What is a medium?
To make a paint with a pigment, you have to mix it in what is
called a medium. This medium is a liquid, creamy or
gelatinous substance. When drying, the medium turns more or less
solid to form a film.
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The mediums of the Old Masters
In Europ, from antiquity till about the 9th or 11th century, the
most utilized medium for painting was hot wax. This method is known
as encaustic painting.
A new technique appeared during the Middel Ages and became the
predominant one in the 12th century: the egg-yolk tempera.
The origins of this technique remain uncertain. But it was
utilized, among others, by the Italian and Flemish Primitives. The
tempera medium is a mixture of egg-yolk and water, thus a very lean
medium, i.e. containing no oil at all. This characteristic is
The freshness and the vivacity of color of the works of these
Old Masters is to be attributed to this lean manner of
painting 1. But it remains a rather limited
technique that prevented them from obtaining smooth gradations of
color, which are easy to realize when painting in oils.
- To the fact that they painted on a white ground too, in
opposition with the later painters as e.g. Rembrandt or Rubens, who
painted on a brown ground, because they had realized its
easier and faster to draw and paint on a more or less dark ground
than on a white canvas. Your drawing will be more lively, less
static, and youll see the colors and, before all, the values
(i.e. the contrasts between darks and lights) better when
Nevertheless, from the viewpoint of luminosity, its a bad
In oil paints, the medium is a mixture of linseed oil with other
substances like turpentine, white spirit, resins and
One of the secrets of the first painters who largely utilized
oil colors, the brothers Johann and Hubert Van Eyck at the
beginning of the 15th century 1, was to begin a
picture in the same way and with the same materials than the
primitives, i.e. with an egg-yolk emulsion, and to finish the
painting in oil, particularly with oil glazes.
- Its generally thought that Johann Van Eyck invented oil
painting. Its not true. Before him, many painters had the
habit of terminating their tempera paintings by oil glazes. But Van
Eyck made more: his whole upperpainting was in oils on a light
tempera underpainting, what gave his works an extraordinarily
luminous brightness combined with very smooth and very gentle color
Painting in pure oils, without tempera underpainting, was rather
uncommon before the 16th century. Its only during the 16th
and the 17th centuries that oil became the prevalent painting
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The new emulsions
At the beginning of the second half of the 20th century, there
have been appeared new emulsions, particularly the vinylics and the
acrylics. Contrary to the egg-yolk emulsion, which becomes harder
and more rigid with age, these new emulsions always remain as
supple as rubber.
In theory, this is not an advantage for the underlying
dead-painting on supple supports like canvas before an
upperpainting in oil, because the oil film turns hard and brittle
with the time, particularly if the oil medium contains hard resins
like copal what was very common until recently. When you
have a brittle film on flexible layers, theres every chance
that cracks and splits will appear. OK: thats the theory, but
in the practice, until now, when the oil film contains no hard
resin, nobody has noted this phenomenon.
On a rigid support, its quite another situation. The
flexible layer acts as a buffer whichs isolates the movements of
the wood from the more or less rigid oil film. Now this flexible
layer prevents to some extent splitting and cracking of the oil
Until recently, some people regarded as hazardous to paint with
oils on a too flexible underground, like canvas prepared with
acrylic emulsions. Nevertheless, there exists now since some
decades an oil medium which remains supple and resilient when
drying: oil-modified alkyd resin, a combination of an alkyd
resin and soya oil (less yellowing than linseed oil, what is
On the other hand (and above all if the support is rigid), the
use of a synthetic white primer covered by a synthetic watery
emulsion underpaint becomes an advantage for ordinary oil painting,
because the rule of fat over lean is perfectly
followed, what is the best way to prevent the formation of
- An evidence of this is what is called crackling
varnish. Its constituted by a lean (watery) varnish
applied on a first coat of fat (oily) varnish!
In addition, the combined emulsion and oil technique will
produce a painting that will keep up its colors better than any
other one painted entirely in oil, because the emulsion
underpainting, contrary to an oil underpainting, will have no
tendency to yellow or darken with age.
With the passing of time, we can judge better the qualities of
the modern resins. We know now that paintings in acrylics, made
fifty years ago, are still as fresh as the first day and have
absolutely not yellowed, what should certainly not have occurred if
they were painted in oil.
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For about 15 years, I have painted my dead-paintings with a
modern synthetic watery emulsion, which permits an easier
expression of the artist than the old egg-yolk dispersion of the
Nevertheless, there are some things impossible to render with
emulsions and it is thus usually necessary to carry on the work
For some years, I have painted the first layer in oils with an
alkyd medium above all because its less yellowing than
linseed oil (See Trials, Mediums).
Then I carry on the following layers with an oily medium which is
my secret 1 because its precisely the core of
the technique of the old Flemish Masters.
- I have written down a complete explanation of this technique
and I have kept this document in a bank safe for my heirs.
The final glazings in oils give the painting a subtle tone
quality impossible to render with other means. Beginning in waters
and finishing in oils is precisely the method employed by the Old
Flemish Masters, particularly the Brothers Van Eyck, to give their
works that extraordinary brightness that has stood the test of
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