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 Why do we enamel?
For two series of reasons:
:: Functional reasons
:: Aesthetic reasons
Functional properties are:
:: Physical
:: Chemical
:: Mechanical
:: Hygienic
:: Environmental 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 | 1016-1017 | 75 | Neoprene rubber | 7 | .02 | 300 | 1012 | 93 | Silicone rubber | 3 | .7 | 260-700 | 1014 | 204 | High voltage porcelain | 6.5 | .8 | 250-400 | 1012-1014 | 982 | | Mica | 5.4-8.7 | .25 | 110-220 | 1013-1017 | 538 | | Fused silica | 3.8 | .07 | 410-1000 | 1019 | 1093 | | Borosilicate Glass | 4.0 | .05 | 400-1000 | 1015-1017 | 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 | 1014-1016 | 315-538 | | 12.’0 | 3.5 | 485 | 1013-1015 | 260-538 | Cover coat enamel over ground coat | 7.9 | 1.0 | - | 1014-1016 | 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. Aesthetic properties
:: Gloss/Brilliance
:: Colour range
:: Decorations and special effects
:: Stability of colours 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. 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. 
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Theo tài liệu của Viện Tráng Men Quốc Tế / IEI - International Enamellers Institute
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