What are the reasons for the partial black-gray phenomenon of bone china tableware?

What are the reasons for the partial black-gray phenomenon of bone china white porcelin dinnerware? The phenomenon that the bone china white porcelin dinnerware is partially or completely black and gray is called smoky. This defect is due to incomplete oxidation or premature reduction of the green body, so that the carbon organic matter and low-temperature carbon deposit in the green body cannot be burned out, and the glaze layer is closed, and the surface of the product is gray. Guangdong Hosen Ceramics Factory analyzes the factors that affect smoking: 1. During the oxidation stage of low-fire and medium-fire heat preservation, the oxidation is incomplete, and some carbon remains in the body, and the section of the body is gray-black, especially when the blank is When using clay containing a large amount of carbon, such as coal gangue, the carbon in the green body should be fully oxidized before the glaze layer is melted. For products that do not contain carbon in the blank, the oxidation should also be strengthened in the medium-fire heat preservation stage, and special attention should be paid to coal-fired intermittent kilns and thick tire products. This is because of the intermittent kiln, the temperature in the kiln is low during ignition, and there is more water vapor. At this time, the flue gas temperature is low and the chimney suction is weak. In the short time when coal is added and the furnace is pried, the nature of the flame in the kiln is a reducing atmosphere. At this time, not only the carbon inside the blank cannot be eliminated, but also the carbon deposits in the blank, especially when the water content of the blanks into the kiln is high, it is easier to produce water black in the product. Therefore, during the medium heat insulation stage (usually between 1000 and 1100°C), a strong oxidizing atmosphere must be maintained and sufficient insulation time must be maintained. That is to say, in the kiln operation, the thickness of the ash layer, fire layer, and coal layer on the grate should be controlled to ensure good ventilation, large excess air coefficient, clear fire layer in the combustion chamber, and every time the coal added must be burned through. , So that the body can be fully oxidized. Otherwise, the amount of coal added at this time is too much, the ash layer is too thick, the ventilation under the grate is poor, and the combustion is impermeable. The porosity of the body is the largest, the adsorption is strong, and it is easy to adsorb flue gas, resulting in carbon deposition. 2. At the end of the reduction stage, the reducing atmosphere is too strong, that is, when the glaze has melted, the kiln still maintains a strong reducing atmosphere, but it is not converted to a weak reducing or neutral atmosphere in time, and the product is still in the reducing atmosphere, then the flue gas The CO decomposes at high temperature to cause surface carbon, especially when calcium glaze is used, because the phenomenon of CaO carbon absorption is more serious, it is most likely to form glazed smoke. 3. The blackening of bone china white porcelin dinnerware analyzes that the blank glaze formula is unreasonable. If excessive CaO or other fluxes are used, the blank glaze will produce liquid phase and melting prematurely, and the vitrification range is too narrow, so that the deposited carbon cannot be burned out in time. , Or the temperature difference of the kiln is too large to ensure that the product atmosphere changes, so that some products are in a chaotic atmosphere or the fuel contains too much sulfur. If the sulfur-containing gas stays in the kiln for too long, the sulfur will strongly seize the flue gas. Oxygen makes the reducing atmosphere thicker and prone to smoke defects.
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