How Enamel Manufacturers Choose Pigments?

The coloring of enamel has two methods: melting and adding colored oxide in the enamel and grinding and adding pigment in the enamel slurry. There are several types of pigments that can be used for enamel:
1. Spinel pigments
Its general formula is XO•Y2O3. In spinel, X is a divalent cation such as cobalt, nickel, copper, zinc, magnesium, iron, manganese, calcium, etc., and Y is a trivalent cation such as chromium, manganese, iron, aluminum and the like. Spinel pigment has high chemical stability and high stability, and can be used for coloring other enamel frit except titanium enamel.

2. Cadmium series pigments
There are cadmium yellow pigments of cadmium sulfide (CdS) and cadmium red pigments of cadmium sulphur selenide (CdSeS). The color of this type of pigment is directly related to the selenium in its composition. When the amount of selenium added increases, the color changes from yellow to orange and then to red. Cadmium pigments are generally only used in transparent enamel frit or translucent enamel frit.

When cadmium-based pigments are used, sometimes a mist-like sulfate film is formed on the surface of the enamel. This is due to the oxidation of part of the sulfide of the pigment to sulfate. It can be solved by appropriately lowering the melting point of the enamel or increasing the firing temperature. If you want to remove the matt film that has been produced, you can rinse the enamel surface with clean water, drain it, and re-fire the product. In addition, for safety reasons, cadmium-based pigments cannot be used in enamel cookware and tableware that come into contact with food.

3. Wrap the paint
Because the high temperature stability of the enamel pigment cadmium pigment in the enamel is relatively poor, it may decompose above 880 °C, and these pigments can be used to coat the transparent zirconium silicate crystal with strong high temperature chemical stability. A cadmium-based pigment that is still hot even at a temperature of 1400°C. Such pigments include red, orange, yellow, etc., which have been widely used in the ceramic industry, and can also be added to enamel frit. The coated pigment should be added to the ball mill 1h before the enamel slurry is ground to the specified fineness. If the ball milling time is too long, the crystal structure of zirconium silicate will be destroyed, and the cadmium-based pigment will lose its protective layer and decompose at high temperature.

4. Zirconium and tin pigments
This type of pigment is formed by the combination of colored oxide, leucosilicate zirconium and tin oxide, and is an important coloring material in ceramics. Because of their light color, colorants are commonly used in light-colored enamels.
The typical colors of zirconium pigments are: zirconium iron red, praseodymium yellow, vanadium blue, vanadium zirconium yellow, fruit green, etc.
Tin-based pigments are: pink, atomic red, and rose red.
Due to various reducing impurities in the enamel frit, such pigments often cause partial fading, and white spots appear on the surface of the enamel after firing, which can be overcome by grinding with 0.5% soda ash in the enamel slurry.

5. Pearlescent pigments
Pearlescent pigments will reflect light multiple times, creating a glittering effect like pearls or fish scales on the porcelain surface. Ordinary muscovite pearlescent pigments cannot withstand the high temperature of enamel firing. The pearlescent pigments for enamel use high-temperature synthetic fluorphlogopite flakes as the base material, and the mica is coated with titanium dioxide hydrate. Due to the difference in refractive index between titanium dioxide and mica, it produces a pearlescent effect, and is coated with tin dioxide and other refractory Oxide, to resist the erosion of pearlescent pigments by enamel at high temperature.

Pearlescent pigments are more sensitive to the firing temperature. A lower firing temperature is beneficial to it. It can only be used in transparent special enamel glazes. Be careful not to introduce opaque ingredients to prevent affecting the pearlescent effect. When used, the pearlescent pigment cannot be ground together with the enamel glaze. After grinding the enamel and glaze, add pearlescent pigment and stir evenly.