In the first half of the semester, students in Chem 550L learned to synthetically produce copper phthalocyanine (phthalo blue), indigo, Prussian blue and cyanotypes, smalt, and YInMn blue. This resource provides background on those pigments and dyes and their synthesis, alongside photographs of lab experiments. All descriptions, notes, and diagrams of chemical reactions were provided by Dr. Jade Fostvedt, unless otherwise noted. The pigments and dyes on view were produced by students in Chem550L.
Researched and developed by Grace White, paper conservator at the Ackland Art Museum
Copper Phthalocyanine (Phthalo Blue)
The pigment copper phthalocyanine (also called phthalocyanine blue, phthalo blue, and Monastral Blue) was first synthesized in 1927 by accident. The reaction of copper(I) cyanide and o-dibromobenzene produced colorless phthalonitrile and an intensely blue side product later identified as phthalo blue. In 1937, DuPont started producing the pigment phthalo blue in America under the trade name Monastral Blue.
Phthalo Blue Lab Notes
The reaction below is called the phthalonitrile process. Students heated phthalonitrile with a copper chloride salt at a high temperature. They added copper(II) chloride to hold the phthalonitrile molecules in position so that they can react to form a ring. Water inhibits the reaction, so they used a water-free solvent.
Indigo
The word “indigo” comes from the Latin word indicum, meaning “Indian,” as the natural dye was first exported to Europe from India. Indigo dye can be obtained from several different plants, including indigofera tinctoria, often called “true indigo,” Japanese indigo, Chinese indigo, Natal indigo, Guatemalan indigo, woad, and the American native species indigofera caroliniana.
Large-scale indigo plantation farming was introduced to the American South in the 1700s. Enslaved laborers enabled indigo farming to dominate in North Carolina, South Carolina, and Georgia, with indigo accounting for more than onethird of the value of American exports. Synthetic indigo was first produced in 1882.
Indigo Lab Notes
Students synthesized indigo using the Baeyer-Drewson reaction, shown below. The reaction combines two equivalents of 2-nitrobenzaldehyde with two equivalents of acetone (as in nail polish remover) to form indigo. Students then used the indigo they synthesized to dye fabric using a vat dyeing procedure.
Prussian Blue
Prussian blue was first synthesized (by accident!) in 1704 in Berlin, Germany. It has an intense dark blue color and is used extensively as a pigment in paints and inks.
Prussian blue is the pigment used in cyanotype printing, a photographic printing process discovered by Sir John Herschel in 1842. Architectural blueprints, for example, are cyanotypes.
Cyanotype Process
Ferric ammonium citrate + Potassium ferricyanide
Prussian Blue
Prussian Blue Lab Notes
In this lab, students first made cyanotypes. Students then synthesized a sample of Prussian blue by reacting ferric chloride, FeCl3, with potassium ferrocyanide, [K]4[Fe(CN)6], as depicted below.
Smalt
Smalt was a ubiquitous source of blue in the sixteenth through nineteenth centuries. It was used in oil paints and ceramic glazes by artists around the world.
Smalt is made from cobalt blue glass and, as a result, has some transparency. The pigment’s particles are large and course and, if ground too fine, the color is weakened. For that reason, artists ground it carefully by hand, and not for too long. Smalt provided a cheaper alternative to the expensive mineral pigment ultramarine.
Smalt Lab Notes
Smalt does not have a precise chemical formula; it contains 2-18% cobalt oxide, 66-72% silica sand, and 10-21% potassium carbonate. In this lab, students used an ancient formula to determine the proportions of reactants needed. They then fired the reaction mixture at 1000°C using a high-temperature oven. Each student was assigned a different postsynthesis grinding time; longer grinding times results in smaller average smalt particle sizes, which affects the color of the pigment.
YInMn Blue
YInMn Blue was discovered in 2009 at the University of Oregon. It is both stable and nontoxic, distinguishing it from other blue pigments. Since its discovery, YInMn blue has been used as both an industrial and artist pigment. It has been called a "perfect blue,” for the absence of red or green tones, its ability to reflect infrared light (heat), and its stability under ultraviolet light illumination.
YInMn Blue Lab Notes
In this lab, students synthesized various metal oxides of the formula YIn(1-x)MnxO3, where the product color depends on the ratio of indium and manganese oxides added. The reaction equation, as well as the crystal structure of YInMn Blue, are depicted below.
