Some glaze suspensions will foam as they are mixed and used. This is problematic since the bubbles prevent the laydown of an even surface. The reason for the foaming relates to issues with particle agglomerations and incompletely whetted particle surfaces hanging on to air bubbles. Glazes containing calcined kaolin, for example, can have this problem. Or glazes containing materials having surface treated particles (e.g. colorants that are normally unstable like red copper oxide). You might try adding excessive water and power mixing the slurry using a propeller mixer, then evaporating the water off. Or, simply power mixing using a machine powerful enough to put considerable energy into the slurry. Or, try mixing the glaze using hot water. It could be that a small amount of deflocculant could disperse the particles also, but you would have to be careful to experiment on a cupful of the glaze before adding it to the whole batch. Foaming is a common problem in industry and defoaming additives are commonly available (after investigating them you might find a common household product that would do the same job).
Defoaming agents at Wikipedia
In ceramics, glazes are suspensions. They consist of water and undissolved powders kept in suspension by clay particles. You have much more control over the properties than you might think.