pigment and chromium oxide
In common inorganic pigments, except chromium oxide and chromium hydroxide, which are found in Cr3+, all chromium are found in the 6-valent chromate (CrO2-4).They are (PbCrO4),Pb(S,Cr)O4,Molybdenum orange [Pb(S,Cr,Mn)O4].The brown and black materials in ceramic pigments are almost all spinels.Chromium is the main component in the Ca-Cr-Si,Cr-Al（Si）, Co(Zn)-(Al)-Cr of green pigments, and in the Zn-Al-Cr red pigments.
Colors like Ca-Sn-Si-Cr maroon, Ca-Sn-Si-Cr-Co purple and some pigments that cannot used for colouring, the chromium is also the main content of them. So does the pigment of Ti-Cr-Sb, Ti-Cr-W yellow for body pigment.In terms of quantity, chromium-soluble chromaticity takes more than the have of the total number of ceramic chromaticity.
Cr3+ is the most difficult cation to integrate in silicate glass.
A glaze may also be considered as a type of silicate glass.So Cr3+ is the most stable cation in the glaze.As a 3-, 4-, 5- valence cation, in coordination with O2-, it requires more than 6 coordination.For example, Cr3+(6)、Sn4+(6)、Zr4+(7 or 8), these oxides are added to glass raw materials and are insoluble in glass liquids.This is the reason why the new color materials of zircon, zircon and Stannite systems are very stable in the glaze.
It should avoid adding Cr2O3 directly into the glaze.It should be first diluted and then added to the glaze with Al203,SiO2 as the main ingredient.For example, Cr-Al and green material and Cr-Al-Si are made by calcining Cr203 together with kaolinite [Al2O3·2SiO2·2H2O] or lasite [Al203·4SiO2·H2O] after mixing them.
Another theory for the dilution of Cr2O3, kaolinite and waxite is the strong colouring ability of Cr2O3.When coloring in the glaze, it is necessary to add a small amount to achieve the effect.
In large quantity of glaze, it is not possible to disperse a very small amount of Cr2O3 evenly.Therefore, it is easy to disperse Cr2O3 when it is diluted into kaolinite and then added into the glaze.It will make the color very uniform.
