CERAMISTS, POTTERS TILERS, DECORATORS, Etc

COOKING OF CERAMIC

HISTORY

It is possible that the discovery of fire was the most important that the humanity has carried out. Among other uses, thanks to it the primitive man realized that the water didn't affect the recipients of cooked clay. This observation was maybe carried out when pouring clay in a bird nest used to transport hot ash. Or perhaps a hole in the floor in which will light fired was covered of clay, which transformed it into a rudimentary recipient. It is believed that the first discoveries that established a relationship between clay and fire were made about ten or twelve thousand years ago. The first ovens were built in Half East. They were built by a home located under a clay "grill" in which the recipients stayed. Then a kind of a camera was built piling old vessels or faces of grass above, while a chimney was left. The heat that retained the camera made possible to reach higher temperatures that those gotten with a blaze. This was the first oven of shot of the History. Gradually the ovens became permanent structures and the recipients were protected from the flames, what made it possible to decorate them and to enamel them without fear to that the fire altered them. It was also possible to control the interior temperature. The Roman ovens were circular; in their interior there was a perforated platform located on a central column. The fuel burned in the camera under the platform, on which the recipients were prepared; the wood used as fuel was introduced by a conduit that also regulated the entrance of the air.

The Chinese potters built different types of ovens that were generally located in the slope of a hill. The fire was lit in the inferior part, the cameras where the recipients were, formed part, in fact, of the chimney. A shot regulator located in the superior camera was enough to regulate the exit of the hot gases, and the oven retained the heat. The ovens of this type provided high temperatures; thanks to the isolation of earth and to the regulator of gases, about 1200º C were reached in next areas of the home. To this temperature, most of the clays were vitrified and they became sandstone. When being made more complex, these ovens were built in a series of steps: part under earth and part in the surface; some were divided in small cameras. Some small holes were opened up in the walls to add fuel, so that the temperatures stayed constant. These ovens spread for the whole Distant East, although presented slight variants from place to place. Until relatively little time ago they continued being more effective and able of reaching higher temperatures that the built in West. In the XVII century ovens of bricks of a single camera were still used in Europe.

GENERALITIES

The cooking constitutes the last step in the process of production of the ceramic. In this operation it is needed to use a bigger quantity of heat than the provided by a domestic oven. It is possible to use, in all ways, a more or less homemade oven, but it is better to acquire a small electric oven. Their price is not prohibitive.

You can cook the pieces in the oven from the shop you go to, but with an own oven, the cooking process is better controlled. To begin acquire an economic model, as the one of 30 x 40 x 30 cm. (Dimensions of the cooking camera).

Make an electrician check the electric connections. Remember that the temperature of the oven is measured with a termopar that constitute an electric group that measures and regulates high temperatures and that should be part of the oven.

Take a registration of the made cookings. For each, write down the temperatures at intervals of one hour. This way you will realize the progresses that you carry out.

The fire is the great teacher or, if we prefer it, the master of the art of the ceramic. The pieces that we have prepared will be truly "ceramic", only when the fire had acted on them. So to consider a ceramic as completely completed, two cookings are necessary. The first one receives the name of "baked". This process transforms the raw earth to cooked earth. The cooked object that the oven leaves is denominated "cake". We mustn’t believe that this first action of the fire is used to make it harder or more resistant to the humidity and, in consequence, more capable to receive the water of the enamels. The true objective of this first action is causing chemical reactions in its interior. These reactions produce gases that come out through small pores that act as discharge valves. The object, in the oven, suffers a true physicochemical transformation. To give an idea, although it is superficial, we will say that during the cooking all the humidity remainder in the object is eliminated, the hydro silicates are dissociated, the oxides are liberated, the carbonates break down and the whole pasta is under imperceptible dilation movements and contraction because of the change of volume of the products of the reactions.

The cooking temperature varies according to the type of used pasta and, in consequence, with the object type that is wanted to manufacture. For the products made in terracotta, the temperature of the oven oscillates between the 850º and the 1000º C; for sandstone and pottery, of the 1000º to the 1300º C; and for porcelain, of the 1300º to the 1500º C, approximately.

When heating the clay, it suffers a series of modifications that, starting from a certain moment, are irreversible. The clay at a temperature of 100º C dries off completely, but it can return to its primitive state if it is soaked with water. If it is warmed up to 600 - 700º C, it begins to take a very dark red colour, and it suffers deep chemical modifications. At this time the clay is soft, crumbly and porous, and it is not plastic again neither it loses its form for the action of the water.

At temperatures around 900 - 1000º C the particles of clay begin to conglomerate and to acquire bigger resistance. All the carbonaceous substances, as vegetable residuals, get burnt and volatilise, giving a pure and brilliant material as a result and sometimes with a colour very different from the original. For example, when arriving to that point the typical flowerpots modelled in red clay, have a brilliant terracotta colour and a resistant and porous structure. Many black clays acquire a whiter colour in the cooking, as a consequence of the disappearance of the burnt organic substances.

BAKED

The recipients after the 1st cooking (baked) must be totally dry and not present protuberances. They can be placed one above the other with the condition that they stay in balance. The recipients of same height are placed in a shelf. Paint the interior part of the covers with mixture of kaolin (80%) and clay (20%) to avoid that they get adhered to the support.

Check if the shelves of the oven are clean of varnish. To eliminate the possible drops, rub them with a carborundo stone. Also, so that the recipients don't adhere to the shelves, paint these with the same mixture used for the covers.

Bolster the shelves of the oven on tubular supports (three for each one) and after that introduce the recipients. If these were of terracotta, place them on refractory lists.

The separation between the recipients and the following shelf must be of 5 mm minimum..

Let the oven warm slowly during two hours, and close the superior chimney then; put the oven to the half of the power during two hours more, and make it then work at full power until reaching the wanted temperature. Wait until it cools down to about 100º C before opening the door to take out the load.

You should resist the temptation to accelerate the cooling. Their consequences could be fateful as much for the recipients as for the oven. When the temperature of the oven is the same as the atmosphere, the door of the oven can be open. It must be done slowly, so that the cooked objects don't suffer the effects of an eventual current of cold air. The opening of the oven, although it is a first cooking, it is touching; the ceramist is attacked for kind of a fever and anxiety, feelings that are even experienced after long years of dedication to the ceramic

We don't insist on the importance of the first cooking and the consequences that can be derived from not well cooked pieces or making bad use of the oven. We have pointed out that, in the first cooking, the slow ascent of the temperature until reaching the wanted grades, according to the used material and the product, produces a special atmosphere and a series of combustion gases, indispensable to achieve a total perfection of the object. Now we face with the problem of cooking of the enamels.

ENAMELED

If the material that we will cook is delicate and it demands cares, even inside the oven, the enamels will need double cares to be able to call it, truly, enamel. Formerly, the problems were of much more difficult solution, since they didn't have the electric oven. At the present time, with this oven type and a minimum of attention and common sense, the mistakes have become very strange. The second cooking, in which the varnish gets fixed, is more complicated than the first one. The recipients are placed so they are not in contact with each other, but at the same time, maximizing the number of recipients in the oven. During the cooking of the varnish, it melts and gets distributed with uniformity on the surface. The cooking temperature must be regulated carefully. If it were too high, the varnish would come off; if it were too low, it would get a very rough texture. The oven is filled carefully; it should not be full neither too many hollow spaces must exist between piece and piece. Diverse layers of objects can be placed, if they are of small dimensions. In this case, it is convenient to isolate the different layers with a badge of refractory material or with small supports of the same refractory material, so that the objects, inside the oven, don't have, at all, the smallest contact among them.

The oven that the ceramist fond of this art will use, will probably be the electric oven. The closed oven is connected to the electric net. When the temperature of the oven ascends, the fear of that the oven will make us betrayal in the crucial moment appear. With only an instant of fire excess, irreparable damages take place in the enamels or in the varnishes. With few grades of difference, they switch from a colour to another. Those stumped disappear and some tonalities lose their intensity. Today, with the technique to the man's service, many of these inconveniences can be eliminated. The electric oven incorporate a temperature control that indicates us the exact temperature of the oven and an automatic switch that, when reaching the wanted temperature, the electric entrance gets closed and the oven is turned down. The temperature will go descending slowly toward zero. It is convenient to wait the perfect cooling of the oven and, if it is possible, to prolong the opening moment. The door opens up slowly, avoiding a quick and abrupt entrance of cold air that could alter the characteristics of the pieces.

It is no needed to add anything else; we abandon the beginner ceramist to their happiness or their disillusion. We only recommend them not to get discouraged if the first pieces cause a disillusion. We insist: in the art of the ceramic the time is the best ally. Some errors, even of great calibre, are good to obtain, in next occasions, exceptional results. Many times from tremendous errors, the ceramist achieves in the following tries formidable effects of colour and decoration.

DECORATION TO THE THIRD FIRE

It is possible to decorate the piece already enameled or porcelain pieces or ceramic bought in specialized stores . For that, we can buy decalcomanias and apply them to the pieces. Placed in the oven and baked to the temperature indicated by the maker of your traces, the effect can be spectacular.

Another option is to buy vitrificables paintings for porcelain decoration and to add borders, names or other symbols.

We can combine both techniques if the cooking temperatures are similar. If the temperatures are different, it will be necessary to carry out different cookings always beginning with the one of higher temperature.

The decoration technique to the third fire is one of the simplest and it provides very satisfactory results.