Three-phase firing
As a three-phase fire, (also: three-stage fire ), a combustion process in the production of ceramics of ancient Greece called. Already Bronze Age vessels, the typical for this procedure three colors ( yellow to orange-red clay ground, etc., decorative brown - red and black) on. Around the 7th century BC in Greece, the process was perfected, so now high-gloss black surfaces were possible, and used until around 300 BC, black-figure and red-figure vases to production.
Oxidation of the iron
All shades of Greek black -red- painting and terra sigillata caused by different iron content of the clay and the different oxidation states of iron in fired 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). What exists of these oxidation states depends on oxygen content and temperature of the reaction mixture: a high proportion of oxygen promotes the production of Fe2O3, lack of oxygen produced tend to FeO. At extremely low oxygen content arises hammer (Fe3O4 ).
The color of iron-containing clays can be in three- phase fire so about controlling the atmosphere in the furnace between " reducing " (that is low in oxygen, carbon- rich) and " oxidizing " (that is rich in oxygen ) affect.
Particle size of the clay: control of the sintering point
However, in order to generate more than one color per vessel, another trick is still necessary: one must prevent the black FeO again converts in matt red Fe2O3, ie, the excess of oxygen must be suppressed in the later black areas, the Fe - oxide particles have "sealed" are. Another property of these clays makes sealing: the sintering point - that is, the temperature at the beginning, the individual clay particles to fuse with each other - depends on the composition of the clay and its particle size. Small clay particles and a high potassium content lower the sintering point. The production of finely dispersed Malschlicker could be done by slurrying and layerwise skimming.
By adding " peptizing " substances ( chemicals that break up the clay particles and prevent them from re- sticking together, thus suspending agent ) the particle size can be further reduced. These are, for example caustic soda (NaOH ), ammonia ( NH3), potash ( K2CO3) and polyphosphates such as Calgon ( NaPO 3 ) 6: these overlap with strong hydrogen bonds to the clay particles and prevent similar surfactants that they connect to each other with each other and glue. The clay particles are thus in colloidal suspension.
The fire
Before firing, the clay pots were stacked tightly in the oven. Since Attic pottery was not completely melting glaze to vessels could quite touching in the oven. However, it was important to make a good Luft-/Gas-Zirkulation to avoid false fires.
Phase 1: initial heating ( oxidizing)
The typical fire was presumably at a temperature of 850-975 degrees Celsius. With continuous firing of the furnace, this temperature has been reached after about 8-9 hours. Accommodated in the furnace vessels lost here any residual moisture of the dried clay. At a temperature of 500 degrees set after 6-7 hours of actual fire of the now glowing vessels. Under the constant supply of oxygen and the temperature rises further oxidized to the highly ferruginous glossy slip and takes the vascular tone as a red color. The iron is transformed into deep red iron (III ) oxide ( Fe2O3).
It is not necessary, but very likely that this heating phase took place in an oxidizing atmosphere can be an oxygen-rich fire already accept only because it generates a lot of heat more effectively. The reducing fire usually allowed no strong temperature increase more, but the ceramic is faster and earlier in this atmosphere ' even '. Therefore, the reduction phase has probably limited to the second firing stage comparatively short.
Phase 2: Reduce ( sintering of Glanzschlickers )
From about 900 ° C, the oxygen supply is cut, it will be reducing conditions created, red Fe2O3 transforms into deep black Fe3O4. This was done in ancient times about by narrowing the exhaust vents and adding plenty of fuel now burned incompletely to carbon monoxide (CO instead of CO2). With presumably about 945 ° C, the temperature was maintained for some time, to ensure complete melting and sintering of fine particles of Malschlickers.
Then, the temperature dropped back to below the sintering point of the Malschlickers still in a reducing atmosphere. Now the Glanztonschicht is "sealed", and prevents the transport of oxygen, so that the stored in these layers Fe3O4 oxides from now on its black color to keep.
Phase 3: Re - oxidation and cooling
In the last phase of the fire, the air inlet openings of the furnace are opened again: it will be created again oxidizing conditions. In the areas of the vessels that were not sealed in the second phase, it is now to re- oxidation: black Fe3O4 is transformed again into red Fe2O3. After the complete oxidation of the red areas oven and contents were cooled slowly and eliminated thereafter.
Concluding Remarks
Prerequisite for the three- phase fire was a controlled kiln. It seems that necessary for this technique was developed BC in Corinth in the 7th century. The cupola now used with drain hole allowed only the black-figure and red-figure production in its impact ceramics. The control of the temperature could be done either optically through a peephole or by smaller specimens in the oven.
Fehlbrand: Terrible reduced or not fired high enough so that the Malschlicker was not adequately sealed and in the third phase reoxidized again, that is was red. For comparison, the bottom left image is a vase with " proper black".
Matching shards in different oxidation states, found at the Areopagus / Athens. Probably used as a fuel sample to check the complete reduction in the oven (left completely right not sufficiently reduced).
Fragment of a red-figure Attic vase. Incomplete painted: black background is still missing. Possibly broken when decorating and used as a fuel sample to check the complete reduction.