***Tenmoku glazes***, with their deep, lustrous colors and distinctive surface patterns, have captivated potters for centuries. Originating from the Jian ware of China’s Song Dynasty, these glazes are characterized by their high iron content, which, when fired under specific conditions, creates a range of visual and textural effects. Think of it as a dance between the clay, the glaze, and the fire, each playing a crucial role in the final masterpiece. The term “Tenmoku” itself comes from the Japanese pronunciation of “Tianmu,” a mountain in China where these iron-glazed bowls were first discovered. Japanese monks, visiting monasteries near Tianmu Mountain, brought these treasures back to Japan, where they were highly valued in tea ceremonies. Over time, Japanese potters began creating their own versions, adapting and evolving the techniques to suit their styles. You can explore more about the history of pottery with traditions from all over the world.
The magic of Tenmoku lies in its variability. *No two firings are ever quite the same*, and the subtle shifts in temperature, atmosphere, and cooling rates can dramatically alter the outcome. This variability is what makes Tenmoku so exciting and challenging. Imagine opening the kiln and discovering a completely unique piece, a testament to the unpredictable nature of the ceramic process. One of the key characteristics of Tenmoku is its color range, which spans from deep blacks and rich browns to occasional hints of blue or green, all dependent on the glaze composition and firing conditions. The surface patterns are just as diverse, with names like “hare’s fur” (fine, streak-like patterns), “oil spot” (circular spots created by iron oxide crystallization), and the highly prized “Yohen” (iridescent, galaxy-like patterns). These patterns are not merely decorative; they tell a story of the firing process, a visual record of the molten dance that occurred within the kiln. A longer cooling time allows for maximum surface crystals. Potters can “fire down” a kiln to help achieve this effect. During a normal firing, the kiln is slowly brought to a maximum temperature by adding fuel, then fueling is stopped and the kiln is allowed to cool slowly by losing heat to the air around it. To fire down a kiln, the potter continues to add a limited amount of fuel after the maximum temperature is reached to slow the cooling process and keep the glazes molten for as long as possible. Understanding kiln types is also important for getting the best results.
Achieving consistent Tenmoku glazes, however, is no easy feat. Potters often face challenges due to the glaze’s sensitivity to various factors. Variability in iron oxide content, firing temperature, atmosphere, cooling rate, and glaze application thickness can all lead to inconsistent results. The type of iron oxide used can also make a difference. As one ceramic artist noted, *”I use only metal oxides in crystalline glaze; and there are several varieties of Iron FE. Spanish Red is 83% pure, Red is over 95%, and yellow which is a weaker version.”* Experimenting with different types and purities of iron oxide can help you find the perfect combination for your desired effect. To overcome these challenges, potters need to be meticulous in their preparation and firing processes. Ensuring even heat distribution within the kiln, maintaining consistent firing schedules and atmospheres, and carefully controlling the cooling rate are all essential. In electric kilns, where achieving reduction effects can be difficult, techniques like adding silicon carbide to the glaze can help induce localized reduction.
The choice of clay body also plays a crucial role in the final appearance of Tenmoku glazes. High-iron stoneware clays are particularly effective, as the iron content in the clay interacts with the glaze during firing, producing rich, dark hues with characteristic crystalline patterns. These effects result from excess iron in the glaze being expelled during firing. Plainsman H443 is a good choice. Using porcelain or low-iron clays may result in less vibrant or muted effects. The glaze thickness is also important, for example: *”To achieve the oil spot effect you must first apply the glaze very thick (¼–⅜ in.) and then fire it in oxidation to cone 10 or higher.”* In the end, Tenmoku glazes are a testament to the beauty and complexity of the ceramic arts. ***They require patience, precision, and a willingness to embrace the unpredictable nature of the firing process.*** You can learn more about the science of glazing with this guide to glaze chemistry.
There are many variations of Tenmoku glazes including:
- Hare’s Fur (Nogime Tenmoku) achieved through firing in an oxidizing atmosphere at temperatures around 1300°C (cone 10). A slow cooling process allows iron crystals to form streaks.
- Oil Spot (Yuteki Tenmoku) glaze is applied thickly (¼–⅜ inch), and during firing, iron oxide thermally reduces, releasing oxygen bubbles that rise to the surface, dragging iron with them and depositing it on the surface. A soaking period at the end of the firing helps smooth over the bubbles.
- Tortoiseshell (Taihisan or Taihi Tenmoku) is achieved by applying the iron-rich glaze over a yellow underglaze, then firing in an oxidizing atmosphere.
- Partridge Feather (Konoha Tenmoku) is made by applying a white glaze over a black Tenmoku base before firing, and during the firing process, it interacts with the black glaze to create the feather-like patterns.
- Iridescent Oil Spot (Yohen Tenmoku) is achieved through Firing in oxidation with precise temperature control and cooling rates to develop the iridescent quality.
Each of these Tenmoku glaze types requires meticulous control over glaze composition, application thickness, firing atmosphere, temperature, and cooling rates to achieve the desired visual effects.
