Shizen-yu: Natural Ash Glaze Explained
Shizen-yu is glaze that arises without anyone applying it. Through a long wood firing, ash from the burning wood drifts through the kiln, settles on the surface of a piece, melts at high temperature and becomes a glassy glaze.
Where ash falls thickly the color is dark, where it falls thinly it is pale — from olive green to amber, varying across a single piece. It forms the core of the scenery of the Six Ancient Kilns — Bizen, Iga, Shigaraki, Echizen, Tamba — and no two pieces are ever the same.
It is glaze the kiln itself applies to yakishime, pottery fired hard at high temperature without any glaze.
How ash becomes glass
In a wood kiln, red pine and similar woods burn continuously for several days to a fortnight. The ash rides the rising air, drifts through the kiln, and settles on the upper faces of pieces and the sides the flame strikes.
Wood ash contains potassium and calcium — the materials of glaze itself. Melting above 1,200°C, that ash turns glassy on the surface. Exactly what happens when a person mixes and applies a glaze happens naturally inside the kiln.
So natural glaze never covers a whole piece. Only the faces where ash settles shine green; the opposite side stays bare clay. That gap between the covered face and the uncovered one is what to look at in a natural-glazed piece.
The characteristic scenery of natural glaze
- Goma (sesame) Ash falling thinly in specks and melting so that it looks sprinkled with sesame. The representative scenery of Bizen.
- bidoro Ash settling thickly, melting and pooling in a vivid, glassy green. Prized on Iga and Shigaraki.
- Koge (scorch) Areas buried in ash that carbonise and burn black. The contrast against green and orange comes out strongly.
- Hiiro (fire color) Faces struck directly by the flame, burnt from orange to red-brown. Beautiful set beside the green of ash.
Natural glaze against applied glaze
| Aspect | Natural glaze | Applied glaze |
|---|---|---|
| How it is applied | Ash settles inside the kiln | Dipped, poured or sprayed |
| Coverage | Only faces where ash falls; often one side | Evenly, wherever intended |
| Repeatability | Never the same twice | Reproducible through recipe and temperature |
| Firing time | A wood kiln, several days to a fortnight | One to two days in an electric or gas kiln |
The expression brought by a chance of flame and ash that not even the maker can predict is the greatest appeal of natural glaze. Few techniques suit the phrase ichigo ichie better.
Related terms and guides
Related terms
- Bidoro: The Glass-Green Drips on Bizen Pottery
- Yakishime: Unglazed Pottery Fired to Its Limit
- Anagama: Inside Japan's Oldest Wood-Fired Kiln
- Noborigama: The Climbing Kiln, Chamber by Chamber
- Hidasuki: The Red Straw Marks on Bizen Pottery
- Medome: How to Seal Unglazed Pottery Before First Use
- Japanese Pottery Glossary | Terms, Techniques & Kilns
Read further
- Keshiki: How to Read the 'Landscape' on a Pot
- Bizen Ware: Pottery With No Glaze at All
- Shigaraki Ware: Why Its Clay Sparkles After 1,200 Years
- Iga Ware: Fire, Ash and the Beauty of Rough Clay
- How to Care for Japanese Ceramics: A Practical Guide
Look at the glaze the kiln applied
Green where ash fell, orange where flame struck. Change the angle of a photograph and a natural-glazed piece becomes another piece. Nokaze introduces unglazed stoneware.
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New to Japanese pottery? Start here — its history, how pottery differs from porcelain, the clays and glazes behind each style, and how to care for a piece. → Japanese Pottery: The Complete Guide