The following metals can be enamelled on an industrial scale:
:: Steel,
:: Aluminium,
:: Cast iron,
:: Aluminized steel

Enamelling Steel
The types of steel that can be enamelled are governed by certain parameters:
:: Chemical composition, for the enamel’s reactivity and adherence;
:: The mass and distribution of carbides and other precipitates, to avoid defects (fish scaling);
:: The carbon content;
:: The mechanical characteristics necessary to form and press the piece.

What types of aluminium can be enamelled?
The aluminium support (pure aluminium, alloys or metal combinations) must have certain prerequisites, which are only found in certain materials. These are:
:: Pure aluminium: Al > 99,5%
:: Aluminium-manganese alloys
:: Various metal supports coated with pure aluminium.

What types of cast iron can be enamelled?
The only cast iron that can be enamelled is grey cast iron in conformity with UNI 8762.
The cast iron must have a pearlitic or pearlitic-ferritic structure.
Any even only localised presence of cementite (white cast iron) must be avoided at all costs.

What types of glass can be enamelled?
All types of glass can be enamelled.
Vitreous enamel does not have any protective or anti-corrosion function on glass, but one that is more functional and decorative (e.g. glazing for oven doors, car windows, decorations for glasses etc.).

How is enamelling done?
There are three or four phases to the enamelling process:
:: Pre-treatment,
:: Drying (if necessary),
:: Application,
:: Firing

Pre-treatment for metal
It is generally necessary to prepare the metal for enamelling.
Known as “pre-treatment”, this operation is crucial to achieving a good degree of adherence between the metal and the enamel and a surface free of defects.
The pre-treatment varies according to the support, the geometry of the product and the application process to be used.

The purpose of the pre-treatment
The pre-treatment has two purposes:
:: To clean the surface of the metal, removing the residues of the oils used in rolling and/or pressing, as well as rust and any other contaminants (such as foundry sand and soil in the case of cast iron);
:: To give the surface of the metal a certain degree of roughness, the better to assist the enamel to adhere mechanically to the support.

Types of pre-treatment
:: Chemical pre-treatment: this is used – as a general rule – to pre-treat steel and aluminium.
:: Physical pre-treatment (sanding): this is used more frequently for cast iron and for thick sheets of hot-rolled steel.

Chemical pre-treatment
Chemical pre-treatment consists of a series of acid and/or alkaline baths, followed by washes (in hot and cold water) and final drying.

The process of spray-based chemical pre-treatment
:: Alkaline degreasing 60 – 80°C,
:: Alkaline degreasing 60 – 80°C,
:: Hot wash 50°C,
:: Cold wash under running water,
:: If necessary, a further wash with demineralised water.

Physical pre-treatment
Physical pre-treatment – which is used almost exclusively for cast iron and for thick sheets of hot-rolled steel – consists of shooting metal granules or sand against the product.

Pre-treating glass
Pre-treatment of glass consists of washing it with a detergent solution, followed by rinsing it with demineralised water.
No special preparations are necessary, because the only purpose of this pre-treatment is to remove any traces of dirt or grease from the surface to be enamelled.

Applying the enamel
A wide range of techniques is used to apply vitreous enamel to metals.
According to whether the enamel was ground in a dry condition (powder enamel) or in a wet condition, in a water suspension (enamel slip), it can be applied in either a dry or a wet process.
There are several variants on both the powder and the wet application.

Enamelling cycles
The first distinction to be made between enamelling cycles is the one based on the number of coats of enamel to be applied. A further classification can be made within this basic division into two major areas:
:: Powder enamelling and
:: Wet enamelling

Vitrification firing
In order to ensure that the enamel applied as a dry or dried coat will adhere to the support, the result must be heated to a high temperature for a suitable period of time.
During this firing or vitrification, important chemical and physical reactions take place which bring about the adherence between the enamel and the metal of the support

The interface between steel and enamel
Vitreous enamel is a solidified vitreous mass, with a solid, prevalently non-crystalline structure, where bonds of an ionic type prevail.
Steel, on the other hand, is a material with a crystalline structure, whose atoms are set in a gridwork pattern with bonds of a metallic type. The chemical and physical bond between these two materials, which are so different from one another, develops in the process of a two-stage mechanism:
:: In the first stage, the metal of the support is oxidised in the course of a heat treatment in the oxidising atmosphere of the vitrification kiln,
:: In the second stage, the enamel becomes liquid and dissolves the ferrous oxide that has formed, which then saturates the enamel and reacts with the nickel and cobalt oxides in the process of oxide reduction reactions. After these reactions on the interface between the steel and the enamel, the surface is left full of tiny rough features where the enamel penetrates while still in liquid form, then solidifies during the subsequent cooling cycle, also developing a physical form of adherence between the steel and the enamel.

The illustration shows how the interface between the steel and the enamel develops.

Firing temperatures

Sưu tầm và biên dịch bởi http://www.congnghetrangmen.com
Theo tài liệu của Viện Tráng Men Quốc Tế / IEI - International Enamellers Institute
Ghi rõ bản quyền WWW.CONGNGHETRANGMEN.COM khi sử dụng lại thông tin.