Application will make a difference. You need to be fairly consistent in applying the soda ash wash. Too much/too heavy will make a spot shinier than others. I usually just dip a small sponge in the solution, wring it out, then wipe. I'd rather do a couple of light solution passes than one heavy one.
I apply oxides/stains/underglazes to greenware at leatherhard so it gets bisqued; that reduces bleeding, etc. from the soda ash was applied for the glaze firing. But, you may get some bleeding. Just have to practice and experiment with application process).
I only do a small number of pieces per load; that is not enough soda to really impact elements. Just run a bisque between glaze loads and your burn off any residual stuff from the elements.