Ceramic production is a very complex and meticulous process. A slight negligence in any process from raw material preparation to firing may cause defects or even scrap. Blistering is one of the common defects after the product is fired, and it is divided into two types: blank bubble and glaze bubble. Among them, the green foam is further divided into oxidizing foam and reducing foam. The following will describe the various foaming situations:
Oxidation bubbles
1. Features
The green body caused by incomplete oxidation is called oxidation bubble. The bubbles are the size of millet, covered with a glaze layer, and are not easy to be broken by hand.
2. Causes
The main reason is that the decomposition products in the porcelain body and the glaze are not fully oxidized, and the disappeared objects are not completely eliminated.
①The oxidation temperature is low, and the oxidation atmosphere in the high temperature part of the oxidation zone is insufficient or fluctuated.
② The preheating zone heats up quickly, the oxidation decomposition stage is short, the temperature in the kiln is too low at the end of oxidation, and the temperature difference between the upper and lower sides is too large.
③The content of carbonate, sulfate and organic impurities in the blank glaze is high.
④ The loading density is inappropriate.
⑤ The moisture content of the green body kiln is too high.
3. Prevention methods
① Properly increase the temperature of the oxidation furnace and adjust the opening of the atmosphere. The air-oil ratio is appropriate. The oxidation stage maintains strong oxidative firing.
②Reduce the temperature difference between the top and bottom of the green body and the temperature difference between the inside and outside. In the oxidation stage, the temperature should be raised slowly or properly kept warm, so that the green body can be preheated evenly and fully oxidized.
③ Reduce the amount of raw materials with more carbonate, sulfate and organic impurities in the billet.
④ The loading density should be appropriate to reduce the water content of the green body in the kiln.
The recovery bubble
1. Features
The green bubbles produced due to insufficient reduction are reduced bubbles. The diameter is larger than that of the oxidation bubble, also known as the overfire bubble, and the cross section is yellow, which is mostly produced at high temperature near the crater.
2. Causes
Mainly due to insufficient reduction of sulfate and high-valent iron in the green body, foaming at high temperature.
①The temperature of the oxidation furnace is too high. When the oxidation is over, the temperature in the kiln is too high.
②The strong reducing atmosphere is insufficient.
③ The power outage time is long, resulting in insufficient product restoration.
④The firing temperature is too high.
3. Prevention methods
① Appropriately reduce the temperature of the oxidation furnace to make the temperature suitable for entering the strong reduction stage.
② Control the reducing atmosphere and firing temperature appropriately.
Glaze bubble
1. Features
The glaze bubbles are generally small and drum on the surface of the glaze layer, which can be easily broken by hand.
2. Causes
The main reason is that the deposited carbon or decomposition products are not burnt out before the glaze is melted, and after the glaze is melted, the gas generated by the burning of carbon is not easy to escape, and is wrapped on the surface of the glaze layer.
①The temperature of the oxidation furnace is improper, and the temperature difference in the kiln is too large at the end of oxidation, which will cause insufficient oxidation of the green body or premature reduction.
② In the strong reduction stage, the atmosphere is too strong, resulting in carbon deposition.
③ The initial melting temperature of the glaze is too low or the temperature rises too fast, and the glaze vitrifies too early.
④The content of high temperature decomposition products (carbonate and sulfate) in the glaze is high.
3. Prevention methods
① Control the temperature of the oxidation furnace to be appropriate, and reduce the temperature difference between the upper and lower preheating zones.
② The strong reducing atmosphere is appropriate.
③ Increase the initial melting temperature of the glaze, or reduce the content of pyrolysis products in the glaze.