The resultant scum that appears is most frequently whitish. It can cause bad adhesion between the dry glaze coat and the ware. This poor adhesion allows the melting glaze to bead up like water does falling through the air. S alts in glaze suspensions can also cause crawling from flocculation. Flocculated glazes require more water to have the same degree of fluidity in application as unflocculated glazes.
More water usually means more shrinkage that can cause cracking in the unfired glaze coat.
More shrinkage also causes a weaker adhesion between the bisqued clay body and the dry glaze coat. Alone or together the cracks and the poor adhesion allow the glaze to pull itself in beads or just crack and fall from the pot. The graduate studios at Arizona State University use rain water collected from the roof downspouts to breed mosquitos and to mix clay. Clay mixed with the rain water is much superior to clay mixed with Tempe tap water. This is the same distilled but dirty water extracted from the air by air conditioners.