Why do preserved eggs contain lead? Thank god, help me.

At the dinner table, I talked about why preserved eggs contain lead. I finally remembered to check it today. When processing preserved eggs, soda ash, lime, salt and yellow lead powder should be mixed in a certain proportion, and mud and bran should be wrapped outside the duck eggs. Two weeks later, the delicious preserved eggs are ready. Plumbum preparatium powder is lead oxide, which can make eggs produce beautiful patterns, but with Plumbum preparatium powder, preserved eggs will be polluted by lead. The functions of lead oxide in preserved egg processing are as follows: ① promoting and guiding the alkaline components in the feed liquid to penetrate into the egg. (2) Make preserved eggs turn quickly, with deep color and long aftertaste. (3) Keep the preserved egg protein to a certain hardness, so that the preserved egg protein does not stick to the shell and is easy to shell. ④ Make preserved eggs have the same maturity and easy to master, and ensure the stability of product quality. In September 2008, the new national standard for preserved eggs was approved. The new standard strictly stipulates that the lead content of preserved eggs should not exceed 0.5 mg/kg, which is significantly lower than the previous 3 mg. For this low value but very desirable "zero", experts explained that lead-free technology means that lead compounds are not used in preserved eggs processing, which does not mean that preserved eggs are completely lead-free, because fresh eggs, air, water and soil all contain a small amount of lead. With the introduction of the new standard, the corresponding production process may need some adjustments, such as replacing lead oxide with zinc oxide or zinc sulfate, and studying copper-zinc mixture and copper sulfate. Zinc sulfate is white, which is basically similar to the color of eggshell, so there are no black spots on the surface of eggshell. The flavor of preserved eggs with only copper is not as good as that with lead or zinc, and there are often copper sulfide spots on the surface of eggshells, which is not beautiful enough. Hehe, I don't really care whether the eggshell looks good or not. The key is that it is healthy, non-toxic and delicious.