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Ceramics

The term ‘ceramics’ refers to any kind of pottery object: vases, plates, bricks, tiles, etc.

Ceramics can be classified into a number of different groups:

  • majolica: red clay fired at 980°
  • earthenware: white clay fired at 1030°
  • porcelain: white clay fired at 1300°
  • stoneware: red clay fired at 1160°; white clay fired at 1240°
  • terracotta: red clay fired at 1000°

There are also a number of different manufacturing techniques that can be sub-divided as follows:

  • colombino
  • potter’s wheel
  • damp press (using a mixture with 20-30% water) and dry press (a powder mixture with 5% water)
  • cast
  • modine
  • plaster casting

The oldest skill is called colombino, which was used in very ancient times when no machines were available at all. A string of clay was formed and was then wrapped around into a spiral to produce the desired object. The exterior finishing was carried out using wood, animal bones or even just hands. It was only with the introduction of the potter’s wheel that it became possible to make more elegant and finely-finished shapes, and the potter’s wheel is still widely used today because excellent results can be obtained, it’s relatively quick and because objects made with it display a marvellous sense of handcrafting.
The modine process involves placing a layer of clay onto a rotating chalk mould. A fixed metal mould is then lowered onto it and the object takes on the desired shape dictated by the space between the two moulds.
The press is perhaps the quickest way of mass-producing shapes and involves pressing a block of clay between two metallic or chalk moulds.
The plaster-casting technique involves pouring liquid clay into a chalk mould divided into a number of parts. The external layer solidifies quite rapidly because the chalk absorbs the water. The form is then turned upside down and the rest of the liquid clay comes out. Finally the form is opened and the result is a fresh but already-shaped object.

The next step is firing in the oven (biscuiting), which is followed by glazing in tanks or by machine: spray-discs-die.
The kind of glaze used depends on the type of clay used:

  • enamel for red clay (covering glass, opaque)
  • crystalline for white clay (transparent glass)

Glazes are silica-based, to which fondants such as lead, sodium and potassium are also added.
Attention! The traditional fondant used in ceramic work is lead but in the production of objects used for food consumption it has been replaced in recent years by alkaline fondants.

Other elements may be added to enamels, including opacifiers (oxides which prevent transparency, for example zirconium, titanium and tin) and characterising elements (zinc and calcium, for instance, but also titanium and tin), which are oxides that characterise the surface of the enamel and the development of any colours there may be.

Important! The products used are not toxic, either during production or during daily use by the consumer.
The glaze stains used to decorate enamelled objects consist of metallic oxides that when appropriately fired and ground form pigments that are used to colour enamels and crystallines.
If fondants are added, this produces colours for decoration.
The most common colours are composed as follows:

  • blue: cobalt-silica; cobalt-alumina
  • turquoise: zirconium-vanadium
  • yellow: zirconium–praseodymium; zirconium-vanadium
  • orange: lead-antimony; zirconium-selenium; titanium-antimony
  • red: cadmium-selenium, chrome-calcium-tin
  • brown: iron-chrome-zinc; manganese
  • black: iron-nickel-cobalt-manganese
  • green: copper; chrome-cobalt

After glazing and decoration, the ceramic item is then fired once again, normally at a temperature 30° lower than the first one.
A third firing may be necessary for particular kinds of decoration, for example those using gold, platinum and silver. The temperatures reached are around 700°.

Ceramic objects can be appreciated both for their beauty and for the fact that they are utterly hygienic. In fact, pottery is covered with a vitreous layer (enamel or crystalline) which besides providing colour also eliminates porosity. This is the reason why almost all objects used for the consumption of food are ceramic. Moreover, their use for domestic purposes requires no particular attention, except the need to avoid chipping. It is a good idea to avoid big and sudden temperature changes as this could lead to the formation of checking (fine cracks in the vitreous surface).

Picture by Kee-Ho Casati
Courtesy of Ceramiche della Valdelsa


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