Three-phase fire

from Wikipedia, the free encyclopedia
View into a Corinthian kiln on one of the pinakes from Penteskouphia (around 575–550 BC).

As a three-phase firing , (also: three-stage fire) , a combustion process in the production of ceramics of ancient Greece is called. Even Bronze Age vessels have the typical tricolor for this process (clay background yellowish to orange-red or similar, decor brown-red and black). Around the 7th century BC The process was perfected in Greece, so that high-gloss black surfaces were now possible, and until around 300 BC the process was perfected. Used to make black-figure and red-figure vases .

Oxidation states of iron

All shades of the Greek black and red painting and the terra sigillata arise from the different iron content of the clay and the different oxidation levels of the iron in the fired clay. Iron has the special property that it can form oxides of different colors, both black iron (II) oxide (FeO) and red iron (III) oxide (Fe 2 O 3 ) as well as deep black magnetite ( iron (II, III) - oxide Fe 3 O 4 ). Which of these oxidation stages is present depends on the oxygen content and temperature of the reaction mixture: a high proportion of oxygen promotes the production of Fe 2 O 3 , while a lack of oxygen tends to produce FeO. A hammer blow (Fe 3 O 4 ) occurs when the oxygen content is extremely low .

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

Particle size of clay: control of sintering point

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 dull red Fe 2 O 3 , so the excess oxygen must be prevented in the later black areas, the Fe -Oxide particles have to be "sealed". Another property of clays enables this sealing: the sintering point - i.e. 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 lower the sintering point. Finely dispersed paint slurries could be produced by slurrying and skimming them off in layers.

The particle size can be further reduced by adding “peptizing” substances (chemicals that break up the clay particles and prevent them from sticking together again, i.e. suspension aids). For this purpose, are, for example caustic soda (NaOH), ammonia (NH 3 ), potassium carbonate (K 2 CO 3 ), and polyphosphates such as Calgon (NaPO 3 ) 6 : this overlap with strong hydrogen bonds to the clay particles and prevent similar surfactants that these mutually connect and glue. The clay particles are thus in colloidal suspension .

The fire

Before the fire, the clay pots were stacked tightly in the oven. Since Attic ceramics did not receive a completely melting glaze , vessels in the oven could well touch. However, it was important to enable good air / gas circulation in order to avoid false fires .

Phase 1: heating up (oxidizing)

Mining of the clay on one of the penteskouphia pinakes (around 575–550 BC).

The typical fire probably took place at a temperature of 850 to 975 degrees Celsius. If the furnace was constantly fired, this temperature was reached after about 8 to 9 hours. The vessels placed in the oven initially lost the residual moisture from the dried-on clay. At a temperature of 500 degrees, the actual burning of the now glowing vessels began after 6 to 7 hours. With a constant supply of oxygen and as the temperature continues to rise, the strong iron-containing gloss tone oxidizes and takes on a red color like the vessel tone. The iron is converted into deep red iron (III) oxide (Fe 2 O 3 ). This first phase of the three-phase fire lasted about 9 hours.

It is not necessary, but very likely, that this heating-up phase took place in an oxidizing atmosphere: one can assume an oxygen-rich fire simply because it generates heat much more effectively. As a rule, the reducing fire does not allow a strong increase in temperature, but the ceramic will 'cook' faster or earlier in this atmosphere. Therefore, the reduction phase has probably been limited to the comparatively short 2nd burning phase.

Corinthian Pinax : stove with hatch and peephole (?), Can be interpreted as a representation of the reducing fire phase: the excess CO causes flashes from the fire hole and vent.

Phase 2: Reduction (sintering of the gloss slip)

From around 900 ° C, the oxygen supply is cut off, reducing conditions are created. In ancient times, the reducing conditions could be achieved by narrowing the exhaust air openings and adding plenty of fuel, which now only incompletely burned to carbon monoxide (CO instead of CO 2 ). During this process, red Fe 2 O 3 is converted into deep black Fe 3 O 4 .

3 Fe2O3 ⇌ 2 Fe3O4 + ½ O2

The oxygen disappears between 850 and 900 ° C. The disintegration of the magnetite in turn forms reactive iron (II) oxide FeO.

Fe3O4 ⇌ 3 FeO + ½ O2

This reacts with the decomposition products of the illite, which are released from the clay from 750-800 ° C, to form hercynite (Fe⋅Al2O3). The amount of hercynite increases with increasing temperature.

FeO + Al2O3 ⇌ FeO⋅Al2O3

These products are black, which is why the ceramic also turns black. The temperature was held at presumably around 945 ° C. for some time in order to ensure complete melting and sintering of the fine particle paint slip.

Then the temperature sank again to below the sintering point of the paint slip, still in a reducing atmosphere. Now the glossy layer is "sealed" and prevents the transport of oxygen, so that the Fe 3 O 4 oxides stored in these layers will keep their black color from now on. In contrast to the long first phase, the second only lasted about 5-10 minutes.

Phase 3: re-oxidation and cooling

In the last phase of the fire, the ventilation openings of the stove are reopened and unburned green wood is removed from the fire: oxidizing conditions are created again. Due to the increased proportion of oxygen, the black iron oxide can combine with enough oxygen to react again to form red iron oxide. Since the fine painting slip sintered in the painted areas during the reducing phase, compacted and thus sealed, the oxygen is unable to combine with the black iron oxide enclosed in it and to reoxidize it to red iron oxide, which is why the painted areas are black After the red areas had completely oxidized, the oven and its contents could be slowly cooled and then emptied.

Kilns

The prerequisite for the three-phase firing was an adjustable kiln . Apparently the technology required for this was developed in the 7th century BC. Developed in Corinth . The dome ovens with drainage holes that were now in use enabled the production of black-figure and subsequently red-figure ceramics.

The ovens could be permanent or temporary and consisted of field stones and bricks that were smeared with clay. The first fire made the ovens permanent. The ovens were rectangular or round, were up to two meters in diameter and often had a dome with a drain hole in the middle and a potato neck with a pot on one side and an opening for inserting the pottery into the chamber on the other . The opening was bricked up after the vessels had been used. In addition, the ovens had fire channels, fire chambers and a second pierced floor (perforated floor) supported by pillars above the furnace, on which the vessels to be burned stood. The stove was heated with bundles of sticks, logs and charcoal. Figure 9: Pottery workshop with two ovens in Seliunt. Figure 10: Cross section of a pottery furnace. The ovens were partially embedded in the earth to make loading easier and to keep the heat better.

The temperature could be checked either visually through a peephole or through smaller test pieces in the oven.

  • Misfire: Reduction is only satisfactory on the left in the picture, on the right either insufficiently reduced or reoxidized due to incomplete sealing. Possibly caused by uneven heat distribution or poor circulation of the reducing gases in the furnace.

  • Incorrect firing: Insufficiently reduced or not fired high enough so that the paint slip was not sufficiently sealed and reoxidized again in the 3rd phase, i.e. H. turned red. For comparison at the bottom left in the picture a vase with “correct black”.

  • Matching sherds in different oxidation states, found on Areopagus / Athens. Presumably used as firing samples to check the complete reduction in the furnace (left complete, right not yet sufficiently reduced).

  • Fragment of a red-figure Attic vase. Incompletely painted: black background is still missing. Possibly broken during decoration and used as a fire test to verify complete reduction.

literature

  • Marie Farnsworth: Draw Pieces as Aids to Correct Firing. In: American Journal of Archeology . Volume 64, 1960, pp. 72-75, plate 16.
  • Ulrich Hofmann : The Chemical Basis of Ancient Greek Vase Painting. In: Angewandte Chemie. No. 1, 1962, pp. 341-350.
  • Joseph Veach Noble: The Technique of Attic Vase-Painting. In: American Journal of Archeology. Volume 64, 1960, pp. 307-318.
  • Joseph Veach Noble: The Techniques of Painted Attic Pottery. Revised edition. Thames and Hudson, London 1988, ISBN 0-500-05047-3 .
  • Ingeborg Scheibler : Greek pottery art. Manufacture, trade and use of antique clay pots . Second, revised and expanded edition. CH Beck, Munich 1995, ISBN 3-406-39307-1 .
  • Theodor Schumann: Surface decoration in ancient pottery. Terra sigillata and Greek black and red painting. In: Reports of the German Ceramic Society. Volume 32, 1942, pp. 408-426.
  • Adam Winter : The basics of the technique of the Greek potter (= technical contributions to archeology. Volume 1). Roman-Germanic Central Museum, Mainz 1959.
  • Adam Winter: The antique gloss ceramic. Practical trials. von Zabern, Mainz 1978, ISBN 3-8053-0333-5 .
  • Adam Winter, Roland Hampe : With potters in Crete, Messenia and Cyprus. Roman-Germanic Central Museum, Mainz 1962.
  • Frank Hildebrandt: Ancient worlds of images. What Greek vases tell. Philipp von Zabern, Darmstadt 2017.
  • Thomas Mannack: Greek Vase Painting - An Introduction. Theiss, Darmstadt 2002, ISBN 3-534-15059-7 .
  • Ingeborg Scheibler: Greek pottery art. Revised and expanded edition, CH Beck, Munich 1995, ISBN 3-406-39307-1 .
  • Toby Schreiber: Athenian Vase Construction. A Potter's Analysis. 2nd pressure. The J. Paul Getty Museum Malibu, California 1999.
  • Walter Noll: Old ceramics and their pigments - studies on material and technology. E. Schweizerbart´sche Verlagbuchhandlung (Nägele and Obermüller), Stuttgart 1991, ISBN 3-510-65145-6 .

Remarks

  1. ↑ The realization that the clay ground and color hardly differ or do not differ in their chemical composition and only differ in their preparation was first published by Schumann (1942). Later picked up and supported by spectrographic analyzes by Noble (1960).
  2. ↑ This and the note that more than just one seal is necessary, i.e. different sintering points of the different colors for the simultaneous display of shiny black, purple-red and rust-red ( intentional red , also coral red, coral red, e.g. to be seen as Flat background color of the famous Exekias bowl with the sailing Dionysus , Munich, Antikensammlungen 2044) comes from Hofmann (1962).
  3. ^ Treated in detail in Winter (1959).
  4. ↑ Schumann (1942) used caustic soda and ammonia for his experiments, Hofmann (1962) tannins , Noble (1960/1965) names Calgon ((NaPO 3 ) 6 ) and potash. In ancient times, the use of potash can be assumed, as it is a natural component of wood ash when wood is burned in a pottery kiln.
  5. ↑ We have received many incompletely reduced vases, especially from the early days, in which parts of the vessel are still red and other parts completely black, even though both were painted with the same paint slip. However, the carbon-rich atmosphere did not reach these areas or the temperature in these areas of the furnace was insufficient to seal the surface.
  6. ↑ Noble (1960) heated ancient fragments on a trial basis; above 975 ° C, the shiny black surfaces melted and re-oxidized. Experiments with modern Attic clays showed that a fire above 1005 ° C gave them a very light reddish color. Firing temperatures below 1000 ° C, on the other hand, produced color tones very similar to the tone of ancient Attic vessels.
  7. ↑ In modern electric ovens, moist sawdust can be poured through the peephole or air inlet / outlet openings in the oven, which, however, significantly reduces the service life of the heating coils. See Gustav Weiß: Reduction in the electric furnace . In: Ceramic Lexicon . Joseph Veach Noble also used sawdust: Noble (1960), p. 310-311.
  8. ↑ Noble (1960) gives a “soaking period” of at least half an hour.
  9. ↑ The exact sintering point varies from clay to clay, Noble ended this phase in his experiments at 875 ° C (Noble 1960, p. 311).
  10. ↑ The differently rough and thus differently permeable surfaces of the sintered and non-sintered vessel parts are very nicely visualized in the electron microscopic images in the article by Hofmann (1962).
  11. ↑ Self-portrayals of potters at work from clay mining to fire, with pictures of ovens on the Corinthian votive tablets from Penteskouphia (today in the Altes Museum in Berlin). Reconstruction of such pottery furnaces in Winter (1959). Descriptions of today's pottery workshops and ovens: Winter / Hampe (1962).
  12. ↑ Noble (1960/65) and Hofmann (1962) argue that the optical control is sufficient. Farnsworth (1960) examined preserved specimens from the surroundings of excavated ancient pottery kilns.