yūyaku 釉薬

Also yū 釉 or uwagusuri. Glaze. In the heat of firing, clay particles vitrify to form a glass matter capsule himaku 被膜 which covers ceramics to make them non-porous, gives added strength, and provides gloss to the surface. Glazes are differentiated in several ways: by firing temperature, kōkadoyū 高火度釉 (high firing glaze) and teikadoyū 低火度釉 (low firing glaze); by appearance, tōmeiyū 透明釉 (transparent glaze), futōmeiyū 不透明釉 (opaque glaze), tsuyakeshiyū 艶消釉 (matte glaze), irogusuri 色釉 (color glaze), and kesshōyū 結晶釉 (crystal glaze); and by ingredient, tetsuyū 鉄釉 (iron glaze), dōyū 銅釉 (copper glaze), enyū 鉛釉 (lead glaze), chōsekiyū 長石釉 (felspathic glaze), kaiyū 灰釉 (ash glaze), and sōdayū ソーダ釉 (alkaline glaze). Basic ingredients are silicic acid, aluminum oxide, sodium, kalium, calcium, and magnesium, with lead, zinc, and other metals added for some glazes. Old pottery glazes are divided into alkaline glaze, lead glaze, and ash glaze. Lead glaze, common since the Han dynasty in China, is found in on ryokuyū 緑釉 (green glaze), sansai 三彩, and Japanese *rakuyaki 楽焼.
Ash glaze, developed as a natural glaze in China, deploys wood ash or lime which melts evenly at 1200 C. The color of most glazes depends on the type and amount of metal content, the oxidation, deoxidization, or neutrality of the firing process, and the properties of solvents. For instance, iron glaze fires to black, brown, amber, or yellow; copper glaze fires to green, red, or blue; and manganese glaze fires to purple, black, cobalt, or indigo blue. Changes in color or texture that take place during firing are called yōhen 窯変 (kiln changes).
In Japanese pottery glaze serves not only a practical sealing function but also is an integral part of the aesthetic appeal. Most Japanese pottery is made watertight by high firing, only the lead glaze on earthenwares serves as a sealant. Early pottery, except for lead-glazed sansai ware of the 8th century, utilizes natural glazes and is thus classified as "unglazed." Although usually created by a mix of glazes, they are generally described as having ash glazes. There are three types of ash glaze: pure wood ash, which may be added by hand or created through natural kiln precipitation or contrived precipitation as in *bizen'yaki 備前焼, *shigarakiyaki 信楽焼 and *igayaki 伊賀焼; ash combined with iron-rich ochre clay to produce reddish brown opaque glazes kakiyū 柿釉; and minerals added to wood ash including wood ash poor in iron oxide for porcelain glazes, wood ash with iron oxide for ki-seto 黄瀬戸, yellowish to reddish brown and black iron glazes ameyū 飴釉, and wood ash with pulverized brass for oribe glaze *oribeyaki 織部焼. Feldspar and pegmatite are also frequently used minerals. Shino glaze *shinoyaki 志野焼 is made only of feldspar. Wood ash and rice-straw ash are also used together in such glazes as dōseiji 銅青磁 (a bluish green opaque copper glaze), and shirohagi 白萩 (the bluish white or sprinkled blue of *hagiyaki 萩焼). Many other glaze effects can be achieved by adding wood charcoal or salt during the firing, or by the arrangement of pots in the kiln.