Additives for selectively influencing the flow properties of engobes and glazes, and thus the surface quality of fired roofing tiles, are described. Method of application and influence of technological parameters are noted. Additives include: deflocculants and dispersants; binders; rheological additives; suspending agents; preservatives; and defoamers. The reason for their use, and their mode of action, are outlined. Their benefits are summarised.