The article analyzed the source of high concentration of residual Ca²⁺ and Mg²⁺ after high temperature pre-processing throughout analyzing the water hardness before and after pretreament. It shows that Ca²⁺ and Mg²⁺ have an great affect on the color shade, especially for light gray, blue and violet colors when the concentration is above 4.5 mmol/L. It is necessary to remove them by adding additional rinsing water.