A funny thing has been happening to us here at the rcboisjoli studio. We've noticed that on the first run of our new Cobalt Slip Cups, the rims are flared. It wasn’t our intention to do this as our models are pretty straight sided, with just a bit of angle to facilitate mouldmaking. Just now, after a well deserved studio beer, our trusty technician Alexx discovered the the cause of the mystery flare. (which we like anyways because it's nice)
Above, you can easily see the flared lip on the finished, fully fired cups, unintentional, but pleasing to drink from. Below, shows the cups at the bisque stage, after one firing, before the glaze. The lips are straight! (check out that cobalt action, such a change when fully matured)
It wasn’t until Alexx spotted this prototype larger Cobalt Slip Bowl where the lip was strangely flared as well. Compared the the same bowl, to the right, it’s really obvious. The bowls are the same mould, the same clay body and the same maturity. The only difference was the thin layer of cobalt tinted porcelain cast into the bowl as a primary layer.
Once you learn about ceramics you learn that you don’t actually know anything at all, really. The more you test and learn and try, the more you know how much more there is to understand. The cobalt oxide added to the slip was causing the outside layer of porcelain to shrink ever so slightly more than the interior body of porcelain. This tension only becomes apparent when both clays reach their maturity in the very hot kiln. At 2165 °F the outside slip has shrunk enough to pull down the lip, at the point where the bowl has the least amount of structure to resist. The resulting flare is caused by two different kinds of porcelain, where one is literally pulling the other one down. Chemically the two slips are almost the same, but the added cobalt oxide is just enough to change the chemical structure of the porcelain, causing it to fuse at a lower temperature, making it shrink just that little bit more. Amazing.