Three-phase firing

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Kiln with opening and viewing hole, perhaps a depiction
of the second or reducing phase: the surplus of CO leads to jets of flame from stoking hole and vent (Corinthian pinax, ca. 575-550 BC) Plaque Penteskouphia MNB2858.jpg
Kiln with opening and viewing hole, perhaps a depiction of the second or reducing phase: the surplus of CO leads to jets of flame from stoking hole and vent (Corinthian pinax , ca. 575–550 BC)

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.


The conventional view, developed in modern times in view of a lack of contemporary accounts, was that painted Greek pottery received a single firing, after the shaped pot had been dried leather-hard and then painted. But the firing had three phases, designed to create the intended colours. Sometimes further painting in other colours was added after firing, especially in white-ground and Hellenistic vases. However, new studies instead provide material evidence that the pottery was made with two or more separate firings [1] in which the pottery is subjected to multiple firing stages. The conventional view is described in more detail below, but the possibility of different firings for the phases described should be kept in mind.

Stages of iron oxidation

A misfired black-figure vessel, with reduction satisfactory only in the left part: the area on the right either reduced insufficiently or reoxidised due to insufficient sealing, perhaps as a result of uneven temperature distribution or bad circulation of reducing gases in the kiln. Misfired black-figured vase from Thebes.jpg
A misfired black-figure vessel, with reduction satisfactory only in the left part: the area on the right either reduced insufficiently or reoxidised due to insufficient sealing, perhaps as a result of uneven temperature distribution or bad circulation of reducing gases in the kiln.

All colours of Greek black-red vase painting 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.

Vitrification and sintering

To achieve more than one colour on a given vase, a further trick is necessary: The black magnetite Fe3O4 has to be prevented from returning to matt red hematite Fe2O3. In other words, the areas to remain black have to be denied access to oxygen, their oxidised particles must be "sealed". This is achieved by using a further property of the clay: the vitrification point, i.e. the temperature at which the individual clay particles irreversibly merge, depends on the composition of the clay and on the particles contained in it. [2]

A misfired red-figure vessel: insufficient reduction or too-low firing temperature caused slip to seal insufficiently and thus reoxidise (return to red), in 3rd phase; compare (at bottom left) vase with "correct" black. Misfired vase, red-figured, not fully reduced in firing.jpg
A misfired red-figure vessel: insufficient reduction or too-low firing temperature caused slip to seal insufficiently and thus reoxidise (return to red), in 3rd phase; compare (at bottom left) vase with "correct" black.

Smaller clay particles and a high calcium content lower the sintering point. [3] The production of finely varied painting slips was achieved through levigation and the subsequent scooping off of various layer. [4] The addition of "peptising" substances (i.e. substances that break up and separate the clay particles and prevent them from coagulating again) can further reduce particle size. Such substances include caustic soda (NaOH), ammonia (NH3), potash (K2CO3) and polyphosphates such as calgon (NaPO3)6: these attach themselves to the clay particles with strong hydrogen bonds and thus prevent them, in a similar way to tensides, from rejoining and coagulating again. In other words, the clay particles are now in a state of colloid suspension. [5]


Before firing, the clay vessels were densely stacked in the kiln. Since Attic pottery contains no glazes proper (i.e. ones that melt and vitrify completely), vessels could touch in the kiln. However, it was of major importance to achieve a good circulation of air/gas, so as to prevent misfiring. [6]

Phase 1: Kindling (oxidising)

Typical firing probably took place at a temperature of 850 to 975 degrees Celsius. [7] With constant firing of the kiln, such temperatures were reached after about 8 to 9 hours. During this process, the vessels in the oven initially lost whatever moisture remained in them. At a temperature of 500 °C, after 6 or 7 hours, true firing of the now red-hot vessels began. With a constant supply of oxygen and a still increasing temperature, the iron-rich shiny slip oxidised and turned red, along with the rest of the vessel. During this process, the iron content is transformed into deep red hematite (Fe2O3). It is not necessary but highly likely that this kindling phase took place in an oxidising atmosphere: an oxygen-rich fire is likely in any case, since it is much more effective in producing heat. Further, the fact that reducing fires are extremely smoky would probably have been considered undesirable, and they were thus limited to the relatively short 2nd phase.

Phase 2: Reduction (vitrification of the shiny slip)

Joining sherds that are oxidised to different degrees, from the Areopagus; probably used as test pieces to check whether full reduction was achieved (left fully oxidised; right insufficient) Matching potsherds from Areopag, Athens in different firing stage, probably used as firing aids.jpg
Joining sherds that are oxidised to different degrees, from the Areopagus; probably used as test pieces to check whether full reduction was achieved (left fully oxidised; right insufficient)

At about 900 °C, the oxygen supply is cut, creating reducing conditions, so that red hematite Fe2O3 turns to matte-black iron oxide FeO, and the black slip turns to deep black magnetite Fe3O4. In antiquity this could be achieved through closing the air supply openings and adding non-dried brushwood and green wood, which would only burn incompletely, producing carbon monoxide (CO rather than CO2). [8] The temperature was held for some time, probably at about 945 °C, to assure a complete melting and sintering of the fine-particled paint slip. [9] Subsequently, the temperature sank below the sintering (vitrification) point of the painted slip again, while still in a reducing atmosphere. [10] Now, the slip is "sealed" and permits no further oxygen to react with its contents, so that the magnetite Fe3O4-oxides within it retain their black colour.

Phase 3: Reoxidation and cooling

During the final phase of firing, the aeration openings of the kiln are reopened: oxidising conditions are restored. Those areas of the vessels that were not sealed in phase 2 now reoxidise: black iron oxide FeO turns back into red hematite Fe2O3. [11] After complete oxidation of the red areas, the kiln could be opened, its contents were then permitted to cool down slowly, and eventually removed.

Kiln control

Fragment of an Attic red-figure vase, probably broken during painting and then used as test piece to check for full reduction Probestuck, Berlin, Altes Museum, Vitrine 12.6-5.jpg
Fragment of an Attic red-figure vase, probably broken during painting and then used as test piece to check for full reduction

A precondition for three-phase firing was a controllable kiln. Apparently, the necessary technology was developed in Corinth in the 7th century BC. Only the domed kilns with vent openings invented then allowed the production of black.figure, and subsequently of red-figure pottery. [12] The control of temperature could be assured visually by using a viewing hole, or through placing test pieces in the oven. [13]

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Hematite, also spelled as haematite, is a common iron oxide with a formula of Fe2O3 and is widespread in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of Fe
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Pottery Craft of making objects from clay

Pottery is the process and the products of forming vessels and other objects with clay and other ceramic materials, which are fired at high temperatures to give them a hard, durable form. Major types include earthenware, stoneware and porcelain. The place where such wares are made by a potter is also called a pottery. The definition of pottery used by the American Society for Testing and Materials (ASTM), is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products." In archaeology, especially of ancient and prehistoric periods, "pottery" often means vessels only, and figures etc. Of the same material are called "terracottas". Clay as a part of the materials used is required by some definitions of pottery, but this is dubious.

Raku ware Type of Japanese pottery traditionally used in tea ceremonies

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Limonite Oxide mineral

Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the three principal iron ores, the others being hematite and magnetite, and has been mined for the production of iron since at least 2500 BCE.

Iron(III) oxide Chemical compound

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition; however, in chemistry, rust is considered an ill-defined material, described as hydrated ferric oxide.

Magnetite iron ore mineral

Magnetite is a rock mineral and one of the main iron ores, with the chemical formula Fe3O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. It is the most magnetic of all the naturally-occurring minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism. Today it is mined as iron ore.

Wüstite oxide mineral

Wüstite (FeO) is a mineral form of iron(II) oxide found with meteorites and native iron. It has a gray color with a greenish tint in reflected light. Wüstite crystallizes in the isometric-hexoctahedral crystal system in opaque to translucent metallic grains. It has a Mohs hardness of 5 to 5.5 and a specific gravity of 5.88. Wüstite is a typical example of a non-stoichiometric compound.

Celadon Term for ceramics with two different types of glazes

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Pottery of ancient Greece ancient Greek artifact made of clay

Ancient Greek pottery, due to its relative durability, comprises a large part of the archaeological record of ancient Greece, and since there is so much of it, it has exerted a disproportionately large influence on our understanding of Greek society. The shards of pots discarded or buried in the 1st millennium BC are still the best guide available to understand the customary life and mind of the ancient Greeks. There were several vessels produced locally for everyday and kitchen use, yet finer pottery from regions such as Attica was imported by other civilizations throughout the Mediterranean, such as the Etruscans in Italy. There were various specific regional varieties, such as the South Italian ancient Greek pottery.

Iron(II,III) oxide chemical compound

Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe2O3) also known as hematite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙ Fe2O3. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic. Its most extensive use is as a black pigment. For this purpose, it is synthesised rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.

Direct reduced iron

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Tin-glazing is the process of giving tin-glazed pottery items a ceramic glaze that is white, glossy and opaque, which is normally applied to red or buff earthenware. Tin-glaze is plain lead glaze with a small amount of tin oxide added. The opacity and whiteness of tin glaze encourage its frequent decoration. Historically this has mostly been done before the single firing, when the colours blend into the glaze, but since the 17th century also using overglaze enamels, with a light second firing, allowing a wider range of colours. Majolica, maiolica, delftware and faience are among the terms used for common types of tin-glazed pottery.

Ceramic glaze layer or coating of vitreous substance fused to a ceramic object

Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic body through firing. Glaze can serve to color, decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding liquids, sealing the inherent porosity of unglazed biscuit earthenware. It also gives a tougher surface. Glaze is also used on stoneware and porcelain. In addition to their functionality, glazes can form a variety of surface finishes, including degrees of glossy or matte finish and color. Glazes may also enhance the underlying design or texture either unmodified or inscribed, carved or painted.

Mineral redox buffer

In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions (or equivalently, oxygen fugacities) at which a rock forms and evolves can be important for interpreting the rock history. Iron, sulfur, and manganese are three of the relatively abundant elements in the Earth's crust that occur in more than one oxidation state. For instance, iron, the fourth most abundant element in the crust, exists as native iron, ferrous iron (Fe2+), and ferric iron (Fe3+). The redox state of a rock affects the relative proportions of the oxidation states of these elements and hence may determine both the minerals present and their compositions. If a rock contains pure minerals that constitute a redox buffer, then the oxygen fugacity of equilibration is defined by one of the curves in the accompanying fugacity-temperature diagram.

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This is a list of pottery and ceramic terms.

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Magnetic mineralogy is the study of the magnetic properties of minerals. The contribution of a mineral to the total magnetism of a rock depends strongly on the type of magnetic order or disorder. Magnetically disordered minerals contribute a weak magnetism and have no remanence. The more important minerals for rock magnetism are the minerals that can be magnetically ordered, at least at some temperatures. These are the ferromagnets, ferrimagnets and certain kinds of antiferromagnets. These minerals have a much stronger response to the field and can have a remanence.

Black-glazed Ware

Black-glazed ware is a type of ancient Greek fine pottery. The modern term describes vessels covered with a shiny black slip.


  1. Walton, M., Trentelman, K., Cummings, M., Poretti, G., Maish, J., Saunders, D., Foran, B., Brodie, M., Mehta, A. (2013), Material Evidence for Multiple Firings of Ancient Athenian Red-Figure Pottery. Journal of the American Ceramic Society, 96: 2031–2035. doi: 10.1111/jace.12395
  2. The realisation that the base clay and the "paint" (slip) do not or only slightly differ in chemical terms was first published by Schumann (1942). It was later supported with spectrographic analyses by Noble (1969).
  3. This, and the fact that various sintering point are necessary to achieve several colours on the same vase, such as shiny black, red and deep red (or coral red, as visible e.g. on Exekias' famous Munich cup with Dionysos on a boat), was first recognised by Hofmann (1962).
  4. Detailed description in Winter (1959).
  5. Schumann (1942) used caustic soda and ammonia for his experiments, Hofmann (1962) tannins, Noble (1960/1965) mentions calgon ((NaPO3)6) and potash. For antiquity, we can assume the use of potash, as it is created as a natural waste product when wood is burnt, e.g. in a potter's kiln.
  6. Especially from earlier periods, there are many incompletely reduced vases, with parts of the vessel remaining red, while other are completely black, although the whole vase is painted with the same slip. This could happen if the atmosphere failed to reach the surface or if the temperature was too low to seal the surface.
  7. E.g. Noble (1969) fired ancient pottery fragments, at above 975 °C the ancient black surfaces melted and reoxidised. Experiments with modern Attic clays have shown that at temperatures over 1005 °C they turn to a very light red colour, whereas below 1000 °C, colours very similar to that of ancient Attic vases are reached.
  8. In modern electrical ovens, wet sawdust can be added for this purpose. See Gustav Weiß: Keramiklexikon, entry "Reduktion im Elektroofen". Joseph Veach Noble also used sawdust: Noble (1960), p. 310-311.
  9. Noble (1960) suggests a "soaking period" of at least half an hour.
  10. The exact sintering point varies from clay to clay, in his experiments, Noble ended this phase at 875 °C (Noble 1960, p. 311).
  11. The different surface qualities of sintered/vitrified and unsintered surfaces are clearly depicted in electron microscope photographs in Hofmann (1962).
  12. Pictorial evidence for this is available in the form of paintings on votive tablets from Penteskoupha (now in the Antikensammlung of Berlin) depicting potters in action, from the building of the kiln to the firing. Reconstruction of a kiln in Winter (1959). Description of modern workshops and kilns: Winter/Hampe (1962).
  13. Noble (1960/65) and Hofmann (1962) argue that visual control is sufficient. Farnsworth (1960) examined preserved test pieces found near excavated potting kilns from antiquity.