CRYSTALLINE GLAZE HISTORY
The early examples of crystalline glazes were discovered by accident in the late 19th century when two chemists, Charles Lauth and George Dutailly, at Sèvres, observed that glazes over-saturated with zinc silicate could develop crystals. Without a clear understanding of what happened, they alarmed other chemists working in other porcelain factories about these contaminants and reported how to avoid this “defect”. Perhaps intrigued, large ceramic industries started comprehending and producing crystalline glazes. A few years later, Sèvres also started to produce crystalline glazes. However, in the early 20th century, most of the ceramic industries that were producing crystalline glazes were experiencing financial problems due to high costs for individually crystalline pieces, apart from being an unpredictable technique. With the beginning of World War I, most of those companies stopped producing these glazes.
It was not until the late 20th century, when automatic and programmable kilns were developed, that crystalline glazes started again to appear. Many potters are now exploring the potential of crystalline glaze technique, making new and exciting discoveries.
MAKING A CRYSTALLINE GLAZE
The macro crystals are formed during the cooling step of the kiln firing process and grow from small nuclei produced during the melting process when zinc oxide and silica come together to form zinc silicate. The most commonly used colorants are iron, manganese, rutile, nickel, copper and cobalt, and the less common are silver, gold, uranium and other rare earth elements. These can be used alone, some in minute amounts, or in a combination with each other.
The firing process of a crystalline glaze requires a complex schedule divided in several temperature ramps that create different crystal growth formations. The kiln is fired to ca. 1280 °C and then cooled to specific holding temperatures for a specific time, depending on the glaze, to allow crystals to be formed in the glaze. The size of the crystals depends on the amount of time held at each temperature. During the firing process, a lot of glaze run off the vases and to protect the kiln a pedestal and dish is used. After the firing, the pedestal is removed and the bottom of each piece smoothed.
Each glaze composition and thickness, combined with the ideal firing schedule, produces a wonderful diversity of glazes that consist in different colors and forms of crystals. Each piece is unique, with crystals occurring randomly in the glaze, therefore impossible to be replicated.
The crystalline glaze technique, apart from being time consuming, has a high failure rate, which can be disappointing and expensive. Even today, most potters do not want to deal with the difficulties that are involved during the production of these glazes.
Francisco developed a special interest in crystalline glazes and began to work on developing new and exciting glazes while facing the complexity and challenges that this technique brings.