burning methods

Out of clay, works must be fired to make them resistant and ready to use. The composition of the material, the further processing and the purpose of the ceramic work determine the firing temperature. Firing furnaces provide the high firing temperature required for the clay. In the following definitions you will learn more about different firing techniques and their properties.

Shrühen

The first firing (approx. 900°C) after the drying process has been completed is called shrinkage firing. The absorbed and chemically bound water evaporates, the organic components burn and the degassing of the body begins. A porous body remains behind, which absorbs the contained water optimally during the later glaze application.

Loading

Before loading the furnace, the furnace plates are coated with release agent or sprinkled with bauxite. This allows the adjusted product to shrink better during firing. When placing the goods in the oven, care must be taken to ensure that glazed goods do not come into contact.

Burning

When firing ceramics, various processes are used with increasing temperatures. First, the water remaining in the body evaporates. From a temperature of 500°C the clay is completely lost and its chemical state is irreversible. The organic components burn – oxidation takes place. After solidification, the glazing phase begins.

oxidizing firing

A burning process with excess oxygen is called oxidation. An electric fire is always oxidizing.

Reducing Firing

A firing process that exposes the ceramic material to reduction with oxygen removal. As the firing temperature is reduced by reducing firing, the firing phases must be alternated with oxidizing firing phases.

Reduction

The reduced firing of the ceramic produces an excess of carbon in the kiln. Carbon monoxide withdraws oxygen from the environment and the body. A strong smoke development is the result. The body changes colour from light grey to black, depending on the thickness and time of reduction.

Firing temperatures

The temperature in the ceramic furnace depends on many factors. For example, the clay mass, the type of glaze, the interaction of the thermal expansion of mass and glaze, the desired effect. Stoneware or earthenware is generally fired lower than stoneware. This in turn is lower than porcelain. Products of technical ceramics or from the refractory industry are fired in firing ranges from about 1500°C.

Tempering

If the final temperature or maximum firing temperature is maintained for a certain time, this is called tempern.

Rakubrand

The Raku firing technique was already practiced in Japan in the 16th century. With this firing method, all processes are shortened. Shrivelled and glazed goods are placed in the raku oven, which is already glowing red. The clay mass must withstand the heat shocks when it is placed in and removed from the oven. The glazes must have a low melting point. The furnace is opened at a temperature of 900°C, the ceramic objects are removed with tongs and placed in a closed container with organic material (e.g. flour) for further reduction. When there is a lack of air, carbon penetrates into the porous body and swells it continuously. The cracking of the glazes produces an experimental, artistic effect.

Field or pit fire

With this firing technique the firing of earlier epochs is simulated. Flames cannot be calculated in advance. This is precisely where the attraction lies. The unglazed ceramic object bears signs of fire, not determined by the Tüpfer. The swollen areas show a reduction during the firing process, i.e. carbon is drawn into the fragments and penetrates them.

Salt firing

The technique of salt glaze was developed in Germany in the 12th or 13th century. In the single firing process, ceramic objects are placed in the furnace in their raw state and fired until the clay mass is sintered. Once the temperature has been reached, common salt is scattered through special openings in the firing chamber. This decomposes quickly and forms a layer of Dämpfe, which binds to the body as an approach glaze. Suitable fuels are gas, Öl or wood.

soda ash

In contrast to salt fires, which produce toxic gases, soda fires are less harmful to the environment. The toxic salt removes. Otherwise, the firing technique is similar to salt spirits. Here, too, the approach glaze is created by a reaction of the vapour-deposited salt with the chemical composition of the body in the fire.

Wood fire

The firing of Töpferware in a wood stove was the most common firing technique until the 19th century. Huge multi-chamber models (Noborigama) or single-chamber models (Anagama) were developed, which required a large team of firing assistants over several days. Today there are small woods that meet the special aesthetic needs of a natural looking body surface. Ash deposits give the ceramic objects in a wood fire their typical appearance.

&&Uround fire

In the case of overfire or overburning, too high firing temperatures are used. Firing errors occur, e.g. the blinding of the glass fragments or the formation of craters on glazes.

Undercutting

If the firing temperature is too low, the glaze does not melt completely, e.g. remains milky. The body does not seal because the melting point has not yet been reached.

Oberzügiger furnace

Ceramic burners with rising flame are called uppermost. The fire is ignited directly under the sole or is passed through under the sole. In this oven, the greatest heat is at the bottom. The heat rises very quickly to the top.

Underburning furnace

In the case of lower furnaces, the flame is drawn off over the bottom of the furnace and “diverted“. The combustion gases do not pass freely through the combustion chamber. The greatest heat is found at the top, below the chamber walls.

Gas burner

Gas burners are used to supply a gas furnace with the required mixture of gas and air. Small orifices located at the burner head increase the surface area of the flame – more oxygen is drawn in, creating a stable flame pattern.

Pyrometer

Temperature measuring instrument for furnaces based on radiant heat with temperature ranges up to 4000°C is called a pyrometer.

Furnace mounting plate

Integral panels for ceramic firing are kiln furniture. They consist for example of cordierite and are highly refractory. They are available in rectangular, round or oval shapes. The plates are built one on top of the other in the ceramic furnace with the aid of inserts. Baking on the plate is prevented with release agents. The mounting plate is coated with it before firing.

Three-point linkage

Three-point kilns are kiln furniture used in ceramic kilns. Objects glazed all around are placed on pointed triples so that they leave only small, inconspicuous scars in the glaze. Stoneware, mayolica or faience are treated like this.

Pyrometric cones

Cones are aids for determining the temperature effect in the kiln. The cones consist of certain mass mixtures and tilt depending on the temperature. Three cones are always placed in the furnace. A distinction is made between different systems e.g. Seger cone, Orton cone and Harrison cone.

carbon monoxide

The reduced firing of the ceramic produces an excess of carbon in the kiln. Carbon monoxide withdraws oxygen from the environment and the body. A strong smoke development is the result. The cullet changes its color.

Mullite

The strength of the fired body is not only based on the glazing, but also on the formation of new crystal structures, especially mullite crystals. The aluminium silicate mullite is characterized by long needle-shaped crystals. Mullite crystals tend to grow at higher temperatures and interlock in the ceramic body with the vitreous matrix.

Cristobalite

A crystal form of pebbles is called cristobalite. It is formed during the firing process from a firing temperature of about 200°C. This results in a volume change of the ceramic object of about 5%. This can result in cracks both during start-up and during cooling down.

Quartz conversion

The crystal structure of quartz ächanges several times during the firing process. At higher temperatures, the crystalline form of the silica is transformed with changes in volume.

temperature shock

This is the rapid start-up or cooling of the ceramic object during the firing process. What is desired, for example, in the Raku firing process – such as cracks in the glaze – is a disadvantage in the traditional dish firing.

Flowing

The bloating of ceramic mass during firing is caused by expanding gases. This should be done by early pore closure, i.e. the surface of the body is already sintered, but the interior is not yet completely gas-free. Clay becomes more permeable to gas by the addition of leaning agents, e.g. chamotte.