I then change, alter and modify the recipes of these potential glazes to see if I can nudge them closer to what I consider a perfect glaze. And what do I consider the perfect glaze to be? That is a very complicated question to try to answer.
I think the first part has to do with glaze mechanics - does it fit the clay body properly? Does it melt at the proper temperature? Is the surface what it is supposed to be; glossy or matte? Does it stay where you put it on the pot or is it fluid and runny? Is the color correct?
The second thing I look at has to do with aesthetics. Does the glaze interact well in combination with other glazes? How does it look on a white clay body versus a dark one? Does it break and highlight edges or raised relief patterns or throw lines?
The last thing I look for as I evaluate glaze tests has to do with firing atmospheres. I fire in an electric kiln (oxidation atmosphere). This gives me a tremendous amount of control and predictability with my glazes and clay bodies. Other ways of firing (gas, wood, coal, oil, etc...) produce different atmospheres and glaze effects. Many (but not all) of the glaze effects produced in fuel burning kilns can be replicated through glaze chemistry and clay body interactions. This is the part of glaze testing that fascinates me the most.