TECHNO TIP Adjusting the Firing Temperature of a Glaze [lis amaeing how many potters push their clay body to the firing limits io get their glaze to melt the way they want, Others don't fire the body to maturity because the glaze doesn’t look right. Such practices are really unnecessary, espectally at higher temperatures. It either wastes energy and prxduces brite, bloated ware or by com- trast weaker and porous wure. [t is really fasy to adjust glaze melting temperature. Glazes don't really melt at one tempera- ture; they usually soften over a wide range. Microscopically, raw glaze powders are mixtures.of different mineral particles that have very different melting temperatures and behaviors. Consider an inventory of some of the particles (we won't look at colourants. opacifiers, amd variegators): |. Some are pure mineral flux particles (e.g. dolomite, whiting, tale containing only C#O and MQ) which melt vigor- ously and over a narrow range. However different minerals melt at very different temperatures. 2. Others are pure Si02 and Al2O3 (e.g. silica, kaolin) and don’t melt at all at stoneware temperatures (they have to be dissalved by other things). 3. Others are in between; they have com- plex chemistries and soften over arange of temperatures supplying fluxes, glass form- ers and stabilizers (e.¢. feldspar, frits) 4, Others like zine oxide and lithium car- bonate are refined super-fluxes that melt very early and vigorously and can have significant melting effects in very small SINUS, Changing the amount of these different types of particles has different implica- lions, You want to change melting tem- perature without changing other fired prop- erties. Thus tt seems logical that you will getihe greatest change in rmelting behavior accompanied by the lewst change in over- all glaze appearance by small changes in the amounts of class 4 or class | materials, By contrast larger changes in certain class 3 matenals has the potential to change other properties the most Pailing that, here are some considerations: * lithe glaze contains a lot of feldspar (e.g. 40%) then simply increase or decrease it Start by adding of removing 5%. “Tithe glaze contains an unusually large amount (e.g. more than 5%) of a powerlul pure Hux like lithium or oxide, it is likely some sorlol reactive variegated orcrystal- line glare and the visual effects may be lost if the wrong type of change is made. «Tf the glaze contains a lot of silica and tlumina and is net melting enough, con- sider removing son. [fF the glaze is ton glossy remove silica; if il 15 too mate remove kaolin; otherwise remove equal amounts, Start with changes of %. How- ever, One caution: If your glaze is tending lo craze removing silica and kaolin will make it worse, adding them will reduce Grazing, cominned nex page Gerstley Borate PLAINSMAN imies Box 1266, Medicine Hat, Alta. T1A 7M9 Phone 403-527-8535 FAX 527-7508 Internet: httpcwaew.plainsmanclays.com Learn all about it at www.gerstleyborate.com Available now through: Greenbarn Potters Supply Surrey, B.C. Phone: 604-888-3411 Vancouver Island Pottery Nanaimo, B.C. Phone: 250-716-9966 Warehouse Inc. Plastic Borate & Potters Guill of British Columbia June 20017