The Alchemy of Ash Glazes: Turning Orchard Prunings into Glass

In February, the fruit growers in the hills west of Lyon prune their apple and pear orchards. For centuries, the tangled bundles of dried cuttings—the sarments—were collected into bonfires at the end of the vineyard rows, their smoke drifting blue and fragrant across the dormant terraces.

While most people see wood ash as agricultural waste, to a studio potter it is a box of precious minerals. Wood ash is what happens when a tree spends eighty years drinking groundwater, extracting dissolved silica, calcium, potash, magnesium, and iron from deep subsoil strata, and storing them within its cellular walls. When the wood burns, the carbon escapes as gas, leaving behind a fine grey powder of pure mineral flux.

The Chemistry of the Bonfire

Commercial pottery glazes are sterile chemical recipes composed of industrially mined feldspar, barium carbonate, synthetic colorants, and toxic cobalt oxides designed to melt at precise temperatures in electric kilns. They produce flat, uniform surfaces that resemble bathroom tiles.

Traditional wood ash glaze, discovered accidentally in China during the Shang dynasty when fly ash from wood-fired kilns landed on raw clay jars and melted into a greenish glaze, operates on organic unpredictability. Ash from an apple tree will not melt like ash from an oak; ash from a grapevine harvested in a limestone-rich parcel will produce an entirely different color than ash from pines growing in granite sand.

“Apple wood is rich in calcium carbonate and phosphate, yielding soft, opaque celadons that pool like melted jade in the carved fluting of a bowl. Pine ash, high in silica and iron, runs down the vessel in transparent, honey-amber rivulets.”

Washing the Lye: The Patient Slaking

You cannot simply mix raw fireplace ash with water and paint it onto a pot. Unwashed wood ash contains potassium hydroxide (caustic potash or lye), which will burn your skin and cause raw clay vessels to slake and dissolve upon contact.

The preparation requires weeks of washing. I dump forty pounds of sifted orchard ash into a large wooden vat, fill it with collected rainwater, and stir it with a wooden paddle. The ash settles to the bottom; the soluble lye dissolves into the water above. After forty-eight hours, I siphon off the caustic water and refill the vat with fresh rain. I repeat this cycle five or six times until the water tests neutral on litmus paper.

The remaining washed ash is dried on linen sheets, weighed, and mixed with equal parts of local feldspar and washed river clay. That is the entire formula: one-third wood ash, one-third stone, one-third earth.

The Melt at Cone 10

Inside the reduction atmosphere of a gas or wood kiln at 1,300 degrees Celsius (Cone 10), something extraordinary happens. The intense heat causes the calcium in the ash to act as a powerful flux, lowering the melting point of the refractory silica in the clay.

The powdered grey coating liquefies into a bubbling, molten glass. Because wood ash contains microscopic traces of titanium and phosphorus, the cooling glaze develops microcrystalline structures that scatter incoming light—an optical phenomenon known as Rayleigh scattering. The finished bowl does not look painted; it looks as though a layer of mountain ice has frozen over the dark clay.

When you hold an ash-glazed tea bowl, you are holding the distilled mineral memory of an orchard that was pruned on a cold winter morning. Craft at this scale is not about manufacturing novel shapes; it is about finding the shortest path between the landscape and the kitchen table.