|Monthly Tech-Tip |
These are GBMF test balls, they dry to about 15mm diameter and melt down onto the tile during firing. These are the same glaze, G2934. The left one was fired using the cone 6 PLC6DS firing schedule. The one on the right got the C6DHSC schedule, it is the same but adds a slow cool to 1400F. The slow cool is creating phase differences, enabling crystallization. This difference in opacity is also evident even when the glaze is used on ware, a special concern if it is employed over underglaze decoration.
Typically the G2934 cone 6 MgO matte recipe fires with a surface that is too matte for functional ware (with cutlery marking and staining problems). This is intentional - it enables users to blend in a glossy base transparent to tune the degree of matteness. However, we have seen variation in the Ferro Frit 3124, serious enough that a recent production batch of glaze came out glossy (upper left in this picture)! This happened despite a C6DHSC slow cool firing. Shown here is a trial with additions of 4% calcined alumina (upper right) and 6 and 8% (bottom). All of these were too matte (1.5% turned up to be good). Although the slow-cool C6DHSC firing is the likely reason for the opacity here, opacity disruption still turned out to be a factor for stain additions (muting the colors slightly) even in faster cool firings. This is a testament to the critical chemistry balance that produces this matte surface. And the need to have adjustment options when inevitable variation occurs. Of course, it is important to use ultra-fine alumina (e.g. 400 mesh) to assure it will dissolve in the melt.
Glaze Melt Fluidity - Ball Test
A test where a 10-gram ball of dried glaze is fired on a porcelain tile to study its melt flow, surface character, bubble retention and surface tension.