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:
- potter’s wheel
- damp press (using a mixture with 20-30% water) and dry press (a powder
mixture with 5% water)
- 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