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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 (Na8-10Al6Si6O24S2-4), 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.

A specimen of lapis lazuli

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

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.

Synthesis of Egyptian blue pigment

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

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 (CoCl2·6H2O). 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, Qing dynasty

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]


  1. Megan Fellman, "Who Knew There Was so Much to Blue? - Scientist studies blue's invention and reinvention throughout history," Northwestern University Press Release, November 5, 2014.
  2. The Natural History of Pliny, John Bostock and H.T. Riley, Trans., (H. G. Bohn: New York, 1857), vol. 6, chap. 57 (via Google Books).

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