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Three-phase firing
Three-phase firing (or three-step firing) or iron reduction technique is a firing technique used in ancient Greek pottery production, specifically for painted vases. Already vessels from the Bronze Age feature the colouring typical of the technique, with yellow, orange or red clay and brown or red decoration. By the 7th century BC, the process was perfected in mainland Greece (Corinth and Athens) enabling the production of extremely shiny black-slipped surfaces, which led to the development of the black-figure and red-figure techniques, which dominated Greek vase painting until about 300 BC.
== Stages of iron oxidation ==
All colours of Greek black-red vase painting, as well as of terra sigilata are produced by the different concentrations of iron in the clay, and the different degrees to which that iron is oxidised during firing. Iron has the special feature of forming oxides of various colours, including grey Iron(II) oxide (FeO), red Iron(III) oxide (Fe2O3), and deep black magnetite Fe3O4). Which of these types of oxidation is achieved depends on the availability of oxygen and the temperature of the reactive mix: a high oxygen content encourages the production of Fe2O3, while a lack of it tends to lead to the creation of FeO or Fe3O4.
Thus, the colour of iron-rich clays can be influenced by controlling the atmosphere during firing, aiming for it to be either "reducing" (i.e. poor in oxygen and rich in carbon) or "oxidising" (i.e. rich in oxygen). This control is the essence of three-phase firing.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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