What a glaze is
A glaze is a thin layer of glassy material that melts onto the surface of a pot in the kiln. It makes the pot smoother, less porous and easier to clean, and it gives the potter a way to add color and texture. Most glazes are made from silica (the glass-forming part), a flux that helps it melt, and often a small amount of a metal compound that supplies color. Because the same recipe can look different depending on thickness, temperature and kiln atmosphere, glaze colors are often described with comparisons to nature and food rather than by exact names.
Celadon: the pale green glaze
Celadon is a family of glazes in soft green, blue-green or gray-green. The color comes from a small amount of iron in the glaze combined with a firing in which the kiln has limited oxygen (a reducing atmosphere). The tradition began in China, and Japan received celadon wares by trade and imitated them in various places over the centuries. Chinese celadon was admired by Japanese collectors, and later Japanese kilns, including some porcelain makers in northern Kyushu, made celadon of their own.
A good celadon is usually valued for depth and calm rather than brightness. Descriptions often mention jade, a still pond or young leaves. The glaze may be glossy and thick, and a pale body beneath can make the green look luminous. Because slight changes in iron or firing shift the color toward yellow, olive or blue, makers aim for consistency, and viewers notice small variations.
White, cream and the feldspar glazes
Many Japanese glazes are based on feldspar, a common mineral that melts to a smooth, opaque or semi-opaque surface. These range from creamy white to milky gray. In Mino ware, the thick white Shino glaze is one example, often showing tiny pinholes and areas of orange-pink where clay shows through. In Hagi, a white glaze over soft clay tends to craze (develop a network of fine cracks), and over time tea can seep into those lines and gradually change the color; this is often treated as part of the bowl’s life rather than a flaw.
Ash glazes: color from the fire
In wood firing, ash falls on the pot, melts, and forms a natural glaze. Potters also deliberately make ash glazes by mixing plant ash with clay and other materials. The results can be green, yellow, amber or gray, sometimes streaked or running in thick drops. These are the surfaces linked to the long firings described in discussions of anagama kilns, and their irregularity is a large part of their appeal.
Iron-brown and black: tenmoku
Iron is also behind the dark glazes. In high concentrations it makes surfaces brown, deep black or reddish-brown. The term tenmoku (also spelled temmoku) is a Japanese reading of the name of a mountain region in China associated with dark tea bowls, and today it is used loosely for glossy iron-rich glazes. Related names include kaki, “persimmon,” for a warm orange-brown, and ame, “candy,” for a glossy amber-brown. Dark glazes were prized for tea, since the green froth of whisked tea shows clearly against them, a point familiar to anyone who has studied the Japanese tea ceremony.
Copper green and Oribe
A bright, translucent green is often made with copper. In Japan this is closely tied to Oribe ware from Mino, where green glaze may cover one half of a dish while the other half carries painted patterns on a white or brown ground. The color is lively, even playful, and quite unlike the restraint of celadon.
Overglaze enamels
On porcelain, glazes can be clear and the decoration added later. Painted enamels in red, green, blue and yellow, fused in a second, lower firing, are typical of the colorful Arita and Imari wares. Under-glaze cobalt blue, painted before glazing, is another widespread option. These decorative colors are in a different category from the body-glaze colors discussed above, but collectors often discuss them together.
How collectors talk about glaze
Words used in praise of a glaze are often quieter than expected: depth, softness, warmth, calm. A glaze may be said to “have a skin” or to look “wet.” These descriptions fit a wider habit of appreciating subtle shifts rather than bold contrast, and they overlap with ideas such as wabi-sabi, where modest, aged-looking surfaces are valued. At the same time, taste varies by region, period and person; some Edo-period wares are bright and decorative, and some modern potters work with glossy, strongly colored glazes.
A note on safety
Glaze materials include metal compounds, and glazes used on dishes need to be formulated and fired correctly to be food-safe. Anyone interested in making glazes should learn from a qualified teacher. As a buyer, it is reasonable to ask the maker whether a piece is intended for food and drink, particularly with older or decorative items.