eoteceramics

Thanks Callie for explaining all that in such detail/ It was my hunch that it was too thick but so glad you have confirmed that and I can get back to work. Have a great day, Jools

Thanks Callie for explaining all that in such detail/ It was my hunch that it was too thick but so glad you have confirmed that and I can get back to work. Have a great day, Jools

Short answer is yes, it’s the thickness of your copper carbonate layer. Rather than giving more blue, it’ll go metallic, like you see here. Too much, and it can make the glaze over top craze like in the third pot from the left.  The metal is just overpowering the glaze, and it doesn’t take a lot to do that.
All glazes rely on the proper proportions of the different ingredients to turn out the way you want them to. Copper by itself, or in enough quantity to overpower the other glaze ingredients, will turn a metallic black like the pot on the right. When you mix sodium (a pretty common glaze flux) and a little bit of copper, it gets that signature turquoise colour you’ve got on the first pot. 
If your goal was to eliminate the dark streaks like the piece on the left, you would have wanted to add slightly more glaze, where the sodium is coming from. Or, if you wanted to get a bit more scientific about it, you could add a little copper (1% or less by weight is usually enough) to your glaze base to make a turquoise. This is done most easily if you’re buying a dry mix, because you just weigh dry ingredients out. But there are ways of getting repeatable results if you have to add dry carbonate to an already fluid glaze.