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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