You have probably heard this term at some stage in your life but you might just have no idea what it means. Here is a brief insight into some of the elements of firing.
When heated at high temperatures clay and mixtures of clay with other materials acquire durability and other favourable qualities. Methods of firing and kiln designs have been developed over centuries. However, in the modern- day, science and engineering have revolutionised ways of firing and kiln design. But it is true to say that not much has changed in the way potters fire their wares.
Traditionally, for many centuries, pottery kilns applied the heat directly to the pots in the kiln. The heat was generated by combustible fuel such as wood and coal. The flames surrounded the ceramic pots or the ‘saggers’, which are the receptacles that contain the ceramics. This type of open flame kiln was used for ceramics that were unlikely to be detrimentally affected by direct contact with the flames. This type of firing is still used when using wood to stoneware temperatures having great benefits to the finish of the ware as ash settles onto the molten glaze and enhances the appearance.
The ‘muffle’ type of kiln was the type of kiln where the flames heated the outside of a chamber that contained the pots and the flames couldn’t have direct contact with them. If you were producing delicate pots with soft colours and glazes then you would choose a muffle kiln. Firings in either of the above type of firings would have been either intermittent or continuous in the way they performed.
Intermittent kilns have the pots placed in them and then the kiln is heated to the correct temperature. Then the kiln is cooled and the pottery ware is taken out when the kiln is completely cool. This type of kiln generally has just one chamber.
A continuous oven consists of a series of chambers which are fired one after the other. It can also consist of a tunnel through which the ceramics travel from the cold entrance, through the firing zone all the way to the cold exit.
However, today the usual firing process takes place in a kiln that is fired by electricity and the application of the heat is controlled more easily.
Pottery ware goes through the firing process twice. The first firing is to make the pots vitrified and this means that the pots can then have a colourful glaze applied. Before the pots go into the kiln they must be completely dry and the heat must be slowly increased once they are in the kiln. This process is called ‘bisque’ firing.
Following the first firing, the potter sets about applying ceramic glaze to the pot to enhance the beauty of the ceramic. It is crucial that individual pots have space between them so that they do not fuse together. Glazing also makes the pot waterproof. After the glaze has dried the pot is ready for its second firing and endures a similar process to the bisque firing. The potter must carefully monitor the temperature of the firing and ensure that the heat is applied at a steady pace. Once the glaze firing is complete the pot that emerges is robust, waterproof and can have a great longevity.