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Today I wanted to write more about giant clams and their astonishing ability to “farm” algae within their body (and then live off of the sweet sugars which the algae produce).  I still want to write about that, but it proving to be a complicated subject: giant clams mastered living on solar energy a long time ago, and we are still trying to figure out the full nature of their symbiotic systems.

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Today, instead we are going to look at the phenomenon which gives the mantles of giant clams their amazingly beautiful iridescent color. It is the same effect which provides the shimmering color of hummingbird feathers and blue morpho wings, or the glistening iridescence of cuttlefish.  All of these effects are quite different from pigmentation as generally conceived:  if you grind up a lapis lazuli in a pestle, the dust will be brilliant blue (you have made ultramarine!) but if you similarly grind up a peacock feather, the dust will be gray, alas! This is because the glistening reflective aqua-blue of the feather is caused by how microscopic lattices within the various surfaces react with light (or I suppose, I should really go ahead and call these lattices “nanostructure” since they exist at a scale much smaller than micrometers). These lattices are known as “photonic crystals” and they appear in various natural iridescent materials—opals, feathers, and scales.  Scientists have long studied these materials because of their amazing optic properties, however it is only since the 1990s that we have begun to truly understand and engineer similar structures on our own.

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Physicists from the 19th century onward have understood that these iridescent color-effects are caused by diffraction within the materials themselves, however actually engineering the materials (beyond merely reproducing similar effects with chemistry) was elusive because of the scales involved.  To shamelessly quote Wikipedia “The periodicity of the photonic crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm (blue) to ~650 nm (red) for photonic crystals that operate in the visible part of the spectrum.”  For comparison, a human hair is about 100,000 nanometers thick.

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The actual physics of photonic crystals are beyond my ability to elucidate (here is a link to a somewhat comprehensible lay explanation for you physicists out there), however, this article is more to let me explain at a sub-rudimentary level and to show a bunch of pictures of the lovely instances of photonic crystals in the natural world. Enjoy these pictures which I stole!

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But, in the mean time don’t forget about the photonic crystals! When we get back to talking about the symbiosis of the giant clams, we will also return to photonic crystals!  I have talked about how ecology is complicated.  Even a symbiotic organism made up of two constituent organisms makes use of nanostructures we are only beginning to comprehend (“we” meaning molecular engineers and materials physicists not necessarily we meaning all of us). imagine how complex it becomes when there are more than one sort of organism interacting in complex ways in the real world!

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Mauve

Mauve

In 1856 the 18 year-old chemist William Henry Perkin was desperately looking for a way to synthesize quinine–since the British Empire relied so heavily on the Peruvian bark as an antimalarial agent throughout its many tropical colonies.  The brilliant young chemist failed to find a replacement for quinine, but he instead found a brilliant purple-pink chemical “mauveine” the very first aniline dye (the toxic aromatic amines today serve as precursors to numerous industrial compounds).

Sir William Perkin (Arthur S. Cope, 1906, oil on canvas)

Sir William Perkin (Arthur S. Cope, 1906, oil on canvas)

Perkin’s discovery lead to a revolution in purple dyes which had historically been costly, rare, and fugitive.  Suddenly cheap synthetic purples were everywhere—particularly mauve, which was named for mauveine.   Perkins named his dye after the French word mauve (French for a particular sort of purple mallow flower).

Victorian style dress with unfaded purple dye

Victorian style dress with unfaded purple dye

Today we understand mauve to be a slightly blue-grayish shade of magenta, but the original usage may have been different.  Mauveine dyes fabrics to a brilliant glowing purple—initially—however the synthetic purples created from this dye are also fugitive.  The fabrics quickly faded and left succeeding generations with a somewhat attenuated color (which is what we thibnk of as mauve). Some of the pre-Raphaelites even painted whole canons of works which soon changed colors as the purples faded.

A contemporary mauve

A contemporary mauve

Many succeeding generations of new artificial dye have long since swept away mauveine (although Perkins became rich and was knighted for his teenage discovery).  We now have brighter purples which do not fade (like the quinacridones and diozanines in my paintbox). Whatever the virtues of the original color, mauve, as it is today understood, is a beautiful purple.

Windflowers (John William Waterhouse, 1902, oil on canvas)

Windflowers (John William Waterhouse, 1902, oil on canvas)

Egyptian Blue faience ushabtis at the Louvre

In ancient Egypt the sky was a gleaming blue, the sacred lotuses had blue petals, the pharaoh’s battle crown was blue, beautiful women wore chokers made of blue stone, and, above all, the life-giving Nile was blue.  The ancient Egyptians needed azure pigment to portray these essential elements of life within their sacred art, but the only natural blue pigments were from turquoise and lapis lazuli—semi-precious stones which were rare and expensive.  To provide a sufficient supply of blue pigment for painting, jewelry, and sculpture, the Egyptians therefore invented the first synthetic pigment which today is appropriately known as “Egyptian blue” (well, it is also appropriately known as calcium copper silicate–CaCuSi4O10 or CaO·CuO·4SiO2—but I’m going to keep calling it Egyptian blue).

Egyptian Blue Faience Votive Cup with Cartouche of Amenhotep III, c. 1391 – 1350 B.C.

Egyptian blue was synthesized in the 4th Dynasty (c.2575-2467 BC) when the newly created pigment was first used to color limestone sculptures, beads, and cylinder seals.  Its use became more prevalent in the Middle Kingdom, and then increased again during the New Kingdom when blue was used for the production of numerous everyday objects.  Throughout the Hellenic and Roman age, Egyptian blue was a mainstay of the nascent chemical industry, and it found its way into all sorts of art, jewelry, crafts, and artisan wares.  Then, in the fourth century the secret of its manufacture was lost.  Only in the beginning of the nineteenth century did interest revive as the English and French pioneers of the chemical trade rushed to synthesize useful compounds.  As one might surmise from the fact that the manufacturing process was lost for a millennium and a half, the method to make Egyptian blue is surprisingly involved.  Citing a British Museum publication, Wikipedia describes it thus:

Several experiments have been carried out by scientists and archaeologists interested in analyzing the composition of Egyptian blue and the techniques used to manufacture it. It is now generally regarded as a multi-phase material that was produced by heating together quartz sand, a copper compound, calcium carbonate, and a small amount of an alkali (plantash or natron) at temperatures ranging between 800–1000 °C (depending on the amount of alkali used) for several hours. The result is cuprorivaite or Egyptian blue, carbon dioxide and water vapor…

The Egyptians were clearly people who took their pigments seriously, and thankfully so–the blue tints they crafted have lasted for thousands of years (and helped us find our way to synthesized pigments).   It is strange to think of the subtle ways that the Nile still flows through our lives.

The mummy wrappings of Ankh Hor (21st or 22nd Dynasty, 1069-715 BC)

One of the Terracotta Soldiers from the tomb of Qin Shi Huang (ca. 200 BC)

In the spring of 1974 a group of farmers digging a well in Shaanxi China about one and a half kilometers (1 mile) north of Mount Li stumbled into an amazing find. A life-sized army of terracotta soldiers numbering over 8000 was entombed in the immense necropolis of Qin Shi Huang, the first emperor of China who lived from 259 BC – 210 BC.  However the story of the despotic Qin Shi Huang (one of history’s most remarkable figures) and his extravagant mausoleum will have to wait.  This post is not about the cruel emperor or his terracotta army, but is rather about the colors found on the terracotta figures, which were originally lacquered with a rainbow of bright colors– pink, red, green, blue, black, brown, white, and lilac.  One of the pigments discovered by archaeologists was Han purple, a manufactured pigment which was in use in China from about 1200 BC to 220 AD.  The secret to making Han purple was lost in antiquity and could not be rediscovered until modern spectroscopy helped chemists rediscover the materials used.

A Modern Reproduction of how the figures originally looked--although no two were painted the same (Credit: British Museum/C Roth)

Many scholars believe that Han purple was accidentally discovered by Taoist alchemists seeking to create synthetic jade.  The compound was a barium copper silicate which was fired for long periods of time at temperatures around 900-1000 °C.  The compound was probably produced in kilns north of the city of Xian (which was once known as Chang’an and was the capital city of China during the Zhou, Qin, Han, Sui, and Tang dynasties).

Modern Han Purple Pigment

Han Blue was a dark bluish purple/indigo.  It became more purplish over time as the barium copper silicate deteriorated and red copper oxides were formed.  The pigment was used for beads, ceramic vessels, paintings, and for octagonal pigment sticks (which may have had a ceremonial value in their own right).

Han purple had a very similar companion compound–Han blue—which was also a barium copper silicate.  Because of certain quirks of chemistry, Han blue was more lightfast than than Han purple and had fungicidal properties to boot.  This allowed Han blue to last for the long centuries, whereas Han purple is now known mostly from faded traces. Han purple was not fungicidal and compounds (namely oxalates) produced by certain long-lived lichen caused the pigment to turn into light blue powder.

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