Egg Tempera is an ancient painting medium using egg yolk or a whole egg. It was famously used in Europe for illuminated manuscripts, and was the most common painting medium until it was surpassed by oil paint during the Renaissance. Once dried, egg yolk tempera hardens permanently and can last for centuries.
Here’s a basic recipe to make it yourself!
1:1 Gum Arabic Solution
Clove oil or vinegar (optional)
Pipette or measuring spoons
Scissors or sharp knife
Whisk or mixing stick
Tiny spoons or spatula
Glass grinding plate
Preparing Egg Tempera Binder:
Use as fresh an egg as possible. Break eggshell carefully, letting egg white drip into a bowl. Move the yolk gently between shell halves; don’t break the yolk.
Carefully roll the yolk onto a paper towel. Roll it back and forth to remove all the egg white.
Rolling the yolk to the edge of the towel, place another bowl under it. Pierce it with the knife and let it drain into the bowl. Keep the yolk sac out, and throw it away.
Use pipette to add enough water for a 1:1 mixture and whisk.
Add 1:1 Gum Arabic solution to the yolk mixture and whisk.
Add a drop of clove oil or vinegar to avoid ‘eggy’ odor and prevent mold. (this is optional)
Using the frosted side of a glass plate, add 1-2 drops of your egg mixture with a pipette.
Add a tiny amount of dry pigment and grind lightly with the muller.
Test your tempera– if it dries dull, add more yolk. If it’s too thin or too glossy, add more pigment.
Without added pigment, the yolk mixture lasts 3-4 days.
Mixed with pigment, it lasts until dried, about 2-3 days.
Make sure to wash your brushes quickly after using. Egg yolk gets very hard! (Remember that egg tempera paintings last many centuries…) Olive oil soap is recommended.
If you’re not part of the solution, you’re part of the precipitate.
The Oldest Chemistry Joke Known To Exist
A common theme you may have noticed in the science articles on the Pandemic Blog is that a near-infinite variety of colors can be found in nature, but not all of the material that gives rise to these colors are suitable for use in painting or dyeing. However, ancient and medieval artists were not limited to the pure, stable colors found in minerals or mineral earths. Many important pigments and dyes were made throughout recorded history by taking natural products (geological or biological or both) and treating them in some way to change their color, or improve their chemical characteristics, or even to synthesize pigments from entirely different things. Many sophisticated techniques to improve the use of natural products were developed far in advance of the modern understanding of chemical principles that became codified from the 17th century on. But the ancient and medieval world nevertheless had access to many reactive chemicals, both from minerals and biological products, which could be used in making or altering pigments. Vinegar contains acid; aged urine, ammonia; Lye can be had from ash and water; salt deposits can be sources of reactive carbonates or metal salts that can be used as mordants. While ancient and medieval artisans had little notion of what we call chemistry, they were perfectly fine at experimentation and developing sophisticated techniques that had no equal until the modern era.
A witty person once said to me that the entire development of science and technology by humans can be summarized by “set fire to it (or, alternately, blow it up) and see what happens.” And indeed, among the earliest pigments used by prehistoric artists was charcoal. Charcoal can be found in the leftovers of wood fires, but from before antiquity it was intentionally made by slowly heating wood twigs to the point of combustion while restricting the access of air. In the absence of a flow of oxygen the wood does not ‘completely’ burn and oxidize to carbon dioxide (a gas of little utility if you are not a plant or a brewer), instead leaving behind a somewhat sticky, hard black residue, nearly pure carbon, that can be used to make black pigment or a very tidy, hot, and nearly smokeless fire when burned in an open fire. A very fine black pigment powder (called lampblack) can be recovered from the smoke of burning of oil in a lamp, another technique known to the ancients. You can also treat some minerals with fire or heat to alter their color. As mentioned previously, the colors burnt sienna and burnt umber are natural earth pigments treated with heat, which causes chemical reactions that reduce some of their more oxidized (and therefore brighter) iron oxides to hematite, a darker mineral. Heating white lead oxide (ceruse) can lead to a yellowish pigment that Theophilus described as a base for making flesh tones (Hawthorne and Smith, 1979). And it should be mentioned at this point that if you treat chalk or limestone with high heat, you get quicklime, a reactive chemical used to make cement or plaster and in some pigment making processes.
Another important property of pigments that artists need to take into account is solubility. Pigments that dissolve readily into water do not necessarily make good paints, as you would rely on the pigment coming out of the water solution to bind to the surface in some way. This is frequently an important part of the fabric dyeing process, where treating a dye solution with a chemical (frequently called a mordant) is required to make the dye turn the proper color and/or bind properly to the fabric. On the other hand, most paints are in fact suspensions of tiny particles of pigment in medium – they do not dissolve, but form an emulsion of sorts with the water, oil, egg yolk, etc. In many cases the same exact pigment can be used for watercolor or oil paints or tempera, because the pigment itself does not dissolve in or react with the medium.
So some natural dyes that dissolve in water can be treated with a mordant to make them insoluble. They then precipitate out of the aqueous solution and become a solid, that can be dried and ground into a pigment powder. These pigments are called lake pigments. The term lake in this context is an English corruption (of course) of the word lac, which refers to the Indian scale insect that makes (like other highly prized bugs) a brilliant red pigment, and also a very useful waxy resin, from which we get lacquer and shellac. Lac in turn has the same root as the Sanskrit word lakh, meaning 100,000 (because there are SO. MANY. BUGS. required to make this). Presumably this also means lac pigments were used in making red lake pigments, but now, like in many bug-derived pigments, there are synthetic alternatives.
A word or two on mordants. Despite what you might think (well; I thought it), the Latin derivation of this word is not “something that makes you dead.” In fact it’s derived from mordēre, which means to “bite.” This makes great sense in the dyeing context, where mordants are used to make the water soluble dyes stay attached to the fabric. A mordant, chemically, is frequently a colorless metal salt that can form a coordination complex (as described in our earlier article) with the organic dye molecule and still have the ability to bind with something else. In the case of lake pigments it changes the dissolved pigment to an insoluble one rather than make it bind to fabric. Common mordants available from ancient times include calcium carbonate (from chalk or bone), lye (from ash), alum (from evaporated salt deposits in Egypt, or ground minerals), and green vitriol (ferrous sulfate, from certain kinds of rust).
The earliest lake pigments known to be produced were indigo lakes made in ancient Egypt from the woad plant. The exact techniques the Egyptians used are not known, but it is thought that they may have used plants grown in chalky, alkaline soil (so there would be calcium carbonate or other light metal salts at hand). Perhaps the most well known lake pigment is made from madder. Madder is well known as a source of red or yellow dye from the purpurin and alizarin found in the roots. The mordants could be chalk, clay, or crushed bone (all containing calcium salts). The ancient Egyptians used gypsum (calcium sulphate hexahydrate, a brilliant white mineral found in evaporated ocean deposits) with madder to make a pink pigment.
A variety of natural pigments were made in ancient and medieval times from the oxidation products of metals. In addition to the familiar iron oxide from rust, blue pigments could be made by corroding copper in the presence of calcium carbonate (azurite is a natural mineral of this nature). Theophilus (I:34, in Hawthorne and Smith) describes a particularly nasty concoction of sulfur and mercury heated over coals produced the pigment Renaissance artists called cinnabar (after the natural mercury ore from Spain or California which can make a fine pigment itself; but the smelted metal was far more valuable and was more widely available). The pigments called “salt green” or “Spanish green” were made from copper, salt, and vinegar to produce copper acetate. Ceruse, the white pigment also used in makeup by the ancient Romans, was made from lead heated with vinegar or urine.
Finally, it was known that some pigments, in particular folium, as described in the previous article, were known to change color in response to the pH, or the acidity, of their medium. Theophilus (I:33) describes making lye and using quicklime to raise the pH of folium to change its color from red to purple to blue. Similar pH sequences are used to adjust the color of indigo as part of the mordanting process for dyeing. It was not until the 17th century that scientists realized you could turn this around and use changes in colors of certain pigments (called indicators) to determine the acidity of a solution.
To conclude, it can be safely assumed that almost any material you can think of that has color to it was subjected to some kind of attempt at alteration by ancient or medieval artisans. Heat, acid, base, and other reactive chemicals were all tried in attempts to improve natural products. In many cases successes resulted in techniques that were used for thousands of years. But many of these techniques that remained important through the Middle Ages became largely extinct, in part from the globalization of trade providing cheaper substitutes for expensive traditional materials – but in the modern era synthetic chemistry has come to dominate the production of pigments for artistic purposes. Next time we’ll talk about why and how this came to pass.
Thank you for joining us as we do Pandemic Projects, meant to keep you energized, curious and learning!
In this article we referred to the translation of “On Divers Arts” by ‘Theophilus’ (possibly Roger of Helmarshausen, ca. 1100 C.E.) by J.G. Hawthorne and C.S. Smith, Dover Publications, New York, 1979.
Last time on the blog we discussed inorganic natural pigments: natural colors from mineral, geological sources. This time we will talk about natural organic (meaning, containing tetrahedral carbon, as opposed to inorganic carbonates or graphite or diamond) pigments, of plant and animal origin.
But first, some breaking news: An organic pigment, thought to be lost to history, has been rediscovered in its plant source, and has been for the first time chemically characterized. Portuguese scientist Paula Nabais, from the Universidade NOVA de Lisboa, and her colleagues reported last week in Science Advances (a publication of the American Association for the Advancement of Science) the results of a thorough historical and chemical investigation into folium, a stable blue pigment used in the Middle Ages for manuscript illumination, but unfamiliar and unavailable to artists in the modern era. Dr. Nabais and her colleagues read primary sources from the Middle Ages up to the 19th century CE to learn how medieval artists identified the plant (Chrozophora tinctoria) and extracted its water-soluble blue color (including some tricks and tips that may have stymied modern researchers trying to recreate the historical pigment). They then used state-of-the-art spectroscopic techniques and computer modeling to elucidate the exact chemical structure of the pigment extracted from the plants and to confirm its absorption spectrum. The pigment thus described was new to science, and was given the name chrozophoridin by Dr. Nabais and her coauthors. This is very exciting, and may lead to this ancient color returning to modern works of art.
Organic (and organometallic) pigments differ from the inorganic pigments because of the presence of organic carbon. Carbon has several chemical forms: carbonate, graphite, diamond, and organic. Organic carbon’s particular chemical properties and reactivity have made it the basis for all biochemicals (at least on this planet). Carbon becomes organic by way of photosynthesis, in which carbon dioxide (a non-ionic inorganic form of carbon) is reduced (meaning, electrons are added), and incorporated into (at first) simple sugars, which then go on in metabolism to make carbohydrates, proteins, lipids, nucleic acids – all the useful biochemicals. Most biochemicals are, however, largely colorless, particularly in solution; think of egg whites (proteins) or sugar syrup (carbohydrates). They absorb ultraviolet light, but we can’t see this (sometimes, if the absorption spectrum is strong enough, these look somewhat yellow, as the absorption peak spills over into visible light). However, some biochemicals, including some that are in coordination with metal ions, have chemical structures that result in absorption peaks in the visible wavelength range, which means they have color. These compounds, or the portions of the compounds that are responsible for light absorption (if it’s a large compound) are frequently called chromophores.
What gives chromophores their optical properties is a particular chemical structure: alternating single and double bonds (often called conjugated bonds) between carbon atoms. This structure creates a ‘cloud’ of electrons with energy levels that can be similar to visible light photons (and thus result in absorption of the photon). The location and shape of the resulting absorption peaks depends on the number of conjugated bonds, and the shape of the structure itself. Orange and yellow carotenes, for example, are long lines of conjugated bonds, with some stuff at either end. Rings are frequently found in these structures. In The Science of Pigments we talked about the porphyrin rings that bind up metal ions to make chlorophyll or heme (found in hemoglobin or cytochromes). Open up this ring, remove the metal ion, and you get a bilin, which is a chromophore found in blue-green algae or, yes, bile. Eww.
An important chemical structure that is often part of chromophores is a ring of six carbon atoms, with alternating single and double bonds that make what is called an aromatic structure. Aromatic structures are very important biochemicals because they not only occur in strong-smelling compounds (thus the name), but they are also very chemically and physically stable. They resist chemical and physical attack quite well. Hold that thought, because one of the main factors that regulate whether a biological pigment is useful for making artistic pigment is its stability. We don’t use chlorophyll a as a painting pigment in part because if you expose it to acidic conditions or even just look at it funny the chromophore will give up its magnesium ion and turn from a lush blue-green color to a dispiriting gray in an instant. And the brilliant colors of anthocyanins of flowers (and purple carrots) can fade quickly (how disappointing it is to cook a purple carrot and see that color lost – in this case the orange or yellow carotenes are far more stable than the purple anthocyanins).
But for a moment let’s consider the role of pigments in organisms. In some cases, but not necessarily all, the color of the pigment plays some sort of crucial role in the life of the organism; sometimes the color is incidental. But, the electronic arrangements that make them able to absorb light makes them uniquely useful as biochemicals. Pigments, such as chlorophylls, absorb light energy for the purpose of photosynthesis; also, pigments can block excess light from damaging cells, like the yellow lutein or flavonoids found in land plants, or melanin in some mammals. Pigments, such as cytochromes (heme containing) are involved in electron transport chains in everything from bacteria on up, used in photosynthesis and respiration to generate chemical energy for cells. Lots of vitamins contain chromophores – and in fact the important role of translating light flux into nerve impules in the eye is played by a carotenoid derivative, retinol. Red colored heme is also found in hemoglobin, where it is used to transport oxygen through blood in many organisms. And, importantly, organisms use pigments as color for recognition purposes (colors and patterns so members of the same species can recognize each other so as to avoid mating … accidents, warning coloration for poisonous organisms), and for stealth. The bright colors of flowers, often from a diverse group of organic compounds called anthocyanins, help their pollinators recognize them (many bees can see into the ultraviolet so some flower pigments have features invisible to our eyes but stand out quite obviously to the right bee). Unfortunately, despite the wide range of colors all across the spectrum that can be produced by organic pigments, not all are suitable for making paints or dyes. The main reason for this is stability. Many of these biochemicals are evolved to be reactive: biochemical processes in the cells transform them into something else, but they can subsequently be recycled by other biochemical processes. Away from the cellular machinery that maintains them, organic pigments can break down, fall into disrepute, become fugitive.
One example of a reputable, upright, good citizen amongst organic pigments is the familiar blue of indigo. Indigo has a chemical structure that contains not one but two of those aromatic rings, in part leading to its resistance to degradation by enzymes and ultraviolet radiation. The exact role of indigo and its precursors in the cell is obscure, but it is derived from the amino acid tryptophan (which does not make you sleepy, and isn’t particularly abundant in turkey, but contains an aromatic ring, and is a component of most proteins and enzymes in all cells). Indigo is also widely distributed, occurring in multiple plant species around the world. The same pigment is found in ‘true indigo’ in the Indian subcontinent, woad in Europe, anil in America, and dyer’s knotweed in Asia. Amazingly enough the important historical color Tyrian purple contains essentially the same chemical compound as does indigo and woad, but its source is the shell of a marine snail.
Indigo requires some chemical gymnastics to properly implement as a dye or pigment (the topic of a future article?) but once extracted and properly prepared, provides a stable and beautiful color.
Another important historical color that is known to be less of a good pigment citizen is madder. Madder is a medium sized (up to 4′) plant whose roots contain a couple different closely related anthoquinones (not terribly different from the quinine in your gin and tonic, cheers), alizarin and purpurin. These can function as antioxidants and antimicrobials in the plant, and incidentally happen to have crimson and purple colors that were much prized in the middle ages for dyeing. Madder is known to be rather fugitive as a pigment for paints (mean it breaks down over time as a result of oxidation by oxygen or sunlight), and has been largely replaced by synthetic dyes. However, madder gives us a good transition to the end of this article, as madder is one of the earliest examples of pigments that were known to have been chemically altered by people to make their properties more useful. Next time we’ll talk about ancient and modern methods of altering natural products to better suit our purposes.
Thank you for joining us as we do Pandemic Projects, meant to keep you energized, curious and learning!
Last time we talked about the physics and chemistry of pigments, both natural and synthetic, and how they come to have the colors that we value. This time we’re going to discuss natural earth and mineral pigments, their origins, and their mineral and chemical composition.
Natural earth pigments are, according to archaeologists, some of the earliest pigments used for art by humans. The earliest paintings known included paints made from earth pigments, and charcoal. Earth pigments are, in their simplest form, soil that lacks organic matter, composed of mixtures of minerals, some of which confer color to the mixture. Soil can be classified by its mineral composition and its particle size, with clay being much smaller than (for example) sand particles. Clays are generally formed in river beds, with the action of flowing water grinding the mineral particles down to tiny sizes. But massive clay deposits, relics of ancient water bodies, are found in rocks on dry land as well. The earliest artists used naturally occuring clay of varying colors as the basis for their pigments, as the fine grains of clay are best for making smooth, regular pigment powders that can be mixed with binders or dispersants to make paints.
The bulk of the material in earth pigments is not the actual mineral that confers the color. The deposits of colorful clay that ancient and modern artists use come from the erosion of rocks into tiny particles, and reflect the origin and chemical composition of the original rocks. For the most part, these are mostly varying degrees of black or white or even crystalline and clear. Take a look at your beach sand. If your beach sand comes from the erosion of mixed continental rocks, you will see individual bits of clear quartz, or black or gray bits of other kinds of rock, mixed in with a few brightly colored grains of minerals. If your beach sand comes from fresh(ish) lava deposits, it could be very black. If your sand comes from coral reefs or limestone, it may be bright white. The famous pink sand beaches in Bermuda are mostly pure white grains of calcium carbonate from coral reefs, with a few bright red shells of a particular foraminiferan (a microorganism that makes a red calcium carbonate shell). If you were to clean and dry and grind Bermuda sand to the consistency of clay, you would have a pale pink pigment powder. Most of the naturally occurring earth pigments are similar sorts of mixtures; a ground of silicate-containing mineral rocks of no particular color, with bright mineral additives. Some synthetic pigments are made this way; a colorless natural earth is doped with a brightly colored synthetic mineral, so that it behaves similarly to a natural earth when used in pigments.
The most common and robust colors in natural earths are of course the reds and yellows and browns. These colors almost all come from various forms of iron oxide, complexed with water and oxygen in various ways that result in different shades, and mixed of course with the background material. Iron is quite abundant in the universe, as it is the heaviest element that can be made by the nuclear reactions of a normal star. To make larger elements requires a supernova explosion, and those are rather rare. So it is no surprise to see the familiar colors of rust and earth pigments in the canyons of Mars, the ice sheets of Europa, and the craggy surface of distant Pluto. Metallic iron, combined with water, leads to the familiar rust, which is one kind of iron oxide. Other iron oxides are formed through geological processes, involving heat or pressure or exposure to oxidixing chemicals other than water.
The mineral that gives yellow ocher its color is called limonite (not surprisingly). It is a hydrated iron oxide, which means the iron oxide (plain old rust) is complexed with water in a more or less permanent way that changes the electronic structure of the metal to change its absorption spectrum. The red in natural pigments often comes from hematite, which is an anhydrous (lacking water) iron oxide. Some hematites look purple (as in the sand at Pfeiffer Beach, in Big Sur), but it is the same mineral, and a feature of the grain size causes the red to have a purplish tone. Grinding the sand down to clay size may, sadly, eliminate the purple. Green colors in some natural earths come from iron in a different oxidation state, complexed with magnesium and aluminum, in minerals like celedonite (also, not surprisingly named).
Historically, natural clays found in the landscape were used by artists for pigments, giving each their local flavor. When trade began moving materials long distances across the Earth, pigments went with them. Desirable colors found in deposits in particular locations the world have become standards. For example, clay deposits found near the city of Siena or the region of Umbria in Italy have given their names to the colors that derive from their pigment powders. Sienna and Umber pigments have more manganese oxides than ocher pigments, leading to darker colors. Heating sienna and umber pigments causes some of the iron oxides to dehydrate, further darkening their color and leading to burnt sienna or burnt umber.
Naturally occuring colored clay is not the only way to obtain a natural mineral pigment, however. Certain minerals that occur in interesting colors and in large enough deposits can be ground finely to make a pigment directly. These minerals generally originate from infrequent and inconveniently located geological processes, deep in the earth. Making them available for use on the surface requires uplift of the regions where they occurred, and erosion to expose them, and perhaps even mining into the earth to find them. Frequently these situations occur in the high mountains of the world, where uplift and erosion can be ongoing processes in the present.
The most precious pigment used in Europe in the ancient world and the middle ages – more expensive than gold! – was ultramarine, made from finely-ground bits of the semiprecious stone lapis lazuli. Lapis lazuli is a metamorphic rock (meaning, transformed by temperatures or pressures deep in the earth) found in several places around the world, but most importantly in the mountains of Afghanistan. Lapis lazuli from Afghanistan traveled very far in the ancient and prehistoric world, with ancient examples of lapis beads found in west Africa, and famously on the coffin of the pharoah Tutankhamen. The mineral that gives lapis lazuli its lovely blue color is called lazurite. The rock itself is often found with inclusions of pyrite (a shiny, gold or brassy iron mineral). The blue itself comes from a particular ionic state of sulfur. Burning sulfur in an oxygen atmosphere can produce blue flames (don’t do this). Another place you can see blue sulfur is erupting from the famous sulfur volcanoes of Jupiter’s moon Io. Iozurite, a blue pigment similar to lapis lazuli, may be a very precious AEP product of the far future.
As you can imagine, from ancient times people have ground up interesting colored rocks and minerals to try and make pigments (at this point I should caution you not to grind up the gorgeous blue-green serpentine rocks of central California as this could be hazardous; the fibrous minerals are similar to asbestos). The ancients were familiar with the bright green mineral malachite as a copper ore: heating malachite over a hot fire causes it to break down and release the copper metal. But they also ground malachite to make a green pigment. Malachite is found in the deposits of ancient limestone caves, where mineral rich water ran over the calcium carbonate, producing reactions that precipitated the mineral into a solid. Deposits of malachite in the Fertile Crescent supplied the ancient world with copper for making bronze, but malachite is found in many places around the world. Malachite sometimes occurs alongside azurite, also a copper mineral, which is a bright blue. Azurite was also used as a pigment (like ultramarine, it is rare and expensive). Turquoise is another mineral rock whose blue color comes from copper, and whose formation involves the weathering of rocks (igneous, rather than carbonate). Deposits of turquoise are found in many places and it is one of the most important gemstones worldwide. Turquoise can be ground to make pigment, but it is more precious than malachite so this was rare.
Of course, artists have never been limited in the colors they use by what they can dig up from the ground. Next time we’ll talk more about organic pigments, derived from biological materials, and their origins.
Thank you for joining us as we do Pandemic Projects, meant to keep you energized, curious and learning!