Chemistry Reference
In-Depth Information
7.6.
COMMON CERAMIC MATERIALS
The physical properties most often used for the characterization of ceramic
materials are
porosity
,
translucency
,
strength
, and
color
.
Porosity
, the relative
amount of voids within a material, is generally measured as the amount of
water that the material absorbs and is expressed as a percentage of the total
volume taken up by the voids. The porosity of ceramic materials is deter-
mined mainly by the nature of the raw materials used for making them, and
the degree of vitrification the objects underwent when fired. The lower the
porosity, the higher the quality of a ceramic material, that is, the tougher,
stronger, and more resistant it is to mechanical and thermal shock.
Translu-
cency
(the opposite of opacity), the property of materials to transmit light, is
related to porosity: the less porous a material, the more translucent it is. Since
ceramic materials become less porous during the vitrification stage of firing,
their translucence is related to the degree of vitrification; highly vitrified
ceramics has no, or little, porosity and is therefore translucent; slightly-vit-
rified ceramics, on the other hand, is porous and consequently opaque. Using
these two parameters, porosity and translucency, ceramic materials can be
technically classified within four basic types:
terracotta
,
earthenware
,
stoneware
, and
porcelain
, as listed in Table 59; within each basic type there are,
however, many variations (Litchfield 1967).
TABLE 59
Ceramics Materials
Material
Firing temperature (°C)
Porosity (%)
Optical characteristics
Terracotta
below 900
greater than 25
Opaque
Earthenware
above 950
up to
10
Opaque
Stoneware
1100-1300
up to
5
Opaque
Porcelain
1300-1450
below
2
Translucent
Terracotta
The simplest and coarsest type of pottery is a lightweight, very porous, and
typically red-colored
terracotta
that is fired at temperatures below 850°C.
Much of ancient pottery, for example, is of the terracotta type. Excavations
in the Near East have revealed that primitive terracotta vessels were being
made there more than 8000 years ago; characteristic types of terracotta were
developed in China by about 5000 B.C.
Because of the relatively low temperatures at which it is fired, terracotta
is only slightly vitrified and consequently is highly porous and totally
