AcrylicThese 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.
Picture-azuriteAzurite ("Blue verditer," "mountain blue," "lapis
armenius," "azurium citramarinum," "blue bice"):Origin and History:
Latin borrowed a Persian word for blue, lajoard, which in the form
of lazurium became azurium, and gave us our word azure. It is
composed of a basic carbonate of copper, found in many parts of the
world in the upper oxidized portions of copper ore deposits.
Azurite mineral is usually associated in nature with malachite, the
green basic carbonate of copper that is far more abundant. Azurite
was the most important blue pigment in European painting throughout
the middle ages and Renaissance by contrast, despite the more
exotic and costly ultramarine having received greater acclaim.
Making the Pigment: To prepare a color from it, lump azurite is
ground into a powder, and sieved. Coarsely ground azurite produces
dark blue, and fine grinding produces a lighter tone; however if
not ground fine enough, it is too sandy and gritty to be used as a
pigment. The medieval system included washing it to remove any mud
and then separating the different grains by some process of
levigation. If plain water is used it is a slow, laborious process,
so they used solutions of soap, gum and lye. When azurite is
washed, the very fine particles are rather pale, greenish sky-blue,
and not much admired for painting. The best grades of azurite for
painting were coarse: not sandy, but so course that it could be
quite laborious to lay them on. Chemical Properties:
Cu3[CO3]2[0H]2, H3.5, SG-3.7, monoclinic. Azurite sometimes looks a
little like lapis lazuli, and the two were often confused in the
Middle Ages. To tell them apart with certainty the stones were
heated red-hot. Azurite turns black when this is done, and true
lapis is not injured. It does not blacken from the effects of
sulphur gases as some chemists have supposed, but from the action
of the strong alkalis improperly used in picture cleaning, and from
the purely optical effect of darkened varnish surrounding its
particles.The color can, however, be ruined by the presence of
acids. Artistic Notes: Glue size was often used as a binder to hold
the pigment grains firmly in place. (Size is more easily affected
by protracted dampness or by washing than egg tempera, and blues in
wall paintings have therefore sometimes perished through the
destruction of their binder where colors in tempera have stood.) It
was necessary to apply several coats of azurite to produce a solid
blue, but the result was quite beautiful. The actual thickness of
the crust of blue added to the richness of the effect, and each
tiny grain of the powdered crystalline mineral sparkled like a
minute sapphire, especially before it was varnished. The open
texture of a coat of azurite blue has often been its undoing on
panels; the varnish sinks into it and surrounds the particles of
blue. As the varnish yellows and darkens, the power of the azurite
to reflect blue light is destroyed, strangled by the varnish---a
large number of blacks in medieval paintings were originally blues,
only obscured by the discoloration of the varnish. It is incredibly
permanent.
Egyptian Makeup Pot Both men and women used special powders and
pastes to cover their skin. The make-up was not only worn to make
them look attractive but also to protect them from the hot climate
- quite similar to how we use sun cream today. This pot may have
stored 'Khol' - black makeup that the Egyptians used to decorate
their eyes.
Vermilion ("Cinnabar"):Vermilion is the standard name given to
the red pigment based on artificially-made mercuric sulfide. The
common red crystalline form of mercuric sulfide is cinnabar, a name
reserved only for the natural mineral. The natural product found
chiefly in Almaden and Idria has been eliminated for practical
purposes (including that it is slightly poisonous).The synthesis of
these mercury and sulphur into cinnabar is accomplished by mixing
them together and heating them; if simply mixed and ground
together, a black sulfide of mercury is formed, but at the proper
temperature this vaporizes and recondenses in the top of the flask
in which it is heated. The flask is then broken and the vermilion
is removed and ground. Upon grinding the red color begins to
appear, and the longer it is ground, the finer the color becomes.
This process was understood before the year 800 AD.The properties
of both natural and artificially prepared are practically
identical. Cinnabar, a dense red mineral, is the principal ore of
mercury or quicksilver. Vermilion is not generally considered today
to be a permanent pigment. It has been known since Roman times that
specimens of vermilion darken when exposed to light. In tests it
has been discovered that impurities in the alkali polysylfides used
to "digest" the pigment, leading to the instability of the red.
This catalyzes the transition of the red to black. Also, we've
found that the darkening of vermilion occurs mainly in paintings in
egg tempera but it is not unknown in oil paintings. It is however
fairly unreactive to other colors' chemical makeups; therefore,
when mixed with lead white to produce flesh tones, it did not
produce the black sulfides.
Verdigris ("green of Greece," "salt green"):Making the Pigment:
Made by treating copper sheets with the vapors of vinegar, wine, or
urine and scraping the resultant corroded crust. Placing copper in
ammonia will cause it to turn blue; adding a few drops of acetic
acid (vinegar) precipitates a light cyan-green salt. Copper sheets
can also be spread with honey and sprinkled with salt before
treated with the acid for a slightly different shade termed "salt
green."Chemical Properties: Copper acetates ranging in color from
green to blue. Reactions with copper acetate vary among substances
such as the following: copper acetates dissolve in mineral acid,
alkalis convert them into blue copper hydroxide, oils, resins and
proteins react to form green transparent copper oleates, resinates,
and proteinates. Of the many different types of verdigris each type
can be classified into either basic or acidic. Neutral verdigris is
Cu(CH3COO)2 H2O, and basic verdigris contains more Cu(OH)2 and H2O.
Neutral verdigris is neutral copper acetate which occurs when basic
acetates are dissolved in acetic acid, or when basic verdigris is
ground up with strong acidic acid. Decomposition of neutral
verdigris occurs when a solution is boiled. This verdigris is
dissolved in acidic acid. The shape of neutral verdigris is
hexagonal and rhombic with distinct boundaries. Basic verdigris
forms from the combination of air, water vapor, acetic acid vapor,
and copper or copper alloy mix. It forms a solid of blue, or
blue-green. It is often made up of fine needles. In the presence of
HCl, verdigris is soluble and forms a green solution. From
interaction with NaOH it is soluble and precipitates. In the first
three months of use the verdigris formulations can change from
blue-green to green. All verdigris reacts with resin to form copper
resinates. This copper resinate is rather transparent and often
used as an overpaint to increase depth of saturation of an opaque
green.Artistic Notes: Verdigris is reactive and unstable, requiring
painters to use isolating varnishes to protect its color. Sulfur
compounds in the air darken all forms of verdigris. It is, however,
lightfast.
Terre verte ("green earth"):Origin and History: The name terre
verte is applied to several different minerals, but most
importantly in medieval painting is the light, cold green of
celadonite, found chiefly in small deposits in rock in the area of
Verona, Italy. The chief deposits of glauconite which yield the
yellowish and olive sorts are in Czechoslovakia. Today the color is
chiefly a durable mixture of chromium oxide, black, white and
ochre, since the natural product is scarcely obtainable, though
possible with effort.Chemical Properties: They are not poisonous,
dissolve partially with a yellowish-green color in hydrochloric
acid, but not in alkalis, and should not discolor water, alcohol or
ammonia.Artistic Notes: Can be rather dull, transparent, and soapy
in texture, like a clay. The color is also not constant, ranging
from a light bluish gray with a greenish cast to a dark, brownish
olive. In manuscripts and on panels they were chiefly used to
underpaint the warm flesh tones.
Burnt Sienna:Making the Pigment: Burnt sienna is prepared by
calcining raw sienna which in the process undergoes a great change
in hue and depth of color; in going from ferric hydrate of raw
earth to ferric oxide, it turns to a warm, reddish brown. Chemical
Properties: Fe2O3 * nH2O + Al2O3 (60%) Manganese dioxide (1%),
calcined natural iron oxide (PBr 7). Microscopically, heating makes
the pigment more even in color and the grains are reddish brown by
transmitted light.Artistic Notes: Because of its transparency,
burnt sienna is used as a fiery glazing color which requires much
binder, about 180%, and as an oil color is apt to jelly. This is
remedied by washing, which however dilutes the intensity of the
color. In 1768, Martin Knoller stated that very strong heat will
produce a sienna resembling vermilion that may be used in fresco
out of doors. American Burnt Sienna is a strong type of ochre and
is neither as clear nor as brilliant as the Italian Sienna. It
supposedly imparts a muddy tone but is very permanent in all
techniques.
Umbers:Making the Pigment: Burnt umber is a combination of iron
oxide, oxide of manganese and clay, made by burning raw umber to
drive off the liquid content. Chemical Properties: An ochre
containing manganese oxide and iron hydroxide (PBr 7), Fe2O3 MnO2.
In acids it dissolves in part leaving a yellow solution;
hydrochloric acid gives it an odor of chlorine. In alkalis it
discolors a little and when heated, becomes a reddish brown. It has
the same properties as natural umber. Completely lightfast and
unaffected by gases, and makes a good glaze when thinned with oil
or varnish.Artistic Notes: Because of the manganese content it is
an excellent dryer. It can be used in all techniques but requires
80% binder, with an additional 2% wax when in tubes to prevent
hardening. Many umbers have a greenish tinge, and in oil, it tends
to turn dark later on, especially if the underlayers were not
thoroughly dried, but this darkening may also occur in alla prima
painting. It is best not to use the color in fresco, as in the open
it tends to decompose and produces a burnt heavy tone. Can be mixed
with all other pigments except for the Lakes. Burnt umber turns
especially dark, surprisingly as the burnt tones are usually more
reliable in this respect. This tendency to darken is increased by
the modern practice of grinding the tube colors too finely.
Roman CosmeticsRoman women used ground chalk or lead to make
their faces look white. Pale skin was very fashionable. It was a
sign that a woman was rich enough not to have to work outdoors in
the fields.Eyeliner was popular to make the eyes look bigger. It
was made from a mixture of soot and olive oil.Metal scoops were
used to mix make-up in small dishes.Roman Cosmetic BoxCosmetic box
with carved bone decoNaples, Archaeological Museum, 1st century
CERoman Cosmetic Secrets RevealedThe fashion conscious women of
Roman Britain used a tin-based foundation to get a pale and
appealing look. The evidence comes from a sealed pot of ointment
found at an archaeological dig in Southwark, south London in
2003.Bristol University scientists analyzed the cream and found it
to be made from animal fat, starch and tin oxide.
Zinc white ("Chinese white," "silver white"):Origin and History:
First introduced in 1840, this white is colder in appearance than
lead white, and doesn't cover nearly as well, yet it is far less
expensive. Zinc has been known as a mineral since antiquity when it
was melted with copper to form brass. It was also known then, as it
is today, as a medicinal ointment. In 1782, zinc oxide was
suggested as a white pigment. Guyton de Morveau at L'Acadmie de
Dijon, France, reported zinc oxide as a substitute for white lead.
Metallic zinc had originally come from China and the East Indies.
When zinc ore was found in Europe, large-scale production of the
extracted metallic zinc began. In 1834, Winsor and Newton, Limited,
of London, introduced a particularly dense form of zinc oxide which
was sold as Chinese white. It was different from former zinc white
in that the zinc was heated at much higher temperatures than the
late eighteenth century variety. By 1844, a better zinc white for
oil was developed by LeClaire in Paris. He ground the zinc oxide
with poppy oil that had been made fast drying by boiling it with
pyrolusite (MnO2). In 1845, he was producing the oil paint on a
large scale. Making the Pigment: The French method of
manufacturing, known as the 'indirect process' used the zinc smoke
derived from molten zinc, which was heated to 150C and collected in
a series of chambers.Chemical Properties: Zinc oxide, ZnO. If you
heat zinc white, it turns to lemon yellow, but will revert to white
when cooled. It differs from lead white in this respect. Since zinc
oxide is derived from smoke fumes, its particles are very fine and
are difficult to observe except at very high magnification. It
readily dissolves in alkaline solutions, acids and ammonia without
foaming.Artistic Notes: It is non-poisonous, permanent and doesn't
yellow, though these factors are true only with pure zinc white. It
also disintegrates quickly out of doors, and increases in volume
causing massive crackling, so it is not useful in fresco. Ground in
oil it dries slowly, especially in poppy oil, where the retarded
drying time is needed. It does not dry as solid as lead white, due
to some transparency in the pigment. A small addition of damar or
mastic varnish speeds up the drying time. As it is very fine in
powder form, it can be sufficiently mixed with only a spatula,
requiring 30% binder and an addition of 2% wax in the tube to
prevent hardening. It is compatible with all other pigments,
including copper-based, but in watercolor it is destructive to the
permanency of coal-tar colors and accelerates the process of fading
(though it doesn't do this in oil.) Zinc is essentially permanent
in sunlight although the yellowing in oil affects its brightness.
It is neither as opaque nor heavy as lead white and it takes much
longer to dry. Because zinc white is so "clean" it is very valuable
for making tints with other colors. Tints made with zinc white show
every nuance of a color's undertones to a degree greater than tints
made with other whites, and the artist has time to complete his
work before the paint dries. Despite its many advantages over lead
white, zinc white oil color also has a drawback; it makes a rather
brittle dry paint film when used unmixed with other colors. Zinc
whites' lack of pliancy can cause cracks in paintings after only a
few years if this color is used straight up to excess.
Vine Black:Charcoal made from young shoots of grape vines were
referred to in medieval times as the best of blacks. It is now
referred to as more of a blue-black, considering the coolness of
the grays that it produces in mixtures.It is important that the
vine sprigs be thoroughly burnt and reduced to carbon, otherwise
the color will be brownish and an unpleasant consistency; but they
must not be burnt in the air or they might reduce to ashes instead
of to carbon. They are packed tightly in little bundles in
casseroles, covered and sealed, and baked in a slow oven. You can
make your own vine black with a similar method. The resulting
charcoal is used in sticks for drawing; for painting it is first
powdered and ground up dry, and then mixed with water and ground
for a long time between two hard stones.
Egyptian blue ("frit," "Pompeiian blue"):Origin and History:
Very stable synthetical pigment of varying blue colour. It is one
of the oldest man-made colors commonly found on wall paintings in
Egypt, Mesopotamia and Rome. Many specimens, well over 3000 years
old, appear to be little changed by the time.Making the Pigment:
Heating a mixture of a calcium compound (carbonate, sulfate or
hydroxide), copper compound (oxide or malachite) and quartz or
silica gel in proportions that correspond to a ratio of 4 SiO2 : 1
CaO : 1 CuO to a temperature of 900C using a flux of sodium
carbonate, potassium carbonate or borax. The mixture is then
maintained at a temperature of 800C for a period ranging from 10 to
100 hours.Chemical Properties: Calcium copper silicate, CaCuSi4O10.
It is insoluble in acids even in warm temperatures.Artistic Notes:
It has a discreet covering power. It can be used in fresco. Not
advised in tempera, oil and encaustic.
Tyrian purple ("Royal purple"):Origin and History: This organic
dye was prepared from various mollusks or whelks, including Murex
brandaris , Purpura haemostoma, Purpura lapillus, and Carpillus
purpura, which can be found on the shores of the Mediterranean and
Atlantic coasts and which excrete the fluid from which the dye is
won. One gram of this dye is made from the secretion of 10,000 of
these large sea snails.The Phoenician cities of Byblos, Sidon and
Tyre were centers of the pottery, glass and purple dye industries.
Because of the Phoenicians' unique purple potion, one possible
meaning for the word Phoenician is "dealer in purple!" Others think
it means "blood-red" because their purple dye had a reddish
undertone.Another name for this superb color was "Tyrian purple"
because Tyre was a famous Phoenician city located on the eastern
shore of the Mediterranean. Everyone called Tyre the "queen of the
seas" because it was the center of the Phoenicians' trade empire.
Soon they realized that they needed a simpler writing system for
their bookkeeping because so many people wanted the dye and their
business exploded. They didn't have the patience to write in
artistic hieroglyphics, so they invented a simple alphabet in the
twelfth century B.C. Tyre was at the height of its influence, and
traders carried the Phoenician alphabet to Greece.From Greece, the
alphabet went to Italy, Egypt, and Eastern Europe. By 900 B.C., the
Phoenician system of writing quickly spread along the Mediterranean
coast and into Asia. The Hebrews and their neighbors used a form of
the Phoenician system in Palestine. Another form of the Phoenician
alphabet spread to the area of present-day Yemen, and on to what is
now called Ethiopia. Unfortunately, just as the unique alphabet
spread around the world, Alexander the Great destroyed the city of
Tyre by filling its prosperous harbors with silt and killing or
enslaving its inhabitants. However, the Phoenician alphabet lived
on.Chemical Properties: This purple color is remarkably stable,
resisting alkalis, soap, and most acids. It is insoluble in most
organic solvents.Artistic Notes: Tyrian purple was used in the
preparation of a purple ink and in dyeing parchments upon which the
codices of Byzantium were written. It was also the traditional
"Imperial Purple" of ancient emperors, kings, and magistrates.
Carmine ("Cochineal," "Crimson Lake"):A dyestuff precipitated on
clay, made from the ground female Coccus cacti, or cochineal,
insect which lives on various cactus plants in Mexico and in
Central and South America. It was brought to Europe shortly after
the discovery of those countries, first described by Mathioli in
1549. The finest quality, known as nacarat carmine, is
non-poisonous and quite beautiful with the peculiarity of being
more permanent in transmitted light as a transparent color, than
when under direct light.Soluble in ammonia. Carmine is an aluminum
and calcium salt of carminic acid, an anthraquinone derivative, and
carmine lake is an aluminum or aluminum-tin lake of cochineal
extract, whereas crimson lake is prepared by striking down an
infusion of cochineal with a 5 per cent solution of alum and cream
of tartar. Purple lake is prepared like carmine lake with the
addition of lime to produce the deep purple tone. Carmine lake is
insoluble in water. It burns completely leaving a white ash, and
smells in the process like burnt horn.According to Maximillian
Toch, it is only legitimate as a food coloring, as exposure to the
sunlight for three months bleaches the pigment completely. Carmine
lake does not behave much better, being even weaker and less
stable; it is of a maroon shade.
At one time, all natural dyes were used such as onion skins or
beets, and the bark of oak or ash trees. Fayum Funeral Portrait,
Mummy Portrait of a Woman, Antinoopolis, End of the Reign of
Trajan, 98-117 A.D., Wax portrait on wood.
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 + CO2The 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 reactionwith 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 +
H2OThis 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 whichhas 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.
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.
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.
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.
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.
