Once the Under Glaze Decoration is over, the actual glaze is applied over the decorated ware which is also known as OVERGLAZE process. This needs a special glaze which can be either prepared in the same factory or can be outsourced from outside specialists who have the necessary technical expertise and control laboratories.
This glaze is mostly a frited one either - lead bisilicate or borax frit or combination of two. The materials are charged into ball mills and wet ground to get a fineness of 75% less than 10 micron. Various types of mills are nowadays available for glaze preparation. Faster grinding and superior fineness can be achieved by using latest new version milling equipments.
It is very much necessary to control the physical characteristics of the slop glaze before using. Some of the important tests like Viscosity, Thixotrophy, Litreweight, Drying time, Cracking resistance and Thickness buildup are usually done before using the glaze.
The glaze can be applied either by dipping process or by spraying process. Both are mechanized and lot of advancements has taken place in both processes. The glaze characteristics varies based on the glaze application process. Dipping needs different glaze like medium litreweight, higher fluidity and medium thickness buildup glazes. However spraying can take care of the quality variations in the glazes. Here a higher litreweight and lesser fluidity glazes can be used. After glaze application, the wares are dried so that the moisture level comes down to less than 1% before firing.
