Well salt is made by mixing black brine and yellow brine at a certain ratio. Black brine is black with plentiful sulfate ions; yellow brine is brown. Once they mixed, the toxicity can be eliminated and salts are obtained with a good taste and white color. 

    As necessities of life, though salt is not expensive, it makes huge profits for the massive demands. So it is monopolized by government in all ages. Salt monopoly dates back to spring and autumn period and there is a person who specially administers salt industry in ancient times. 

    In ancient times, people who live near the sea boil seawater for salts; people who are near salt lake dry the lake water in the sun for salts; while without convenient transportation and topographical advantages, peoples in the mainland have to dig a well and extract brine with their hands. Then extract and purify the brine to obtain salts. We call the salt prepared in this way well salt. Actually, between 255BC and 251 BC people have already known systematically digging salt well and refining well salt. Well salt is mainly produced in the ancient Bashu area, southwest of China. This area has special geological conditions and buries abundant salt mines. Brine is formed by underground water and finally natural salt spring comes into being. For the color difference between salt spring and common spring, people are easily attracted to be here to exploit resources.