Pigment History



Pigment History

Europe

Recipes for blue pigments were mentioned extensively in medieval artists' manuals

The recipes varied from some rather straightforward methods of making copper acetate to more mysterious "silver blue" recipes for which, as Cyril Stanley Smith has said, "the chemistry escapes us."

The proliferation of these recipes is understandable in light of the fact that the only two blue pigments available to the medieval artist (between the eighth and the sixteenth centuries) were the very expensive azurite and ultramarine (3).

Recipes for making artificial blue colors are very old. They are embedded in the literature of a technical tradition dating from the 3rd century CE that managed to survive five centuries of "dark ages" to reemerge in the late 8th or early 9th century in two Latin manuscripts, which contain recipes for making blue pigments from both copper and silver.

Other blues with intriguing crystalline forms are still unidentified, including a beautiful blue with a morphology resembling rosettes (Fig. 4). Could it be that the medieval artist was such a good synthetic chemist that, to this day, we have failed to synthesize and characterize compounds produced many centuries ago? The mystery surrounding the blue pigments remains unsolved.

an iron, or Prussian, blue (Fig. 5). The iron blues are the first of the artificial pigments with a known history and an established date of first preparation. The color was made by the Berlin colormaker Diesbach in or around 1704. Moreover, the material is so complex in composition and method of manufacture that there is practically no possibility that it was synthesized independently in other times or places.

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all paints have three types of components:

Pigments, consisting of small particles of colored compounds

Media, serving to suspend the pigments and bind them to the surface of the object painted

Diluents, allowing the painter to thin the paint to the best consistency for the work.

Pigment, binder or medium, thinners

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Until the mid-Ninteenth Century, most artist's pigments were derived from finely ground naturally occurring minerals:  rocks and ores.   Most of the dyes came from organic sources:   mostly plants like indigo for blue or madder root for red but also a few animals like cochineal beetles for carmine. (Hampden-Sydney's "garnet and grey" colors date back to the Civil War when the students dyed their civil war uniforms with pokeberries and butternut hickory husks.) With the rise of modern chemistry in the Nineteenth Century, some new "inorganic" or mineral-like compounds like "chrome yellow" and many, many new organic dyes were prepared.

Shown below are some examples of a few of the inorganic or mineral pigments used in paints. Many of these have been in use for centuries.

|COMMON NAME |CHEMICAL NAME |FORMULA |COLOR |

|Cinnabar |Mercury(II) Sulfide |HgS |Vermillion |

|Cobalt Blue |Cobalt(II) Oxide |CoO-Al2O3 |Bright Blue |

| |Aluminum Oxide | | |

|Prussian Blue |Potassium Iron(III) Ferrocyanide |KFe(Fe(CN)6) |Deep Blue |

|Verdigris |Copper(II) Acetate |Cu(CH3CO2)2 |Green |

|Chrome Yellow |Lead(II) Chromate |PbCrO4 |Yellow |

|Burnt Sienna |Iron(III) Oxide |Fe2O3 |Reddish Brown |

| |in clay | | |

|Malachite |Copper(II) Carbonate |CuCO3Cu(OH)2 |Green |

| |Copper(II) Hydroxide | | |

|Cadmium Yellow |Cadmium(II) Sulfide |CdS-ZnS |Lemon Yellow |

| |Zinc(II) Sulfide | | |

|Rutile |Titanium(IV) Oxide |TiO2 |White |

|Chinese White |Zinc Oxide |ZnO |White |

|Lead White |Lead(IV) oxide |PbO2 |White |

|Vine Black |Carbon |C |Black |

ORGANIC COLORANTS

Among the new organic colorants were the "Azo Dyes" which were discovered by accident by Perkin in England in 18xx. Perkin had hoped to make a synthetic version of quinine, the only known anti-malarial treatment which was an extract of a South American tree Because those countries that were the sources of quinine were in the hands of the Spanish and Portuguese, English colonists across the topics were vulnerable to malaria. Perkin used coal tar, an ample and cheap byproduct of the coal and steel industry, to provide as the starting materials for his experiments. The resulting compounds that he made gave colors more brilliant and varied than those available from most vegetable dyes. They were also inexpensive so that the average man could afford colored clothes that had previously been beyond the reach of all but the rich. Though Perkin himself became rich from his patents, the British did not capitalize on their early technological lead in dye-making. The color chemistry industry moved to Germany were several companies, notably I.G. Farber, made huge advances in chemistry and wealth.

Shown below in a skeleton of the common diazo "kernel" that all azo dyes have. It has two benzene rings (six carbons in a ring with three double bonds) joined by a double-bonded pair of nitrogen atoms. Shown along with this central piece are several organic dyes that are also used as pigments in paints. You might try to identify those that are azo dyes.

 PIGMENTS.

Most traditional artist pigments are derived from finely ground minerals or inorganic compounds. (Ultramarine, from "across the sea", is the pigment from ground lapis lazuli, a semiprecious stone.) Most traditional dyes came from organic sources such as berries and insect bodies. (The garnet of Hampden-Sydney's colors came from using the juice of the pokeberry to dye their civil war uniforms; the gray came from butternut husks.) With the advent of modern chemistry, many colored synthetic organic and inorganic compounds have become available.

The chief class among these, is the group of diazo compounds that became available in the nineteenth century. Some of these compounds have been used as both dyes and pigments. Shown below is one of the diazo dyes,

All diazo dyes have benzene rings separated by a double bonded pair of nitrogen atoms. You should be able to recognize a diazo dye by this substructure.

TYPE OF PAINT BINDER DILUENT DESCRIPTION OF MATERIALS AND PROCESS

Encaustic Beeswax None 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

redominantly of hydrocarbons.

Fresco Plaster Water

Ca(OH)2

CaCO3 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 Egg Yolk Water 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 Linseed or Turpentine

Walnut Oil Mineral Spirits

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 Gum Arabic Water 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

sap 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 a 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 Acrylate Esters Water 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

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