yuuyaku　釉薬

KEY WORD :　art history / crafts

Also yuu 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, koukadoyuu 高火度釉 (high firing glaze) and teikadoyuu 低火度釉 (low firing glaze); by appearance, toumeiyuu 透明釉 (transparent glaze), futoumeiyuu 不透明釉 (opaque glaze), tsuyakeshiyuu 艶消釉 (matte glaze), irogusuri 色釉 (color glaze) and kesshouyuu 結晶釉 (crystal glaze); and by ingredient, tetsuyuu 鉄釉 (iron glaze), douyuu 銅釉 (copper glaze), enyuu 鉛釉 (lead glaze), chousekiyuu 長石釉 (felspathic glaze), kaiyuu 灰釉 (ash glaze) and soudayuu ソーダ釉 (alkaline glaze). Basic ingredients are silicic acid, aluminumoxide, 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 ryokuyuu 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 youhen 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 8c, 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 *bizenyaki 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 kakiyuu 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 ameyuu 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 douseiji 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.

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