Burning analysis

Ceramic blanks are heated to a high temperature according to a certain rule, undergoing a series of physical and chemical reactions, and then cooled to room temperature. The mineral composition and microstructure of the blanks are the final key steps in the ceramic manufacturing process. Improper firing will directly affect product quality, not only ruining the results of previous processes, but also making it difficult to recycle fired waste products, resulting in a huge waste of resources. Therefore, it is crucial to formulate a reasonable firing process.

Section 1: Single Firing and Double Firing

The production process of ordinary ceramics is divided into single firing and double firing. Single firing, also known as bisque firing, refers to the process route in which green bodies after molding, drying, or glazing are fired into ceramic products in a firing kiln in one go. Double firing refers to the process route in which green bodies after molding and drying are first bisque fired in a bisque kiln—the first firing—and then, after inspection and glazing, are fired in a glaze kiln—the second firing.

In double firing, the bisque firing temperature is sometimes lower than the glaze firing temperature; that is, low-temperature bisque firing (600~900℃) is performed first, followed by high-temperature glaze firing, so that both the blank and the glaze reach the highest firing temperature (vitrification) simultaneously, as is the case with the firing of general ceramics. The main purpose of bisque firing in this case is to give the blank sufficient strength to allow glazing, reduce breakage, and have good glaze adsorption capacity; in addition, some oxidation decomposition reactions, such as the oxidation of carbon and organic matter, dehydration of kaolin, and thermal decomposition of magnesite, can also be completed in this stage, reducing the amount of material exchange during glaze firing. However, low-temperature bisque firing should be distinguished from drying. Although the drying process also heats the blank, and the strength and glaze adsorption capacity of the dry blank are improved, drying is a physical process without chemical reactions, so it cannot be called bisque firing.

For general fine ceramic products, when double firing is performed, the bisque firing temperature is often higher than the glaze firing temperature. In this case, bisque firing is the main process, and the final temperature of bisque firing is the firing temperature of this type of ceramic. The role of glaze firing is to melt the glaze material with a lower melting temperature, distribute it evenly on the surface of the blank, and form a tight glaze layer.

Some fine ceramic products, such as glazed tiles, also have bisque firing temperatures close to or even slightly higher than glaze firing temperatures. This difference also exists in porcelain. The bone china trial-produced in China is low-temperature (850～900℃) bisque firing and high-temperature glaze firing; the UK adopts high-temperature bisque firing and low-temperature glaze firing. When determining whether to use high-temperature or low-temperature bisque firing, the composition of the glaze, the sintering (vitrification) temperature of the blank, and the suitable melting temperature of the glaze used should be considered. When the melting temperature of the glaze is low and the sintering temperature of the blank is high, high-temperature bisque firing and low-temperature glaze firing are preferred.