Ceramic glazes can be bought ready made up and raring to go - just order online, put in the studio, dip your wares in and fire - often to get a characterless finish no different from everyone else who buys the same glaze, and with no real input from you as maker. This may be fine if you are a painter, and the glaze is a tool to help you create your images on your pots, but for those like me where the glaze is the finish, with no painterly effects, this doesn't let you create any individual effects. The approach I take is to develop my own glazes from the raw materials, though often building on the works of others, be they other contemporary makers, or Chinese potters from a thousand years ago.
Of course, thought also needs to go into the purpose of the glaze. Should it be seen as a glossy decorative finish, with the underlying clay just providing the form and the canvas, or should the clay and the glaze be seen as an integral whole, the glaze reinforcing the earthy nature of the clay? And, for functional pieces, of course the glaze must be food safe, durable, resistant to cutlery marks, and fine for the dishwasher and microwave.