Ceramic glazes are glass, but if the glass is not melted enough or if its chemistry is not balanced the fired surface will not be micro-smooth. That means it will trap particles and stains and make them difficult or impossible to clean off. Glazes fire the way they do because of their chemistry. They need enough flux to melt, enough SiO2 to develop a good glass and enough Al2O3 to be hard and durable. By comparing the chemistry and recipes of glazes that do and do not stain (in your account at insight-live.com) you can develop understanding of how to resolve this issue with any recipe.
Two cone 10R bamboo glazes. One stains, one does not. Why?
These mugs are Plainsman H443. The bamboo glaze on the left (A) has 3.5% rutile and 10% zircopax added to the base G2571A dolomite matte. The one on the right (B) has the same addition but in a base having slightly less MgO and slightly more KNaO. B stains badly (as can be seen from the felt marker residue that could not be removed using lacquer thinner). Why does A stain only slightly? It has an additional 4% Gerstley Borate (GB). GB is a powerful flux that develops the glass better, making the surface more silky. The differences in the recipe provide another advantage: A has a much lower thermal expansion and is less likely to craze.