Introduction

Vitreous enamel

A natural material born from fire, immune to corrosion, as pure as glass, available in a huge range of colours that remain stable over time and in any atmospheric conditions, recyclable…
A coating that offers a highly versatile, global response to many different sectors of industry.

…a natural material
Although it is not exactly technical, this definition tells us many important things. The starting material for vitreous enamel, called the “frit”, is a mixture of different substances found in nature: silicates, borates, carbonates, oxides etc.

…born from fire
The frit is the product of smelting these materials and subsequently cooling them back down. According to how it is cooled (in water or between two metal rollers), it takes the form of either crumbling granules or small flakes.

…immune to corrosion
The composition of the frit, i.e. the vitreous component of the enamel, varies considerably, according to:
:: the support to which it is applied (steel, aluminium, cast iron, glass);
:: the functionality to deliver to the enamel, for fitting the usage requirement.
As a general rule, all vitreous enamels can be said to be resistant to:
:: All organic solvents,
:: All normal detergents (pH>7),
:: All neutral saline solutions (pH=7),
:: All acids (at room temperature) except hydrofluoric acid.

…as pure as glass
Enamel is a borosilicate glass with an amorphous structure that gives it a hard, compact surface, with no porous areas and impermeable to liquids. It neither absorbs nor transmits odours or flavours.
It inhibits the proliferation of bacteria.
It is easy to clean.

…Available in a huge range of colours
Enamels are generally coloured using inorganic pigments or metal oxides added to the frit during the grinding phase.

…Easy to decorate
In addition to lending themselves to all sorts of colours, enamelled surfaces can also be decorated using various industrial systems (decalcomania, serigraphy) and artistic methods

A natural material,
environmental friendly,
recyclable

…Recyclable
Both the elements that are used to make the frit and the supports (steel, cast iron, aluminium and glass) on which the enamels are applied are recyclable without any problems, as they contain no polluting substances.
Once an enamelled product’s life cycle is over, it can be used again to produce steel or aluminium.

Vitreous Enamel Typical Composition
Component	Mineral	Functions
SiO₂	Quartz – Feldspar	Refractory component, hardens the vitreous system, adds chemical resistance and increases viscosity.
B₂O₃	Borates	Flux that produces the vitreous matrix, reduces viscosity and increases surface hardness.
Na₂O K₂O LiO₂	Albite Feldspar – Microcline Spodumene – Petalite	Alkaline components that lower the temperature at which glass softens, reduce its elasticity and increase its brilliance.
Al₂O₃	Feldspar – Corundum	Increases viscosity and chemical, mechanical and thermal resistance, reduces the expansion coefficient and favours an opaque finish.
ZrO₂	Zirconium	Improves resistance to acids, to knocks and to shocks, acts as an opacifier.
ZnO	Zincite, Blende, Wurzite	Excellent flux, lowers the expansion coefficient and improves brilliance and surface quality.
CoO	Cobaltite	A very strong adherence agent, it produces a structure with lots of small, well distributed bubbles.
NiO₂	Niccolite	A second adherence agent, it produces a structure with a small number of large bubbles.
CuO	Copper	When combined with primary bonds, it triggers the adherence reaction at lower temperatures.
MnO₂	Manganese dioxide	Produces brown-coloured frits, intensifies dark colours and acts as an oxidant and as a weak bond.
Sb₂O₃	Antimonite	Produces a high degree of opacity and improves resistance to acids.
TiO₂	Rutile, Anatase, Brookite	The epitome of opacifiers, it increases whiteness, brilliance and resistance to acids and heat.
BaO CaO MgO	Barite Dolomite-Limestone Periclase	Alkaline-earthy components that integrate anti-acid enamels, increasing their resistance, their smelting facility and their viscosity.
P₂O₅	Apatite	Alters the opacity of the finish, improves colour stability and reduces chemical resistance.
F₂	Fluorite	Softens glass and influences the opacity.

Table a – Typical composition of a ground coat enamel
Felspar	21.0
Borax	35.0
Quartz	28.0
Sodium hydrogen carbonate	4.5
Sodium nitrate	4.0
Potassium and Sodium Fluosilicate	5.0
Cobalt oxide	0.5
Nickel oxide	0.5
Manganese dioxide	1.5

Table b – Typical composition of a white Ti cover coat
Felspar	4.5
Quartz	25.6
Sodium hydrogen carbonate	24.2
Sodium nitrate	2.4
Calcium carbonate	3.4
Cryolite	2.6
Sodium antimonate	0.8
Titanium dioxide	17.2
Zinc oxide	19.5

Properties

Why do we enamel?
For two series of reasons:
:: Functional reasons
:: Aesthetic reasons

Functional properties are:
:: Physical
:: Chemical
:: Mechanical
:: Hygienic
:: Environmental

Resistance to sudden changes in temperature

Physical properties are:
:: Incombustibility
:: Resistance to high temperatures
:: Resistance to thermal shock (sudden temperature change)
Vitreous surfaces do not deteriorate in any way when they come into contact with heat, with naked flames or with fire. In case of fire, they give off no toxic gases.

Chemical properties are:
:: Resistance to corrosion
:: Resistance to chemical agents
:: Resistance to atmospheric agents
:: Impermeability to liquids

Chemical resistance
When exposed to atmospheric agents, ferrous metals oxidise, rust rapidly and deteriorate to the point of destruction.
The function of vitreous enamel is to protect metal products against this inexorable process, lengthening their durability by decades.

Mechanical properties are:
:: Hardness
:: Resistance to scratches
:: Resistance to abrasion
:: Resistance to impacts and knocks

Mechanical resistance
The hardness of enamelled surfaces is comparable to the hardness of glass. As a rough guide, it is somewhere between 5 degrees (apatite) and 7 degrees (quartz) on the Mohs scale.
Enamel’s resistance to abrasion and scratches is closely related to its hardness.

Hygienic properties
Comparative bacteriological research undertaken on enamelled surfaces and on other materials and coatings has demonstrated enamel’s greater propensity to inhibit the development of bacteria: this propensity is greater than that of stainless steel.

Enamel and the environment
:: Enamel contains no toxic substances;
:: Enamel does not pollute the environment;
:: Enamelled products are recyclable.

The properties of enamel … in brief

Characteristic	Property	References
Resistance to fire	Incombustible	UNI ISO 1182
Resistance to high temperatures (400°C)	Naked flames do not damage enamel, turn it yellow or leave any burn marks.	UNI ISO 4530 UNI 10822
Resistance to sudden changes in temperature	Enamelled surfaces undergo no alterations, even when the temperature changes by 320°C.	UNI ISO 9227
Resistance to corrosion	Vitreous enamel protects the support against corrosion.	UNI ISO 9227
Resistance to acids	T.A. acids do not damage enamel. Hydrochloric and hydrofluoric acids should not be used on enamelled surfaces.	UNI EN 14483-1 ISO 2722
Resistance to normal detergents pH >7	Enamelled surfaces can be cleaned with liquid and cream detergents, because they do not scratch the enamel or alter it.	UNI 8026
Adherence	The enamel must adhere to the metal support.	UNI 8883 UNI 8527
Hardness	The enamel coating is hard and does not mark easily (5-7 on the Mohs scale).	EN 101
Resistance to scratches and graffiti	The enamel coating does not scratch easily.	UNI EN 15695
Resistance to impacts	Only knocks of a certain force will chip enamel.	UNI 9613 ISO 4532

Dielectric properties
Enamel has a high degree of electric resistance and acts as an excellent insulator, especially at room temperature.
This property enhances an enamelled product’s characteristics of protection against corrosion, preventing or blocking stray currents.

Electrical properties of certain insulating materials

Material	Electrical properties
	Dieletric constant K a	Dissipation factor δ a	Dielectric rigidity (volts/mil)	Specific resistance (ohm-cm)	Max. use temp. °C
Teflon	2.0-2.2	.02-.05	450-900	10¹⁶	204-260
Natural rubber	2.6-5	.25-5	600	10^16-10¹⁷	75
Neoprene rubber	7	.02	300	10¹²	93
Silicone rubber	3	.7	260-700	10¹⁴	204
High voltage porcelain	6.5	.8	250-400	10^12-10¹⁴	982
Mica	5.4-8.7	.25	110-220	10^13-10¹⁷	538
Fused silica	3.8	.07	410-1000	10¹⁹	1093
Borosilicate Glass	4.0	.05	400-1000	10^15-10¹⁷	371-982

Electrical properties of vitreous enamel

Material	Electrical properties
	Dielectric constant K a	Dissipation factor δ a	Dielectric rigidity (volts/mil)	Specific resistance (ohm-cm)	Max. use temp. °C
Ground coat enamel	6.2	.8	345	10^14-10¹⁶	315-538
Cover coat enamel	12.’0	3.5	485	10^13-10¹⁵	260-538
Cover coat enamel over ground coat	7.9	1.0	–	10^14-10¹⁶	260-538

Note:
a) Measured at 103 cps, T.A.
b) As the “maximum use temperature” depends on the frequency and type of use, a temperature range is given in each case.

Gloss/Brilliance
The availability of a high gloss finish is a unique property of vitreous enamel in contrast to other finishes. Organic finishes can not achieve the glass like smooth surface which can be obtained with vitreous enamel. However, vitreous enamel is also available as matt and semi-matt enamels, which are generally favoured for applications in the building industry, street furniture and interior design.

Aesthetic properties
:: Gloss/Brilliance
:: Colour range
:: Decorations and special effects
:: Stability of colours

It can be colored in
a huge color range.

Colour range
Vitreous enamels can be coloured in a huge range of hues and shades.
Enamels are generally coloured using inorganic pigments or metal oxides, which are added to the frit during the grinding stage. The full colour will develop during firing.

Decorations and special effects
Enamelled surfaces can be decorated using special enamels by variety of industrial processes including serigraphy (screen printing), tampo printing, decalcomania (transfers) or also hand painted by arists. This printing is as durable as the vitreous enamel to which it is applied.
This ability to be decorated also has a functional side, as in the case of sign systems, for example. Special effects (metal, granite etc.) are also possible.

Stability of colours
Time, atmospheric agents, light and UV radiation do nothing to alter the colours of enamel, which remain unchanged over time, without fading, turning yellow or losing their enamelled sheen.

Applications – Building, Exterior and interior architecture, Street furniture

In recent years, applications of vitreous enamel have expanded to include:
:: Civil and industrial buildings,
:: Exterior and interior architecture,
:: Street furniture,
all sectors where the material’s ability to provide a specific performance constitutes a crucial aspect of the design content.

What led to these new applications?
Several gradual transformations, some large, others small:
:: The increase in the cost of labour involved in maintenance and cleaning, which has made it a sound economic approach to make a slightly higher initial investment, on condition that those costs are subsequently kept down;
:: The increasing aggressiveness of meteorological agents, brought about by environmental pollution;
:: A cultural deterioration that is manifest in the abuses made of architecture and of the furnishings set out for public use; these abuses take the tangible form of graffiti, spray writing and, more generally, the tendency to make a mess of everything.

The solution to these problems
Vitreous enamel can solve these new problems: it is resistant to atmospheric agents and the aggressiveness of chemical products, its working life is practically unlimited, it requires no special maintenance and it is extremely easy to clean.
When coated with vitreous enamel, a simple sheet of steel can be transformed, as though by magic, into a panel in a continuous or ventilated façade, into one of the panels used to line the inside of an operating theatre or an underground railway station, into a tabletop, into the structure of a door or into the seat for a park bench.

Building, Exterior and interior architecture, Street furniture
Vitreous enamel has all the right characteristics to be a star player in current and future building design, for all kinds of applications: residential, commercial and industrial. Its use depends exclusively on the creativity of the architect, who can rely on a material that brings together extraordinary functional fidelity with infinite decorative potential. Vitreous enamel offers the architect a global, extremely versatile response: all he then has to do is “discover” the huge range of different applications made possible by this material.

Technical applications
:: Heat exchangers in desulphurisation plants in thermoelectric power stations,
:: Silencers for industrial vehicles,
:: Silos,
:: Reactors for the chemicals industry etc.

Home applications
:: Sinks and basins,
:: Water boilers,
:: Bathtubs,
:: Shower plates
:: Free-standing cookers,
:: Cooking hobs,
:: Built-in ovens and microwaves,
:: Saucepan supports and gas flame heads,
:: Saucepans
:: Stoves and tubes

It is quite natural that vitreous enamel has acquired an extensive range of applications in domestic environments: it is its unique capacities to resist heat (both naked flames and sudden changes in temperature) and to remain immune to the aggressions of chemical reagents and mechanical abrasives (detergents) that make it a material suited to environments – such as the kitchen and the bathroom – where it comes into contact with water every day and hygiene is of paramount importance.

Technology of enamelling

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 steel can be enamelled?
The types of steel that lend themselves to being enamelled can be summarised in three categories:
:: Carbon steel (C 0.035 ÷ 0.065 %)
:: Decarburised steel (C < 0.0040 %)
:: Interstitial-free steel (C 0.0015 ÷ 0.0050 %)

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

Enamelling cycles	N° of coats of enamel applied and firings
Multi-coat	2 Coats / 2 Fires 2 Coats / 1 Fire 3 Coats / 2 Fires
Single coat	1 Coat / 1 Fire

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
Support	Temperature°C
Steel	790 – 860
Aluminium	500 – 550
Cast iron (powder)	900 – 950
Cast iron (wet	730 – 800
Glass	550 – 700

Design – Construction details

Design
Designers are well advised to contact the enamelling technician in advance, indicating the product’s prerequisites as a function of its use.
It is worth remembering that:
:: Once a product has been enamelled, no more mechanical processes (such as cutting, bending and the like) can be undertaken;
:: To obtain good enamelling results, you must comply with certain important rules

Bends
The internal radius of the fold must be greater than 4 mm.
Sharp edges created by cutting cause problems when enamelling, unless due consideration is given to them.
For reasons of aesthetics, they should be rounded off, rolled over or finished with open curls.

Ribbing
If the edges are folded over as a way of making the product more rigid, it is essential to ensure that these folds do not generate problems of retention of oils, detergent liquids or vitreous enamel that could cause defects during the phases of coating and firing.
Beadings is recommended as an alternative.

Folded joints
Folded joints may also generate puddles where liquids are retained.
They also create areas where the thickness of the coating is not homogeneous, and this is undesirable during firing.
Whenever possible, end-to-end welds should be preferred to folded joints.

Welding
When joints are made using electro-welding, the metallographic structure of the steel is altered in the area immediately surrounding the weld: this may lead to the formation of aesthetic defects on the enamelled surface.

To avoid this, electro-welded joints should be made in areas that do not affect the final appearance of the finished object.

Joining attachments
To safeguard the appearance of the finished object, it is advisable to conceal these joints as much as possible.
The type of welding joints recommended in any case is the projection type of welding, using the same material as the object itself and ensuring that the attachments is thinner than the object to which it is being welded or of the same thickness .

Holes
Holes are generally drilled for functional and/or process purposes.
They should not have any burrs.
To avoid the possible formation of cracks, it is advisable to make all holes either circular or elliptical in shape.
The holes necessary for process purposes (balancing, grasping and hooking the object during the various phases of the process) must be drilled in positions that will not be visible when the product is finished. This way, they will avoid disturbing its appearance.

Cutting and Drilling

Bends and beadings
The internal radius of the fold must be > 4 mm

Principles to be considered
These are the factors that make the difference when designing products to be made of steel, aluminium, cast iron and glass to be coated with vitreous enamel:
:: The characteristics of the support,
:: Its weight and dimensions,
:: The design,
:: The manufacturing process
…In particular,
it is worth remembering the following:
:: The geometric characteristics of the support,
:: The deformations due to the temperatures generated in the enamelling process itself.
In addition, you also have to ensure:
:: the continuity of the coating, to achieve maximum protection.

Characteristics of the support
The support (steel, aluminium, cast iron or glass) must be suitable for enamelling, in order to guarantee:
:: good adherence between the enamel and the support,
:: the absence of defects.

Weight and dimensions
There is no limit of dimensions and/or structure to products that are destined to be enamelled: the only restrictions are those imposed by the manufacturing process.
For this reason, in the case of very large products, it is advisable to reduce them to their constituent parts for enamelling and then assemble them afterwards.
When designing products, it is worth remembering the following points:
:: the dimensions of the firing kiln,
:: how the piece will be handled during the manufacturing process,
:: any deformations that may occur during the process of firing.

Geometric characteristics
The product’s geometry:
:: must be of a kind that enables it to be coated completely and make allowance for potential serigraphic applications,
:: must make allowance for certain requirements of the enamelling process.

The phases of the process
Each individual phase of the enamelling process:
:: Pre-treatment of the support,
:: Applying the vitreous enamel coat(s)
:: And the vitrification firing,
requires special design solutions of its own.

Pre-treatment of the support
It is essential to remember that the object must be designed with drainage points that allow for both the easy entry and the easy drainage of the products and/or materials used in the process.

Applying the vitreous enamel coat(s)
In order to achieve maximum coverage for the coating, it is essential that the entire surface to be enamelled be easily accessible by the enamel, as a function of the technology being used.

Vitrification firing
It is essential to remember that deformations may be caused by the high temperature used for firing.

Easy curves	Sharp bends

Adopt	Avoid

Rolled edges	Enclosed spaces

Adopt	Avoid

Beadings	Folded ends

Adopt	Avoid

Welded joints	Folded joints

Adopt	Avoid

Continuous welding	Spot welding

Adopt	Avoid

Properties

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