Types of Paint
Encaustic
The Egyptians, Greeks, and Romans often used beeswax as the medium for pigments.
The encaustic method was in very common use until the 8th century A.D. and is still
used by a few painters today.
In this technique finely ground pigment is mixed in melted wax and applied to the
surface.
Waxes are polymers composed predominantly of hydrocarbons.
Fresco
In fresco painting, the medium and the surface are the same.
An aqueous suspension of the pigment is applied directly to a wet plaster of calcium
hydroxide and fine sand.
The pigment is absorbed and is bound into the surface as the plaster dries.
Several processes involved in making a fresco are of chemical interest.
Limestone or marble which is made up from calcium carbonate is heated in a lime kiln to
produce quicklime, CaO.
CaCO3 --> CaO + CO2
The quicklime is then "slaked" with water to produce lime, Ca(OH)2, which is used to
prepare the plaster.
CaO + H2O --> Ca(OH)2
As the plaster dries and ages, the calcium hydroxide in it undergoes a chemical reaction
with CO2 in the air changing it from Ca(OH)2 to CaCO3, which is the mineral that
comprises limestone and marble.
Ca(OH)2 + CO2 --> CaCO3 + H2O
This calcium carbonate matrix, which now contains the pigments of the painting, is
insoluble so that the painting is almost impervious to water.
However, calcium carbonate in all its forms reacts with sulfuric acid to give CaSO4 which
has a much greater solubility in water than does aCO3. Thus acid rain, which contains
sulfuric acid, slowly converts the CaCO3 in frescoes and in marble sculpture to CaSO4.
The CaSO4 then flakes of or washes away eventually destroying the artwork.
Egg Tempera
Until the 15th century, egg yolk was used as the
most common binder and medium
for paints.
Egg tempera is prepared from the separated yolks of eggs mixed with a slurry of artist's
pigment in water.
Enough water is added to provide the proper consistency for painting.
This paint dries extremely rapidly, and when applied thinly, it gives a translucent glaze
that allows either a white surface ground or an undercoat to show through.
The drying and hardening process of the medium involves the both the denaturation of
the proteins from the egg and polymerization of the fats in the yolk.
The proteins form many hydrogen bonds with each other and with the surface, locking
the pigments into a solid matrix. As they age, these proteins form covalent bonds with
each other, making the matrix very stable and permanent.
Oil
By the 15th century, oil paints, using vegetable oils as the medium, replaced egg
tempera as
the most common paint.
The oil most commonly used is linseed oil which is obtained from the seed of the flax
plant.
The oil does not dry but rather is cross-linked where there are carbon-carbon double
bonds in the oil.
This process is initiated by oxidation by oxygen in the air or by metal oxides.
Early oil paints were very slow "drying" because the initiation step of air oxidation is quite
slow.
However, it was soon discover that adding some metal oxides like ZnO or MnO2 could
also start the cross-linking process and speed up this hardening process.
Ironically, the relative slow pace of drying compared to that of egg tempera was
considered an advantage since paintings could be reworked and the composition
modified before the paint hardens.
Watercolor
In water paints the pigments are usually very finely ground mineral based transition
metal compounds and the vehicle is an aqueous solution of gum arabic, a resin
prepared from the ap of the African acacia tree.
This resin is a translucent water-soluble polymer.
The resulting paintings usually retain a translucent quality; they appear bright in part
because the whiteness of the paper is reflected through layers of the paints.
A much more opaque water based paint, Gouache, has more coarsely ground pigment
and sometimes incorporates CaSO4 as an opaque whitener. The medium is still gum
arabic.
Acrylic
Since 1945, plastic media such as acrylics have become popular.
These man-made media have not replaced oil paint as the vehicle for pigments but
rather have provided an alternative method.
These paints use an aqueous suspension of both the pigment and monomers of
compounds such as methyl acrylate and vinyl acetate.
The paint does not become plastic until the monomers combine.
In a process similar to the "drying" of oil paints, these monomers are linked together by a
chain reaction to form a polymer molecule that is insoluble in both water and most
organic solvents.
methyl acrylate vinyl acetate