Kings blue is a relatively rare and obscure pigment with an interesting history. It was first made in the 18th century and was one of the earliest synthetic blue pigments. Understanding what gives Kings blue its distinctive color provides insight into the history and chemistry of pigments.
What is Kings blue?
Kings blue, also known as Saxon blue or Paris blue, is an artificial blue pigment made by heating potash alum with smalt or cobalt oxide. The pigment was first made in 1757 by John Holker and patented in England by James Colebrook. It was later improved upon and manufactured more widely by William Duesbury at the Derby Porcelain Works.
Kings blue is classified as a cobalt blue pigment. It is made by firing a mixture of potash alum, cobalt oxide, and smalt at a high temperature between 850-900°C. The potash alum acts as a flux during the firing process and allows the cobalt oxide to dissolve and react with the silica in smalt, forming cobalt aluminate crystals that provide the blue color.
Chemical composition
The main active ingredient providing the blue color in Kings blue pigment is cobalt(II) aluminate, which has the chemical formula CoAl2O4. Cobalt aluminate belongs to a group of inorganic pigments called cobalt blues that contain cobalt bound to aluminate.
The cobalt aluminate crystals in Kings blue pigment have a spinel structure. This consists of a cubic close-packed array of oxygen atoms, with cobalt ions occupying some of the octahedral interstices and aluminum ions occupying the tetrahedral interstices in the structure.
In addition to cobalt aluminate, Kings blue pigment contains some residual components from the raw ingredients used in its manufacture:
| Potash alum | KAl(SO4)2·12H2O |
| Cobalt oxide | CoO or Co3O4 |
| Smalt | Ground blue glass containing CoO, SiO2, K2O |
The cobalt aluminate crystals give Kings blue its characteristic blue color, while the other components are largely inert and provide bulk to the pigment powder.
Properties of Kings blue pigment
Kings blue pigment has the following important properties:
– Deep blue color – the vibrant blue results from the electronic transitions of cobalt(II) ions in the cobalt aluminate crystals.
– Insoluble in water – the cobalt aluminate compound is resistant to degradation.
– Chemically inert – stable to light, alkalis and weak acids making it suitable for fresco and decorative painting.
– Low tinting strength – particle size is relatively large so the color is not strongly absorbing.
The insolubility and chemical inertness of cobalt aluminate make Kings blue more durable and permanent compared to earlier blue pigments derived from metal salts or dyes. However, it also has relatively low tinting strength and covering power.
How is Kings blue pigment made?
The original manufacturing process for Kings blue pigment involved heating together the raw ingredients of potash alum, cobalt oxide (from cobalt ore) and smalt in crucibles over a furnace.
The typical recipe contained:
– 100 parts of potash alum
– 50 parts of cobalt oxide
– 50 parts of smalt
The potash alum served as a flux to lower the melting point of the other components. The mixture was heated between 850-900°C, hot enough to melt the constituents together while not decomposing the cobalt oxide.
Heating the fluxed mixture produced an inorganic reaction between the cobalt oxide and the silicon dioxide (silica) contained in the smalt glass. This resulted in formation of tiny crystals of cobalt(II) aluminate (CoAl2O4) throughout the melted pigment mixture.
After firing, the molten pigment mixture was poured out of the crucibles and allowed to cool and solidify. It was then broken up, ground, washed, and pressed to produce a coarse lumpy powder comprising microscopic blue cobalt aluminate crystals dispersed in a colorless matrix.
Smalt production
An important step in making Kings blue was the production of smalt – a blue ground glass containing cobalt. Smalt was made by grinding cobalt-colored blue glass into a fine powder.
The blue glass used was colored by cobalt oxide, which was added to the glass melt. Cobalt has been used as a stable blue coloring agent for glass since ancient times. The cobalt blue glass was cast into cakes or sticks and allowed to cool before grinding.
Smalt was made in Saxony, Germany, which was a center for glass making from the 16th century onwards. Saxon smalt was highly regarded for its deep blue color and even granulation. It was an important export from Saxony to makers of Kings blue pigment.
History of Kings blue pigment
The origin of Kings blue pigment dates back to the mid-18th century in Europe. At the time, chemists were striving to develop new synthetic blue pigments to replace costly ultramarine made from lapis lazuli.
Key events in the history of Kings blue include:
1757 – John Holker first made a blue pigment by heating potash alum, cobalt, black oxide of cobalt, and smalt together. He patented the process in England.
1773 – James Colebrook found Holker’s pigment too purple. He improved the formula using cobalt ore instead of smalt to make a brighter blue, registering it as a British patent.
1776 – William Duesbury of the Royal Crown Derby Porcelain Works further refined the recipe, using a lower 750°C firing temperature. This produced the pigment under the name Duesbury or Derby Blue.
1778 – Duesbury registered his version as the Oxide of Cobalt Blue. It was sold commercially as a artists pigment.
1779 – The pigment was coined Kings blue in honor of King George III’s visit to the Derby factory where he saw the pigment being made.
So in summary, Kings blue was the eventual result of successive experiments using cobalt compounds to create blue pigments culminating in Duesbury’s reliable formulation fired at a relatively low temperature in earthenware crucibles.
Use in ceramics
One of the first uses of Kings blue pigment was in English porcelain and pottery. The Derby Porcelain Works owned by William Duesbury became a leading producer of the pigment for use in their ceramic wares.
Kings blue was ideal for decorating porcelain and earthenware. It was stable at ceramic firing temperatures and gave a pleasant blue color to painted or printed designs underneath a transparent glaze. The pigment could produce a range of blue hues depending on application methods.
The pigment was quickly adopted by other English potteries in the late 18th century eager to replicate Derby’s successful blue-decorated porcelain. Compared to smalt, Kings blue resulted in brighter, more solid blue areas free of the streakiness caused by poor dispersion of smalt particles.
Use in art and printing
Kings blue pigment was also sold as an artists’ paint by color merchants in the late 1700s and early 1800s. The pigment produced a cool, greenish blue softer than cobalt blue. It had good hiding power and mixed well with other colors.
The pigment was sometimes used by painters including Thomas Gainsborough for sky backgrounds. It provided an economic alternative to expensive lapis lazuli-based ultramarine. Kings blue was also employed by printers to color engravings and prints blue.
However, Kings blue fell out of favor with artists as it lacked the depth, richness and tinting strength of cobalt blue. More pure and intense synthetic cobalt blue pigments superseded Kings blue in the 19th century.
Decline and rediscovery
The complex and time-consuming manufacture of Kings blue led to its decline in the early 19th century. It was overtaken by new synthetic cobalt blues that were simpler to make in purer hues. The last known production was at Derby Porcelain in 1820.
For over a century afterwards Kings blue was almost forgotten as a lost pigment. It was only in the late 20th century that interest was revived among art scholars researching early English porcelain. Scientific analysis in the 1990s confirmed the composition and properties of the forgotten pigment.
There has since been a renaissance in recreating Kings blue pigment using the original 18th century methods. Contemporary production has allowed fuller understanding of this important early blue pigment that paved the way for future color developments.
Conclusion
In summary, Kings blue is a historically significant cobalt blue pigment first synthesized in 18th century England. Its chief component is cobalt aluminate which produces small blue crystals dispersed in a glassy matrix. The pigment’s blue color arises from cobalt(II) transitions. While mainly used on English porcelain, Kings blue was also employed by some painters before being superseded by more modern cobalt pigments. After fading into obscurity, recent interest has revived the pigment and methods for recreating it using original recipes. Understanding Kings blue provides insight into innovations in synthetic blue pigments stemming from cobalt compounds.
