Test to find out whether a more even coverage can be obtained over, under, or within the glaze. Mix stain with medium for more even application. Spray stain onto ware for more even application. Use a stain of the same color from a different oxide system to see if one is available that covers better. Frequently, a stain or stain-medium mix is applied too thickly under or over a glaze.
Some stains require a medium with higher flux.
Pure stains will almost always cause this, and the solution is to mix them with a medium or apply more thinly. Porcelain ware provides a base for colors with maximum brightness, whereas buff or red colored clays mute the colors of overlying stains or stained glazes because of the iron content. The slightest imperfection in a clear glaze will show up when viewed against the background of a colored stain. Glazes must be screened finely and mixed well. Escaping gases of decomposition in the body can bubble an underlying glaze up through an overlying stain.