The earliest means of firing we know of consisted of firing the vessels over an open fire. Amazingly, this procedure was still used up until the nineteenth century in a town near the Basque Country-Ordizan, in the province of Bearne. R. Coquerel says that a layer of dry straw was laid on an area about two metres in diameter on clay ground or gravel. A few thin branches were placed on top. A covering of ferns and unfired vessels was placed on top of these. This was followed by another layer of ferns and more vessels, and so until, to form a cone. The potters then set fire to the entire pile. Once the pieces had been fired, the heap was covered in ashes and sods to ensure slow cooling.
One of the most important developments in the firing process was the separation of the combustion and firing chambers, introduced some time in the fourth millennium BCE in Mesopotamia.
In his fascinating book, "Les Fours", Daniel Rhodes writes:
While there were of course subsequent improvements, this arrangement was used in most kilns in Mediterranean countries and Europe until modern times.
Pictures on some Greek glasses show the kilns that were used in the fifteenth century BCE, and they are very similar to some in Navarre (Irunberri, Atarrabia, the old kiln in Tutera, etc.).
These are known as vertical kilns with an ascending draught. In the Iberian Peninsula they are also known as "Arab" kilns, since it appears that it was the Moors who first introduced them. In Asia (China, Japan, Korea, etc.), on the other hand, horizontal kilns were developed, in which the fire or combustion chamber and the stack were practically at the same level. These kilns were of better quality because they attained higher temperatures with a more uniform distribution of the heat.
The kilns we have seen in the Basque Country are practically square in plan, except for the one in Marino Gonzalez's pottery in Tafalla, and Carmelo Añon's modern kiln in Tutera, which were circular. Sadly, both of these constructions have since been demolished.
They were all open, except for those in Irunberri, Atarrabia and the old kiln in Tutera, whose firing chamber had a brick vault, with holes to let out smoke and fumes, and create a draught.
As we have said, the vessels were placed on tacas -cylindrical clay blocks-separating the enamelled or glazed vessels by means of fired clay trivets so that they would not stick together during firing. These were called txakurrek (dogs) in Zegama and some other potteries in the Basque Country. On fired plates and cups we can see three small unglazed patches. These were the palces were the vessel was supported on the legs of the trivets.
The charge of vessels normally protruded out the top of the kiln, and this additional part was called the colme .
Once the kiln had been charged the doors of the firing chamber were bricked up and the colme was covered in broken roof tiles, remains of vessels from previous firings, etc.
The fire was then lit, gently at first, and later raised to a temperature of between 900º and 1000º C.
The most widely-used procedure for determining the condition of the vessels, consisted of removing some enamelled cups from the colme with a metal hook.
The vessels were thus checked about five times before the firing process was considered complete. They were usually fired for between 16 and 17 hours, and in some larger kilns, up to 23 hours.
In other potteries, as well as checking the test pieces, they also used small viewing holes ( visteros ) in the bricks used to block off the doors of the firing chamber.
When the potter saw that the pieces of broken pottery in the colme began to turn white, he removed the plugs blocking the holes and inserted a very dry stick. By the light of the burning stick he checked the condition of the enamel or glaze on the vessels. Once firing was complete, the mouth of the combustion chamber was blocked off to stop the draught and put out the fire.
It took several days for the pieces to cool sufficiently to be removed from the kiln.
Cracked pieces were called apeladas and were piled in a heap near the kiln (the testar), to be used in the colme in later firings.
Unsightly pieces were called reuses , ereuses or rauses . These were sold privately at a lower price.
Firing was a delicate operation. A south wind, the wrong distribution of the fire, incorrect tempering, etc. could put paid to hours of work, and mean a waste of materials: clay, enamels, fuel, etc. Daniel Rhodes says
Potters did not look on their kilns as others craftsmen looked on their tools. Their kiln was the altar of a holocaust, a potential enemy, a destroyer, but also an ally with whom they must work.
Consequently the potters sought protection from on high, making crosses out of pottery shards on the inside walls of the firing chamber before charging the vessels, crosses on the top of the combustion chamber when lighting the kiln or at the end of the process, and crosses and on the rubble in the colme etc. They also said prayers or crossed themselves at certain points during the process, and it was quite frequent to hear an exclamation of "praise God" at the end of a successful firing.
In Ordizan, where firing was communal, families that had pieces in the fire knelt around it, and said certain prayers throughout the firing process. Whenever they heard a noise that suggested that something had broken, they all said in unison soule assieu -may it be the only one-and redoubled their prayers. Coquerel says it was quite a pagan ceremony and met with complete opposition from the village priest. The origins of the ritual
must be as old as the potter's art.