Tag Archives: chemicals

The impermanence of color: the treachery of entropy

Color seems like an easy, marvelous thing when you get that 64 color box of Crayolas as a kid. 64 sticks of pure color. But, of course, color is complicated. It can be impermanent, difficult to obtain, and toxic. To understand the life and chemistry of colors is to peek under the hood. It’s not what catches your eye, but it’s the heart of the drama.

Many paintings are known to be fading; it’s the newer paintings that draw the most concern. To some extent, the older paintings had probably already faded, but the older paintings also used old tried-and-true methods. The works of Jan van Eyck (1390-1441) are considered to be about as colorful today as when they were painted. Van Gogh’s daisies are fading. Renoir’s red has been digitally re-envisioned to show its pre-faded look.

The 1800s were a boom time of chemistry and industrialization, and the art world participated in this expansion too. 12 elements, including sodium and potassium, were discovered between 1800 and 1810. As Chemistry exploded, and new colors exploded. Mauve, the first synthetic dye, was produced in 1856 from coal tar. Renaissance painters (or their apprentices) prepared their own dyes and pigments (think of those scenes from “The Girl with the Pearl Earring” where Scarlett Johansson is grinding various things); 19th century painters bought paint from chemists. Like the disintegrating trade paperbacks of the early 1900s, when industrialization took over an old process, it was faster and cheaper, but took a while to match other characteristics. Books from the early 1800s are often far more intact than the wood-pulp books that followed them.

Artists like Van Gogh knew the strengths and limitations of their new tools. Van Gogh wrote to his brother, noting that the Impressionist paints “fade like flowers,” so he used the brightest colors he could, doing what he could to counteract impermanence. Even now, not all paints are equally durable; here’s a table for watercolors including such measures.

Today, scientists are studying the precise chemistry that causes fading using X-rays. So far, nothing can be done to stop or reverse the fading; they can only be kept away from light. At least we have the tools to imagine their former glory.

Further reading: Victoria Finlay’s Color is a great read on the chemistry of color without diving too deep technically. I reviewed it on this blog a couple of years ago. This article about the history of oil colors is also really fascinating. And finally, if you’re a chemistry buff, the scientific article about Van Gogh’s fading yellow is open source, and available to the public here.

 

 

Food and science: Why are peppers hot?

Chili peppers, such as jalapeños and serranos and habañeros, are hot because they contain a chemical called capsaicin, which is an irritant to humans.

Why capsaicin?

If it is advantageous to plants to spread their seeds, why do the fruits (peppers) contain a chemical that repels animals? It turns out that birds are not sensitive to the effects of capsaicin. Thus, capsaicin repels animals whose chewing action may destroy the seed while not repelling birds. Additionally, capsaicin may function as a antifungal.

What does capsaicin do, chemically?

Capsaicin binds to receptors for heat and pain called vanilloid receptors. Capsaicin causes the receptor’s neuron to fire, which normally occurs at higher temperatures; thus, the brain interprets this neuron signal as sensing heat.

Capsaicin is just one kind of vanilloid compound; vanilla is another member of the vanilloid family, with a similar structure, but it does not act on vanilloid receptors. Now I wonder if peppers taste at all like vanilla to birds. A mystery for the ages.

Capsaicin in peppers

As you may have noticed, some peppers are hotter than others. This is because some contain more capsaicin and related chemicals than others. The relative hotness of peppers is measured by the Scoville scale. According to this scale, habañeros are a few times hotter than ají peppers which are a few times hotter than chipotle peppers. Bell peppers actually don’t contain capsaicin due to a recessive trait.

Contrary to what I learned growing up, the seeds don’t contain capsaicin. However, the pith that surrounds the seeds has the highest concentration. This may be why the hottest peppers look like wrinkly sphinx cats; they are just packed with pith (such as the Carolina Reaper pepper, bottom). You can reduce the hotness of a pepper for cooking by preferentially removing the white spines, which is the pith.

Habañero peppers have a Scoville rating of roughly 500,000. Pure capsaicin has a rating of 16 million. Some chemical called resiniferatoxin found in Moroccan cacti has a rating of 16 billion (1,000 times higher!). 40 grams of the stuff can kill a person.

So, there’s your overview of peppers for this Friday. Don’t consider this a challenge to go find that Moroccan cactus, but when your lips tingle after eating salsa, perhaps think of capsaicin.

Carolina reaper peppers, one of the hottest according to the Scoville Scale. Just look at the wrinkles! (Wikipedia)

Fun Science: Crystals Everywhere!

I went on a trip to DC last fall. Almost accidentally I ended up in the Natural History Smithsonian Museum. Wow! Especially worthy is the section on minerals. I assume there are other museums with such displays, but I hadn’t been to one. The Hope diamond is displayed also in the minerals section, but fancy jewels I can’t touch are way less interesting than all the minerals and natural crystals.

I find crystals fascinating because they tell you so much about the microscopic structure of the material. Where else in life can you just look at an object and see what it does down to the nanometer? So naturally the camera came out. Below are a few favorites, and some comments about what we can infer from the pictures.

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Fluorite (CaF2): As you can see, Fluorite has a cubic crystalline structure. Fluorite can come in basically any color. Color can be due to impurities, exposure to radiation, or defects in the crystalline structure. Fluorite was originally so named due to fluorescent properties; fluorite can fluoresce in a variety of colors depending upon the impurities present.

IMG_2139Beryl (Be3Al2(SiO3)6): You might be more familiar with other names for Beryl, such as aquamarine or emerald or morganite. Beryl is naturally clear, but takes on color in the presence of impurities. Emerald, for example, has chromium or vanadium present. Aquamarine coloration results when the Fe2+ oxidation state is present. Fe3+ results in yellow coloration. You can see in the image above that beryl has a hexagonal crystal structure. You can also see that this is one big hexagonal crystal, unlike the population of cubes in the fluorite picture. This tells us a lot about how the crystal grew. If the crystal grew very fast, there would be a number of columns, because crystallization would be faster than the time for the mineral components to diffuse to one specific column. So this crystal grew pretty slowly.

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Wulfenite (PbMoO4): Wulfenite is often found around lead deposits, which makes sense since it contains lead. It has a tetragonal crystal structure, and tends to be yellow or orange or brown in color. You can see that the crystals are much smaller in this picture than the beryl crystal. Clearly these crystals grew quickly from many nucleation sites. The size to which crystals tend to grow is a property of the crystal too; some only form a ton of small crystals, some form a few very large ones. It depends whether it is lower energy to just form another crystal, or if it is lower energy to allow diffusion to an already established crystal. This is related to thermodynamics. Wulfenite seems to favor lots of small crystals. Some wulfenite has a really cool property called piezoelectricity; when there is the right kind of pressure on the crystal, an electric charge accumulates.

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Manganese dioxide (MnO2): This manganese dioxide has grown in a dendritic fashion. It might look like frost or snowflakes, which grow in similar ways. These dendrites are very fractal, a favorite topic of mine. Here diffusion was definitely limited, so crystals grew where the materials were present.

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Chalcedony (SiO2): Chalcedony is a type of silicon dioxide, which is the chemical composition of most sand. Chalcedony is composed of two different silicon dioxide minerals: quartz and moganite. Quartz and moganite have different crystalline structures which grow together at a fine scale in chalcedony, which is probably why it looks far less geometric than the other crystals I’ve shown. Agate is a type of chalcedony.