Breviary Technical Ceramics


      From Powder to Part




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.


<< back   home   next >>