Description
The formula of glycerin is CH2OH-CHOH-CH2OH
(two primary
alcohol functions CH2OH and one secondary CHOH, i.e. C3H8O3).
It has then three alcohol functions (it is a
trialcohol) and by combining itself with
acids (a triacid or three
different acids) it forms
esters that are nothing
else but
fats (triglycerids).
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Forming of a triglycerid in Saponification and
Esterification.
The fats that are in us living
beings, the fat we eat (see
lipids,
glyceride),
the oils among which the ones used in painting, result from these
combinations (see
esterification).
Glycerin gives esters which
have generally the characteristic of being supple.
It is itself a softener (see below
Used in painting as an adjuvant), but in
the context of fine arts, it is possible and often preferable to soften the
products with the help of glyceric esters when it is feasible.
Indeed, glycerin alone is not binding. It only takes away
the molecules of binding agent, pigments, resins, etc. (see below,
used as an adjuvant). To become binding, it must be
combined with an
unsaturated acid, in the form of an ester.
The artificial making of triglyceridic esters is
relatively difficult - although industry has a perfect control of this
process - because the efficiency of glycerin in the slow process of
esterification is rather
poor comparatively to other alcohols.
Concretely, this product is available in pharmacy.
Called glycerol by the
chemists, it leads to a large number of pharmaceutical, cosmetical and
industrial products. Moreover it is itself a by-product of
soap and
candle-making. As a
residual, it is supposed to have been, according to some sources, an
important water contaminant before being used for industrial purposes.
Glycerin would be
hygroscopic according to
some authors. After checking, it seems that precise conditions must be met
in order to get a reaction with water. It is necessary to consider, notably,
the electrochemical charge of the base, which is quite determinant, as we
noticed. There is really no hygroscopy that is
specific to glycerin or to its esters on the whole. However, some
esters can be hygroscopic because of the properties of their acids.
It is a lot more important to
emphasize the sometimes radical effect of
alkalis
on glyceric esters.
Used in painting as an adjuvant
Glycerin, as we said, is used as a
softening agent but also as a
drying retardant for different
water-based paintings and
distempers. Very thick (see
the lense effect on the picture), it "reduces" considerably while drying,
forming finally, when used pure, a very thin layer without creases, cracks,
or accidents. Its drying time is not the same as linseed oil but is still
rather long. It has no efficiency by itself as a binding agent. When added
to a paste, it has to be considered as a thinner, i.e a product that will
take the "binding" molecules (unsaturated)
away and reduce the viscosity of this paste. It may then weaken, impoverish
the paint.
Glycerin polarizes fleetingly the light in surface
when agitated. We note incidentally some prismatic effects near the
bottleneck on the picture. This property is unnoticeable when glycerin is
added to paint in small quantity.
For all these reasons but also because, in addition,
it shows an important drying shrinkage, glycerin must not be highly dosed
(at most about 1/20th of total weight in order to avoid problems, or more,
until 1/10th, which is considerable, if you want to "compel" a substance to
become more supple or if you want to delay the drying a lot). Of course,
these figures must be adapted to the nature of the paste and of the base.
They are presented here only as an
example.
Dilution,
soap
making
Glycerin can be
dissolved into
alcohol and also into water
although rather limply, in the manner of a syrup, with a tendency to
concentrate at the bottom of the container if the blend is not shaken. This
is because it does not blend naturally that the extraction by decantation in
"glycerinous water" (which is a soap-making residue) is possible. The
parting in differentiated heights on different stages of the process would
be one of the main keys to success for the fabrication process from
Marseille.
The reputation of a launderer
Concerning its use in plastic arts when pure, glycerin
gained, wrongly and rightly, a partially bad reputation: it is supposed to
"bleach". Actually, alterations can be caused by an inadequate use
because of the interaction with the acids that
are present in the pictorial coat or in the support.
Neutral or feebly charged paints and supports will not cause any problem.
Glycerin can even "neutralize" partially the acidity of some substances.
However, this slow process can give various and not always desirable
results. See
Saponification.
Glycerin-based modern
products (paint)
Glycerin, as said previously, is found in its natural
state in traditional “fat” paints. Since it has been synthezised, producers
have been able to create new products such as
"glycerophtalic" binders
and
alkyd binders.
Acrylic and
vinyl do not contain
glycerin. They are esters as well but their alcohols are not glyceric.
See
alcohol,
glycol,
polyalcohol.
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