Ceramic - Pottery Dictionary

by Susan Mussi



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Umbrella stand: It is usually round and tall and placed near the front door to keep umbrellas and walking sticks. The photo shows a large jar, 46cm high and 22cm in diameter made for this use. It is decorated with the Majolica Method. Read more about: Majolica: 5g – Painting jars



Under firing is when the ware being fired does not reach the correct heat. This can be caused by the kiln not working properly or a product not being correctly prepared.

Bisque: It is very difficult to know when it is under fired, but if sounded it will be flat and the color might be different from other pieces, so if in doubt, fire it again. If it is decorated and then fired, as the clay will continue shrinking and the glaze won’t, this will cause faults in the glaze, making it matte and cracked.

Glazes: if under fired a piece will have a matte surface and if fired again to the right temperature it will vitrify correctly. This can be caused by something wrong with the elements inside the kiln or with the products being used. Sometimes a fired piece can be half matte and incorrectly fired. If this occurs, cover it with a thin layer of glaze and re-fire it at the same heat. The transparent glaze, when crude, has an uneven surface but when fired it will integrate and leave a shiny surface. With modern industrial tiles there are many problems one being that when they are decorated and fired to the correct heat they are matte, which gives them the appearance of being under fired. Read more about: Tile Problems



Underglaze; is when ceramic colors are applied on to bisque or clay and covered with transparent glaze before firing. The glaze on top gives it a strong, permanent finish and is used in the mass production of pottery for hotels and restaurants as it can sustain the heat and the powerful substances used in industrial dishwashing.

Colors can be bought as powder or as liquid ready to use. If powder, the potter can create his own colors. The colors are usually made with a small portion of flux to help them integrate with the clay below and the glaze that is put on top. They must be mixed with varnish or oil if applied to bisque and gum if to unfired clay.

Mixing the colors is hard work, the powder must be very dry as water cannot come into contact with the liquids it is going to be mixed with, if there is any doubt spread the powder over a piece of newspaper and leave it in a warm place to dry. On top of a nonabsorbent surface, such as glass or china put the powder and mix it slowly with the liquid, pressing and stirring with a pallete knife until it becomes a smooth liquid of the correct consistency for your work.

The suppliers of the colors usually specify in their catalogues the best way and temperatures they should be fire at. Ware can be fired in two ways; first the colors then covered with a glaze and fired again or the colors and the glaze can be fired together. The firing should bring out the strength of the colors and melt the glaze to a clear, shiny layer of glass. Read more about: In-glaze / On-Glaze

ca: SOTA

es: BAJO

Unglazed: clay or bisque without a glazed surface.



Up-draft kiln: It is used in one of the oldest methods of firing. This kiln is round, made with bricks and narrows at the top to form a chimney. It has a floor and underneath which the wood is burned. The air enters through the doors, the pressure of the air pushes the heat and the gases up through the floor, round the ware being fired and up and out of the vent at the top. The vent is narrow so the heat collects and intensifies round the work and fires it.



Uranium dioxide: is an oxide used to form and modify colors such as red, yellow, orange and green. It is also called: urania, uranic oxide, uranium oxide.
A black, crystalline compound, (UO2), insoluble in water, used in nuclear fuel rods, in ceramics, and pigments.