High and middle temperature glazes with little or no flint or kaolin need an explanation. Firing Temperature Higher temperature glazes contain less flux and more silica and alumina. Since silica and alumina are so closely related to glaze stability it follows that high temperature glazes are intrinsicly more stable. At high temperatures experimenters can mix a far greater range of materials and get a good melt because temperature is on their side. Thus it is easy to get 'sloppy' and adopt the attitude that if a glaze looks good it is also safe.
Some potters even assume that any glaze is safe as long as it is fired at high temperature.
It is common to see popular high fire recipes with very high metal oxide and barium contents. Matte glazes often contain abnormally large magnesia contents. These are destabilizing factors that temperature cannot counteract. With a knowledge of glaze chemistry you can create stable glazes at low fire, without it you can make unstable ones at high temperatures. If a glaze recipe contains materials that are 'surrounded in controversy' it might be best to eliminate or reduce these if they are not necessary.