This a flow tester fired at cone 6. On the right is G1214W, a typical cone 6 transparent fluxed by a boron frit (google it). On the left is typical of what many use to achieve a matte. The recipe is: Nepheline Syenite 40, Dolomite 15, Whiting 10, Ball Clay 20, China Clay 10, Silica 5.

This simple comparison teaches us many things about glazes and even ourselves. Here are some of them.

-Cone 6 glazes require boron (from frits or gerstley borate) or zinc to melt (the one on the left has neither). Materials like feldspar, dolomite, whiting and talc just do not melt well at all at cone 6. If a glaze is not properly melted then it is not hard and strong and resistant to leaching and cutlery marking.
-Textbook glaze recipes often are junk. Unquestioned use of them demonstrates our willingness to accept something ridiculous just because it is in a book or on a web page. We need use common sense when looking at glaze recipes for cone 6 to determine if they are for real or impostors (the glaze on the left is actually really not a glaze, it is a porcelain). If there is no frit, no zinc, no gerstley borate, then a red this-likely-does-not-melt light is flashing.
-Glazes with high feldspar or nepheline syenite (40% is really high) are likely to craze. If this one does not craze it is simply because it has not melt enough for the sodium to impose its high thermal expansion on the glass.
-A cone 6 glaze with 30% clay (in this case ball clay and kaolin) and no boron is certainly not going to melt well because clay is refractory.
-True matte glazes have high alumina and low silica (which this has) but they also need to melt well to form a glass (which this does not). So this is just a matte because it is not melted.
-If a white-firing glaze has no zircon or tin oxide, then how can it be white? Something is wrong. In this case it is white simply because it is not melted.
-Since this does not melt, evaluating it on the basis of its chemistry would be invalid. Ceramic chemistry theory depends on the glaze being melted to a homogeneous glass. Thus, in addition to knowing all about the chemistry, you need to have common sense on the recipe level also.