated particles up to 20 pm) then it turns out that a dispersion of fine powder in linseed oil gives it a strong elasticity. The coarse particles have no effect on the flow behaviour, they merely heighten the viscosity. Small particles can give to linseed oil a considerable yield value and may lead to the formation of thread-like structures. On the basis of the absence of {low and the presence of very small particles, the paint would seem also to have a considerable yield value. A large variation in particle sizes would favour flow, certainly under the high speed (shear rate) with which a painter works his brush.

SO… they seem to find that the white impasto was a relatively fine grind of lead white, with added egg, not likely to be in pre-treated oil. A mix of particle sizes is not favorable for Rembrandt’s thick impasto.

They also found possibility of cherry gum, a water soluble emulsifier like gum arabic, in some glazes (possibly to stabilize smalt used as an extender or transparentizer), and an indication that water may have been emulsified in the oil. Contemporary sources mention cherry gum, gum arabic, and honey sometimes emulsified into paint.  In the red glaze of the Jewish Bride, cherry gum was found, and:

It seems we may be dealing with a two-phase syslem, for example water emulsified in oil. The paint seems to have congealed before enclosed air bubbles, or water droplets or solvent, could escape….The addition of gum could well be the explanation for the cavities seen in the paint layer. The light microscopic image of the cavities certainly looks like water droplets emulsified in oil. A water solution of the polysaccharide obtained as a gum from cherry trees easily emulsifies oils and resins, sticks very well to oii paint layers and dries ro a clear translucent layer.

Rembrandt’s quartz/chalk/pigment grounds include quartz in the range of 5 – 6o microns. It appears that coarser sand, up to 2 mm, was crushed. This may have been for economy as well as to fill loose weave canvas.

Regarding particle size, Tad Spurgeon says modern paints are generally ground between 0.5 to 2 microns. He suggests adding ground calcite in the range of 20 microns for smaller paintings, and up to 50-100 microns for larger sizes, if you wish to experiment with the effect of particle size. He also suggests adding cristobalite in the 7 micron range to modern synthetic pigments, which have little body.