(Source: China ceramic net)

In terms of some material properties involved in ceramic materials, mechanical properties and optical properties are undoubtedly the two most important elements. Mechanical properties determine the basic performance of materials, while optics is the embodiment of decorative properties. In building ceramics, the optical properties are mainly reflected in the glaze. The corresponding optical properties can be basically divided into three reference elements: glossiness, transparency and whiteness.

Glossiness

When light is projected on an object, it will not only reflect in a certain direction according to the law of reflection, but also scatter. If the surface is smooth and flat, the intensity of light in the specular reflection direction is greater than that in other directions, so it is much brighter, which is reflected in strong glossiness. If the surface is rough and uneven, the light is diffusely reflected in all directions, and the surface is semi matte or matte.

It can be seen that the luster of an object is mainly caused by the specular reflection of the object, which reflects the flatness and smoothness of the surface. Glossiness is the ratio of the intensity of light in the specular reflection direction to the intensity of all reflected light.

The gloss of 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 surface, because the high refractive index increases the reflection component in the mirror direction. The refractive index is directly proportional to the density of the glaze layer. Therefore, under the same other conditions, the ceramic glaze contains oxides of Pb, Ba, Sr, Sn and other high-density elements, so its refractive index is larger and its luster is stronger than that of the porcelain glaze.In the aspect of preparation, the glaze surface can be finely polished to obtain a high specular surface, so as to improve the gloss of the glaze.

Transparency 

The transparency basically depends on the content of glass phase in the glaze.

Generally speaking, the higher the content of glass phase, the less the content of crystal and bubble, and the higher the transparency of glaze.

Therefore, from the aspect of formula design, a large number of fusible elements are used in the formula, and controlling the content of aluminum is conducive to the improvement of transparency. From the perspective of preparation, the rapid cooling of glaze at high temperature and avoiding glaze crystallization are conducive to the improvement of transparency. The three main raw materials for glass preparation, soda ash, limestone and silica, are white and low iron raw materials in appearance, the prepared glass has high transparency and very low whiteness. However, once the internal crystallization becomes glass ceramics, it will become white products and high white products.

Whiteness 

Whiteness is caused by diffuse reflection of light on the product. For household porcelain, sanitary porcelain and building ceramics, whiteness is an important index to evaluate their appearance performance. This is because consumers are easy to associate white with clean.

The white color of the object is caused by its less selective absorption of white light, low transmittance and large scattering.If an object has less selective absorption of white light and less scattering, the object is transparent. It can be seen that the whiteness of the glaze mainly depends on the low white light absorption, low transmittance and strong scattering ability of the glaze.

In terms of composition, the influence of whiteness mainly depends on the content of colored oxide and fusible elements in glaze. Generally speaking, the lower the colored oxide, the higher the whiteness; The less fusible elements, the higher the whiteness.

In terms of preparation, whiteness is affected by firing system. The raw material has more iron and less titanium, Firing in reducing atmosphere can increase the whiteness; On the contrary, the use of oxidizing atmosphere will increase the whiteness. If the product is cooled or insulated with the furnace, the number of crystals in the glaze will increase, which will lead to the increase of glaze whiteness.

When testing the whiteness of raw materials, there is often little difference between the dry white and wet white data of porcelain and stone raw materials, while the dry white and wet white data of clay materials are often very different. This is because the glass phase fills the gap in the sintering process of porcelain and stone materials, and light reflection often occurs on the surface. The glass phase of the clay fired plate is less, and the light is also reflected inside the plate. After immersion treatment, the light cannot be reflected from the inside, resulting in an obvious decline in the detection data, which is particularly prominent in the kaolin containing mica. At the same time during firing, the firing atmosphere should be controlled and prevent the decrease of whiteness caused by carbon deposition.

 

On building ceramic glaze, the effects of three kinds of light will occur. Therefore, during the process of formulation and preparation, it is often considered in production to highlight one item and weaken others in order to improve some effect.