The photos below are all of the same glaze fired to the same temperature. The only difference between them is the cooling rate and thickness of application as described below:

The tile above is Variegated Slate Blue (see pages 86 and 87 for the recipe) cooled very quickly. The kiln was simply turned off when maturity was reached. While the photo does not show it well, this glaze is quite glossy. Only a few scattered crystals managed to form during cooling.		This tile is Variegated Slate Blue cooled slowly by the program we recommend in our book (about 125 to 150ºF per hour down to 1500ºF). The glaze was applied on the thick side and is similar to the effect we got in Figure 6-4 (p. 88)		This tile is Variegated Slate Blue also cooled slowly by the program we recommend. The glaze was applied more thinly than the one directly to the left. Crystals have developed across the entire surface and it is an attractive semimatte glaze which is similar to the mug we show in Figure 6-1 (p. 87)		This tile is Variegated Slate Blue cooled very slowly--at 50ºF per hour from 1900 down to 1500ºF. The crystals (or clusters of crystals) have changed in nature to give this glaze a lighter shade and it is more matte than the one directly to the left.
These 4 test tiles illustrate why you must control the cooling of your kiln if you are to get attractive, durable matte glazes. You also must pay attention to thickness of application if you want reproducible aesthetics. We find these difference quite startling and were very surprised, in the early stages of our work, to see how big a difference cooling rate and glaze application thickness can make.