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.
G2934 is a popular matte for cone 6 (far left). The mechanism of the matteness is high MgO content (it produces a more pleasant surface that cutlery marks and stains less than other mechanisms such as crystallization). But what if it is too matte for you? This recipe requires accurate firings, did your kiln really go to cone 6? Proven by a properly set firing cone? If it did, then we need plan B: Add some glossy to shine it up a bit. I fired these ten-gram GBMF test balls of glaze to cone 6 on porcelain tiles, they melted down into nice buttons that display the surface well. Top row proceeding right: 10%, 20%, 30%, 40% G2926B added (100% far right). Bottom: G2916F in the same proportions. The effects are similar but the top one produces a more pebbly surface.
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.
This production batch of G2934 cone 6 MgO matte glaze is firing almost glossy (upper left). Matte glaze chemistries are generally sensitive - big variations in surface character can result from small changes in firing or material chemistry/physics. This recipe relies both on high MgO and lots of Al2O3 (from high dolomite and kaolin in the recipe). A change in the frit has crossed a tipping point. But there is an amazing fix: A small addition of super fine calcined alumina (400 mesh). How small? only 1%! A small change in the frit turned it glossy so this small change has fixed it. Another factor is that greater additions of alumina (shown here are 1.5, 2.0, 2.5, and 3.0%) progressively matte it more (the slow-cool C6DHSC firing is the reason for the opacity). If the alumina was not dissolving we would expect cutlery marking and surface staining. But neither is happening, even with additions of up to 8%, achieving stoney matteness.
The glaze is G2934 cone 6 matte base. Because it was not firing matte enough additions of 2, 4, 6 and 8% super fine calcined alumina were tested (Al2O3 is the key to matteness and this material is a pure source, if kaolin were added it would also source SiO2 and require much more). Each addition made it progressively more matte. But with the mattness comes increasing susceptibility to cutlery marking and staining. To test the latter we marked each using a felt pen and then cleaned off the black ink using Acetone. The only one with noticeable staining is the 8% addition (the 6% addition has a slight stain also). The testing also showed no obvious cutlery marked on any of them. The results are reassuring since only 2% or less alumina is needed to achieve the degree of matteness desired so no danger of either problem is indicated. In addition, the integrity of the fired glass suggests that the alumina is dissolving in the melt - that means it is likely contributing to increased surface hardness and durability.
G2934 - Matte Glaze Base for Cone 6
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
G2934Y - Cone 6 Magnesia Matte Low LOI Version
The same chemistry as the widely used G2934 but the MgO is sourced from a frit and talc instead of dolomite. It has a finer surface, less cutlery marking and staining.