Home / Wood-Ash Glaze in High-Fire Ceramics

Wood-Ash Glaze in High-Fire Ceramics

Hadar Iron

Ceramic Artist & Fermentation Educator


It is such a pleasure for me to write about the wood-ash glazes I use here in the studio – about how the beautiful surfaces of these vessels come to be, and about how that process is connected to our community and to the natural world around us. Central to this story are the trees that give us the wood which is at the heart of the process. I’d like to share some knowledge about glazes, high-fire ceramics, and wood-ash glazes in particular, so that you can appreciate with me the wonder of these processes and the beauty of the results.

Forest wood ash glaze on Sake set made by InFerment

Sake Carafe and cups glazed with inFerment’s studio wood ash glaze called Forest

What is glaze and why do we need it?

Glaze was developed primarily to make ceramics more hygienic. Unglazed ceramics are very porous, so that cleaning them well is difficult. The glaze provides a smooth non-porous surface that doesn’t retain particles, and which can be cleaned without deteriorating. Glaze is a chemical composition of raw materials such as minerals and oxides. It is based on silica, a glassy material, which covers and bonds to the clay during the firing. The three essential elements are: silica and alumina, which create the glossiness and provide the structural parts of the glaze; feldspars, which act as fluxes; and colorants. Silica and alumina have very high melting temperatures, so we add feldspars to lower their melting temperature. Once melted, the molecules can separate and bond with the molecules of the clay and the colorant. The colorants in the glaze are usually oxides of iron, cobalt, copper, chrome or nickel. Some fluxes affect the color as well. Iron is the main contributor and can produce a wide range of colors, depending on its concentration – a small amount of iron oxide will result in light blue glazes such as the Chinese blue celadon, and as we add more iron to the formula, we can create greens, yellows, reds, browns, and even black as in Tenmoku, the Japanese glaze.

Desert wood ash glaze on bowl with lid made by InFerment

Bowl with lid glazed with inFerment’s studio wood ash glaze called Desert

Low fire glazes and high fire glazes.

As the names suggest, low and high fire glazes are glazes that are fired at lower and higher temperatures. Low fire ceramics, such as terra cotta, are fired at temperatures between 1700 and 2000 degrees Fahrenheit (927-1093 Centigrade). High fire ceramics are fired at temperatures between 2012 and 2336 degrees Fahrenheit (1000-1280 degrees Centigrade). Commercial tableware is usually fired in between the two, in what is called the mid fire range. When glazed ceramics are fired, the silica based surface material covers and bonds to the clay. In the low fire process, the glaze becomes an exterior coating, which protects the clay underneath. The clear glaze can be chipped off, revealing the decorated or undecorated clay beneath it. In a high fire process, the glaze chemically bonds with the clay body. It rarely chips or separates from the clay it protects. Clay that is high fired actually becomes itself vitreous, and thus nonporous, but without glaze the surface is rough and hard to clean. The glaze creates a smooth surface and also, because of the strength of the bond between the glaze and clay, actually makes the vessel stronger. A vessel that is high fired and glazed is more durable – resistant to extreme conditions such as heat, freezing, pressure, and the forces brought to bear in a dishwasher.

Variation of wood spices ash stored in separate buckets in inFerment studio
Variation of wood spices ash stored in separate buckets in inFerment studio

What is wood ash glaze?

Wood ash is what is left after wood is burnt. Present in that ash are minerals that have been drawn by the tree from the earth up into its trunk and branches. These minerals include silica, as well as fluxes such as calcium, sodium, and potassium – and sometimes a colorant, like iron oxide. Because these minerals are already in the wood ash, it can function as a glaze just by itself in high-fire ceramics. Each tree has its own mineral composition that affects the character and appearance of the glaze. Since it is hard to know the exact amount of each mineral in a particular batch of ash, the results are always to some degree unpredictable. In order to develop a durable and reliable glaze, we test each wood ash species we use in our glaze mixes. Often, we modify the formula – to minimize a property in the glaze that is undesirable, such as a too glossy look, or too drippy– or to maximize a feature that we like.

Mixing glaze test based on a formula we develop in inFerment studio

Mixing glaze test based on a formula we develop in inFerment studio

Why wood ash glaze is beautiful and fascinating.

Inferment bowl with Glacier glaze

Bowl made at inFerment’s studio wood ash glaze called Glacier

Wood ash glazes create expressions that are not found in other glazes. The surface of a wood ash glaze is complex and rich with layers of colors and textures, as if there were not one but several glazes on the surface. The impression it gives me is like that of a musical instrument that produces several sounds at the same stroke, or of two hands playing simultaneously on a piano. In ceramics jargon we call these phenomena “phase separation” and “crystallization.” Rather than going further into descriptions of the aesthetic qualities of wood ash glazes I want to direct your attention to the beauty of the ash glazes actually in the studio. 

Bowl with lid glazed with inFerment's studio wood ash glaze called Moss
Bowl with lid glazed with inFerment’s studio wood ash glaze called Moss
Look at the “Moss” glaze, at the black and brown spots floating on the green surface, and at the occasional golden yellow spots, which appear at a certain temperature. Look at the “Glacier” glaze, with its soft white clouds and blue streaks – or at the texture of the “Forest” glaze with vertical trails of liquid glassy material flowing down amid shadows of yellow, green, and brown – or at the “Desert” glaze with its arid and austere surface. All of these effects are very particular and unique to wood ash glazes created in high fire.
Waxwing wood ash glaze on mixing bowl by inFerment
Bowl with lid glazed with inFerment’s studio wood ash glaze called Waxwing

Wood ash in the studio – connecting to the natural environment.

Here in the studio we have a collection of wood ash from a variety of trees. Each variety is stored in a labeled bucket because each one of them is a unique substance. We have ash from apple, maple, fir, pine, tamarack, and even compressed sawdust logs. Each kind offers us a different set of properties which result in a different palette of color, sheen, and texture. We get the ash, which is mixed with coal, either from the local pizzeria, with whom we have formed a friendly collaboration, or from other studio partners’ fireplaces. The mixed ash needs to be sieved into a fine powder, with no coal chips in it. This powder contains the basic minerals, and it is added in a specific portion to the glaze mix, based on the formula we have developed in the studio. Each mix has gone through several tests before we make a big batch to glaze our pots.

Sustainability, and using material resources consciously.

Pick up a bucket of coal and ash from the local pizzeria Mioposto in Seattle
Pick up a bucket of coal and ash from the local pizzeria Mioposto in Seattle

It is fascinating to be able to make good use of a substance that is considered waste, but which has such value to offer in another form. From their birth the trees, as they breathe and grow, slow climate change by removing carbon dioxide from the air, storing the carbon, and releasing oxygen into the atmosphere. Their shade, their forms, and the beauty of their leaves and flowers inspire us and make our lives pleasant. Even when it no longer lives, the tree offers us this wonderful substance with which we can create a useful and beautiful glaze. This awareness and understanding of the natural sources of the materials we work with can deepen and broaden our experience. Thinking about the origins of all of our materials, including the earth itself and the minerals that are mined from that earth across the world – mined in places far away by people who have lives as we do – can help us be more responsible and humane.

Forest wood ash glaze on a miso vessel made by inferment
Two gallon gallons Miso fermenting vessel glazed with inFerment’s studio wood ash glaze called Forest

My own experience with these glazes, their origins, and the larger world.

In addition to these ethical and humanizing possibilities, for me there is also a poetic dimension to my experience with wood ash glazes. Though we work hard to develop a practical and repeatable product, there is always some unpredictability in the outcome. This unpredictability is intriguing and inspiring to me as an artist who works with natural substances – minerals, rocks, wood, fire, and water. The elements have their own expression. I’m also inspired by working with wood that has been collected in my own region – the mountains, the forests, and the orchards around Washington State. Working with the ash, the remains of the trees that surround me, connects me to the place in which I live, think, and feel. When I drive to the wood oven pizzeria to collect ash from their ovens and get a chance to chat with the cook, I feel a part of my neighborhood. This dynamic – in which elements flow into the studio from my local environment and the larger world, are transformed there, and then are shipped out to other localities and other countries as sculptural forms and functional vessels – is something I actually experience as a wider aesthetic experience. Thank you for being a part of it.

Desert wood ash glaze on vessels inferment
Two gallon gallons water seal fermenting crock glazed with inFerment’s studio wood ash glaze called Desert, Condiment dish glazed with Moss and Garlic Holder glazed with Desert wood ash glaze.