The blue color seen on many famous Chinese porcelains is known as “blue and white porcelain” or “underglaze blue porcelain”. This distinctive cobalt blue coloring has become iconic and synonymous with Chinese porcelain. But where does this vibrant blue color come from?
The blue pigment used is cobalt oxide, derived from the mineral cobalt. Cobalt compounds have been used to create blue coloring in ceramics and glass for thousands of years, not just in China but in the Middle East and Egypt as well. However, the Chinese potters were able to refine and perfect the use of cobalt pigment in porcelain to achieve the desired blue tones that were stable at high kiln temperatures.
Some key questions around the blue coloring in Chinese porcelain include:
– What is the history and origins of the blue pigment used?
– How and why did it become so prevalent in Chinese porcelains?
– What production and firing techniques allow the vibrant blue color to be achieved?
– What are the unique properties and chemistry of the cobalt pigment that enable its use?
This article will explore the fascinating history, chemistry, and significance of the beloved blue and white wares of China.
History and Origins
The use of blue pigment in ceramic glazes dates back thousands of years. The earliest known examples are from ancient Egypt and the Near East around 2,500 BC. Cobalt compounds were used to create blue glass as early as the Bronze Age circa 5,000 years ago.
In China, cobalt blue decorations started appearing on pottery during the Tang Dynasty (618-906 AD). Decorated Tang pieces included blue paintings or blue-outlined engraving. The vivid blue color stood out boldly against the white porcelain background.
The technique of using cobalt blue designs under a transparent glaze was perfected during the Yuan dynasty (1271–1368). This allowed the cobalt blue decorations to remain crisp and vivid, compared to overglaze enamels used previously. By the early 14th century, quality blue and white porcelains were being exported from China to Europe and the Middle East, where they were highly prized.
Prevalence in Chinese Porcelains
Blue and white porcelain came to define the public perception of Chinese ceramics from the early Ming dynasty onwards. Chinese emperors and royal courts particularly prized the cobalt blue and white wares.
Several factors contributed to the prevalence of blue and white porcelain in China:
– Cobalt minerals were available in China allowing domestic production. Major deposits were found in Jiangxi, Jiangsu and Shandong provinces.
– The vivid blue stood out on the white porcelain background, creating desirable contrast.
– The color blue symbolized virtues in Chinese culture like peace, calmness, and eternity.
– Technical mastery was achieved in utilizing cobalt pigments at high kiln temperatures.
– Intricately painted blue and white designs became associated with the highest quality porcelain wares.
As Chinese porcelain gained fame internationally, cobalt blue decorations had become the norm by the 15th century. Chinese potters catered to export markets in Europe and elsewhere that desired blue and white wares.
Production and Firing Techniques
Producing high quality blue and white porcelain required precision and skill at every step:
Pigment preparation – The raw cobalt ore was processed by roasting, crushing, washing, and grinding to produce a fine purified cobalt oxide powder. Careful washing and grinding ensured uniformly sized particles.
Pigment application – The cobalt oxide powder was mixed with water to create the blue pigment mixture. Brushes applied the pigment directly onto the unfired white porcelain body. Frequent mixing prevented settling.
Firing – The key challenge was keeping the cobalt blue from turning grey, faded or blotchy during the high temperature kiln firing. Careful control of furnace atmosphere and temperature changes was needed to achieve vibrant blue hues.
Multiple firings – On finer quality porcelains, multiple firings at different temperatures were done. First at 800-900°C to fix the cobalt pigment, then at 1,260-1,360°C to vitrify the glaze and complete the fusion.
The ability to consistently produce vivid even blue tones that didn’t fade or run during firing separated the best quality porcelains. It required deep knowledge and skill from the potters.
Properties of Cobalt Pigment
The chemistry behind how the cobalt oxide pigment imparts such a stable vivid blue color in porcelain glazes is fascinating. Some key properties include:
Cobalt alumina spinel – When cobalt oxide is heated, it forms a crystalline compound called cobalt aluminate spinel (CoAl2O4) which is very stable even at high temperatures. This spinel structure is key for achieving the desirable blue color.
Inertness – Cobalt oxide is resistant to chemical decomposition and light fading. It does not react with the glaze ingredients, allowing the blue color to persist undiminished.
Heat stability – The spinel cobalt compound retains its blue color up to very high temperatures, unlike other blue pigments. This enables it to be used in porcelain glazes fired around 1300°C.
Alumina interaction – Cobalt aluminate spinel forms readily in porcelain glazes due to their high alumina content. This ensures evenly dispersed blue coloring.
Color intensity – The right cobalt concentration creates an intense blue color without appearing too dark. Higher cobalt levels result in undesirable purple hues.
Conclusion
The vibrant blue color that graces many famous Chinese porcelains relies on the exceptional heat stability, vivid hues, and intense color imparted by cobalt oxide pigments. Centuries of innovation perfected the production and firing process to maximize the benefits of cobalt’s properties. The blue and white wares pioneered in ancient China continue to be admired worldwide as examples of skillful ceramic artistry.
