Porcelain Enamel Frit

Description

Enamel (or vitreous enamel or porcelain enamel in U.S.) is the colorful result of fusing powdered glass to a substrate by firing, usually between 750 and 850 degrees Celsius. The powder melts and flows and hardens to a smooth, durable vitreous coating on metal, glass or ceramic.

Synonyms

porcelain enamel frit, vitreous enamel, porcelain enamel, blue porcelain, white porcelain, paste porcelain, meissen porcelain, glaze, clay, glass, soapstone, lime,

Chemical Properties

1) Enamel powder often is applied as a paste, and may be transparent or opaque when fired; vitreous enamel can be applied to most metals. It has many excellent properties: it is smooth, hard, chemically resistant, durable, can assume brilliant, long-lasting colors, and cannot burn. Its disadvantages are its tendency to crack or shatter when the substrate is stressed or bent. Its durability has found it many functional applications: early 20th century advertising signs, interior oven walls, cooking pots, exterior walls of kitchen appliances, cast iron bathtubs, farm storage silos, and processing equipment such as chemical reactors and pharmaceutical chemical process tanks. Commercial structures such as gas stations, bus stations and even Lustron Houses had walls, ceilings and structural elements made of porcelain-enamel steel.

2) Color in enamel is obtained by the addition of various minerals, often metal oxides cobalt, praseodymium, iron, or neodymium. The last creates delicate shades ranging from pure violet through wine-red and warm gray. Enamel can be either transparent, opaque or opalescent (translucent), which is a variety that gains a milky opacity the longer it is fired. Different enamel colors cannot be mixed to make a new color, in the manner of paint. This produces tiny specks of both colors; although the eye can be tricked by grinding colors together to an extremely fine, flour-like, powder.

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Enamel frits for glass and ceramic decorating

Decorating Enamels Frits

Properly fired enamels form a permanent coating that provides scratch resistance, chemical durability, gloss and color, all at the desired levels. Figure 1 shows some examples of fired-on enamel decorations that use glass frit. Each enamel component plays a specific role in enabling proper application and firing.

Decorating enamels are composed of glass frit, ceramic pigments and an organic medium, which have been mixed together under high shear to attain a uniform dispersion. Figure 2 shows a three-roll mill, which is one type of machine used to accomplish this mixing and dispersion step. Enamels firts are carefully formulated to obtain the properties required for successful application, firing and in-service performance. Application methods range from manual to fully automated, and common techniques include screen printing, spraying, rolling or using pre-printed decals.

Enamels frits typically contain 10-15% of ceramic pigments by weight. These pigments belong to a class of complex oxide colorants, used as fine particles typically less than 8 microns in diameter. Pigments lend color to enamels and, barring unwanted reactions during firing, are inert particles suspended in a layer of glass formed by the frit component. For most decorations, this glass layer is transparent, and the enamel frits color results from the reflection of incident light from the surfaces of the opaque, colored pigment particles.

The medium is a liquid carrier into which the frit and pigments particles are dispersed, and enamels firts are typically 15-25% by weight medium. The medium enables the delivery and retention of the frit and pigment particles onto the substrate in the desired pattern. Once the firing process has started, the medium must completely burn out, leaving only the frit and pigment behind. Important medium properties include viscosity, surface tension, drying behavior, strength or adhesion, and burn-out temperature. Most mediums are blends themselves, containing additives to precisely tailor the properties listed previously to optimize the blending, application and firing of the enamel mixture.

Glass frit is usually more than 60% by weight in the unfired enamel mixture, comprising about 70-80% after firing. The frit particles soften and fuse during firing, creating a continuous glass layer bonded chemically and mechanically to the substrate. Pigment particles are dispersed inside this glass layer, and both pigment and glass contribute to the color of the decoration. Gloss level is directly related to the glass refractive index and the fired surface smoothness, while the ratio of frit to pigment is a factor too, with high pigment content leading to a matte appearance. Superficially, glass frit is a high-temperature “glue” that holds the pigment in place. In reality, proper frit selection is crucial because the frit properties profoundly influence both the enamel behavior during firing and the fired decoration properties.

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Classification and marking of direct enamel frit

Direct enamel frit are used for preparation of glazes on floor and wall tiles and other products of architectural, sanitary, utility and art ceramics. Frits are divided according to their characteristics and use into main groups, secondary groups and subgroups.

In agreement with this classification, the marking of direct enamel frit consists of two letters, a group of 5 digits and a letter at the end of the marking.

Letters specify the classification in the corresponding main group: the first letter indicates the use of the frit (C stands for ceramic), the second letter indicates the characteristics of the fired glaze which was prepared from the indicated frit.

In the following group of five digits, the first digit specifies the colour of the glaze. The next two digits indicate 1/10 of temperature in a hot-stage microscope, at which the pellet prepared from the milled frit has the shape of a hemisphere. The determined temperature is rounded off upwards or downwards within a range 200C (at a temperature over 10000C, the value 10000C is subtracted). Further digits indicates the use of the frit, the last digit indicates the serial number in the corresponding group or subgroup.

The letter at the end of the frit marking indicates the form in which the frit is being supplied.

Direct enamel frit are divided into the following main groups according to their use and visual characteristics:

direct enamel frit for preparation of glossy glazes – marking CL

direct enamel frit for preparation of semi-glossy glazes – markingCS

direct enamel frit for preparation of matte glazes – marking CM

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