Three-phase firing

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.

Only the domed kilns with vent openings invented then allowed the production of black-figure, and subsequently of red-figure pottery.

Since Attic pottery contains no glazes proper (i.e. ones that melt and vitrify completely), vessels could touch in the kiln.

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.

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.

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).

Those areas of the vessels that were not sealed in phase 2 now reoxidise: black iron oxide FeO turns back into red hematite Fe2O3.

[13] 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 with opening and viewing hole, perhaps a depiction [ citation needed ] 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)
Fragment of an Attic red-figure vase, probably broken during painting and then used as test piece to check for full reduction
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.
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.
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 reduced; right insufficient)