Three-phase firing
Encyclopedia
Three-phase 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.
(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 if iron-rich clays can be influenced by controlling the atnmosphere 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.
point, i.e. the temperature at which the individual clay particles irreversible merge, depends on the composition of the clay and on the particles contained in it.
Smaller clay particles and a high calium content lower the sintering
point. The production of finely varied painting slips
was achieved through levigation and the subsequent scooping off of various layer.
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 polyphosphate
s such as calgon
(NaPO3)6: these attach thrmselves to the clay particles with strong hydrogen bond
s 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
.
. Since Attic pottery contains no glaze
s 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.
. With constant firing of the kiln, such temeperatures 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 oxides 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 as undesirable, and was thus limited to the relatively short 2nd phase.
(CO rather than CO2). 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.
Subsequently, the temperature sank below the sintering (vitrification) point of the painted slip again, while still in a reducing atmposphere. Now, the slip is "sealed" and permits now further oxygen to react with its contents, so that the magnetite Fe3O4-oxides within it retain their black colour.
. 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. The control of temperature could be assured visually by using a viewing hole, or through placing test pieces in the oven.
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
pottery production, specifically for painted vases. Already vessels from the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron 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
Corinth
Corinth is a city and former municipality in Corinthia, Peloponnese, Greece. Since the 2011 local government reform it is part of the municipality Corinth, of which it is the seat and a municipal unit...
and Athens
Athens
Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...
) enabling the production of extremely shiny black-slipped
Slip (ceramics)
A slip is a suspension in water of clay and/or other materials used in the production of ceramic ware. Deflocculant, such as sodium silicate, can be added to the slip to disperse the raw material particles...
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) oxideIron(II) oxide
Iron oxide, also known as ferrous oxide, is one of the iron oxides. It is a black-colored powder with the chemical formula . It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron oxide should not be confused with rust,...
(FeO), red Iron(III) oxide
Iron(III) oxide
Iron 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 oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
(Fe2O3), and deep black magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
Fe3O4). Which of these types of oxidation is achieved depends on the availability of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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 if iron-rich clays can be influenced by controlling the atnmosphere 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 vitrificationVitrification
Vitrification is the transformation of a substance into a glass. Usually, it is achieved by rapidly cooling a liquid through the glass transition. Certain chemical reactions also result in glasses...
point, i.e. the temperature at which the individual clay particles irreversible merge, depends on the composition of the clay and on the particles contained in it.
Smaller clay particles and a high calium content lower the sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
point. The production of finely varied painting slips
Slip (ceramics)
A slip is a suspension in water of clay and/or other materials used in the production of ceramic ware. Deflocculant, such as sodium silicate, can be added to the slip to disperse the raw material particles...
was achieved through levigation and the subsequent scooping off of various layer.
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
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3), potash
Potash
Potash is the common name for various mined and manufactured salts that contain potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains, and this was the major historical source for it before the industrial era...
(K2CO3) and polyphosphate
Polyphosphate
Triphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 structural units linked together by sharing oxygen atoms. When two corners are shared the polyphosphate may have a linear chain structure or a cyclic ring structure. In biology the polyphosphate esters AMP, ADP...
s such as calgon
Calgon
Calgon is a brand registered trademark of different corporations. The original product consisted of powdered sodium hexametaphosphate , which in water would complex with ambient calcium ion and certain other cations, preventing formation of unwanted salts and interference by those cations with the...
(NaPO3)6: these attach thrmselves to the clay particles with strong hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s 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
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
.
Firing
Before firing, the clay vessels were densely stacked in thr kilnKiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
. Since Attic pottery contains no glaze
Glaze
Glaze or glazing is a thin shiny coating, or the act of applying the coating; it may refer to:In materials or engineering:* Architectural glass, a building material typically used as transparent glazing material in the building envelope...
s 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.
Phase 1: Kindling (oxidising)
Typical firing probably tookm place at a temperature of 850 to 975 degrees CelsiusCelsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
. With constant firing of the kiln, such temeperatures 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 oxides and turned red, along with the rest of the vessel. During this process, the iron content is transformed into deep red hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
(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 as undesirable, and was thus limited to the relatively short 2nd phase.
Phase 2: Reduction (vitrification of the shiny slip)
At about 900°C, the oxygen supply is cut, creating reducing conditions, so that red hematite Fe2O3 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 monoxideCarbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO rather than CO2). 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.
Subsequently, the temperature sank below the sintering (vitrification) point of the painted slip again, while still in a reducing atmposphere. Now, the slip is "sealed" and permits now 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 magnetite Fe3O4 turns back into red hematite Fe2O3. After complete oxidation of the red areas, the kiln could be opened, its contents were then permitted to cool down slowly, and eventually removed.Closing remarks
A precondition for three-phase firing was a controllable kilnKiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
. Apparently, the necessary technology was developed in Corinth
Corinth
Corinth is a city and former municipality in Corinthia, Peloponnese, Greece. Since the 2011 local government reform it is part of the municipality Corinth, of which it is the seat and a municipal unit...
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. The control of temperature could be assured visually by using a viewing hole, or through placing test pieces in the oven.