ROMSO Cyprus Knowledge Base

--- CONTENT ---
Three-phase firing, (also: three-stage firing), is a firing process in the ceramic production of ancient Greece. Already Bronze Age vessels have the three-coloredness typical for this process (clay background yellowish to orange-red or the like, decoration brown-red and black). Around the 7th century BC, the process was perfected in Greece, so that now high-gloss black surfaces became possible, and until around 300 BC used for the production of black-figure and red-figure vases.

Oxidation states of iron
All shades of Greek black-red painting and Terra Sigillata are formed by different iron content of the clay and the different oxidation states of iron in the burned clay. Iron has the special property that it can form different colored oxides, both black iron(II) oxide (FeO) and red iron(III) oxide (Fe2O3) and deep black magnetite (iron(II,III) oxide Fe3O4). Which of these oxidation states is present depends on the oxygen content and temperature of the reaction mixture: a high proportion of oxygen promotes the production of Fe2 O3; if oxygen is lacking, FeO tends to form. At extremely low oxygen content, hammer strikes (Fe3O4).

The colour of iron-containing clays can thus be influenced in three-phase firing by controlling the atmosphere in the furnace between “reducing” (i.e. low-oxygen, high-carbon) and “oxidising” (i.e., high-oxygen).

Particle size of clay: sinter point control
However, in order to produce more than just one color per vessel, another trick is necessary: you have to prevent the black FeO from converting back into matte Fe2O3, so the excess oxygen must be prevented in the later black regions, the Fe oxide particles must be “sealed”. Another property of the clay enables this sealing: the sintering point – the temperature at which the individual clay particles begin to fuse together – depends on the composition of the clay and its particle size. Small clay particles and a high potassium content reduce the sintering point. The preparation of finely dispersed paint slips could be effected by slurry application and layerwise skimming.

By adding “peptizing” substances (chemicals that break up the clay particles and prevent them from sticking together again, i.e. suspension aids), the particle size can be further reduced. Suitable for this purpose are, for example, caustic soda (NaOH), ammonia (NH3), potash (K2CO3) and polyphosphates such as calgon (NaPO3)6: these attach to the clay particles with strong hydrogen bridges and, like surfactants, prevent them from bonding and sticking together. The clay particles are thus in colloidal suspension.

The fire
Before the fire, the clay vessels were stacked tightly in the oven. Since Attic ceramics did not receive a completely melting glaze