4.2.4 Glazing
and Enamelling (“engobe”)
Depending on the grain size of the starting
materials and the crystalline phases created during the firing,
the ceramic product possesses a certain surface roughness,
as a result of which the surface can more easily become soiled.
By applying a glaze, the surfaces are made
smooth and visually more attractive. Above all, however, the
glaze may noticeably improve many technically important properties
of the ceramic product (for example, electrical behaviour,
mechanical strength, resistance to chemicals, etc.).
The glaze contains a higher amount of flux than the fired
ceramic body. Thus at high temperatures it has a corrosive
effect on the body. The result is the creation of an intermediate
layer, leading to a strong bond between the glaze and the
ceramic material underneath. A wide variety of glaze colours
can be created by mixing in colorants (metal oxides).
In order to achieve the increase in strength that is possible,
the thermal expansion coefficient of the glaze must be matched
to that of the body very precisely. A slight compressive stress
in the glaze increases the strength of the finished product.
Tensile stress reduces this effect, and is thus undesirable.
A thin mineral “engobe” or non-vitreous
enamel layer is applied to ceramic surfaces by dipping,
rolling, spraying or brushing.
In contrast to glazing, an engobe is porous and largely free
from glass phase material. They usually consist of fire-resistant
oxides (Al2O3, SiO2, MgO, ZrO2), mixtures of these, or of
fire-resistant minerals such as mullite, spinel, zircon silicate,
or even kaolin or clay.
Engobes are used in furnace engineering to protect ceramic
surfaces from mechanical or corrosive attack. Applied to fire-resistant
kiln furniture such as plates, beams or cases, engobes prevent
both contact reactions with the supported firing materials
and adhesion from any glaze running off the fired items that
are standing on the coated kiln furniture.
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