Color does not exist. It’s all in your head. Or as Kurt Nassau so elegantly states in The Physics and Chemistry of Color, “If one is not confused by color mixing, then one does not really understand it!”

In the field of paint and coatings, I interact with artists and conservators on a daily basis, and our conversations often address how to achieve a specific color and/or how to make colors last longer. The latter question is usually worded as, “Will it be okay over time?” In this article, I will speak to color on the basic molecular level, its stability, and how the viewer interacts with energy to create the lush world we see. In particular I will focus on how we create and perceive color in handmade paper. Let’s begin by looking at color stability.

Scientifically, color is a very unusual and fascinating phenomenon because it is a visual interpretation of a narrow band of energy. In figure 1, we see that light comes into the eye, hits a receptor containing a protein sensitive to a wavelength range, and is converted to an electrical impulse that moves down the optical nerve into the brain, where the color is actually created. This means that color is created by the viewer, and relates to the age-old question: Do we see the same colors as others? On that, we are still unsure, but we do receive the same vibrations.

Energy is the “fuel” for color. Without a light source color cannot exist. To better understand the color perception of our eyes, try this fun experiment: get dressed in very low lighting such as the twilight of the morning. At first, you will only see in black and white so picking out clothing can lead to some very wild combinations! As the day breaks, observe how much light is required before you see color. It is surprising how much is actually required. In low light the black and white sensors in our eyes lump all wavelengths into an either “on” (white) or “off” (black) signal. The portions of our eyes that pick up individual wavelengths require substantially more energy to see color.

The electromagnetic spectrum, as pictured in figure 2, runs from very short wavelength energies, like cosmic rays and/or X-rays that can pass through matter, to longer wavelength energies, such as radio waves. There is a narrow band of energy between ultraviolet energy and infrared energy that our eyes have evolved to convert to color. Many animals have less color perception than we do, and some have far more (e.g., the mantis shrimp). The band of energy most humans can see, with a dramatic drop-off in sensitivity on either side of the spectrum, runs from wavelengths around 380 nanometers to 750 nanometers. Energies with slightly shorter wavelengths than the visual spectrum tend to break apart matter (i.e., UV degradation) and energies with moderately longer wavelengths tend to heat things up (i.e., infrared or microwaves). The visual band conveniently excites many molecules to a non-destructive resonance state. This excitation by a light source is mostly converted to heat, but some structures called “chromophores” emit some of the energy back out in the visible spectrum, and this is the light that we convert into color in our minds.

As shown in figure 3, color can be mixed in an additive process with light or in a subtractive process with pigments and dyes. When we add together all the narrow wavelengths of light, we combine energy, making white. When we mix together pigments, we lose energy because each pigment absorbs most of the energy and converts it to heat. When all pigments are added together, we make black.

In the realm of subtractive color mixing (with pigments and dyes), there are two main classes of materials that exhibit chromophoric structures: inorganic molecules (made of rocks or minerals) and organic molecules (made of carbon-based chemistry).

Inorganic colors are made out of rocks and minerals, everything from mud to lapis lazuli. To describe the color production of this class of colorants in a wildly oversimplified way: Inorganic colors are basically crystal structures where minute amounts of impurities allow the crystal to have an excited state; see figure 4. That is to say that when a photon of light hits the crystal, it rattles around the electrons at a resonance rate, emitting a pattern of energy back out.

Organic colors are made of carbon-based chemistry, encompassing all the natural colors of plants and animals. Some examples are Alizarin Crimson PR83 or walnut ink as well as synthetic pigments made in a lab such as Quinacridone Magenta PR122 or dyes such as Synthetic Indigo FD&C Dye #2. In these molecules, the chromophores are resonant structures that juggle energy at a certain rate. To illustrate this, let’s look at beta carotene, in figure 5, as we have all experienced (or made up) the color of a carrot. In the diagram, do not be scared by the shape of the molecule; just notice the double lines (points of desaturation) and how the light causes them to flip. Then, the molecule is excited, having unbalanced charges (grumpy energetic state), and releases this energy back out to our eyes as a visible vibration.

Regarding paper, artists and conservators frequently look to either treat previously discolored paper or create paper that will not change over time. Treating discoloration is very situational and a full discussion is beyond the scope of this article. For interest, though, I will share one of the preferred practices of the paper conservation world. Often when stains are removed from old papers, there is a halo around the treatment where the paper looks dramatically lighter than untreated areas. Conservators collect paper samples (most commonly blank business ledgers) that were produced at a similar time and composition as the objects they are treating. They shred them and boil them down in water until a syrup is formed, and strain off the pulp. The resulting material, which they call “paper dirt,” is diluted down with alcohol or other solvents and used like watercolor to tone surfaces.

Paper dirt is made up of soluble ingredients such as lignin, tannin, and a vast array of other possible constituents of the historical paper that would be considered dyes. Dyes, as a single molecule or few clustered molecules, are very sensitive to degradation from a wide range of causes such as heat, pH, oxidation, and UV exposure. In the carrot molecule we examined earlier, if any one of the double bonds is broken, the color goes away. For example, if clothing were stained with carrot dye (juice) and left in the sun it would change to a brown vibration before slowly disappearing. This is the same mechanism that laundry bleach uses to remove stains.

One of the main distinctions between dyes and pigments is whether or not the material is soluble in the solvent used for delivery. This is kind of like how sugar is soluble in water while sand is not. Pigments are insoluble crystallites (made of identical repeating units) that are larger in size than dyes. The larger size of the pigment allows for absorbed energy (the colors from the white light that are not reflected back to our eyes) to rattle around and convert to heat before breaking chemical bonds. A good analogy is if a pigment is a boulder, then a dye is pea gravel. We can think of UV energy as a hammer. When the hammer hits a boulder, there is minimal damage, if any. When the same hammer hits fine gravel, fractures occur and dust is formed, causing the material (or color) to deteriorate.

Made up of photons, UV energy (a.k.a. the hammer) has the energy equivalent of 315–400 kJ/mol, meaning it has enough energy to break many chemical bonds. Breaking these bonds leads to free radical oxygen that goes on to break more bonds. Having a larger color source, such as a pigment instead of a dye, means that it will take much longer to see damage, or if the bonds are strong enough, UV energy will not cause any damage.

This lengthy sidenote gives us some background to understand how to make color-stable papers. It really comes down to using pigments instead of dyes. This could be as simple as adding dry pigments or pigment dispersions into the pulp slurry, or post-treating the paper surface with a dilute application of a pigmented paint, commonly achieved by dipping, spraying, or painting.

ASTM (American Society for Testing and Materials) has a number of standards regarding pigments in artist materials that one can refer to (such as D4302, D5067, D5098). There are some great online resources such as artiscreation.com or handprint.com that pull together pigment listings so that a papermaker can determine the stability of one pigment over another. Also, justpaint.org from Golden Artist Colors (where I work) offers lots of information on lightfastness in addition to many other interesting articles.

Some pigments have sensitivity to acids or bases, so checking the pH of your system and making a small test before progressing with larger projects is highly recommended.

Pigments (with some exceptions) have little to no hydrogen bonding to the pendant hydroxyl groups on cellulose chains (these are the connected Oxygen and Hydrogen atoms sticking out all over the fiber chain that allows paper to hold together when dry). So instead, a papermaker is relying on the entanglement of the pulp to capture the pigment particles and the translucency of the fiber in front of the pigment to allow for the reflected energy (color) to be observed.

To create a very rich, deep tone, the papermaker can add retention aid (cationic polymeric binder) to hold the loose pigment. This works because a cation is a positively charged molecule (meaning that it desires more electrons) so it attracts electrons from neutral or negatively charged species (anions) such as cellulose fiber or pigment particle. One side of the cationic polymer interacts with the pulp and the other side draws pigment to it strongly. One can think of molecular charges behaving like the positive and negative sides of metal magnets. Opposite charges attract and like charges repel, and both the positive and the negative side of a magnet can pick up iron filings (neutral charge). One could also use acrylic mediums, for instance matte medium, for this purpose as they will surround the pigment and then bond to the fibers in the paper. Note that adding a polymer reduces the absorbency of the paper.

Here are two simple ways to test if the pigment is overloaded in the pulp: wipe the dry paper surface with a white cloth and see if any color rubs off; or quickly dip the paper in water to see if the water picks up color.

1. Kurt Nassau, The Physics and Chemistry of Color (Wiley-Interscience, 2001 second edition), 8.

2. Three excellent, general reference books on color are: Gary Waldman, Introduction to Light: The Physics of Light, Vision (Dover, 2002 revised edition); Michael Mark Woolfson, Colour: How We See It and How We Use It (World Scientific, 2016); Kurt Nassau, The Physics and Chemistry of Color (Wiley-Interscience, 2001 second edition).
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3. There are many great resources regarding the topic of restoration such as the American Institute for Conservation (culturalheritage.org), Canadian Conservation Institute (CCI), https://www.canada.ca/en/conservation-institute/services/care-objects.html and the American Library Association, https://www.ala.org.

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