Oil Spot Glazes
Rachel Davis
Glaze Formulation
Matt Katz
August 9th, 2019
The aesthetic of the oil spot glaze has been captivating viewers since it was developed during the Song Dynasty (960-1279 CE) in the Fujian province of China. Tea bowls with this highly prized surface were traded throughout east Asia, and the Japanese began developing their own glazes to mimic them.[1]In Japan, they were called Temmoku in reference to the Chinese mountain range Tianmu Shan. Generally speaking, the image that comes to mind when referring to a Temmoku glaze, is a deep brown or black that breaks over edges with a lighter reddish brown. But Temmoku can also refer to an entire glaze category, which includes glazes with striking effects such as the oil spot.[2]The potters of the Jianyang kilns in the Fujian province had specific methods for achieving this alluring glaze, which have been deciphered through excavation and analysis of sherds from the original kilns. Before attempting to understand how to create oil spot glazes today with modern technology, it is imperative to learn about this history and their technical choices. Much of the knowledge they had can be applied or modified to suit the needs of the modern potter.
In 2014, Pamela Vandiver and Chandra Reedy published an article on their findings from analyzing 30 sherds excavated by J.M. Plumer in 1935, from three kilns in Jianyang. They were able to determine information about glaze composition, application, and the firing process. It is important to note that the pieces they analyzed were not exclusively oil spots, but also included hare’s fur glazes. The significance of this fact is that no variation was found between the two in terms of glaze composition, meaning that the difference in the end product was due to variations in the firing process. All the glaze samples had a high iron content with calcium oxides, which had been applied to an iron-heavy body.[3]A high percentage of iron is one of the major keys to producing an oil spot glaze. When red iron oxide (Fe2O3) is heated, it converts to black iron oxide (FeO), releasing oxygen in the process. The calcium that is present in the glaze aids in breaking apart the iron and the oxygen.[4]When the oxygen bursts the surface of the glaze, it creates a crater. This occurrence is normally avoided by putting high iron glazes through an early reduction during the firing process. Starving the kiln of oxygen forces the chemically bonded oxygen in the red iron oxide to be released. When this happens before the glaze has begun melting, no oxygen bubbles will form and burst through the surface.[5]However, this “glaze flaw” is essential for creating the oil spot effect, which is why they must be fired in oxidation. The red iron oxide molecule will naturally release oxygen atoms at about 1230°C. This disrupts the homogeneity of the glaze, causing what is called phase separation.[6]When the oxygen leaves the glaze, a “spot” of iron remains.[7]As the firing continues, a layer of silica covers the blemished area, healing it. The potters in Jianyang had to be meticulous in their glaze application and firing techniques because viscosity and thickness plays an important role during this part of the glaze development in the kiln. Glaze thickness was around 3mm or more. If the glaze was applied too thinly, it would not produce a successful oil spot effect.[8]A certain balance must be achieved with the glaze viscosity. It must be stiff enough not to run, taking the iron spots with it. On the other hand, it cannot be so viscous that the oxygen bubbles fail to break the surface.[9]Historically these wares were glazed as greenware, placed on setters, and protected with a saggar as they were fired to 1230-1300°C. The saggar served to regulate both the temperature and the oxidation state.[10]
Today, it is more common to apply the glaze to bisque ware rather than leather hard pots for increased durability during the final firing. And although the saggar method is still valid, it is not always necessary. For example, firing in an electric kiln provides an oxidation atmosphere without the threat of debris from the firing melting onto the surface. Another major development in recent years is the ability to obtain an oil spot effect with mid-range firing, albeit with some drawbacks. Spotting at this temperature tends to be smaller because of the lower temperature.[11]Another method for obtaining the oil spot look that works for both mid-range and high fire temperatures is using a slip-based glaze with a cover coat glaze. In this scenario, the cover glaze must melt first so the oxygen bubble can burst through it.[12]Traditional oil spot glazes tend to be variations of brown or black depending on the colorant. Brown, rusty colored spots come from iron oxide and darker blacks come from the addition of cobalt carbonate.[13]Although the cover glaze method is not the traditional way of creating the oil spot aesthetic, it uses the same principle of forming two glasses at different times during the firing. One glass must create bubbles to break through the other before it heals over. The difference here is that the two glasses come from two different glazes and is not due to phase separation of one glaze. This alternation in the process makes it simpler to explore a wider range of color possibilities with the cover coat.[14]Another interesting oil spot development is the usage of opacifiers such as zirconium silicate in glaze recipes. When the bubble bursts, the opacifier collects in the crater, creating white spots.[15]This goes to show how a bit of knowledge and creativity can create a whole new branch of a glaze type.
During the Song Dynasty, the Jianyang potters mastered the oil spot glaze using the resources they had. The foundation of knowledge they amassed is one that potters continue to build on today. In the modern world, we are not limited to local materials as they were. Material accessibility makes it possible to explore the potential of oil spot glazes in ways that could not be done before. Continuing to analyze ceramics of the past while testing new glaze formulas and firing methods is certain to lead to more exciting developments for oil spot glazes in the future.
Bibliography
Britt, John. “Oil-spot Glazes.” Ceramics Technical , no. 21 (2005): 15-18.
Britt, John. 2004. The Complete Guide to High-Fire Glazes: Glazing & Firing at Cone 10. New York: Lark Books.
Britt, John. 2014. The Complete Guide to Mid-Range Glazes: Glazing & Firing at Cones 4-7. New York: Lark Books.
Hansen, Tony. “Phase Separation.” Digitalfire. Accessed August 08, 2019. https://digitalfire.com/4sight/glossary/glossary_phase_separation.html.
Katz, Matt, and Mackenzie McDonald. Glaze formula and Test Tiles. March 08, 2019. Raw data. Alfred University, Alfred, New York.
Instagram post by @ceramicmaterialsworkshop.
Katz, Matt. “Special Effects.” Lecture, Glaze Formulation Online, August 06, 2019.
Vandiver, Pamela B., and Chandra L. Reedy. “Traditional Craftsmanship and Technology of Jianyang Black Wares from Fujian, China.” Studies in Conservation , no. 59 sup1 (2014): S169-S172.
Vickery, Frank R. “A Potters Oil-spot Marx.” Master’s thesis, Western Carolina University, 2012.
[1]Vandiver, Pamela B., and Chandra L. Reedy. “Traditional Craftsmanship and Technology of Jianyang Black Wares from Fujian, China.” Studies in Conservation , no. 59 sup1 (2014): S169-S172, S169.
[2]Britt, John. 2004. The Complete Guide to High-Fire Glazes: Glazing & Firing at Cone 10. New York: Lark Books, 70.
[3]Vandiver and Reedy. "Traditional Craftsmanship,” S169.
[4]Vickery, Frank R. “A Potters Oil-spot Marx.” Master’s thesis, Western Carolina University, 2012, 22.
[5]Katz, Matt. “Special Effects.” Lecture, Glaze Formulation Online, August 06, 2019.
[6]Hansen, Tony. “Phase Separation.” Digitalfire. Accessed August 08, 2019. https://digitalfire.com/4sight/glossary/glossary_phase_separation.html.
[7]Britt, John. “Oil-spot Glazes.” Ceramics Technical, no. 21 (2005): 15-18, 16.
[8] Vickery. “A Potters Oil-spot Marx,” 21-22.
[9]Britt. The Complete Guide to High-Fire, 75-76.
[10]Vandiver and Reedy. "Traditional Craftsmanship,” S170.
[11]Britt, John. 2014. The Complete Guide to Mid-Range Glazes: Glazing & Firing at Cones 4-7. New York: Lark Books, 89.
[12]Vickery. “A Potters Oil-spot Marx,” 23.
[13]Britt. The Complete Guide to High-Fire, 75.
[14]Britt. “Oil-spot Glazes,” 18.
[15]Katz, Matt, and Mackenzie McDonald. Glaze formula and Test Tiles. March 08, 2019. Raw data. Alfred University, Alfred, New York. Instagram post by @ceramicmaterialsworkshop.
