In a single cooking, the green pasta is glazed, dried and then baked in a high temperature oven. Secondary firing refers to the unglazed green body, after drying, the first firing in bisque, then the glazing and then the second firing of the product. After cooking, the process is simple and the heat loss is reduced. The advantages of the second firing are even more evident:
(1) Cooking in bisque oxidizes organic matter, escapes crystalline water, carbonate and sulfate decompose before glazing, which is beneficial for eliminating glaze defects.
(2) The pores of the green body are dilated, the absorbency of the enamel is improved, the low concentration enamel can be hung, and the flatness of the enamel surface is improved.
(3) Enhance the strength of the green body, which promotes the subsequent process, promotes automatic operation and reduces damage.
(4) After the crude is baked with the biscuits, it is easy to select and remove the unskilled products and improve the qualification rate of the finished products.
Many famous porcelain in the old country, such as Tang Sancai, Yaozhou kiln, and Tianqing glaze, were fired twice. The first time the bisque is cooked at a higher temperature (1200-1500 ° C) and then the icing is cooked at a lower temperature (950-1050 ° C). A second firing is also often used in modern industrial ceramics.
It should be noted that most of the utility ceramic firing methods in our country use a single muffle firing. The fuels used are mainly coal and heavy oil (heavy oil content about 30%), the main cooking systems are tunnel ovens and reverse flame ovens, cooking is always controlled manually. The national average standard coal consumption per 10,000 pieces of porcelain is more than 10 tons, and the gas and open flame rapid combustion system is widely used abroad. The oven has basically realized the automatic control of the microcomputer ratio, and the thermal efficiency and cooking quality are very high. The table below reflects the difference of the main technical and economic indicators of ceramic firing processes between China and abroad. Comparative table of Chinese and foreign ceramic firing process name firing system kiln level thermal fuel efficiency Kg heat consumption of porcelain porcelain box weight ratio degree of automation kiln cleaning
China, flame insulation degree in the early 60's Coal and heavy oil 20-30% about 12000 kcal 1: 6-7 No automatic control means 90%.
Abroad, the open flame ignites quickly. Advanced level in the 1980s. Natural gas and gas 50-60% 5000 kcal approximately 1: 2.2 Automatic control has been achieved 98%.