Here's a vase with a beautiful iron red glaze and streaks of black temmoku glaze over that. Putting a large amount of iron in a glaze will, in some formulas, bring out a reddish brown color like this. The effect of the soft black lines worked so well on this vase, I spent the next six months trying to duplicate this piece. I couldn't do it. This piece had the luck of just the right glaze in just the right fire to come out perfectly.