Why are there defects in the firing process of bone china mugs? | Win-Win

February 10, 2023

1. Deformation: Deformation of bone china cups after firing is a very common and serious defect in the ceramic industry, such as crooked and out-of-round calibers and irregular changes in how many styles. The main reason is improper kiln installation method. If the column of the sagger is not straight, the bottom of the sagger or the gasket is not flat, the operation of the kiln car will be touched, and the deformation of the product will be affected. In addition, when the green body is preheated and heated quickly during firing, the temperature difference is large and deformation is likely to occur. Excessively high firing temperature or too long holding time will also cause a large number of deformation defects. The high-temperature strength of the used sagger is poor, or when the paint is not smooth, it will also cause deformation of the fired product.


2. Cracks: Cracks in bone china cups refer to cracks of different sizes on the finished product. The reason is that the water content of the green body in the kiln is too high (more than 2%), and the preheating rises softly and cools down too quickly, resulting in uneven shrinkage of the finished product. In some cases, the green bodies have been impacted and have internal injuries before they are placed in the bowl. The uneven thickness of the body, excessive weight of accessories (such as pot handles, spouts, etc.) or poor bonding will also cause cracking of the finished product. The preventive measures are: (1) The water content of the body entering the kiln is less than 2%, and the speed of the vehicle is appropriate to reduce the cooling capacity. (2) When installing the kiln, the suit is carefully controlled, and the formula of the gasket is consistent with that of the green body. The size, weight and bonding position of the accessories are appropriate. Some add 10-15% glaze to the bonding mud, which can make the nozzle, the handle and the main body firmly welded together, so that the defect of cracking can be overcome.


3. Blistering: There are two types of foaming in the fired bone china cup: base foam and glaze foam. Blank foam is divided into oxidation foam and reduction foam. Oxidation bubbles refer to the glaze layer covering the surface of the billet bubbles, and the cross-section is gray-black, mostly formed in the low-temperature parts of the kiln. The main reason is that the decomposed matter in the porcelain body and glaze has not been fully oxidized, and the burnt matter has not been completely removed. The preheating temperature rises quickly, the time of the oxidation decomposition stage 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 is too large. In blank glazes, carbonates. High content of sulfate and organic impurities are the main causes of product foaming. In addition, attention should be paid to the improper density of the fashion car and the high water content in the kiln. Reduction bubbles, also known as extreme bubbles, break and turn yellow, and mostly occur in finished products near the crater at high temperature. It is mainly caused by insufficient reduction of sulfate and high-valent iron in the green body, insufficient strong reducing air, and excessive firing temperature. Glaze blisters are deposited charcoal and dissociated matter that cannot be completely burned and volatilized before the glaze is melted, and the gas is blocked in the glaze layer to form. It can be solved if the glaze melting time is prolonged or the flat firing is done properly.


4. Yin yellow: the surface of the finished product is yellow or patchy yellow, and some sections also have yellow, mostly appearing at high fire positions. The main reason is that the temperature rises too fast, the glaze melts too early, and the reducing air is insufficient, so that the fe2o3 in the porcelain body cannot be reduced to feo. In addition, if the bowl column is too low, the temperature of some products on the top of the kiln is too high and the reduction is not enough, which will also cause the defect of cloudy yellow. If the content of tio2 in the raw material of the product is too high, it will also lead to yellowing of the product. If a small amount of coo is added to the blank, the yellowing of the product can be concealed.


5. Smoke: No matter what kind of fuel is used, smoke will occur. Smoke refers to a product that has a gray or impure white finish. Mainly because the body is not completely oxidized or reduced too early, the carbon, organic matter or low-temperature carbon in the body cannot be burned out before the glaze layer is closed. Occasional backflow of smoke will also erode the glaze. If the calcium content in the glaze is too high, it is easy to cause smoky defects.


6. Pinholes: Refers to small dents or small holes on the glazed surface of the product. One such shortcoming is the organic matter in the billet. The content of carbon and iron oxide is relatively high. When the temperature rises rapidly, the burnt matter cannot be completely burnt and volatilized, and only escapes from the glaze surface at the later high temperature stage, forming microscopic volcanic pinholes. In addition, the reducing air in the high-temperature furnace is too weak, and the re-oxidation of the product at the crater will also cause pinholes. Furthermore, pinhole defects can also occur when the glaze is poorly mobile or the glaze is too thin.


7. Orange glaze: The glazed surface of the finished product is unsatisfactory and looks like orange peel. It is generally produced in plates, dishes or porcelain tiles. The main reason is that the temperature rises too fast when the glaze surface corrugates, and the firing temperature is too high, which causes the glaze surface to boil. In addition, the uneven thickness of the glaze slurry, poor high-temperature fluidity and improper grinding of the glaze are all the crux of the defects of the orange glaze.


8. Shocking glaze: There are thick cracks on the glazed surface of the product. The main reason is that the expansion coefficients of the billet and glaze are quite different. This requires readjustment of the blank. Glaze ingredient recipe. In addition, if the firing temperature is too high, the cooling system is unreasonable or the glaze layer is too thick, it will also cause glaze defects.


9. Raw and over-fired: The surface of raw-fired finished products is yellow, the water absorption rate is high, the glaze surface is poor and rough, the strength is low, and the sound is dirty when knocked. When overfired, the product is deformed, and the glaze surface is blistered or flowed. The main reasons are high or low firing temperature, improper control of high temperature heat preservation time, unreasonable loading density or large firing temperature difference, etc., resulting in partial overburning or raw burning.


10. Matte: also known as dull. The reason for the lack of light on the glaze is that the formation of fine bodies on the glaze and the poor melting of the glaze layer lead to the lack of light on the glaze. Rapid cooling can be adopted in the early stage of cooling to prevent crystallization of the glaze layer. Improve glaze gloss.

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