Tikalon Blog by Dev Gualtieri

The Color, Blue

February 9, 2015

Because of my Italian ancestry, my veins are not that visible beneath my dark skin. However, fair-skinned people have visible veins, and that's apparently where we get the term, "blue blood," to denote aristocracy. Today, however, the skin of the average one-percenter has become somewhat darker, so "blue-blood" is destined to become an archaic term.

The color, blue, is used to signify a melancholy mood, and the word appears often in popular culture. The following is a list of a few popular songs with blue in their titles. I selected ones that are most familiar to me, and I played many of these in my short, pre-scientific, career as a top-40 DJ.

	Artist		Song
	Tony Bennett		Blue Velvet
	David Bowie		Blue Jean
	Neil Diamond		Song Sung Blue
	The Doors		Roadhouse Blues
	Bob Dylan		Subterranean Homesick Blues
	Bob Dylan		Tangled Up In Blue
	Johnny Cash		Folsom Prison Blues
	Crosby, Stills, & Nash		Suite: Judy Blue Eyes
	Fats Domino		My Blue Heaven
	Foreigner		Blue Morning, Blue Day
	Tommy James and the Shondells		Crystal Blue Persuasion
	The Marcels		Blue Moon
	Roy Orbison		Blue Bayou
	Carl Perkins		Blue Suede Shoes
	Elvis Presley		Blue Suede Shoes
	Linda Rondstat		Blue Bayou
	Bobby Vinton		Blue Velvet
	The Who		Behind Blue Eyes

One look through a window will confirm that half our visible world is blue, at least on the nicer days. Artists and artisans have added blue to their palette through the use of several natural pigments. The most historically famous of these is ultramarine (Na_8-10Al₆Si₆O₂₄S_2-4), first obtained by grinding the mineral, lapis lazuli. Lapis lazuli was imported into Renaissance Europe from Afghanistan, so ultramarine was expensive until its chemical synthesis in 1826.

Lapis lazuli was first mined in Afghanistan, circa 4500 B.C.[1]

(Northwestern University image.)

Azurite, another blue pigment, is chemically Cu₃(CO₃)₂(OH)₂. Azurite has been known since antiquity, and it's mentioned in Pliny the Elder's Natural History.[2] Our word, "cyan," comes from the Greek word for azurite, κυανος (kyanos). I've often mentioned Pliny's Natural History.

Azurite specimen in the Melbourne Museum.

(Photo by Graeme Churchard, cropped, via Wikimedia Commons.)

Egyptian blue is calcium copper silicate (CaCuSi₄O₁₀). Unlike both ultramarine and azurite, it doesn't exist as a mineral. This first synthetically-produced pigment, known by the Latin name, caeruleum, was produced in Egypt, circa 2500 B.C. It's formed by the high temperature reaction of malachite (copper carbonate hydroxide, Cu₂CO₃(OH)₂), quartz (SiO₂), and calcite (CaCO₃); viz.,

Cu₂CO₃(OH)₂ + 8SiO₂ + CaCO₃ -> CaCuSi₄O₁₀ + 3CO₂ + H₂O

Often, a small quantity of sodium carbonate was added to aid in the fusion of the reactants.

Laboratory synthesis of Egyptian blue pigment by firing sodium carbonate, quartz, malachite and calcite (right).

(Northwestern University image.)

As its chemical formula indicates, Prussian blue, Fe₇(CN)₁₈, is an inexpensive blue pigment. This chemical was the blue in the now rare blueprints (cyanotypes), and it was also one chemical used in laundry bluing before the current era of optical brighteners (a.k.a., whiteners). Prussian blue is widely used in dyeing, and in the manufacture of inks and other artist's colorants.

Mrs. Stewart's Bluing, first produced in 1883, was a fabric bluing agent made with Prussian blue.

I remember doing chemistry experiments with my mother's laundry bluing in the 1960s.

(Photo by Joe Mabel (modified), via Wikimedia Commons.)

One chemical stocked in my childhood chemistry set was cobalt (II) chloride (Co Cl₂·6H₂O). Today, this chemical would not be sold to children, since it's an allergen, but it has a blue color in its anhydrous form. The transition between its hydrous (red) and anhydrous (blue) forms allowed its use as an inexpensive humidity indicator. This one example illustrates the possibility that other cobalt (II) compounds would have a blue color.

And Lo! (I must be channeling Pliny the Elder, here), there are Cobalt blue, which is cobalt(II) aluminate, CoAl₂O₄, and also Cerulean blue, which is cobalt(II) stannate, Co₂SnO₄.

Cobalt blue dish, Europeans Playing Musical Instruments, Qing dynasty, Kangxi period (1661-1722).

(The Art Institute of Chicago, Bequest of Henry C. Schwab, via Northwestern University.)

Han blue, BaCuSi₄O₁₀, also called, Chinese blue, was used in China from about 1000 B.C. Han blue was only discovered in a mineral form, effenbergerite, in 1993. It has always been synthesized from chemical reactants; for example, a fusion of malachite ( copper carbonate hydroxide, quartz), and barium carbonate.

Cu₂CO₃(OH)₂ + 8SiO₂ + 2BaCO₃ -> 2BaCuSi₄O₁₀ + 3CO₂ + H₂O

You can see that this is the barium analog of the Egyptian blue reaction, using barium carbonate instead of calcium carbonate.

Marc Walton, a senior scientist at the Northwestern University, Art Institute of Chicago, Center for Scientific Studies in the Arts, has been intrigued by blue pigments for the past fifteen years.[1] It started when Walton, as a graduate student, found that the word, "blue," didn't appear until centuries after the first blue pigment, Egyptian blue, appeared.[1] Blue has a special place in pigments, since the color appears often in nature, but it doesn't appear that often in minerals.[1]

The Egyptians made extensive use of blue, but Roman art used very little blue; so, the art of manufacturing blue pigment was lost.[1] Around the 6th century A.D., only lapis lazuli was used as a blue pigment, but it was used sparingly because of its cost.[1] Azurite became a cheaper blue pigment, and it was generally used as a foundation layer for the more vibrant lapis lazuli pigment. In modern times, Prussian blue was used by Picasso, and synthetic chemistry has given us many blue pigment alternatives.[1]

The Color, Blue

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