Prussian Blue

Considered the first artificial pigment, Prussian Blue was created at the turn of the eighteenth century, rather ironically by an artist seeking to create a new source for red paint.  There are varying accounts as to the exact story behind the color, but the most interesting is from German physician Georg Ernst Stahl (1659–1734). He says that a pigment maker in Berlin named Diesbach was making a batch of a Red Lake pigment using potash, crushed cochineal insects, and iron sulfate. Having run out of potash, he went to local pharmacist Johann Konrad Dippel to purchase some more. Dippel, possibly looking to fleece Diesbach, sold him potash that was contaminated with dried cattle blood. When Diesbach tried to make his red pigment, it made a deep blue pigment instead. Dippel knew that it was his adulterated potash that had caused this reaction and saw a business opportunity, so he conducted further experiments and commercialized the color under the name Berlin Blue. Dippel kept the composition of the pigment a close secret, thus amassing a considerable fortune. Unfortunately for Dippel, an English chemist reverse-engineered it in 1724 and published the formulation.

Prussian blue was named “Preußisch blau” in 1709 by its first trader and that name has stuck unto today. The pigment was significant because it replaced the expensive Lapis Lazuli based paint which was purported to have cost more than gold.   In 1752, the French chemist Pierre J. Macquer made the important step of showing the Prussian blue could be reduced to a salt of iron and a new acid, which could be used to reconstitute the dye. The new acid, hydrogen cyanide, first isolated from Prussian blue in pure form and characterized about 1783 by the Swedish chemist Carl Wilhelm Scheele, was eventually given the name Blausäure (literally “Blue acid”) because of its derivation from Prussian blue, and in English became known popularly as Prussic acid. Cyanide, a colorless anion that forms in the process of making Prussian Blue, derives its name from the Greek word for dark blue.

Prior to the use of Prussian blue for clothing dye, both indigo and woad were used to achieve a similar shade. Indigo was particularly expensive to import and farmers in England and the American Colonies began growing it at home around the mid-1700s.  Another commonly used blue pigment was extracted from the semi-precious limestone rock mixture ‘lapis lazuli’. This pigment was regularly imported from Asia, across the Silk Road, from the remote Sar-e-Sang valley in the Badakhshan mountains in Afghanistan, because of its rarity and the expense in bringing lapis lazuli to Europe, this blue pigment often cost more than gold.  Because it is not a common “earthy tone” like umber or sallow, the color was associated with royalty and high wealth.  The discovery of Prussian Blue, however, as a synthetic dye, decreased cost to produce this particularly fashionable color.  Soon even those of moderate wealth could afford to display blue and it became a very sought-after color.  During the American Revolution, the leader of the Whig Party in England, Charles James Fox, wore a blue coat and buff waistcoat and breeches, the colors of the Whig Party and of the uniform of George Washington, whose principles he supported. The blue of the lottery coats of the Continental Army are dyed not with indigo but rather Prussian Blue.

When Prussian blue is oxidized, it transforms to Prussian yellow, which has the same structure as Prussian blue but with all the iron sites occupied by Fe(III) ions. Pure Prussian blue is inherently stable to light and the yellowish-grey appearance of aged Prussian blue oil paints has been attributed to oxidation, rather than light fading.  This is why you sometimes see 18th Century surfaces painted with Prussian Blue as having a green color today. 

How to make Prussian Blue Paint:

1. Prepare a saturated iron(III) chloride solution, dissolve 92 g of iron(III) chloride hexahydrate, in enough distilled or deionized water to make 100 mL of solution.
2. Prepare a saturated potassium ferrocyanide solution, dissolve 30 g of potassium ferrocyanide trihydrate, in enough distilled or deionized water to make 100 mL of solution.
3. Mix 20 mL of saturated iron(III) chloride solution with 20 mL of saturated potassium ferrocyanide solution. 
4. Filter the precipitate
5. Scrape the precipitate from the filter paper onto a watch glass and allow it to dry completely. It may take up to a week for the pigment to dry completely. This is the Prussian blue pigment.
6. Place 10 mL of linseed oil into a mortar with 2 g Prussian Blue pigment and grind with a pestle until a uniform paste is obtained. Add additional pigment to achieve the desired color.

Why is all this significant?  Well on 12/31/22, I plan to deliver a musket that I acquired from an estate sale in Bucks County to Fort Mifflin.  Like many reproduction muskets from India, it was very functional but the hardwood stock needed many repairs.  Fortunately, we have a precedent for this, the Crown frequently sold muskets that were repaired or reconditioned to its Indian allies (both in India and America).  Frequently, was used by the Crown to mark items as having been given rather than sold (see What’s with the Blue Musket).   I have reconditioned the musket and then painted it with Prussian Blue paint.  We believe this is the only blue trade musket available for public display (and firing demonstrations) in the Philadelphia region.