How to improve inclusion colors?
The inclusion pigments
Ceramic colors are known for their unique stability in glazes and ceramic body at high temperatures. The stable temperature range of different colors in the glaze is 800~1300℃. The maximum firing temperature depends on the composition of the colors. According to the temperature range and other properties of color stability. It is suitable for hard enamel, soft enamel, wall tile glaze or sanitary enamel respectively. The stable temperature range of color in the body is 1100~1400℃. The specific temperature depends on the composition of the pigments.
It is found by electron microscope that uniform color is formed by fine particles with average particle size of micron. In order to improve the ability and stability of colorant, a basic requirement is to reduce the solubility of colorant particles in glazed glass as much as possible. Tin oxide (SnO2), zirconia (ZrO2) and zirconium silicate are all opacifiers in glass fluxes. Therefore, they have the basic conditions to be used as color coating.
This series of colors are applied to wall tiles and sanitary porcelain decoration.
One of its basic components is Cd(SxSe1-x), which changes color from bright red to orange and then to yellow depending on the x value. The colour of the series is bright (there is an obvious absorption peak in the spectral reflectance curve of the system, which moves with different x, resulting in a change in colour).
However,the stability of the CdS/CdSe color in the glaze at high temperature is very poor. It usually break down above 800℃. By wrapping these colors in stable ZrSiO4 crystals, a stable color can be produced up to 1400 ℃.
When the nucleation of Cd(S,Se) microcrystalline is controlled, the number of crystal nuclei is large and the growth rate is getting slow. When the growth rate of ZrSiO4 microcrystalline getting fast,the inclusion colors with high wrapping rate, high intensity and bright color can be produced.
Since Cd(SxSe1-x) cannot be completely wrapped during synthesis, any unwrapped Cd(SxSe1-x) left on the surface of ZrSiO4 must be washed off by chemical means.
Properties of ZrSiO4/Cd(S,Se) inclusion pigment
Color Performance
Analysis in theory, because Cd-S-Se system can form a complete series of mixed crystal (solid solution), ZrSiO4/Cd(S,Se) inclusion pigment can present transition from all bright red - orange - yellow color.
High temperature stability
When ZrSiO4/Cd(S,Se) is applied to the glaze, the high temperature viscosity of the molten base glaze has a great influence on its color stability.
The very soft and flowing glaze will dissolve in the CdS/Se ZrSiO4 protective layer. Which causing the unstable Cd(SxSe1-x) colors to decompose. And this will cause the color of the coating to change. The stability of the inclusion color is to maintain its color stability in the glaze at the longest firing cycle and the highest firing temperature.
In order to test the high temperature stability of the inclusion pigment material, the colorimetric analysis is carried out by means of the ladder temperature method. The specific method is as follows: Take a long and narrow ceramic chip (50mm × 400mm). Evenly coat the surface of the chip with a layer of colored glaze. Then put the chip into the tube ladder furnace. The temperature gradient in the furnace can be controlled. There is 200C temperature difference along the length of the furnace, such as 950~1150℃, 900~1100℃. The ladder temperature strip is obtained after the sample is burned in the ladder temperature furnace. The range of firing temperature can be determined by color contrast along the length of the strip. When the color difference △E=0.9NBS, the corresponding temperature range is the firing temperature range.
Chemical stability
Besides to improve the high temperature stability of the color, the zirconite surrounding CsS/Se can also provide a good protection against chemical corrosion of CdS/Se colors.
In order to have better performance of inclusion color in the frit glaze,the following measures are suggested to obtain bright red ZrSiO4/Cd(S,Se).
Major performance requirements for frit glazes (base glazes).
Select: RI=1.9~2.0; tg>500℃; t>600℃; a=(60~80) ×10-7℃-1
Chemical composition requirements for frit base glazes (percentage, %)
Alkali content < 5%; The SiO2 content < 50%; PbO content ~ 50%; Alkaline earth metal and boron content should not be too high.
Requirements for ball milling process of the mixture of color and base glaze
In order to prevent the destruction of zircon inclusions, the time of ball grinding for the inclusion colors and frit glaze should be reduced as much as possible. The basic principle is that the mixture is well mixed. For this purpose, the base glaze and inclusion colors should reach the required fineness before mixing.
Application of inclusion color in sanitary and other fields
Sanitary ware is fired at 1200~1300℃. Since ZrSiO4 is usually used as opacifier for sanitary glazes, then the stability of Cd(SxSe1-x) inclusion color is guaranteed.
The inclusion color applied in sanitary enamel is mainly Cd(SxSe1-x) red inclusion color. The following points should be noted when using:
①When the fineness of sanitary enamel reaches the requirement, then add the inclusion color. The mixing ball grinding time should not exceed 2h to avoid damaging the ZrSiO4 inclusion.
②No ZnO should be added to the glaze to prevent pinholes.
③Reduce the amount of BaCO3 to reduce its side effect on color.
④Do not add lithium feldspar.
⑤Use High gradient strong magnetic separator to conduct magnetic separation on glaze slurry to remove harmful impurities.
The applications of ceramic color, such as color glaze, on-glaze, under-glaze, color painting, ceramic stained paper, trademark and so on, can also use Cd(SxSe1-x) inclusion color. But it should avoid reducing atmosphere, especially strong reducing atmosphere.
Study on the formation mechanism of ZrSiO4-Cd(SxSe1-x) inclusion colors
The formation of ZrSiO4-Cd(SxSe1-x) inclusion pigment can be divided into two stages. They include the formation of ZrSiO4, the appearance of liquid phase, and the formation of Cd(SxSe1-x）.
①The initial stage
ZrSiO4 was obtained by reaction of SiO2,ZrO2 and mineralizing agent. The crystals formed at this stage are small and there are six-sided Cd(SxSe1-x) microcrystals formed.
②Major formative stages
At this stage,Cd(SxSe1-x) microcrystals are sandwiched between ZrSiO4. This phenomenon can be explained by liquid phase sintering theory. The formation of Li2SiO4 glass phase causes the zircon particles to rearrange. They grow by dissolving， precipitating. At the same time, a certain amount of Cd(SxSe1-x) solid solution is wrapped in ZrSiO4. Usually the encapsulation rate is not very high because only small cadmium selenide particles can be completely encapsulated by ZrSiO4 microcrystals. The key to the synthesis of inclusion pigments is to improve the coating rate of colorants.
Quality control of inclusion pigments
(1)Determination of particle
It need to test each lot of the inclusion pigment. Laser particle analyzer was used for testing. The deviation between the average particle size of the colorant and the standard average particle size should be controlled within ±25%.
(2)The testing of the color
It need to take this test of each lot of the inclusion pigment.Using a spectral colorimeter, the wavelength interval 2 is 5nm or 10nm.The 1976 CIE Lab color space was used to represent the results and calculate the color difference. And the deviation should not be greater than 1NBS.
(3)Control of synthesis temperature
The temperature of the color material calcination, the starting temperature and the terminating temperature should be strictly controlled. Deviation from the standard shall not be greater than 15%.
Chemical coprecipitation, a new preparation method for inclusion pigments
In order to obtain high quality high temperature incluson color for under glaze, it is necessary to significantly increase the amount of permeating agent in zirconite crystal. That is, to improve the coating rate.In the past solid phase method, the effective rate of colouring agent infiltration is not more than 5%~10%, which cannot meet the requirements.In order to obtain the superior package color with high wrapping rate,the liquid phase method must be used. Such as chemical coprecipitation method and sol-gel method.
In this method, the lead substances, zirconia hydroxide and silicic acid of the colorant are precipitated successively from the aqueous solution of salt.With this method, the wrapping efficiency can be up to 30%~50%.The cadmium selenide (CD) can infiltrate into the zirconite crystal lattice by 7%~12%.At this level, the color strength of the protected material is sufficient.In addition to producing excellent color effects, the wrapping method also gives these colors good thermal stability up to 1400℃.But its enlightenment and influence are far greater than the color itself.Because of the universality of the wrapping method, it is not only suitable for cadmium selenide color system, but also suitable for other colors with poor color stability.
The high temperature stability and chemical stability of coating colors make them available for decoration under and in glazes.
