It’s Not Easy Being Green

The following is a post by Cindy Connelly Ryan, Preservation Science Specialist, Preservation Research and Testing Division. Cindy analyzes, identifies, studies the chemistry of, and makes reconstructions of historic artists’ materials.

Hues of green surround us in the natural world, and the human eye has evolved to discern subtle differences between greens with extra acuity. In honor of spring, as green sprouts re-emerge from the snow and ice, this post discusses some of the pigments and dyes that have been used over the centuries to make the green colors found on items in the Library of Congress’ collections.

The magic lantern slide shown here was hand painted, requiring transparent green colorants of high-tinting strength that could withstand having bright light projected through them. But there are few historic pigments that give a vibrant, satisfying, and durable green ink or paint. What options were available, before the 20th Century?

 

The gemstone malachite offers a range of rich green to paler sea-green hues, depending on how finely it is ground. Deeper tones thus tend to be coarse, gritty, and somewhat challenging to paint with. Despite this, and its expensive nature, malachite green has been used as a pigment since the Egyptian Dynastic era, and was particularly popular in Asian art. For example, when analyzing early 19th Century Chinese export paintings on pith for a recent project we identified malachite in numerous examples, such as the lovely example shown here.

 

Malachite is a carbonate salt of copper, and copper forms numerous other green salts, several of which are nicer to paint with, giving sheer translucent washes of color or vibrant opaque paints. Hundreds of recipes are known from antiquity through to today for reacting metallic copper with table salt, acidic vapors, soap, and other materials to make green pigments of varied hues.

You can watch me making some copper-green paints using recipes for translucent inks from a revolutionary-war era collection of color recipes held in the Library’s Manuscript Division.

 

Far and away the most common of these copper-based green pigments is verdigris, which forms when copper meets acetic acid. Relatively inexpensive and easy to make, it was particularly recommended for tinting maps and prints with translucent, thin washes, as well as thicker applications in printing ink, oil paintings, manuscript illumination, and more. Unfortunately, it is prone to altering over time. It can turn brown, as seen in this example from a 1513 atlas of hand-colored maps. It can damage the underlying paper, and even discolor adjacent pages in a book. Our division has published several studies on the chemical alteration of verdigris and worked with our colleagues in the Conservation Division to explore possible stabilization treatments.

 

Verdigris is a cool blue-green. It was often mixed with some sort of yellow pigment to give a wider range of hues, sometimes with the hope of taming its tendency to degrade. These could be yellow ochres, plant-derived organic yellows, or mineral pigments such as lead-tin yellow and orpiment. Mixtures of blue with yellow to make green are also quite common, and are found often in book, print, and manuscript decorations – more so than the use of a single green pigment. Perhaps the most common of these was the mixture of orpiment (yellow) with indigo (blue), which was so popular during the late Middle Ages that it acquired its own name, “vergaut.”

 

Another historic pigment is green earth, a naturally occurring silicate mineral that derives its green tone from the iron and magnesium ions included in the soft, easily crumbled mineral.

Deposits are known from multiple locations across Europe. The hue can be yellowish, dark olive, or a pale greyish green, depending on where the sample is sourced from. Green earth has been used since the Roman era, and was famously identified on the wall paintings at Pompeii. It is often identified in paint under-layers, used to model the flesh tones in icon paintings and Renaissance panel paintings. It appears less often in books, manuscripts, and prints, where the paint layers are more thinly applied, perhaps because when used by itself the hues are less vibrantly “green” than malachite or verdigris.

 

Well, what about all of those fabulous green hues that plants manifest? Can’t any of those be turned into a nice paint? Chlorophyl is the source of green in most plants and algae, and unfortunately, chlorophyl does not ‘fix’ well onto fabric as a dye or onto a manuscript page as a paint, by which we colorants scientists mean that it readily washes out, fades, or discolors with time and light exposure. Green textile dyes, and organic green colorants for ink and paint, are almost always also mixtures of two dyes, one blue and one yellow. An exception is the color “sap green,” which is extracted from the ripe berries of a spiky shrub common across much of Eurasia and introduced into North America. Preparation of this colorant is described in several surviving texts on making colors from the Middle Ages through the early modern period, although it has not been identified very often on surviving artworks or manuscripts. I’ve tried making this colorant following nearly all of them, and while they look promising during extraction, these colors dry to a disappointing greenish brown. Mixtures of sap green with verdigris, however, give a range of vibrant grassy greens, and this appears to be how sap green was most often used.

Around the year 1800, a group of brilliant green pigments appeared, based on arsenic salts. These pigments’ vibrant green hues made them quickly popular, but as mystery fans reading this know, many forms of arsenic are highly poisonous!  First Scheele’s Green, and then Paris Green, a.k.a. Emerald Green, were used for book and journal illustrations, for printing wallpaper, for toys, and even for dyeing textiles, until it was realized that poisonings could occur from wet mold induced deterioration of Scheele’s green, releasing the toxic gas arsine. This can occur when wallpapers experience consistent damp weather, or when wearing textiles as garments. Although Paris Green was developed as a (somewhat) more stable alternative, the popularity of the arsenic green pigments waned later in the 19^th Century. Arsenic green textiles found their way into book bindings, as well. Awareness of this has grown among librarians and archivists in recent years, accompanied by recommended metrics for risk assessment, handling, and storage.

No need to panic and throw away all your green books, though – the toxicity of heavy metals depends on the bio-availability of the chemical form that it is present in. For these arsenic-based pigments, the primary risk vector is inhalation or ingestion of loose fibers or particles from worn and abraded items, which can be controlled by wearing gloves, washing your hands, bagging and isolating items, and similar good housekeeping protocols. As we like to say around the lab, “don’t lick the books.”

 

To sum up – most of the green colorants available before the 20^th Century were unstable, expensive, gritty, poisonous, or not a very nice shade of green. Until the introduction of viridian green in the 1850’s, the pthalo greens in the 1930’s, and the development of modern synthetic green dyes, it was, indeed, not easy being green!

 

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