In the firing method of enamel, the firing of the ground coat is one of the most important links. The combination of metal and enamel occurs during the firing process, which determines the strength of the coating. In the firing process, the binding oxide occupies a special position. With the effect of oxygen, the enamel can produce a firm bond in a short firing time.

When the ground coat is fired, it can be divided into two steps:

1. The first step is the process of heating the substrate to the ground coat melting, and the process of enamel melting. In the first type of procedure, when the enamel layer is heated, there is residual moisture in the enamel layer, and it begins to evaporate. At 500°C, the component water is separated and precipitated as a result of the decomposition of kaolin and other clay minerals. In the process of clay shrinkage, cracks are formed, which increases the porosity of the enamel layer. Oxygen in the air will also take advantage of the vacancy and enter to the surface of the steel, which will cause iron oxidation in the following ways:

2Fe+O2→2FeO 3Fe+2O2→Fe3O4

The rust film is produced. As for the amount of oxide and the structure of the film, it depends on the oxidation amount and heating time of the steel billet under the enamel layer. The oxidation amount of the steel billet is composed of the chemical composition and structure of the steel and the fired enamel. Depends.

2. The second step is that when heating continues, the salt in the enamel slurry will begin to melt, and then the ground coat itself. At this time, oxygen can no longer contact the surface of the steel billet, mainly because the dewatering effect of the clay continues. The water of the clay reacts with the iron in the following ways:

Fe+H2O→FeO+H2↑

At this time, hydrogen bubbles are formed, some escape through the enamel layer, and some are melted in the metal. Finally, this melted hydrogen is present in the enamel coating and may cause defects such as fish scales, bubbling and scales. The carbon element in the steel billet reduces the composition of the enamel according to the following reaction:

C+MgO→CO↑+Mg

C+MnO2→CO2↑+Mn

The above two products also form bubbles during the normal firing process. The gas formation develops rapidly, and continues until the firing is completed, and the molten enamel underglaze smoothly overflows to the metal surface.