Viridian Pigment

Viridian pigment is a blue-green hydrated chromium sesquioxide specifically manufactured for use in artists’ materials, personal care, and cosmetics. It is moderately opaque in oils and lightfast is most artists’ mediums.

This high purity pigment meets the specifications of both the Food and Drug Administration (FDA, 21CFR73.2326) and the former Cosmetic, Toiletry and Fragrance Association (CT FA) for use in externally applied cosmetics. It is widely used in bar soaps, detergents, eye shadow, face powder, and other cosmetic materials.

For a detailed explanation of the terms in the table above, please visit Composition and Permanence.

Origin and History

The element chromium was discovered by Vauquelin in 1797. The opaque chromium oxide (Cr₂O₃), was known at that time and in 1809 was suggested as a ceramic glaze colorant. The transparent hydrated chromium sesquioxide (Cr₂O₃.2H₂O or viridian), may have been first introduced as an artists’ color in 1838; however, both pigments were apparently not widely available until after about 1862.

Hydrated chromium sesquioxide is an intense, transparent, bluish-green color. Chromium oxide was referred to as viridian when it was first discovered. It earned its name from its chemical composition, which is 55.4% chromium, 42.5% oxygen, and 2.1% hydrogen. The official pigment code of hydrated chromium sesquioxide is Pigment Green 18 (Colour Index 77289).

It has been written by several sources that chromium oxide was not used as a pigment until 1862, but there is evidence to suggest that it was used earlier. The pigment has been identified on a J.M.W. Turner painting which dates back to 1812. An 1815 journal entry by George Field included a homemade example of the pigment, and, in 1831, Vergnaud discussed two different preparation methods, but said that the pigment was not widely used because of its high price. A catalog of pigments printed around 1840 lists a green oxide of chromium, which is believed to be chromium oxide. Also, in 1969, Kühn, using microscopy and emission spectroscopy, found chromium oxide in three paintings that dated between 1845 and 1850. It is widely accepted that chromium oxide was used before the hydrated version. Their use has been limited, though, since these greens are much more expensive than the emerald greens or chrome greens, due to the preparation process. It is sometimes used to make lightfast paints when mixed with yellows. In the past, it was used in automotive finishes and to make banknotes.

Source

The first to prepare chromium oxide was Pannetier in Paris, but his process was kept hidden. Guignet invented a two-step process for preparing the chemical. His first step was to heat boric acid and potassium bichromate, a process known as calcining, which produced a porous mass. The second step produces hydrated oxide, boric acid, and some boron, after the mass is washed in cold water, or hydrolyzed. This is only one way of several for the preparation of chromium oxide. There is no specific chemical composition for hydrated chromium oxide since different preparation processes produce the chemical with boron, while other processes produce the chemical without boron.

Permanence and Compatibility

For an artist, hydrated chromium oxide is permanent, but chromium oxide is more stable and is one of the most permanent pigments an artist uses. It will not react with hydrochloric acid or with sodium hydroxide. Hydrated chromium oxide has a much higher spectral reflectance than chromium oxide, with a maximum of 510 nanometers. Hydrated chromium oxide has lower infrared reflectography than chromium oxide, but it is much higher than other green pigments like chrome and emerald.

Due to the low refractive indices of hydrated chromium sesquioxide, it has moderately good hiding power. Viridian is unsuitable for ceramic glazes since it is unaffected by temperature up to 260º C. When properly prepared, hydrated chromium oxide is resistant to alkalies and acids, except for highly concentrated acids. It is insoluble in water and compatible with many organic materials. It is inert except under very vigorous conditions. Reacts slowly in hot concentrated sulfuric acid. Oxidizes in hot perchloric acid. Also reacts with molten alkali at high temperatures, specifically under oxidizing conditions. Not readily reduced by hydrogen or carbon monoxide high temperatures and pressures are required.

When it comes to mixing pigment with media, hydrated chromium oxide is compatible with all pigments in all media.

Oil Absorption and Grinding

Viridian absorbs a moderately high amount of oil; about 55 grams of linseed oil per 100 grams of pigment to make a paste. When mixed with gum arabic 1 gram of hydrated chromium sesquioxide required 25 drops to obtain a fully saturated watercolor paint.

Chromium oxide disperses with difficulty, making it difficult to grind hydrated chromium oxide with a glass muller. It is easier to disperse when mixed with barium sulfate. When mixed with lead-tin yellow, chromium oxide watercolor paint turned a sea-green hue. The watercolor paint mixed with smalt just turned a darker shade of blue-green. When mixed with vermillion, the watercolor paint turned olive red. The chromium oxide oil paint mixed with gamboge to form a hue about the same as our original color. When mixed with titanium dioxide, the oil paint turned a baby blue hue.

Toxicity

As a by-product of manufacture, viridian may contain trace amounts of soluble hexavalent chromium, a suspected carcinogen, considered a known carcinogen in California, and may require a warning label. It should be safe in normal artist use, avoid dust, wear a mask working with the dry powdered pigment or spraying, limit exposure to skin and eyes or other mucus membranes, don't eat it, or point your brushes with your mouth.

Ecology

Since viridian is inert and practically insoluble in water, it does not pose a hazard to the environment. The pigment can be removed mechanically from effluents. On controlled dumpsites, no dissolved heavy metals are released into the seepage water.

For more information on how to handle pigments safely, please visit How to Safely Handle Art Materials and Pigments.