[Pigments and Binders]

Prussian Blue

Prussian Blue has several names. The original name of the pigment came from its use as a dye in the German Army uniforms. The name of the pigment had different names due to the different locations of its manufacture, the process of the manufacture itself, the result of varied physical properties, or the components through which it originated. The generic name for Prussian Blue is iron blue. Iron blue has a variety of hues due to the different processes of manufacture. Some of its commonly used names include Hamburg Blue, Paris Blue, bronze blue, celestial blue, cyanine, Haarlem blue, oriental blue, and potash blue. The general pigment code is CI Pigment Blue 27. Since Prussian Blue has two formulas, then the pigment code varies for each formula. The code for Fe₄[Fe(CN₆]₃ xH₂0 is CI no. 77510. The code for KFe[Fe(Cn)₆] xH₂0 id CI no. 77520. (Berrie 191)HCO₃^-

History of Use by Artists

Often called the first of the modern or artificial pigments, Prussian blue was introduced in the early part of the nineteenth century. A German color maker or dyer in Berlin named Diesbach accidentally discovered the black-blue of Prussian blue in 1704, thinking that his pigment would be red since it was made from cattle blood. Prussian blue was the earliest of the modern synthetic colors. After its discovery in 1704, Diesbach spread the manufacturing process of his pigment on to his pupil, de Pierre, who in turn began manufacturing Prussian blue in Paris. Simon Eikenlenberg, a Dutch painter wrote on the knowledge of Prussian blue in his Notes on Paint and Painting in 1722. By 1724 the manufacturing process of the pigment had spread to England and appeared in an artists' manual by Woodard. InThe Handmaid to the Artists, Dossie quoted the preparation of Prussian blue in its entirety in 1764.

The pigment Prussian blue has many uses. The color was a precursor to blueprints and an early form of the photograph known as cyanotypes. In Japan, the popularity of Prussian Blue soon took the place of indigo. Commonly the pigment became used as an ink by dissolving it in oxalic acid and to color paper. "Prussian blue is used as a pigment in printing inks, paints, typewriter ribbons, and carbon paper. In the nineteenth century, it was the principal pigment of the dyes used in German army uniforms, hence the name"
(Caceci 1). One of the most commonly used pigments until 1970, Prussian blue was popular because of its high tinting strength and absorbance spectrum for the photograph. Due to its lack of toxicicity, Prussian blue could also be found in cosmetics and as a medicinal antidote to radioisotope and heavy metal poisoning in the eighteenth century. Prussian blue was widely accepted and used throughout the world until 1970 when it was replaced by another pigment, phthaloryanine blue. (Berrie 191-201)

Source/Preparation of Pigment

Prussian blue was manufactured first in Germany by a color maker named Diesbach. The process included heating equal amounts of saltpeter (KNO.3) and potassium tartrate in a red-hot crucible. Dry powdered cattle blood was added and the mixture was heated to an incadescence then washed with water and treated with a solution of alum [K.2SO.4; Al.2(So.4).3 x24H.2O] and ferrous sulfate. A green precipitate was formed which turned blue with the addition of hydrochloric acid. Prussian blue is manufactured through the action of an oxidizing agent, such as potassium bichromate and sulphuric acid, upon a mixture of copperas (ferrous sulphate), sodium ferrocyanide and ammonium sulphate. The pigment precipitated from dilute solutions of those salts is a deep blue, finely divided compound which once settled is washed, filtered and dried. Although Prussian blue is deep blue, it can have reddish or green undertones, depending on the preparation conditions, oxidizing agents, pH, temperature and batch size. (Berrie 194-95; Gettens 149)

Chemistry of the Pigment

Prussian blue is a very complex compound of ferric ferocyanide or a closely similar compound. Prussian blue can either be an insoluble polymeric ferric ferrocyanide or an alkali ferric ferrocyanide. It is a stavle inert transition metal comlex. "The origin of color of Prussian Blue is an electronic transition from a low-spin Fe2+ ion in a carbon-coordination center to a high spin Fe 3+ ion in a nitrogen-coordination enter that occurs when visible light is absorbed at 680nm. This absorption is called the intervalence transfer band which produces the intense color of Prussian Blue" (Berrie 195). The appropriate formula is Fe₄(Fe[CN]₆)₃ Prussian blue dissolves in concentrated mineral acids, but is resitant to dilute (10%) mineral acids, polar and nonpolar solvents, fatty acids, drying oils and plasticizers. Prussian blue is amorphous in colloidal aggregates and so finely divided that it almost has the characteristics of a dye. The pigment is permanent in light and air over long periods of time, but can sometimes acquire a metallic bronze cast when exposed to outdoor weathering. Prussian blue is sensitive to alkalis which turn the pigment brown. This landmark characteristic proves Prussian blue not to be a true fresco. (Gettens 150, Berrie 199)

Binders

In lab, we mixed our pigment with three different binders. The three are egg to make egg tempra, gum arabic to make watercolor, and linseed oil to make oil paint. Much research has been done into which binder works better than others.
Prussian Blue with Gum Arabic

Prussian Blue with Egg Tempra

Egg tempera can be made with the egg white, yolk, or the entire egg. In Painting Materials, Doerner recommends a way to produce egg tempera. He suggests using the entire egg mixed with one part oil and two parts water. These ingredients are to be added separately snd shaken well. The freshness of the egg makes a difference in the quality and permanence of the paint. Other suggestions given in this book include one from Wehlte and Thompson. Wehlte agrees with Doerner in using the entire eggbut with 3/8 part linseed oil varnish, 3/8 part dammsar resin in Turpentine, and 1 part water. Thompson uses only the yolk mixed with with 2 or 3 drops of vinegar or some other 3% acetic acid. This is supposed to preserve the paint and make it less greasy. In lab, we used the yolk directly added to the pigment. The paint was too thick and dark to deal with. The water must be necessary to make the paint mangeable. We used 1/2 egg yolk to 2.0 grams of pigment.

Oil paint made with linseed oil was another choice. Linseed oil used by artists is preferable to be obtained by cold expression. The oil must be refined. The preferred method for this is alkali. The alkali refined oil is more desirable and expensive because it does a better job preserving the paint. In lab, the linseed oil we used produced a creamy paint. It looked like a manufactured paint. The proportions we used were 35 drops linseed oil to 2.0 grams of pigment.

The third type was watercolor produced by adding gum arabic to our pigment. In Painting Materials, Church believes the most desirable gum is Senegal gum. This is good for paint production. Church also believes that in order for gum arabic to be good it has to undergo a certain process. This process is to be slow boiled in distilled water in the proportion 1 powdered gum and 2 parts distilled water solution. This solution is to stand a minumum of one day. The gum arabic we used in lab made a thick paste that was hard to work with. The appearance did not look like watercolor. The proportions we used were 18 drops of gum arabic to 1.1 grams of pigment.

Optical characteristics: Looking through the microscope

Prussian blue has extremely small particle sizes which are approximately 0.01 um in diameter. Like color, the size and shape of the pigment are dependant upon the manufacturing process and will vary. The particles smear or clump when dry as one can see in my picture of the pigment. The pigment is also transpanrent and may appear to be flakes or streaks. The clumps of the particles, which one will also find in the slide, are opaque. The particles are isotropic and are not cross polar. Prussian blue is hard to distinguish between similar pigments such as ultramarine and indigo. The pigment is a transparent color with a high tinting strength. In oil film, discrete particles cannot be seen even at a high magnification. (Gettens 150, Berrie 204-205)

 

Prussian Blue Pigment

Health Issues

"Only slightly toxic, but can emit highly toxic hydrogen cyanide gas if exposed to acid, high heat, or strong ultraviolet light" (M. Rossol107)

Links to other Web sites

The Framework of a Prussian Blue Crystal

Prussian Blue Stain

Other

Prussian blue has been identifies in paintings around the world since the eighteenth century. The pigment has been identified in paintings by artists such as Pablo Picasso, Anish Napier, Christen Kobke, Albert Bierstadt, Giovanni Antonio Canal, Antoine Watteau, William Hogarth, Thomas Bardwell and William Blake--just to name a few. (Berrie 211-212)

Jessica Copelandand, Christy Rochelle, 1998.