Vitreous Enamels

Topics Covered

Background

Composition of Enamels

Key Properties

Thermal Expansion

Colour

Durability

Maturing Range

Applications

Background

Vitreous enamels are very similar to ceramic glazes in many ways.  They are applied and fired to form a more or less glassy layer.  They are familiar in household items particularly those coming into contact with heat and aggressive environments, such as enamelled cookware and enamelled ‘white goods’ such as cookers.

The difference is that enamels are applied to metal items rather than ceramic and as such must meet the peculiar constraints set by these materials e.g. high thermal expansion, low firing temperatures.

Composition of Enamels

Many of the oxides are common with glaze composition, however, enamels are necessarily richer in the fluxing (modifiers) oxides and the materials are largely fritted.

Although several glass-forming systems exist, enamels are almost exclusively based on alumino-silicate glass systems.  The main glass forming oxide, silica (SiO2) is modified by the addition of a range of other oxides.  The modifiers act to alter thermal, chemical, and physical properties (see table 1).

Glass Formers

Intermediates

Modifiers (melters)

SiO2

Al2O3
ZrO2
TiO2

CaO
BaO
SrO
ZnO
PbO
Na2O
K2O
Li2O
Bi2O3
B2O3

Enamel batches are high in frit content due to the requirement for low temperature melting.  Additions are made to the frit(s) to modify fluid properties and alter colour and improve application.

Key Properties

There are no fixed properties for enamels as their composition and use is so varied.

The application of enamel may be purely for aesthetic reasons, functional purposes or both.

The following properties are considered of importance, depending on the application and product/process types.

Thermal Expansion

Enamels are normally required to be in compression in the fired state to avoid the fault of crazing. Expansion is controlled by the composition of the fired enamel on the metal.

Colour

The colours available will depend upon the firing temperature, the chemistry of the enamel, and the composition of the chosen colour addition. 

Durability

Depending on the application, chemical and physical durability may be of importance.  Various tests are employed for the different product types to assess durability.

Maturing Range

Enamels must mature within an appropriate temperature and time interval. There must also be a suitable range over which maturity is reached in order to allow for process variables. 

They are applied by spraying and dipping, or by dry dusting or electrostatic methods.  In some cases, enamels are applied in single layers or, in order to obtain suitable finishes and improve adhesion on certain metals, in two layers, a base coat and a topcoat.  Certain pre-treatments of the metal such as ‘pickling’ in acid baths to remove grease, and pre-oxidation to create an oxide bonding layer may be required.

Enamels are usually fired between 700°C and 1000°C depending on their composition, the substrate metal and the length of time in heating.

Applications

As mentioned earlier, enamels are usually applied to metal substrates that are exposed to high temperatures and/or corrosive environments. Some applications include:

•        Stoves

•        Ovens

•        Cookware

Primary author: CERAM Research Ltd

For more information on this source please visit CERAM Research Ltd.

 

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