What are the common defects on ceramics?

Ceramic production is a very complicated and meticulous process. A slight negligence in any process from raw material to firing may cause defects or even scrap. Some of these defects were discovered before firing, some were exposed during firing before firing, and some were directly caused by improper firing. Because the reasons for the defects are complex and the production conditions of each kiln are not the same, it is difficult to make a comprehensive and universally applicable solution. 1. Cracking: Cracking refers to cracks of different sizes on the product. The reason is that the moisture of the green body entering the kiln is too high (more than 2%), and the preheating and cooling are too fast, resulting in uneven shrinkage of the product inside and outside. In some cases, the green body has been collided with internal injuries before loading the bowl. Uneven thickness of the blank, excessive weight of accessories (such as pot handles, spouts, etc.) or poor bonding can also cause the product to crack. The preventive measures are: (1) The green body must be dry when entering the kiln; (2) When installing the kiln, operate the suit carefully, and the gasket and the green body have the same formula. The size, weight and bonding position of the accessories are appropriate. Some add 10-15% of the glaze to the bonding slurry, which can make the nozzle and handle firmly welded to the main body, so that cracking defects can be overcome. 2. Foaming: There are two types of foaming for burnt products: 'green foam' and 'glaze foam'. The green bubbles are divided into two types: 'oxidized bubbles' and 'reduced bubbles'. Oxidized bubble refers to the glaze layer on the outside of the blank bubble, the cross section is gray and black, and it is mostly formed in the low temperature part of the kiln. The main reason is that the decomposition products in the porcelain white porcelin dinnerware body and glaze are not fully oxidized and the burning loss is not completely eliminated. The preheating temperature rises quickly, the oxidation decomposition stage time is short, the temperature in the kiln is too low at the end of the oxidation, and the temperature difference between the upper and lower sides is too large. In the blank glaze, carbonate. Sulfate and more organic impurities are the main causes of product foaming. In addition, the improper density of the fashion kiln and the high moisture entering the kiln must also be paid attention to. The reduction bubble is also called the overfire bubble, which breaks and turns yellow, and it mostly occurs in the products near the vent of high temperature. It is mainly caused by insufficient reduction of sulfate and high-valent iron in the body, insufficient strong reducing atmosphere and excessively high firing temperature. The deposited charcoal and decomposition products of the glaze bubble system cannot be burnt and volatilized before the glaze is melted, and the gas is blocked in the glaze surface layer. It can be solved if the glaze melting time is prolonged or the flat firing is appropriate. 3. Pinholes: Refers to tiny dents or small holes on the glaze of the product. One such defect is the organic matter in the blank. The content of carbon and iron oxide is relatively high. When the temperature rises quickly, the burning loss cannot be completely burnt out and volatilized, and it will not escape the glaze until the later high temperature stage, forming pinholes like microscopic volcanoes. In addition, the reducing atmosphere of the high-temperature furnace is too weak, and the product at the vent is oxidized again, which will also cause pinholes. Furthermore, pinhole defects can also occur when the glaze has poor fluidity or the glaze is too thin. 4. Orange glaze: The glaze of the product is uneven and orange peel-like. Generally occurs in plates, dishes or porcelain tile products. The main reason is that the heating of the glaze is too fast during waveization, and the sintering temperature is too high to cause the glaze to boil. In addition, uneven thickness of the glaze slurry, poor high-temperature fluidity, and poorly ground glaze are the crux of the formation of orange glaze defects. 5. Surprised glaze: There are thick cracks on the glaze of the product. The main reason is that the expansion coefficient of the blank and the glaze are quite different. This requires re-adjustment of the billet. Glaze ingredients recipe. In addition, the firing temperature is too high, the cooling system is unreasonable, or the glaze layer is too thick can also cause glaze defects. 6. Deformation: Product firing deformation is the most common and most serious defect in the ceramic industry, such as distorted and out-of-round caliber and irregular changes in geometry. The main reason is improper method of kiln installation. If the column of the sagger is not correct, the bottom of the sagger or the gasket is not flat, the kiln car will vibrate, which will affect the deformation of the product. In addition, when the product is sintered, when the green body is preheated and the temperature rises quickly, the temperature difference is large and it is easy to deform. The firing temperature is too high or the holding time is too long will also cause a large number of deformation defects. The poor high temperature strength of the used saggar or uneven paint will also cause the deformation of the fired product. 7. Yin yellow: The surface of the product is yellowish or spotted yellowish, and some sections also have yellowish phenomenon, which usually occurs at high fire positions. The main reason is that the temperature rises too fast, the glaze melts too early, and the reducing atmosphere is insufficient, so that the Fe2O3 in the porcelain tableware body cannot be reduced to FeO. In addition, the filling of the bowl column is too low, the local product temperature of the kiln top is too high and the reduction is insufficient, which will also cause the defect of yellowing. Too high TiO2 content in the product raw materials will also cause the product to turn yellow. If a trace amount of CoO is added to the blank, the yellow color of the product can be covered. 8. Smoke: No matter what kind of fuel is used, smoke will occur. Smoke refers to the appearance of gray or impure white on the surface of the product. Mainly due to incomplete oxidation or premature reduction of the green body, the carbon, organic matter or low-temperature carbon in the green body cannot be burned out before the glaze layer is sealed. Sometimes the backflow of smoke will also erode the glaze. If the calcium content in the glaze is too high, it is easy to form smoke defects. Nine. Burning and overburning: Burning products have yellow appearance, high water absorption, poor and rough glaze gloss, low strength, and muddy sound when hit. When over-firing, the product deforms and the glaze blisters or flows. The main reasons are that the firing temperature is too high or low, the high temperature holding time is not properly controlled, the loading density is unreasonable, or the firing temperature difference is large, etc., which will cause local overburning or burning. 10. Matte: Also known as Xiaoyan. The reason for the glaze matt is the formation of fine bodies on the glaze surface and poor fusion of the glaze layer, so the glaze matt defect is formed. Rapid cooling can be adopted in the initial cooling stage to prevent chromatographic crystals on the glaze surface. Improve glaze gloss.

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