The vessel is now ready to be fired: this is the process by which the chemical and physical make-up of a clay is unalterably changed into a form like artificial stone. If the vessel is fired at high enough temperatures, the clay vitrifies and becomes very hard and translucent, especially if there are glassy inclusions in the clay and/or glaze. Depending on the desired end-product or the nature of the potters’ workshop, the potter may choose to fire in a kiln or on an open bonfire.

The vessel can be fired on a huge bonfire open to the elements. This is called open firing or clamp firing, and is normally never used for glazed vessels as the firing duration is very short (from fifteen minutes to a couple of hours) and only reaches relatively low temperatures. The pots are heaped on a bed of fuel (which may consist of wood, dung, bark, charcoal, palm fronds, straw) then covered with more of the same, and the fire is lit from below. The fuel on top and at the sides retains heat around the vessels, and ultimately reduces to ash which continues to keep the vessels hot. The advantages of open firing are that it is cheap, and does not require the heavy capital investment needed to construct and maintain a kiln. It has thus been the predominant firing method for low-fired unglazed wares for utilitarian purposes (cooking and storage) for thousands of years.

However, there are disadvantages to this method, mainly that the pots are not protected from contact with the fuel or drafts, thus the heat is uneven, and rapid temperature changes and shifts in position of the fuel as it burns, can easily lead to cracks or over/under-firing. Furthermore, much heat is lost to the atmosphere, and it is thus not an efficient use of the fuel. The potter also has to restrict his craft to favourable seasons, as open firing can be severely disrupted by wind, humidity and rain. Heavy smoke results from open firings, as from kiln firings, and thus potters’ quarters are invariably situated on the outskirts of a settlement.

If the potter runs a more professional workshop with a regular market, he may have invested money in building a kiln. Firing in a kiln needs to be very carefully controlled, as time, temperature and atmosphere are equally important, and need careful regulating. The "atmosphere" in a kiln consists of the presence of gases, especially oxygen, while the clay is heated and cooled. An "oxidising" atmosphere is where the air is allowed to circulate freely within the kiln, with ample oxygen free to bind with elements in or on the clay. On the other hand, a "reducing" atmosphere is one which lacks freely circulating oxygen, and is often smokey: the air-supply to the kiln is restricted thus creating carbon monoxide which is an unstable gas that will extract oxygen from any available source. See section D.7.a (lustre) for an idea of the different effects on pots of these firing atmospheres.

Abu’l-Qasim describes a traditional updraught kiln, a type which seems to have continued in use in Persia from pre-Islamic times: kilns have been excavated at Tureng Tepe in north-eastern Iran, and at Tel-i Malyan in the Marv Dasht region. This type of kiln is still used today. It usually consists of two chambers, a lower (combustion) chamber where the fuel is placed and the fire is stoked, and an upper chamber where the pots are placed. The heat rises from below through flues in the floor so that it passes up through the pots and is drawn out of the top of the kiln by vents or chimneys.

A variant design of kiln is the down-draught kiln, where there is a difference in the location of the vessels relative to the movement of the flames and heat from combustion: the heat is directed upwards into the kiln by a bag-wall between the heat source and the firing chamber, then moves down through the pots and is vented out through a chimney. This design uses the heat more effectively and avoids hot-spots in the kiln, where a concentration of heat in certain parts of the kiln causes some vessels to be overfired, and others to be underfired. It is the kind of kiln used by the Chinese to make porcelain, and reaches much higher temperatures than the updraught kiln.

Piccolpasso, who wrote on the Italian ceramics industry, circa midC16th, said that Italians believed that kiln-design was the crucial key in the secret of making lustre, and claimed that they even kept their kilns in locked cellars, only passing on the secret to their heir on their death-bed.

Loading the kiln was a careful and skilled job: because of the expense of fuel, the potter has to accommodate as much pottery as possible in a limited space, but so arranged that the kiln draught spreads evenly among the pots; if the pots are too close together, the draught will get blocked and the required temperature will not be reached, and if they are unevenly spaced parts of the kiln will get too hot and other parts will not be hot enough. The potter also has to be careful not to smudge the raw glaze or the painted decoration.

Abu’l-Qasim describes how the kilns are lined with rows of earthenware pegs fitted into holes in the wall forming shelves on which the pots are placed for firing. Kilns such as this were excavated at Nishapur. Particularly high status pots are placed in saggars, individually made earthenware cases with fitting lids. Remnants of such kiln equipment as pegs and saggars have been excavated at Siraf and Takht-i Sulaiman. Saggars protected these pots from hot-spots and ash that may have been blowing around in the kiln, and meant that the glaze would not stick to any other vessels when it melted. Lower status vessels could be stacked one within the other in columns, separated by small tripods called stilts: as a result, the glaze on the interior of bowls/plates etc stacked in this manner often shows three small dimples or stilt-marks.

Different chemical and physical changes take place at different times within the clay body, which may give rise to stresses in the clay body and these show as small cracks in the surface of the fired vessel, or in extreme cases the pot may explode! The effect may be exacerbated if there is too much water left in the clay, because it was not dried for long enough, or if the pot is heated too rapidly.

The effect of firing is that, first, water and, second, organic matter (which all clays contain to some extent) volatilise (ie. evaporate into gases) from the clay body: the net effect of volatilisation is weight loss and shrinkage, resulting in a body that is denser and less porous. This process accelerates at high temperatures as the material melts and forms a glass: at this stage new minerals, which take the form of crystals, develop in the clay. Changes in the atomic structure of some of the ingredients (especially quartz) also take place.

If the potter is high-firing his vessel, several stages occur. A glassy phase begins with "sintering", when the surfaces of particles begin to stick and fuse together with other particle surfaces, and some constituents of the body begin to melt and form a liquid. The particles draw closer together and the pores between them get smaller, meaning shrinkage, less porosity and increased density. The second and final stage is vitrification, resulting from continued heating, when the particles melt and form a bond for the body, pores are eliminated, the body becomes increasingly dense and the mass shrinks considerably. The potter has to know when to stop this process, as the vessel may warp or slump if vitrification goes on for too long.

Glazes are very sensitive to changes in firing temperature and atmosphere, and different textures or colours may be created as a result. The glaze also shrinks during firing, but this enhances the fit of the glaze to the body, especially if the glaze contracts a little more than the clay as this compresses and strengthens the body.

The initial fuel has to be light brushwood or straw, and once the requisite temperature is reached then it is maintained with softwoods, ie. willow, poplar, pine, which have a long flame and don’t make the glaze boil or become rough. A lot of fuel is required, especially since firing duration often lasts for several days. Abu’l-Qasim describes how the tin-glazed Kashan vessels were fired for twelve hours at a hot even temperature then left in the kiln to cool and taken out after a week. For lustre wares, the second firing in a reducing kiln lasted for seventy-two hours. As seen above, there are also different types of kilns, based on differences in construction, which alter the atmosphere or circulation of gases in the kiln; however, according to Abu’l-Qasim, there are also purpose-built kilns for making glass-frits for a stonepaste body, or glaze-frits for tin + lead glazes.