Saturday, January 29, 2011
Cassius Basaltic Questions
Has anyone used cassius basaltic clay or know anyone who does? I'm in a little bit of dilemna with the pieces I've been making over the last several months. The beauty of this clay and the draw for me is the smooth ebony black color it fires at Cone 4. My pieces are all bisque fired and are still the brown color, but once fired the clay will turn black like the white feather vase completed early last year.
I'd like to do a test fire at Cone 4 for a clear glaze I want to use on them. I have a test tile but it's flat. I've glazed cassius basaltic clay vase shaped pieces before and the glaze bubbled. Remember the moon crater pieces. I'm pretty sure the glaze bubbled because it was poured on the inside and was too thick and it was fired quickly (at another studio). I think this clay would be better fired slower during a glaze load.
I did do one test tile to Cone 5 and the glaze was fine, but the tile was very thin and the clay pieces I've made are much thicker. By the way I happened to put the test tile in a kiln load as greenware and once fired it by mistake and it turned out ok. What a lucky one that was. I worried the whole time it would blow up and ruin other pieces in the kiln.
That got me thinking about once firing though. More experiments are in order for that. I just looked at that tile closer. Although the glaze didn't bubble or pit, the clay color is more brown rather than black, I may have just solved my dilema. If I want the black smooth color of the clay to show I probably shouldn't put a glaze over it.
I could sacrifice several pieces to see how various clear glazes fare. Which would mean firing my kiln for only one piece. That seems like a waste of electricity, but I can't think of another way to do it without risking the nice pieces I have all in one load. I no longer have my test kiln.
I may have to consider all of the cassius basaltic clay pieces as sculpture and not water tight. Since there are several which are tall with narrow openings and I can't see how I could hand glaze those to make them water tight for flowers. But then there is the large slip decorated bowl (12 w x 3 H inches) in the first couple of photos, wonder if I can put some type of coating over it after it's fired, like verathane, clear spray paint? More testing needed there.
I think I've decided to leave the clay natural. No sense using a beautiful smooth black clay and covering it up with glaze. I notice on the back of the test tile there is minor white speckling which happens at cone 5, so I'll fire my pieces to cone 4. Hope you don't mind, I guess this turned into a thinking-out-loud type of post; thanks for listening. Comments and suggestions are always welcome. Have a great weekend.
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