Base and direct enamel frits have the function of establishing a firm bond between the metal workpiece and the vitreous enamel layer. The enamelling should withstand firing over a wide temperature range, have a smooth, flawless surface and undergo no loss of bonding even after repeated stoving. The development of these direct enamel frits has hitherto taken place empirically. So-called network forming oxides such as SiO2, TiO2, ZrO2, B2 O3 and Al2 O3 were combined with network migrators such as Li2 O, Na2 O, K2 O, MgO, CaO, BaO or fluorides and phosphates and varied until the enamels were satisfactory in the required commercial properties such as fluidity, surface tension, thermal expansion, surface quality and bonding. To improve the bonding of base enamels, heavy metal oxides such as CoO, NiO and CuO and occasionally iron oxide, manganese oxide, molybdenum oxide or antimony oxide were added in varying quantities.
For direct enamel frits, the addition of TiO2 increased the resistance of the enamelling to acid attack, while the addition of ZrO2 increased the resistance to alkalies.
Molten mixtures of base direct enamel frits and enamel frits containing more than 12 oxidic constituents are therefore no rarity. In all multicomponent systems, their development is difficult to oversee and complicated and therefore expensive. The effect of any one component on certain properties of the frits is rarely proportional to its ratio by weight and is independent of other components. An incremental calculation which is recommended for simple types of glass is only rarely possible, e.g. in the case of thermal expansion, and then only approximately. An added difficulty is that several direct enamel frits are usually mixed together and inert substances may be added for enamel commercial requirements or reasons of economy.
For conventional two-layered or multi-layered enamelling, frits with different viscosities and melting properties were combined for the base enamel and inert substances were added to facilitate the gas reactions proceeding from the sheet steel.
The base enamels which are viscous and hard at the stoving temperature are described as filling bases, while low viscosity, soft frits which wet readily and are capable of dissolving iron oxides are known as network base enamels. The proportions in which they are mixed and the amount of quartz added depend on the quality of the steel and the thickness of the steel sheet as well as on the stoving temperature and the dwell time. Here again, the proportion of frits and the additives used for the milling process are selected empirically and either accepted or varied under practical conditions. Predictions as to the optimum properties such as compatibility of the frits with one another, bonding to differently cast qualities of steel and flow properties of the slips were rarely possible.