|Monthly Tech-Tip |
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.
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.
True functional mattes have fluid melts, like glossy glazes. They need this in order to develop a hard, non-scratching durable glass. The mechanism of the G1214Z1 matte on the right is high Al2O3, it is actually melting more than the glossy glaze on the left (G1214W).
A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.
G2916F - Cone 6 Stoneware/Whiteware transparent glaze
Crystal clear industrial dinnerware glaze
G2926B - Cone 6 Whiteware/Porcelain transparent glaze
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
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.
Stop! Think! Do not get addicted to the trafficking in online glaze recipes. Learn to make your own or adjust/adapt/fix what you find online.