Techno File: Controlling Shino Joe Hicks

Appears in the November 2023 issue of Ceramics Monthly.

Experiences with carbon-trap shino glazes can be simultaneously exhilarating and tumultuous. Learn how you can focus on a set number of variables to gain more control over your shino-glazed surfaces.

Defining the Terms 

Carbon Trapping: The inclusion of atmospheric carbon in the glaze surface due to the combination of melting sodium and heavy reduction, resulting in patterns of black and gray.  

Over Trapping: Purely aesthetic, referencing the elimination of preferred soda-ash characteristics and color due to an abundance of carbon trapping.

Shino Glaze: Developed in Japan during the Momoyama period, this white glaze had various characteristics, including flashing due to iron. It was adopted by 20th-century American potters who, with additions of soda ash, developed carbon-trap shino.

Soda-Ash Characteristics: All of the surface effects related to abundant levels of soda ash in a shino glaze recipe, including crystal formulations and the inclusion of atmospheric carbon in the fired glaze.

The wide variety of surface characteristics that shino glazes produce has gifted me personal joy alongside complete misery for the past 15 years. The chaos surrounding carbon-trap shino-glaze behavior creates challenges for obtaining consistent results inside of any particular aesthetic range. My aesthetic range is a balance between color, soda-ash crystals, and carbon trapping. Though achieving this range has tested my resolve and dedication to using these types of glazes, I’m completely captivated by the endless variety of surfaces they offer. 

Like many other fired surfaces potters employ, I’ve accepted the fact that I’m only able to encourage results, due to the vast number of variables associated with accomplishing successful carbon-trap shino surfaces. Recently, I’ve been able to acquire consistency and predictability by identifying and experimenting with a specific set of four variables: iron, glaze thickness, timing, and reduction.

Iron

Iron is the prevalent cause of color in shino-style glazes. The amounts of iron in the clay body and in the glaze collaborate to affect the colors appearing on the eventually fired surface. In very simple terms, higher amounts of iron in the glaze may push the color response to favor brick reds and golds, whereas brighter oranges and peach blushes may become more prevalent as the amount of iron is decreased, especially when using clay bodies such as porcelain or white stoneware. Understanding how the amounts of iron between the clay body and the glaze recipe coordinate will offer the opportunity for wide experimentation and discovery of varying color responses. 

Kaolin, ball clay, and Cedar Heights Redart are typical clays used in American shino glazes. Adjusting the amounts of these materials in glaze recipes is a simple technique for altering the color response in relation to the clay body hue. Because kaolin contains only trace amounts of iron, you can encourage brighter oranges on a light brown stoneware by increasing the ratio of kaolin to ball clay in the glaze recipe. Or, you might experiment with additions of Cedar Heights Redart to the glaze recipe to achieve brick reds and golds on a porcelain body. Regardless of your choices, it’s important to understand the collaboration between the clay body and the glaze layer, because it will provide direction on what materials to use for achieving your aesthetic goals. 

Glaze Thickness

Glaze thickness might be the most important variable in establishing consistent results. Specific gravity measurements help obtain a desired glaze thickness, and I’m convinced that glaze application directly influences color, crystal development, carbon trapping, and the quality of the melted surface. 

I’ve been experimenting with specific-gravity measurements ranging between 1.33–1.58, and precisely documenting the results in an effort to chart outcomes and relationships. These efforts have paid off, and the data I’ve collected offers guidance on particular aesthetic outcomes with consistency. Developing a data set may take several firings, and I would suggest experimenting with a range of specific gravity measurements for each firing. 

Timing

Along with glaze thickness, choosing when to mix and apply glaze to bisqueware in relation to actual kiln ignition will affect the nature of soda-ash characteristics. I can’t yet offer empirical evidence based on chemical behavior, but I have arrived at the following conclusions based on countless experiments and observations. 

Carbon-trap shino glazes have the potential to develop soda-ash characteristics regardless of how old the glaze is, but freshly mixed glaze batches have the tendency to create crystal patterns with more frequency and explosiveness. I’ve experimented with both fresh and aged batches extensively, and the fresh batches (within 1–3 weeks of mixing) seem to be more responsive with crystal production. Using a fresh batch of glaze is ideal, but can be quite wasteful. This is why I’ll add a minimum of 50% fresh glaze to an older batch, which will reinvigorate its potential to offer crystal patterns. 

I also glaze within 1 day of kiln ignition to encourage higher soda-ash crystal development. This practice seems to be more important than using fresh glaze to promote crystals. I’ve achieved interesting results with pots that were glazed at least 30 days prior to kiln ignition, but their surfaces are generally fields of carbon trapping without any crystal development. In contrast, pots glazed just before loading and ignition have consistently produced more crystal patterns, and typically respond with more fascinating results overall. 

Reduction

Carbon-trap shino glazes prefer strong reduction over a longer period of time to develop good color. Reduction can be relative between potters, and levels will vary depending on kiln design and fuel source, but consistent reduction from body reduction to 2100°F (1149°C) will help provide good color responses. In my experience, color development is occurring throughout this entire temperature zone.  

During this temperature zone, I’m also balancing surface color with building a carbon-trap layer. I’ve discovered that soda ash is most active for carbon trapping between cone 08 and cone 04 when incorporated into the glaze recipe. The glazes I use start carbon trapping at cone 012 and stop by cone 02, which offers great insight for developing a firing strategy. I try to pass cone 02 within 70–90 minutes after starting a heavy reduction cycle by cone 08. This is my attempt to prevent over-trapping, but black pots are inevitable in my kiln because of the substantial amount of carbon produced from my preference for heavy reduction. I also use stacking as a defender against over-trapping by positioning pots closely together in open areas of the kiln. This strategy can help shield pots from too much exposure to carbon in the atmosphere. 

Courageous Experimentation

It’s important to emphasize that I’ve catered this research to achieve surfaces related to my personal aesthetics. Different potters have different goals, with different facilities and materials. Therefore, I’m offering my process as merely information to be appropriated, and not to follow precisely like a recipe. Potters still need the experience of experimentation and learning the nuances associated with carbon-trap shino glazes before achieving successful results.

I’ve failed countless times in exploring this glaze, including losing entire kiln loads. The loss of kiln loads is quite devastating and humbling, but I embrace that failure as a learning experience. Failure often provides the most important instruction, and often establishes the limits I need in order to effectively navigate toward my goals. 

I also try to employ courage when faced with tough choices, especially during the firing process. It’s hard to subject an entire kiln load to experimentation in order to isolate the effects of one damper position, but for me, the knowledge is more important. 

Maintaining the courage to experiment is fundamental in driving innovation, gaining knowledge, and building sophistication in my fired surfaces. I’m constantly experimenting with processes and materials in order to keep things fresh and inventive. I’m persistently inquiring about the collaboration between the clay body and the shino-glaze layer, which provides incredible depth and complexity to the surface of my vessels. What I love about shino is that nothing will ever repeat itself, and each pot is a unique artifact with the capacity of providing infinite possibilities. 

the author Joe Hicks is a potter and associate professor of art at Marymount University. He teaches workshops throughout his community. Learn more at www.joehicksceramics.com.