I have been using soda ash and/or baking soda solutions sprayed on areas of bisque and green ware for more than 10 years.   The fired color surface depends on the clay body. the appearance change of the fired surface is more a change in the value of the surface color of the fired clay body;   generally the color becomes more dark or stronger of the orange and yellow surfaces when treated with the soda application.   a very white porcelain surface will show a faint graying in the whit