The previous article introduced the various defects of enamel products and their causes, including bubbles, scorching, peeling, copper heads (copper spots), tearing and bursting, but these defects are only part of them. Covering other defects in the production of full enamel, we will continue to introduce other defects in the finished enamel.

6. Hairline (silk pattern)

Enamel silk patterns are continuous cracks caused during the second firing, and generally occur in the cover coat. The reasons may be:

①The vessel is poorly designed and the internal stress is uneven

②Poor fixture

③Applying enamel many times and firing (the enamel is too thick)

④The expansion coefficient of enamel is too low (the difference between the expansion coefficient of enamel ground coat and cover coat is too large).

⑤Uneven drying

⑥ Local temperature unevenness in heating and cooling during firing

Bubble hairline (chicken paw print)

There has always been a special bubble type hairline called chicken paw print. In addition to the above-mentioned reasons, most of the bubble-type hairline occurs in the large R angle part, mainly due to poor mold design and excessive stretching; it should be Three stretches are changed to two stretches, or the internal stress is uneven due to the improper mold of the steel plate. Or excessive rust may also cause chicken paw prints.

It is not complicated to solve the production defects of chicken paw prints, and the production process is controlled according to the following points:

① Speed ​​up pickling

②Low temperature and slow burning

③Add Sb203 in the ground coat to increase adhesion

④ Try to complete it at once to avoid rework

7, boil

When the temperature of the cover coat firing process reaches about 600°C, the ground coat will sometimes boil, and the overflowing gas will damage the cover coat and expose black spots. Reboiling and scaling are related to the formation of bubbles in hydrogen gas. When the pressure of hydrogen is insufficient, it cannot escape during the first firing. During the second firing, hydrogen bubbles expand and reboil. If the ground coat is left for too long after firing, applying the cover coat will aggravate the reboil. The enamel must be applied after empty firing.

8. Black spots

There are black spots on the surface of the finished enamel, which are mostly caused by contamination by impurities, and some are caused by scales and reboiling, and even the ground coat is thin and penetrates into the cover coat.

9. Waterline

The waterline is a defect caused by the precipitation and volatilization of the salt in the grinding admixture during drying. This precipitated salt causes the porcelain surface to lose its color or reduce the gloss.

10. Warpage

When firing or cooling, the green body deforms due to the following reasons:

① Poor design

②Improper tool support

③Uneven heating of utensils

④The expansion coefficient of enamel and steel billet is too different

11. Eggshell noodles

The small bubbles in the enamel are caused by breaking down during cooling. Such defects are caused by too high alkali (potassium, sodium) in the enamel.

12. Orange peel

The orange peel of the finished enamel greatly affects the aesthetics, usually caused by too high spray pressure, too large muzzle, and too close distance.

13, no light

①The composition in the enamel is incorrect; fluoride and sulfide are too high

②Insufficient firing

③There is sulfur dioxide (SO2) gas in the atmosphere in the furnace

④ Excessive or inferior clay

14. Pinhole

The firing temperature of the ground coat and the cover coat is too high or too long, causing needle-like holes on the face.