Rembrandt’s Medium – A Study of Rembrandt’s Painting
Technique via Copying
OBI, Osamu
Summary
Perhaps no other artist’s painting technique has attracted as much attention as
Rembrandt’s who splendidly combined special qualities of oil painting
characterised by colour transparency seen in the Flemish school and colour
opacity and plasticity of pastose paint developed after the Venetian school. It is
no exaggeration to say that Rembrandt’s technique, brought to fruition in his
works, was a milestone in the history of painting.
The chemical analysis of paintings has immensely advanced in recent years. It
has brought to light facts that completely differ from the previously widely
accepted theories of what Rembrandt’s technique might have been. The
contemporary research through copying based on the newly available
information presents significant value in elucidating his technique.
This paper focuses on researching Rembrandt’s medium. The author first
discusses the making of paint mixtures using data gleaned from the literature.
Next, he discusses the outcome and issues encountered in the process of copying
the original at the Louvre. Finally, he presents a hypothesis drawn from theabove and discusses how it applies to the actual paintings. Through this step-bystep discussion, the author verifies the information he used and, from aperspective of a painter, explores the aspects of Rembrandt's technique thatcannot be learned from chemical analysis alone.
Introduction
Historical Significance of Rembrandt’s Technique
Oil painting techniques that have been developed in Flanders in the 15th century
took advantage of transparency and viscosity that tempera or fresco paints did
not have, and thus enabled a subtle tone control in transitions between light andshadow, and spatial depth achieved by multi-layered glazing. Later, when the useof canvas was introduced, pastose paints, which had been improved by the
Italian (i.e. Venetian) painters, acquired an added characteristic of plasticity on
top of transparency, an already known quality of oil paint. Until then, luminosity
had been achieved by exploiting the whites of the underpainting. Application of
thick and opaque paint allowed to express luminosity by pastose white paint,
and at the same time, facilitated transformation of shapes in the middle of thepainting process. Thus, the inherent rigidity of the outline-based painting
techniques was gradually replaced by a freer and more direct representation oflight and shadow through coloured surfaces and their volume. Rubens (PeterPaul Rubens, 1577-1640) and Rembrandt (Rembrandt Harmensz Van Rijn, 16061669) in North Europe, particularly in Flanders and the Netherlands, re-instatedthe original transparency of the new techniques of the Venetian school in theirworks. By applying transparent colours on top of a vivid bottom layer and using
opaque and thick paint for increased contrast, these painters of the Flemish
school were able to create impressive spatial effects and light with a material
feel. In a sense, they can be seen as the artists who brought the oil painting
technique to perfection.
This is why Rembrandt’s technique has always been a subject of great interest
for many a researcher and painter. In recent years, the advance of scientificanalyses that assisted restoration efforts at many national galleries meant thatsophisticated scientific methods could be used to analyse painting media where,
until recently, the naked eye had been the only tool available. Data gained fromthese new research methods fundamentally questioned the theories we thought
to be commonsensical.
By copying the original painting using the newly available information and themethods as close to the original as possible, I attempted to verify findings
reported in the research literature. In my research, I mainly focused on thepainting medium to discuss from a painter's perspective the analytical results
and the issues observed during the process of copying Rembrandt's original
artwork.
Copied Artwork

Image 1. Portrait de lʼartiste à la toque et à la chaîne dʼor (Self-Portrait Wearing a
Toque and a Gold Chain)Rembrandt created the subject of my copying experiment, Portrait de lʼartiste à latoque et à la chaîne dʼor (1633) (Self-Portrait Wearing a Toque and a Gold Chain,
Image 1), in the early period of his career. Its owner is the Louvre in Paris, France.
Throughout his lifetime, Rembrandt painted on both wood panels and canvas, and, in
his early life, more often than not, he preferred wood. The above portrait was painted
on a wood panel and took advantage of a transparent brown imprimatura1 that wasapplied as an underpainting. The effect of allowing the chalk ground to show throughin the slightly lighted areas of the face reflecting light and as the base colour of thehair was characteristic of Rembrandt’s works painted on wood. This method smartly
exploits paint’s transparency, which was a common technique of the early Flemish
school. On the other hand, lead white paint was thickly applied in the lightest areas tocreate a strong texture. An opaque and thick application of paint in the lightest areas
demonstrates the effect of impasto, the use of which had been pioneered by the
Venetian school. This technique enabled sculpting the lights with thick white paintand transformed the way by which light and shadow were separated. From reliance onoutlines, the painters progressed to a more audacious use of paint to create coloured
patches. Caravaggio (Michelangelo Merisi da Caravaggio, 1571-1610) furthered thisapproach even more, and his influence is clearly seen in Rembrandt’s work.
Rembrandt also used paint’s plasticity to depict lights. He promoted this technique todefine texture and even increase the material strength of a paint layer. The copying
subject I selected for my current research is representative of these characteristics insuch places as the gold chain where the artist used decorative thickness of paint as ameans of expression.
Study of Rembrandt – Paint Plasticity
Rembrandt’s Medium
In the beginning of the 20th century, a German author, Max Doerner (1870-1939)
wrote in his book entitled Malmaterial und seine Verwendung im Bilde that
Rembrandt had used “a medium containing Venetian balsam, thickened oil, andmastic resin.” This suggested that Rembrandt used a resin-rich medium 2 . Doernerpresented a theory that resin was the key to achieving viscosity and plasticity ofthickly applied paint layers and transparency of glazes, which the Rembrandt’s
artworks are famous of. For a long time, this was accepted as a convincing theory.
However, in the days of Doerner, the science of organic material analysis still was in
its infancy, and painting media was mainly analysed by the naked eye. In recent years,
the scientific analysis of painting media undertaken by national galleries in the courseof various restoration projects has yielded analytical data that were inaccessible in
Doerner’s days.
According to an analysis conducted by the National Gallery in London, Rembrandt’s
1

A technique of applying a neutral colour to the entire surface to increase the effect of whites in lightareas. A transparent neutral colour base is achieved by spreading a thin solution of paint over thesurface with a flat brush.
2
Malmaterial und seine Verwendung im Bilde, Max Doerner, p.153

medium was surprisingly simple: it was found to consist of a single drying oil, such as
linseed or walnut oil (the latter was used rarely and only with pale colours)3. The resinthat Rembrandt had supposedly used was not detected in any of Rembrandt’s
paintings although there were a few instances when other artists associated with
Rembrandt used small quantities of resin in their media. This begs the question: is it
possible to create a work displaying plasticity and transparency inherent in
Rembrandt’s paintings by using paints mixed with drying oil and nothing else? Paint
transparency can also be explained by aging: the refractive index of drying oils isknown to increase over time, and as a result, paint layers may become moretransparent. But what about plasticity in the thickly painted areas? To find out, I
made an experiment mixing paint and linseed oil.
The Experiment of Mixing Lead White and Oil
In Rembrandt’s days, colours were hand-mixed by apprentices in his workshop.
Tubed paints would appear only in the middle of the 19th century after the metal tube
had been invented. As the mass production of artist paints began, their most desired
quality was stability. In order to be sold on the market, tubed oil paints needed to be
storable meaning that oil should not deteriorate or separate from pigment. To achievethis, manufacturers use various additives: emulsifiers are added to prevent separationor leakage of oil; cobalt, manganese or lead driers to aid the drying process and levelout drying speed of different colours; extenders to reduce the costs. Naturally, thepresent-day artist paints that contain all the above additives have a very different
quality than paints used by Rembrandt. Therefore, in my research, I used lead white
pigment, which was crucial for achieving plasticity in Rembrandt’s work, and mixed
it with several media that presumably were available at Rembrandt’s time. By mixing
the paints with the media, I tried to gauge the true potential of Rembrandt’s medium.
First, I mixed raw linseed oil and lead white. I put the resulting mixture on a brushand tried to paint with it. There was no problem with its plasticity, and the brush
moved freely. However, the oil did not have much viscosity; therefore, the mixture
had a much thinner feel than the sticky, bulky and viscous substance unique toRembrandt. In addition, the biggest problem was that it took seven to ten days before
the paint mixture was feeling dry to the touch even though I used lead white known
for its fast drying. This meant that it would have taken weeks before the entire paintlayer was completely dry in places where the mixture was applied thickly. It is
impossible to know how many days Rembrandt’s paint mixture took to dry, but
judging by the number of works he left behind and his workshop’s high production
rates, the time the paint took to dry in my experiment was definitely too long. Also,
slow drying meant that painted surface would remain sticky for a long time andcollect a lot of dust. However, excessive dust was never found on Rembrandt’s
paintings. Therefore, it is logical to assume that some measures were taken toaccelerate the drying process. There are many possible methods to accelerate thedrying process of the oils, and analytical data cannot reveal what exact method(s)Rembrandt used. With these limitations in mind, I compared two methods that arethought to have been available in Rembrandt’s days: 1) use of black oil (Image 2), and
2) use of sun-thickened linseed oil (Image 3). I made these oils from linseed oil,mixed each of them with paint, applied the mixture onto a surface and comparedresults (Image 4).
3

Art in the Making: Rembrandt – New Edition, National Gallery, London

Image 2. Hand-made black oil
Image 3. From right to left: raw linseed oil; homemade sun-thickened linseed oil;commercially available sun-thickened linseed oil

Image 4. Lead white painting experiment. From left to right: linseed oil; black oil;sun-thickened linseed oil; sun-thickened linseed oil with egg yolk.
Black oil is oil with an addition of lead to accelerate drying. I added lead white
pigment to linseed oil in a ratio of 1:10 and heated the mixture at a temperature notexceeding 150 degrees. The oil colour gradually changed to that of coffee. I adjusted
heating time using oil colour as a guide. The darker the oil colour is, the faster itdries, but if the oil dries too quickly, it will make the wet-in-wet4 work more difficult,and if the oil is too dark, it will interfere with the whiteness of lead white. After sometrial and error, I decided to use black oil that was not completely black but rather
caramel in colour (Image 2).
The sun-thickened linseed oil is processed oil that contains no additives and was not
exposed to heat. Exposing the oil to air, water and sunlight makes it dry faster. First
you pour several centimetres of linseed oil on a quantity of water in a basin (the oilwould float on the surface of water). This exposes the oil to both oxygen and water.
The oil and water need to be mixed daily; the mixture must stand for one and a halfmonths in summer, or more than three months in winter. Exposure to sunlight makes
the oil turn pale yellow and more transparent. (However, the commercially available
sun-thickened linseed oil is heat-processed at the final stage to ensure all residual
water completely evaporates, and is therefore caramel-coloured.) Also, due to
4

A technique of flexible mixing or blending the oil paints on a wet surface. It can be described as anoil painting technique that most efficiently utilises characteristics of drying oils (those that dry slowly
by the process called oxidative polymerisation).

oxidative polymerisation from exposure to the air, the oil gains a honey-likeconsistency (Image 3).
I mixed the lead white pigment with each of the two oils and applied the mixture witha brush.
The points to be observed during this experiment were:
1) drying time of the paint mixture,2) plasticity, and3) brushability (Table 3).
The mixture containing black oil produced a surface that was dry to the touch after
one to one and a half days. This drying time was the fastest among all the media I
experimented with. The purpose of heating the oil below a temperature of 150 degrees
is to dissolve lead, which serves as a siccative. At this temperature, the oil does notpolymerise and retains viscosity of raw linseed oil. The mixture of paint and black oil
is slightly viscous, but it has almost the same plasticity as the mixture containing raw
linseed oil. This paint mixture can be easily spread with a brush. However, due to low
viscosity, it does not produce a sufficiently viscous substance, and one can clearly see
the brush marks. It is closer to the present-day artist paints than the classical paint of
the old masters.
The paint mixture containing sun-thickened linseed oil produced a surface that felt dry
to the touch after one and a half to two days. This was a remarkably higher drying
speed than that of the one the mixture containing raw linseed oil could produce, which
made it suitable for professional painting. However, this oil is very viscous, andmixing it with paint required quite a lot of physical effort. When I was putting thispaint mixture on a brush, I could sense its stickiness and heaviness. It was extremely“long”. Once I applied it onto a surface, my brushstroke instantly lost its shape. Overtime, impasto sunk in and moved under its own weight. Two weeks later, after thesurface fully dried, I could observe fine wrinkles appearing in thickly painted areas(Image 5). In order to increase plasticity, I tried to apply a thicker mixture, but it was
nearly impossible to move it around with the brush. I found it totally unworkable.
This mixture was not suitable to reproduce Rembrandt’s typical thick but spreadable
brushstrokes.

Image 5. The mixture of lead white and sun-thickened linseed oil (shown on Image 4)
two weeks after application.

Medium – Another Possibility
My experiment with two media followed a learned assumption derived from the
National Gallery’s analytical data yet failed to achieve a satisfactory result.
Although black oil was impeccable in terms of drying speed, plasticity andworkability of paint mixture, its viscosity was too low and did not match the levelof viscosity unique for Rembrandt. From experimenting with the medium, I
learned that paint viscosity is relative to the level of polymerisation of drying oil,but I also learned that plasticity rapidly declines in a manner almost inversely
proportional to the degree of polymerisation. Was it really possible to combine
two conflicting qualities, viscosity and plasticity, without adding any resin or
beeswax?
The National Gallery also reported another finding: some of the samples
contained white chalk mixed with lead white. In Rembrandt’s time, there were
two types of lead white, and one of them had white chalk added as a bulking
agent. This lead white was mainly used for underpainting and never in the top
layers. Use of white chalk may give paint some degree of plasticity and alsotransparency. Could it be that Rembrandt added white chalk to increase paint
plasticity? But on the other hand, the National Gallery’s analysis of the Portrait ofPhilips Lucasz revealed that the lace collar was painted with a pure lead white
impasto, which demonstrates that paint plasticity could be achieved without
addition of white chalk.
Ernst Van De Wetering5 reported in his book entitled Rembrandt: The painter at
Work published in 1997 that egg was detected in Rembrandt’s medium when the
Rijksmuseum in Amsterdam conducted restoration and scientific analysis of
Jewish Bride and Syndics of the Drapers' Guild. The book mentions that no
additives other than drying oils were found in the medium samples that wereanalysed by the National Gallery and described in the book Art in the Making:Rembrandt. It further points out that only a trace amount of egg was found in theactual Rembrandt’s medium whereas the analytical method chosen by the
National Gallery was designed to detect resin and wax, which, according totraditional beliefs, should have been part of Rembrandt’s medium. Other thanegg, vegetable gum was detected in the medium used with red lake, and pinholes
were observed under the microscope hinting at evaporation of water throughthe paint layer. Based on the above, the master’s medium could have been an
emulsion containing some kind of protein. The book states that it was possible
egg was needed as an emulsifier, once water was added to prevent uncontrolled
spreading of poisonous lead white. The book also argues that plasticity andsmooth fluidity, or in other words, improved brushability probably were side
effects of medium emulsification through addition of egg. If this theory is true,
applying this method to sun-thickened linseed oil (which demonstrated highviscosity but low plasticity) could prove to be an effective solution. Therefore, I
added a small amount of egg yolk to the above described paint mixture of leadwhite and sun-thickened linseed oil. The amount of egg yolk was just a tiny scoop
5

Head of the Rembrandt Research Project (RRP); Professor Emeritus of Art History, University of
Amsterdam; and author of Rembrandt: the Painter at Work, 1997.

at the tip of a painting knife. After mixing it in, I brushed the resulting mixture
onto a surface. Remarkably, the brush moved much more smoothly while both
plasticity and an adequate level of viscosity were retained. It was finally possible
to re-create Rembrandt’s voluminous texture. Adding a trace amount of egg
presented a possibility to resolve the conflict between viscosity and plasticity.
Taking this method a step further, I created a new medium by mixing lead whitewith sun-thickened linseed oil and adding an adequate amount of egg yolk for
improved plasticity.
Table 1. Media Experiment Results
Drying time (until
dry to touch)
Linseed oil

Over 10 days

Black oil

1 to 1 ½ days

Sun-thickened
linseed oil
Sun-thickened
linseed oil + egg
yolk

1 ½ to 2 days
2 daysPlasticity
Good (lowviscosity)
Good (lowviscosity)
Good (highviscosity)
Good (medium
viscosity)

Brushability
Good (light)
Good (light)
Brush is too heavy
to move
Good (viscous)

Verification Test – Copying Rembrandt’s Work
Colours Used
Research made it possible to establish pigments Rembrandt used to create his
masterpieces. The choice of pigments is far greater now than in his time. He wasmainly using inexpensive natural earths plus several minerals and some manmade pigments such as transparent lakes, lead white and vermilion. His palette
included a number of pigments that are no longer manufactured or used, such as
lead-tin yellow and smalt.
For my experiment however, I wanted to emulate Rembrandt’s method as close
as possible; therefore, I decided to grind the colours myself. Where the required
amount was very small (in case of vermilion) and where the colours needed tobe diluted with a large quantity of medium (in case of lake pigments), I assumed
that the impact of medium contained in tubed colours could be ignored, andopted for commercially available artist paints. This way, I used both hand-madeand mass-manufactured colours. Colours I selected for this experiment arepresented in the list below:
White:
Black:Red:
Yellow:Blue:

Lead White
Ivory Black
Vermilion, Alizarin Crimson
Yellow Ochre, Italian Pink, Lead-Tin Yellow
Azurite, Smalt

Brown:Burnt Sienna, Burnt Umber, Van Dyke Brown, English Red

Of the above, I eventually dropped lead-tin yellow and azurite as I realized, in theprocess of copying, that these were unnecessary for my project.
Substrate and Preparation
Rembrandt, depending on his objective, mainly used two different types ofsubstrates: wood panels and canvas. He used oak boards primed with animalglue (size). On top of the glue layer, he would apply a thin layer of chalk ground(gesso) using glue as a binder. On top of this layer, he would apply a very thinunderpainting, a mixture of lead white, chalk and small amount of umber tocreate a light-coloured preparation (imprimatura), using drying oils as a binder.
Two main purposes of the imprimatura were to create a neutrally coloured
background so that whites applied in light areas would become more prominent,
and to control the absorption rate of the chalk ground to prevent oil from being
sucked out of the subsequent paint layer.
There are two basic types of canvas grounds, and with both types, drying oils
were used as a binder. The first type is called “double ground”. First, a layer of a
relatively cheap earth pigment is brushed on to fill the canvas texture; thesecond layer consists of a grey, light brown, or dull grayish yellow colour, withlead white as its main component. This seems to be the most common type ofcanvas ground. Rubens and Van Dyke also used it.
The other type appeared in the 1640s – a single-layer ground with a roughsurface, with silica as its main component, and containing brown ochre and asmall amount of lead white. Why silica was used is unknown but it is assumed
that its low price and easy availability were behind it.
Looking at the subject for copying, Self-Portrait Wearing a Toque and a Gold
Chain (1633), from up close, we can see a brown imprimatura showing throughglazes in the parts of the face which reflect light, and being the base colour of thehair. However, we don't see the imprimatura directly; it only shows throughglazes. Therefore, it is impossible to judge its exact colour. A Woman Bathing in a
Stream owned by the National Gallery in London (see Image 6) was painted on awood panel. It reveals the imprimatura colour directly in the gaps between
overlaying brushstrokes (Image 7). I decided to use the imprimatura colour seenin this painting in my copy (Image 8).

Image 6 and 7. A Woman Bathing in a Stream (National Gallery, London). You
can see the imprimatura colour in the gaps between overlaying brushstrokes.

Image 8. Coloured base for my copy (brown imprimatura on wood)

Image 9.
Image 10.

Image 11.
Image 12.

Image 13. Completed underpainting
Image 14.

Image 15. Completed copy
Process of Copying
I made my copy directly from the original painting at the Louvre, which owns theoriginal.
1) Wash Layer
After priming the panel with a water-based chalk ground, I applied a brown
imprimatura to create a coloured base.
In the beginning, I painted roughly with Van Dyke brown to capture larger
shapes and major contrast between lights and darks (Image 9).
2) Dead Layer
Instead of a true black-and-white monochrome called grisaille, I used a
monochrome layer made with earth pigments. This is a stage where a “deadlayer” is created that will serve as a foundation for glazing.6 If warm colours ofunderpainting are allowed to show through, it will greatly assist with thepainting’s temperature or variation between warm and cold colours (Image 10).

6

To be precise, the term ‘grisaille’ means a black-and-white or monochrome painting. However, itappears that a broader definition of this term is sometimes used. For example, the passage referring toRembrandt’s grisaille in Art in the Making: Rembrandt reads, “[He] used various tones of brown
directly in the first layer of grisaille.” In this case, it seems that the term’s definition is closer to that ofcamaïeu.

Lead white was applied opaquely and thickly in the lights on the face (Image 11)
while the underpainting shines through a thin, semi-transparent colour layer indark areas (Image 12).
The lights on the hair were painted without use of white. Dark colours containing
no white pigment were brushed on top of the lighter underpainting – nothingelse. To depict black garment, again, no opaque grey was used. Instead, I useddarker and paler versions of black without addition of white pigment. However,a small amount of white was mixed in light areas to create a sense of volume.
Similar to lights on the face, the gold chain was painted with a mixture of a small
amount of egg yolk and lead white to give it more body. This enabled me toreproduce thick application of paint that is regarded Rembrandt’s signature style(Image 13).
3) Glaze
After the underpainting has dried, glazing was done to give intensity to colours
and a feeling of depth to the surface. According to the chemical analysis, acomplex composition including vermilion, several earth pigments, lakes, whiteand black was used for wet-in-wet glazing of skin parts (Image 14). Typical for
Rembrandt and Rubens’ works is application of cold colours in halftones
between lights and darks. Use of azurite for glazing in the Portrait of Philips
Lucasz, owned by the National Gallery, London, has been reported. I was thinking
of using azurite in my copy, but an effect similar to the original was alreadyachieved by using grey (mixture of white and black), and I decided to exclude
azurite this time (Image 15).
Discussion on and Application of Glazing as a Means to Create Texture
My Reflections after Copying
By making a copy while standing in front of the original painting, I was able tocheck out some hypotheses but also stumbled upon some new questions. I
confirmed that the use of sun-thickened linseed oil and a small amount of egg
yolk rightfully reproduced the substantial texture of the light areas, a major
characteristic of Rembrandt’s work, by taking advantage of paint plasticity(Image 16). On the other hand, I found it surprisingly difficult to glaze over darkareas. A basic principle of the post-Venetian school of oil painting is that paint isapplied opaquely and thickly in lights and transparently – as a thin glaze – indarks. However, a careful observation of Rembrandt and Rubens’ works reveals
that some dark areas have a surprisingly thick impastoed texture. For example,
Slaughtered Ox owned by the Louvre shows a thick application of transparent
colour in dark areas of the hanging ox’s flesh (Image 17). The Angel Raphael
Leaving the Family of Tobit shows relief-like bulges of paint in dark areas onangel’s both wings (Image 18). The method used in my experiment was unlikely
to reproduce this effect. The same can be said of the black areas of the artist’s
gown in the Self-Portrait Wearing a Toque and a Gold Chain, subject of thisresearch paper. Paint remains sufficiently transparent to slightly reveal the
imprimatura, but at the same time, it was applied thickly enough to retainbrushstrokes.

As for the dark parts of the human figure, semi-transparent greyish colours wereused to let imprimatura and whiteness of the chalk ground show through. At thesame time, semi-transparent grey paint has retained enough body to form abulky texture. However, opaque grey, if used as a semi-transparent colour rather
than a thin glaze, would entirely obscure the colour of the underpainting. In
order to create a substantial texture of note, one must use an essentiallytransparent colour.
It is a known fact that oil paints become more transparent with age. If we assume
that transparency seen in Rembrandt’s work has resulted from aging, we have toconclude that he applied paints thickly and opaquely. However, if you apply
black thickly and opaquely, the way it was used in the gown, with no regard tobrowns of the underpainting, the resultant black colour would look extremely
flat, and the sense of spatial depth will disappear. The same can be said of thedark areas of the human figure. Therefore, we must presume that Rembrandt
applied paint thickly yet was sure he would achieve a certain level oftransparency.

Image 16. A section of the copy

Image 17. Slaughtered Ox

Image 18. A section of The Angel Raphael Leaving the Family of Tobit
Description of Medium Found in Oil Sketches
Rembrandt and Rubens left behind many oil sketches that were completed in avery short time. When you paint an oil sketch or anything that needs to befinished quickly, you inevitably make ease of painting your first priority.
Transparent thick application can also be found in that type of work. Portrait of aYoung Jew (1948) is an example of such an oil sketch (Image 19). This painting
was probably completed within one day except for glazes that were quickly
applied after the painting had fully dried. If you study it carefully, you will see
ridges of opaque paint in the light parts of the face whereas the clothing wasquickly finished with a small amount of semi-transparent paint applied on top ofa brown imprimatura. If you look at those parts, you will find them so oil-richthat it appears as if the pigment was floating in the oil. If that much oil were used,brushstrokes would have become completely obliterated. Yet brushstrokes arevery prominent here, and the paint layer has a substantial body in the dark areasof the hair, for example. If you complete a painting within a few hours using alarge amount of oil, painted areas lying next to each other would amalgamate
and outlines will become blurred. None of this can be observed in the original
artwork – it displays crisp brushstrokes. Of course, it is also a matter of painter’sskills, but it is fairly obvious that the painter managed to create a paint ofparticular quality, which made this effect possible.
From these observations, we can deduce the following medium characteristics:
1) The medium itself offered some level of plasticity,2) The medium allowed the painter to apply different colours in adjacent areas ina short period of time without a risk of unintended blending.

Image 19. Rembrandt’s Portrait of a Young Jew
Transparent Thick Paint Layer
I can think of two possible ways how a substantial texture that is thick andtransparent at the same time could be achieved. One is to use a less staining
pigment as an extender. Another is adding more medium to paint.
I tried to verify these assumptions by using ivory black, based on the datacollected from the available literature.
The added extenders were:
1) white chalk, and2) smalt.
The media the volume of which was increased were:
3) Sun-thickened linseed oil, and
4) Emulsion of sun-thickened linseed oil and egg yolk.
And the results were as below (Image 20):

Image 20. Comparison study using ivory black. From left to right: mixed withwhite chalk, smalt, sun-thickened linseed oil, and emulsion of sun-thickened
linseed oil and egg yolk.

Use of white chalk as an extender was mentioned in the final chapter of the
National Gallery’s research report. In my experiment, white chalk did increasetransparency and plasticity to some extent, but to achieve a satisfactory level oftransparency, I needed nearly a 70% mix of the white chalk. When applied
thickly, the paint mixture started drooping. In my copy of Rembrandt’s work, I
used this method to paint black garment. To some extent, I could achieve thedesired effect. However, if white chalk had been used for this purpose, thepainter would have used large amounts of it in many other areas as well. The
analytical results indicated that a limited amount of chalk was used as anextender for lead white in the underpainting and in some parts of glaze layers.
The analytical results do not seem to agree with this theory.
Smalt can be also used as an extender. Smalt is ground cobalt glass; it is atransparent blue pigment of low tinting strength. Other than being used as a blue
pigment, it was frequently added as a siccative to the slow drying paints. The
National Gallery’s analysis showed that it was also used as a means to increase
thickness of dark glazes. However, use of smalt seems to be mostly associatedwith Rembrandt’s later works. It is questionable if smalt was frequently used inpaintings produced in his early career, which I selected for my study. The
attempt to use smalt had a similar outcome as the use of white chalk. I tried thismedium in some areas of the toque but was not satisfied with the results.
As for the option of increasing the medium-to-pigment ratio, if you look at theold masters' works with this idea in mind, you will notice that classical paintings(not only Rembrandt’s) are more oil-rich than you previously thought. However,while adding more drying oil increases transparency, it decreases plasticity.
Excessive amounts of oil always make paint runny. My experiment equally
proved that adding more medium reduces plasticity.
The last remaining possibility was the use of egg. For my copy, I added thenecessary amount of egg yolk to lead white to achieve desired plasticity, but wasit possible to add egg yolk to a medium as a thinner?
I first mixed a small amount of egg yolk with the same amount of vinegar. Then I
slowly added sun-thickened linseed oil while blending and emulsifying the
mixture. When the quantity of added oil reached about five parts of egg yolk, theemulsion suddenly started forming mayonnaise-like peaks. Using this concoction
as a medium, I mixed in the pigments and made a paint mixture. Taking a small
amount with the tip of a brush and mixing it with paint, I found that I could freely
control transparency. During my experiment, I also found that the paint mixture
would retain brushstrokes if applied thickly but nevertheless provide a desireddegree of transparency and plasticity.
From these experiments and building on a theory that Rembrandt used small
amounts of egg yolk in the areas where plasticity was required, I deduced thefollowing hypothesis: Rembrandt used an emulsion containing egg yolk as a
medium and paint thinner. The advantage of this approach is that it “kills two
birds with one stone”: we can obtain a medium that facilitates both goodbrushability and transparency. By using this medium as a thinner, the paintercan also effortlessly achieve desired transparency and plasticity. There is noneed to take an unnatural step of adding white chalk or smalt just to achieve thethickness and plasticity of transparent paints.
The problem is that using a thinner containing a large quantity of drying oil may,from the very beginning, create a very oil-rich surface. This would be contrary to
the oil painting principle of “fat over lean.” However, Rembrandt’s approach
naturally followed the above principle because he painted lower layers opaqueand upper layers transparent. Where a lower layer was made using a transparent
paint mixture, the same level of transparency in most cases was also maintainedin the subsequent layers. In other words, an opaque layer containing more
pigment was never sandwiched between two transparent layers. Therefore, the“fat over lean” principle was never breached.
Next experiment was to create an oil sketch to check how effective this emulsion
was as a medium.
Painting an Oil Sketch with the Emulsion as a Medium
First I prepared a water-based white chalk ground and applied a brown
imprimatura.
I completed the underpainting in about three hours. I made a rough sketch withstrongly diluted Van Dyke brown, and without waiting for the paint to dry,
proceeded to applying a lead white paint mixture in lighted areas thickly. In darkareas, I spread a thin layer of transparent paint to let the underpainting show
through. For black hair, I used paint thinned with the medium to a transparent
state without much thinking about how I was going to move it around with thebrush (Image 21). A few days later when the underpainting was dry, I spent
about two hours glazing it (Image 22). Brushstrokes that appeared in light areas(Image 23) and substantial texture created by the paint plasticity in dark areas(Image 24) were effortlessly achieved by this working method.

Image 21. UnderpaintingImage 22. Glazing
Image 23. Thick paint in light areas

Image 24. Substantial texture in dark areas
Image 25.
Image 26. Enlarged view of the hair in the painting depicted in Image 25
One thing I noticed while running this experiment was that I was able to apply
different layers of paint without accidentally intermixing colours despite
working wet-in-wet. For example, when you paint a highlight in the hair, it ishard to draw thin sharp lines on a wet surface because colours tend to blend.
While experimenting with this approach, I found that the highlight colour could
spread over the wet lower layer without blending with it, and to some extent
remained transparent (Image 26). This matches the qualities of a medium I
specified in the chapter entitled “Description of Medium Found in Oil Sketches”.
Based on the results of my experiment, I concluded that my hypothesis that
Rembrandt used an emulsion containing egg yolk as his medium at least
deserves a discussion. Further study of existing literature and analytical data will
be required to verify my hypothesis.
Conclusion
Rembrandt’s Medium and Flemish School of Painting
Once again, I would like to revisit the position Rembrandt holds in the history ofpainting techniques.
As I stated in the beginning of this paper, a prominent mark of Rembrandt’s
technique is that it integrated transparent painting found in the early Flemish
school with exploiting paint plasticity that can be seen in the Venetian school.
When I just started out, I was looking at this as a question of paint layer structure.
I was asking myself whether expression of light should rely on the luminosity of
the underpainting or the whiteness of the paint. However, when I actually stoodwith a paintbrush in my hand and copied the master’s original work, anotherquestion came to the surface: “Of what was his medium made?”
A recent medium analysis established that Jan van Eyck (1395-1441) (Image 27),
Rogier van del Weyden (1399/1400-1464) (Images 28 and 29) and other early
Flemish painters of the 15th century also used egg and glue in their creations.
This suggests that their medium may also have been an emulsion containing
protein7. In order to discuss this possibility, I actually mixed crimson lake, atypical transparent red paint, and found that mixing egg and glue changed therefractive index of the entire mixture and lessened the medium’s transparency
proportionally to the concentration of egg and glue. However, if a very small
amount of them was added, it stopped the paint mixture from drooping. With
this effect, an artist can dramatically increase the thickness of the paint layer andstill easily control transitions. Armed with this hypothesis, let us have anotherlook at the works of early Flemish painters, Rembrandt and Rubens. We can seethat they had a common quality – a unique glazing technique, which enabled
them to create dark areas, transparent and richly textured at the same time.
When we look at the works of Rembrandt and Rubens, we can say that they
demonstrated more of the Venetian school techniques represented by Tiziano
Vecellio rather than transparent glazing techniques observed in the early
Flemish school. In other words, the Venetians improved the thick paint mixture
to create an opaque and substantially thick application of paint in light areas.
They expressed contrast between lights and darks by applying transparent paintover an opaque layer. When we look at this in terms of the medium and its
composition, however, the rendering of dark areas clearly reveals theuniqueness of the Flemish school’s approach. In this sense, I propose that we
reposition Rembrandt as one of the last painters of the Flemish school foundedby van Eyck and others.
Anthony van Dyke (1599-1641), a Rembrandt’s contemporary, started his
painting career as Rubens’ apprentice. However, when you look at Van Dyke’s
work, you will notice that a thick layer of paint in dark areas, a telltale sign of theFlemish tradition, is no longer there despite the fact that he was, without any
doubt, strongly influenced by his teacher. The question of how Van Dyke selected
his ingredients remains a mystery worthy of further study. Whatever the truth
may be, this interesting example suggests that it was the time of transition fromthe Flemish painting techniques to those of subsequent schools of painting.

7 Rogier van der Weyden – St. Luke Drawing the Virgin, the Museum of Fine Arts,

BostonRembrandt: The painter at Work, Ernst van de WeteringImage 27. Thick application of paint in dark areas seen in a painting of van Eyck.

Image 28 and 29. Thick application of paint in dark areas seen in a painting of
Weyden.

References
1. Art in the Making: Rembrandt - New Edition, National Gallery, London, 2006
2. Ernst Van de Wetering: Rembrandt: The Painter at Work, 1997
3. A Study of Techniques by Copying Tiziano Vecellio and Rembrandt, Kuniharu
Saito, Research Journal No. 27, Musashino Art University, 1996
4. Rembrandt and the Face of Jesus, Lloyd Dewitt, 2011
5. Rogier van der Weyden - St. Luke drawing the Virgin, The Museum of Fine Arts,
Boston, 1997
6. Malmaterial und seine Verwendung im Bilde, Max Doerner, Japanese translation
by Ichiro Sato, 1980