Kohiki Pottery: The White Slip Technique, Explained

Kohiki is the technique of applying white slip to a red-brown or grey body high in iron, then covering it with a clear glaze and firing.

The name comes from the look of it: soft and beautiful, as though dusted with white powder. It began in Korea, in the buncheong ware of the Joseon dynasty around the 15th century: potters who admired the pure white porcelain of China found a way to make the iron-rich clay at hand look white. From 1592, Korean potters brought to Kyushu carried the technique into Japan, and kohiki-garatsu became one of the faces of Karatsu ware in Saga. It is still made across Japan today — the kohiki we carry is fired by Furutani Pottery in Shigaraki, Shiga, a workshop that has worked there since 1970.

Within the larger technique of white slip, kohiki is the version applied evenly over the whole. Leave the brush marks and it is hakeme; squeeze it from a tube and it is icchin.

A structure of three layers

Seen in cross-section, a kohiki piece has three layers: body, slip, clear glaze. That structure decides every property kohiki has.

The clay itself is not white as porcelain is. A white layer is laid over colored clay. So at the rim and around the foot ring the slip thins and the red-brown body shows through. That red appearing at the edge of the white is exactly what to look at in kohiki.

And the middle slip layer is less dense than glaze. It holds moisture. That is the true source of its ability to develop.

Why it develops

Fine crazing enters the surface glaze through the difference in shrinkage after firing. Tea and coffee seep through those gaps and spread into the slip layer.

• The day you buy it　A cool, even white. Nothing has begun to change.
• Six months　It settles a little, and the crazing lines begin to show faintly.
• Some years　Crazing like leaf veins appears, and the white turns a soft grey-cream.

Uneven absorption can also raise cloud-like mottling, called amamori, rain-leak. This is keinen bika — received not as dirt but as a record of time.

Making it last

Absorbency means it takes a little knack. Nothing difficult.

• Do medome at the start　Simmering gently in rice-rinsing water for 20–30 minutes lets starch fill the crazing, keeping odour and staining out.
• Pass it through water before use　Pour soy sauce or coffee straight into a dry piece and that color alone goes in dark. Letting it drink water first makes it develop evenly.
• Dry completely after washing　Put away damp and mould appears in the slip layer as black spots. Standing it upside down overnight is reliable.
• Avoid dishwasher, microwave and bleach　High heat and sudden wet-dry cycles load the boundary between slip and glaze.

Related terms and guides

Related terms

• Shirogeshou: How Japanese Potters Make Dark Clay White
• Hakeme: The Brush-Marked White Slip Technique
• Icchin: Slip Trailing, the Raised Dot Technique
• Keinen Bika: Pottery That Gets Better With Age
• Medome: How to Seal Unglazed Pottery Before First Use
• Japanese Pottery Glossary | Terms, Techniques & Kilns

Read further

• Crazing (Kannyu): Cracks in Glaze That Aren't Damage
• Pottery That 'Grows': How Ceramics Change With Use
• Reading Wabi-Sabi in a Single Bowl
• How to Care for Japanese Ceramics: A Practical Guide