[Pigments and Binders]
[Pigments and Binders] |
Natural Red Earth |
Pigment Labsetup
"iron oxide pigment"
"earth colors"
Earth pigments are inorganic and derived from mineral sources, river
beds, and volcanic areas. 
(Thomas,
8) They are mined specifically in France, Cyprus, Italy, the Persian Gulf,
Andalusia, the Province of Jaen, and mainly in Spain (not necessarily on
the plain). Earth Pigments are also found in the US in New York State.
Once the pigment is mined in Andalusia and Province of Jaen, it is shipped
to Málaga for grinding. (Thomas,
13-14)
Earth pigments can also be manufactured synthetically, and often are
due to the high cost of transportation. The red hue is reproduced by using
a dry process called thermal decomposition. In this process, ferrous sulfate
is put into a two-step calcination process. The final product is calcium
sulfate and iron oxide (Fe2O3) (Thomas, 17)
The principle earth colors have been used since the dawn of man. In the Paleolithic and Primitive Cultures, natural red earths were used in cave paintings and for ritualistic purposes; specifically death ceremonies, because the red color can easily symbolize blood. They are also used because of their permanence. Some paintings with red earth hues in the Italian city of Pompeii have withstood wind and rain for over a millennium. In fact, this pigment was so important that the people took risks in locating the it.
In Ancient times, Red Earth was used inside the Egyptian pyramids and
temples. It was also utilized in India with vegetable or animal glues as
binders, as well as China. (Thomas,
20-22) They were usually prepared by grinding the earth on a rock and using
binders to make the paint. (Thomas,
19-20) Interestingly enough when samples were prepared in the pigment lab
the same technique, save grinding, was used.
Red Earth was put to use in the Classical period because it was perfect
for fresco painting. In particular, the Minoans, Etruscans, and Greeks
used this technique. The pigment was mixed with water and applied to a
wall surface covered with wet lime plaster. As the plaster dried, the pigments
bonded to it and became part of the wall's surface. Only pigments that
are compatible with lime could be used in this way, and earth pigments
are not affected by alkalies, so they make a good pair. (Thomas, 22)
Medieval and Renaissance painters used Red Earth pigments for fresco,
tempera, and oil painting. Using the pigment in the form of chalks was
a new technique that came out of the Renaissance. Chalks are made by cutting
sticks directly from the deposits of colored earth. Conté pencils
are a modern commercial equivalent to the chalk used during the Renaissance.
(Thomas,
23-24)
In the modern world, Red Earth pigments are used in paints and stains.
They are also used in the manufacture of rubber, plastics, concrete products,
paper, magnetic inks, and fertilizers. Because of their non-toxic nature,
they are legal to use in drugs, foods, and cosmetics. (Thomas, 24-25)
In any earth pigment, there are three components that are necessary.
The first is a principle color producing agent, which is iron oxide. The
second component is a secondary color producing agent, which can be either
calcium, manganese, carbon, or an organic material such as silica or limestone.
The third component is a base filler, diluent, or carrier of color, which
is usually clay. Natural Red Earth is possibly a product of low humidity
and high temperature. (Thomas,
8-9) Red Earth is also a hematite, which is anhydrous ferric oxide. (Thomas,
10) On a side, they are resistant to acids and bases, this attributes to
their permanence.
Before a pigment can be added to a particular binder, the pigment must
first be prepared. Once it is mined, the pigment must be dried and sifted.
Then, it is washed by a process of levigation which is the separation of
fine and coarse materials into a suspension. Then the pigment is ground
up. (Thomas,
28-30)
Pigments can be mixed with many different binders. One in particular is oil. Vegetable oils are best for binders because they dry by oxidation, not by evaporation which makes tough adhesive films. The best to use is linseed oil, but some other good oils to use are poppy, walnut, soybean, and tung. Linseed oil was the binder used in the pigment lab. Different oils also yield different levels of permanence. Different pigments have varying rates of oil absorption, so it is important to consider the specifics of the pigment when preparing the paint. Natural earths are thirsty for oil. Furthermore, the pigment needs at least forty percent oil in order to become a useful paint. This is congruent with the lab observations made, for the oil paint created required morethan the initial amount of Linseed oil to be added so that the paper could be painted. A good test for proper oil amount is to spread the paint on a glass. If the paint cracks or rolls, then there is not enough oil present to bond with the pigment.
Another type of binder used is an emulsion, which is any mixture of
oil and water, such as egg. These paints tend to dry very quickly. This
mixture of pigment is most commonly made when the egg yolk is separated
from the white. The yolk skin was then poked with a needle and the excess
yellow solution was added to the pigment to make a tempera paint. It is
important that the pigment be ground as fine as possible. If the pigment
is ground with water, it can be stored indefinitely. A preservative can
be added to the emulsion and pigment. An example of such a preservative
is vinegar, but a better one to use is oil of clove. To test the ratio
of binder to pigment one would paint on a piece of glass and let dry overnight.
The next day, if the paint comes up together, the ratio was good. If the
paint crumbles and powders, then the ratio was bad. (Thomas, 53-56)
Microscope pictures of egg Tempera at 20x magnification
The last type of binder creates a watercolor and gouache paint. To make
a pure watercolor, one would use vegetable gum (or gum arabic), in a powder
that dissolves in water to form a one part gum, two parts water solution.
When one dissolves the gum arabic, it needs to be put in a mesh bag and
suspended in cold water. To make a gouache paint one must combine a plasticizer
found in a sugar such as glucose/dextrose or honey/glycerin and a preservative.
The transparency of this particular kind of paint depends on how finely
the pigment was ground. To make a gouache, though, the grain does not need
to be as fine. The advantage of using this type of binder is that the paint
can be reworked after for several years of dry existence. All that needs
to be done is to spray the surface of the painting with water until it
softens. Then, add the next layer of paint, it will bond easily to the
original layer. (Thomas,
56-60)
The Natural Red Earth pigments are not dangerous. However, if a large
amount of the aforementioned chalk is inhaled it could lead to silicosis.
(Rossol,
114)
When the unbound pigment was examined with a microscope it was found that the individual particles are rough in shape. An interesting property of this pigment is the fact that it appears in a cross-polarized environment. Cross polarization is the exclusion of light on the x-axis as well as the y-axis. For example if one were to look at a light through cross-polarized "film" they would find that no light gets through. However, the red earth pigments bend the light so that some light makes it through the "film." This property makes the microscopic examination of the red earths interesting as well as exciting.
Raw pigment Crosspolarized
Raw pigment
Katie Nard, John Wallace, 1998.