Historically, ultramarine was derived from lapis lazuli, primarily mined in the Sar-e-Sang mines of present-day northeastern Afghanistan. However, in 1828, Jean-Baptise Guimet revolutionized the production by perfecting a method to create artificial ultramarine. This involved heating a blend of china clay, soda ash, coal, charcoal, silica, and sulfur. Termed "French ultramarine" due to a specific request from the French government, it served to distinguish the artificial variant from its natural counterpart.
In contemporary paint-making, the terms ultramarine and French ultramarine are employed to denote distinct blue hues. Beyond the renowned blue pigment, ultramarine can be meticulously crafted into redder shades. Ultramarine violet is fashioned by heating a blend of ultramarine blue and ammonium chloride, while ultramarine pink is derived from ultramarine violet through heating with gaseous hydrochloric acid. However, as the ultramarines venture into the realm of red shades, they progressively lose their tinting strength and opacity.
In German folklore, the 'kobold' was a mischievous sprite believed to haunt and torment miners, with a reputation for causing the mines' poisonous nature. Cobalt, naturally occurring in the ore smaltite – a blend of cobalt and nickel arsenides found in silver mines – manifested as a brilliant blue crystal containing lethal arsenic, known to miners as 'cobalt bloom.'
Although cobalt had been utilized as a coloring agent in pigments and ceramic glazes since ancient times, the 19th-century saw the creation of more vibrant and stable cobalt colors. Modern cobalt blue, created by the French chemist Louis Jacques Thénard in 1802, surpassed its predecessors in purity of color, swiftly embraced by artists. Cobalt exhibits chameleon-like qualities, allowing it to be combined with other elements to produce green, violet, and yellow hues.
Cobalt green, sharing a similar composition with cobalt blue, replaces the alumina that imparts cobalt blue's distinctive color with zinc oxide. While cobalt green was discovered in 1780, it wasn't until 1835, with the industrial production of zinc oxide, that its commercial practicality was realized. The Impressionists and Post-Impressionists enthusiastically embraced these cobalt hues. Cobalt's dazzling qualities meant it was ideal for the Impressionist palette and, in way, is an important pigment in the context of art authentication for Impressionist art.
Cerulean blue, a compound of cobalt and tin oxides (cobalt stannate), derives its name from the Latin word 'caeruleus,' where 'caelum' translates to the 'vault of heaven.' In ancient times, 'caeruleum' served as a general term encompassing various blue pigments. Despite the Roman author Pliny referencing a diverse range of 'caeruleum,' his descriptions primarily aligned with variations of Egyptian blue, still in production during his era.
At one point, the term cerulean was also applied to mixtures of copper and cobalt oxides, including natural azurite and synthetic salt. Exhibiting a slight green hue, cerulean blue serves as an ideal complement to the red tones of cobalt blue. Upon its introduction, as with Cobalt, the Impressionists promptly integrated it extensively into their works and, as such, is commonly found in conservation, restoration and authentication records for Impressionist painting.
Both versions of cerulean blue remain pivotal for artists today, with their distinct characteristics and historical significance in the realm of pigments.