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Autumn Color Chemistry

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It’s Fall! Time to break out those comfy sweaters. Time to hold a mug of warm cocoa in two hands and watch yellow tinted leaves slowly topple to the ground. Perhaps some of us are looking at the rich textures and thinking “I wonder if I could get a great color out of that leaf?”

In the Preservation Research and Testing Division (PRTD) we often look at natural sources, like plants, leaves, roots, and flowers, since these have been used as color sources for centuries. A significant component of PRTD’s analytical research relates to the identification of colorants, sometimes centuries or hundreds of years old. Understanding these colors, where they come from and how they change over time, is an exciting and challenging area of research. Read more about our bug crushing antics and reference colorant samples for a look at how re-creating historic pigments are central to investigating the Library’s collections.

Digital photograph of red, orange, and yellow pigment powders.
A digital photograph of red, orange, and yellow pigment powders. Photo Credit: PRTD

Let’s take a closer look at a few favorite fall time colors.

Photograph of cochineal used in an object and a microscope image of cochineal pigment particles.
Cropped photograph of one of the Huexotzinco paintings (top) and a microscope image of the red pigment. (bottom). Photo Credit: Betsy Haude and Tana Villafana

If there is one color that reminds me of autumn, it’s red and I can’t think about red without thinking cochineal. Thought to be one of the first cultivated crops, cochineal is a bug native to South America. When dried and crushed, this cactus lice releases carminic acid, a vibrant red that truly revolutionized the use of the color red across the world. Cochineal became one of the first materials exported back to Europe after the Spanish invasion of Mexico in the 1520’s in quantities similar in amount to gold and silver.

As an organic red dye, cochineal has many interesting chemical properties. Once the bug has been crushed, the ‘leftovers’ are typically washed and filtered. As a free dye (what comes straight from the bug), carminic acid is sensitive to changes in pH, which means by controlling how acidic or basic the wash is, you can control the final color! Cochineal, though most widely known for being red could also be a deep purple (from basic conditions, such as ammonia) or even orange (acidic conditions, like vinegar). Typically, during the extraction process, a metallic salt is added (such as alum), which creates a coordination complex with the dye. A dye-metal complex is typically more stable than the free dye and can either be dried to make pigment powders or fixed onto textiles for longer lasting colors.  Once fixed in this way, cochineal is relatively stable (resistant to fading). This red dye is still widely used today, not only as a beautiful red artist material, but also as a red colorant in foods and cosmetics.

You want an orange as bright as the pumpkin on your porch? Let me introduce you to the pigment red lead (and yes, it is orange). Minium is a mineral that occurs naturally in the Earth’s crust in lead mineral deposits throughout the world in the form of lead tetraoxide. Although minium is found in nature, it is thought to be one of the first manufactured pigments, with the synthetic version being termed red lead. Red lead can be created by roasting lead white in air over a fire, gradually turning from white to yellow to orange; this process has taken place since before the 5th century BCE and involves changing a lead hydroxide into different forms of lead oxide. Care must be taken not to apply too much heat for too long, else the orange will turn white again.

Photograph of red lead in an object and a microscope image of red lead pigment particles.
Cropped photograph from the Art of Memory (left) and a microscope image of red lead particles (right). Photo Credit: Meghan Wilson and Tana Villafana

Much like the falling leaves, red lead isn’t immune to changes over time. Lead oxides can be fairly chemically reactive, and heat isn’t the only thing that can cause chemical reactions resulting in color changes. Red lead is known to darken to brown or black when exposed to high humidity, used as a watercolor, or with an egg tempera binder. Lead sulfide, a black compound, is formed when red lead is exposed to sulfur, an element that can occur naturally in air, water, and egg yolk. These are the type of changes that can cause issues during the analytical analysis of collection items; we have to be on the look-out for signs of degradation caused color changes. However, the distinctive color of red lead makes it generally easy to spot for proper care and can be found in many medieval writings (including this Hellmouth).

Photograph of bark in a vacuum extraction, extraction yellow dye.
Photograph of a yellow dye being extracted from tree bark using a vacuum filtration system. Photo Credit: Gwenanne Edwards

Now to yellows, like those gorgeous autumnal leaves on a sugar maple tree. There are hundreds, if not more, natural sources for organic yellow dyes. From tree resins to bark, roots to flowers, and more, it seems like yellow dyes can be extracted from almost anywhere. Turmeric is a perfect fall yellow; it’s the warm, earthy spice in your golden latte and could be used to make that soft yellow scarf. In use as a spice, a pigment, and medicinally for thousands of years, turmeric is derived from a root vegetable similar in appearance to ginger, native to India.

As a spice, the root is cut and ground, creating a yellowish powder used to flavor curries, beverages, and as a food coloring for mustards and cheeses. The main coloring component of ground turmeric is a compound called curcumin, which can be further extracted from turmeric powder and concentrated into a strong dye. Curcumin is not very soluble (meaning it does not dissolve completely) in water but will dissolve in alcohols. The trick with maintaining a bright yellow color with turmeric dyes is to keep the extraction wash neutral (most alcohols have a nearly neutral pH, similar to water). Dying fabric becomes as simple as putting your cloth in a pot with turmeric powder, water, and alcohol then simmering. For a bright yellow, cotton is preferred, and turmeric is known to hold well to cotton without any mordants. Similar to cochineal, mordants can be added to create a shelf stable pigment and is frequently added to textiles to help fibers hold onto dyes easier. With turmeric, common mordants such as alum, tin, or copper causes shades from orange-yellow to brown and using iron will create a greenish hue. Turmeric’s pH sensitivity has actually been utilized as an indicator and turmeric dyed papers were used as litmus tests; when the paper came into contact with acid it would turn bright red.

Sometimes colors that appear blue or red today used to be green or orange! This is because the colors used were a mixture, and the yellow in the mixtures have faded, leaving behind only one color. Organic yellows are especially difficult to identify because of this poor lightfastness, which is where PRTD’s scientific reference samples become useful. We’ve created an array of organic yellow samples in different binding media for testing and comparison to collection items. We can age or purposely fade the samples in order to investigate how to identify lost yellows and to study degradation processes that happen in general to historic coloring materials.

Cropped photographs of a faded Japanese print, one in white light and the other under ultraviolet light.
Visible light photograph of a faded Japanese print (left) and the same print under ultraviolet light (right) showing areas where faded yellow pigment is glowing. Photo Credit: Gwenanne Edwards

While we watch the trees changes, remember that leaves aren’t the only thing that can change colors. Artist’s pigments and materials weren’t meant to last forever, but the Preservation Directorate of the Library of Congress works together to keep the colors of the collection alive and vibrant.

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