This kind of black spot will appear when the colored uranium on the surface of the ceramic cup falls off after a long time. Ceramics are colored with iron, but black spots are actually iron spots. Because the glaze powder is made of pure natural minerals, it is fired by the ancient reduction firing method, just like the proportion of gold is 99.9%. Even if the technology is improved now, it can't be completely purified, so there will be black spots. These black spots are harmless to the body and are iron in the embryo.
Tips: If there are black spots on the ceramic cup, try to wipe it gently with sandpaper and steel balls, and don't care too much. Remember to clean up after wiping. You can also scrub with toothpaste, which is also the simplest and most practical method.
In the industrial system, the wear resistance of general porcelain is expressed by Mohs degree, and the value range is 1 ~ 10. The higher Mohs degree, the better the surface wear resistance and hardness.
The Mohs value of daily-use ceramic tableware is different. For good quality, the Mohs degree of glaze can reach 7~8, but for ordinary quality, or too delicate to give up hardness, the value is only about 5~6, or even lower.
Under normal circumstances, the hardness of steel balls is 5~6, but the glaze of porcelain is not absolutely uniform, so the hardness is high or low. The hardness of each part of the steel ball is basically the same, but the hardness of porcelain is local, and the parts with low hardness are prone to scratches, which are called black marks.
This kind of black mark, the substance of which is mainly silicon oxide exposed after the glaze is destroyed, will appear black because iron enters the gap of silicon oxide as an impurity, thus showing a series of black scratches.
Therefore, the essence of black mark is silicon oxide containing iron impurities, which is non-toxic and will not affect human health.
However, this iron impurity, silica, is an acidic oxide and will not react with ordinary acids. It will only react with hot concentrated phosphoric acid or hot high-concentration alkali, and react with alkali to produce only salts and water oxides, namely silicates and water.
Whose plate will be filled with hot concentrated phosphoric acid, right?
Therefore, although the black scratches are not good-looking, they are very safe to hold all kinds of food and will not cause harm to human health.