Источник: Digitalfire Ceramics Technical Articles. Section: Ceramic Tile, Subsection: General Proc. 2003 Winter Simulation Conf., New Orleans, LA, 2003.
http://digitalfire.com/4sight/education/glaze_and_body_pigments_and_stains_in_the_ceramic_tile_industry_342.html
Description
A complete discussion of how ceramic pigments and stains are manufactured and used in the tile industry. It includes theory, types, colors, opacification, processing, particles size, testing information.
Article
A ceramic pigment particle is an inorganic white, black or colored solid that is insoluble in the matrix into which is incorporated and does not react chemically or physically with it. Thus one of most important characteristics of ceramic pigments is their thermal stability at high temperatures and their chemical stability in respect to phases, even liquid, forming during firing of glazes or bodies as a result of the sinterisation process and melting.
Non oxides
Cadmium sulfoselenide, Cd(Sx,Se1-x), is a very important pigment because it is the only one available to obtain true red and orange colors for glazes (depending on the amount of selenium). The old production method was to trap cadmium sulfoselenide in a sintered matrix of a refractory material, thereby stabilizing it. Another method was to mix it with a batch to produce a frit under specific melting conditions. Both methods ensured good stability up to 1100oC.
However these pigments were not suitable for single firing so we overcame the problem by encapsulating the coloring oxide in a stable vitreous or crystalline matrix. The crystal responsible for color is thus occluded in the matrix during the process of sintering (two phases are formed). The most important examples are Cd(Sx,Se1-x) red and Fe2O3 pink, occluded in a matrix of zirconium silicate.
In the first step of this process, at about 900 C, the initial formation of ZrSiO4 occurs via the reaction between SiO2 and ZrO2(and mineralizers). The result is the formation of hexagonal crystals of Cd(Sx,Se1-x) from the reaction of CdS and Se or CdCO3, S and Se. As noted, a liquid vitreous phase of low melting compounds (mineralizers) are employed to help in the growth of zircon around the sulfoselenide crystals. These stains are very expensive due to this production process and the range of colors isn't wide (dark red tones are not possible).
Metallic Colloids
The most important color is the pink given by colloidal metallic gold (selenium is often used for glasses, not for glazes, other colloids give less interesting colors. This material is synthesized by adding tin(II) chloride to a solution of chlorine acid gold colloidal metallic, the gold particles settle. The color ranges from pink to violet, depending on the ratio of tin/gold (Cassio's purple). To produce a stable color at high temperatures the settling of purple is carried out in a slip of kaolin or clay to avoid coagulation (particles of metallic gold are separated by the clay particles). Additions of silver chloride alter the color towards reddish, additions of cobalt oxide change it toward violet. Unfortunately the use of this technique is limited by cost.
Metallic oxides
Synthetic oxides are predominantly used, together with some natural ones like iron oxide and manganese oxide. They usually dissolve in the vitreous matrix exhibiting their coloring function in the ionic form and for this reason they give to glazes a pleasant deep and transparent appearance.
The disadvantages of using these pigments are numerous:
Complex Inorganic Colored Pigments
These materials are obtained by solid state reactions at high temperatures using metallic oxides or salts, generally in the presence of mineralizers such as halogenides of alkaline metals, borates, carbonates etc. These substances can thus be considered colored artificial minerals resulting from reactions in the temperature range 800 - 1400C. Pigments remain unaltered in the glaze during firing (even finely dispersed ones) and they enable us to obtain a wide range of shades.
Crystalline structures suitable to produce ceramic stains are of a limited number. Good pigments are of high purity, uniform and chemically inert and do not decompose at high temperatures or react with or dissolve in glazes; that is often not true of this class of materials. Still, some are stable enough, notably rutile, zircon, zirconia, corundum and sphene.
Metallic oxides
Solubility and Particle Size Distribution of Pigments
A few pigment crystal structures are very stable, like spinel, because they have high melting point and low solubility in molten silica glass. Others are more soluble because of mobility of their elements. When compounds have a partial solubility and crystallize again during cooling we can loose the coloring ability (if the colored elements remain dissolved in the glaze). Solubility also depends on granulometric sizes (this variable plays a very important role). Stains with large particle sizes have a reduced coloring power because the number of coloring particles is less. Smaller granulometric sizes tend to decrease the color intensity and/or produce different shades (because particles of smaller size tend to easily dissolve into the glaze). Furthermore there is an increase in the scattering of white light by small particles, this results in a decrease of saturation (dilution of light). Thus, generally speaking, every type of stain has its own optimal particle size.