My celadon green glaze is one of my favorites, but I've had to give up all expectations of consistent performance when I use it. I never know what this color will do. Take this photo, for instance: some of the pieces seem blue-celadon, some have a definite jade green quality. Sometimes I am able to tell customers why different batches turn out differently- for example, the two vases with a row of dots on their shoulders were glazed together, and I'm fairly certain that while I used my usual combo of a bronzey green over plain celadon, the bronzey green really stole the show. They are a completely different color from the rest of these pieces. And there's a 10% chance that I accidentally used a different green, one that I usually use to paint my ferns, on top of the base color. Experienced (more experienced than little old me!) potters know that kilns typically have warm and cool spots, and temperature definitely effects how a glaze's colors develop. That's one explanation. Using the wrong glaze would be another!
Sometimes two pieces glazed at exactly the same time, in the same manner, and fired right next to each other turn out completely differently. Take this honey pot and bud vase. The bud vase (and the shorter bud vase in the background) show more of the bronzey-green characteristics than the honey pot, especially where the glaze has pooled in carved lines. The honey pot is pale, smooth, with little variation in color. All three of these pieces were treated in exactly the same way, fired on the same shelf. As I look at these photographs, the color seems more consistent than it does in person, perhaps because of my blue drop. I'm happy with all three pieces, but every time I open the kiln I'm the tiniest bit perplexed with how this glaze behaves.
I freely admit that I use commercial glazes because I have neither the knowledge nor the time to mix my own (not to mention the patience to weigh out grams)- and because glazing isn't my favorite part of the pottery-making process. Even with the chemical variables removed, pottery frequently feels like dabbling in alchemy. It certainly is an adventure!