In terms of some material properties involved in building ceramic materials, mechanical properties and optical properties are undoubtedly the two most important factors. The mechanical properties determine the basic performance of the material, while the optical properties reflect the decorative properties. In building ceramics, the optical properties are mainly reflected on the glaze. The corresponding optical properties can be basically divided into three reference elements: gloss, transparency and whiteness.
Glossiness
When light hits an object, it will both reflect in a certain direction according to the law of reflection and scatter. If the surface is smooth and flat, the intensity of light in the direction of specular reflection is greater than in other directions, so it is much brighter, which is reflected in strong gloss. If the surface is rough and uneven, the light will be diffusely reflected in all directions, and the surface will be semi-matte or matte.
It can be seen thatthe gloss of an object is mainly caused by the specular reflection of light from the object, which reflects the flatness and smoothness of the surface. Glossiness is the proportional coefficient of the intensity of light in the direction of specular reflection to the intensity of all reflected light.
The gloss of the glaze is directly related to its refractive index. Generally speaking, the higher the content of high-refractive elements in the formula, the stronger the glossiness of the glaze, because the high refractive index increases the reflection component in the mirror direction. The refractive index is proportional to the density of the glaze layer. Therefore, under other conditions being the same, ceramic glazes contain oxides of Pb, Ba, Sr, Sn and other high-density elements, so their refractive index is greater than that of porcelain glazes. The luster is also strong. In terms of preparation, the gloss of the glaze can beimproved by fine polishing of the glaze surface to obtain a high specular surface.
Transparency
Transparency basically depends on the glass phase content in the glaze.
Generally speaking, the higher the glass phase content, the lower the crystal and bubble content, and the higher the transparency of the glaze.
Therefore, from the perspective of formula design, a large number of fluxing elements are used in the formula, and controlling the content of aluminum elements is conducive to improving transparency. From a preparation perspective, rapid cooling of the glaze at high temperatures to avoid crystallization on the glaze surface is beneficial to improving transparency. The three main raw materials for making glass, soda ash, limestone and silica, are all white and low-iron raw materials in appearance. The transparency of the prepared glass is very high, but the whiteness is very low. But once it crystallizes internally and turns into crystallized glass, it will become a white product, and it is a high-white product.
Whiteness
Whiteness is caused by diffuse reflection of light on the product. For daily porcelain, sanitary porcelain and building ceramics, whiteness is an important indicator to evaluate their appearance performance. This is because consumers tend to associate white and clean with each other.
The white color of an object is caused by its small selective absorption of white light, low transmittance, and large amount of scattering. If an object selectively absorbs less white light and scatters less, the object is transparent. It can be seen that the whiteness of the glaze mainly depends on the glaze's low absorption of white light, low transmittance, and extremely strong scattering ability.
From a composition perspective, the whiteness mainly depends on the content of colored oxides and fluxing elements in the glaze. Generally speaking, if the coloring oxide is lower, the whiteness is higher; the fluxing elements are less, the whiteness is higher.
In terms of preparation, whiteness is affected by the firing system. The raw material contains more iron and less titanium, so firing in a reducing atmosphere can increase the whiteness; conversely, using an oxidizing atmosphere can increase the whiteness. If the product is furnace-cooled or heat-insulated, the number of crystals in the glaze increases, which will lead to an increase in the whiteness of the glaze.
When testing the whiteness of raw materials, it often appears that the dry white and wet white data of porcelain stone materials are not very different, while the dry white and wet white data of earth materials are often very different. This is because the glass phase fills the gaps during the sintering process of porcelain stone materials, and light reflection often occurs on the surface. The earth-fired plate has less glass, and the light is reflected inside the plate. After the water immersion treatment, the light cannot be reflected from the inside, resulting in a significant drop in the detection data. This is especially true for kaolin containing mica. protrude. At the same time, during firing, the firing atmosphere should be controlled and carbon deposition should be prevented, which would lead to a decrease in whiteness.
In building pottery glazes, the effects of three types of light will occur. Therefore, in the formula and preparation process, one item should be highlighted and others weakened to obtain a certain effect. , is something that is often considered in production.
