Friday, January 1, 2010

Question re: Cobalt Carbonate in glaze

I have a bowl that I brushed with cobalt carbonate (or is it carbonate cobalt... time for my 'up too late on New Years Eve nap ;-) that I'd mixed with some water. I wiped it off, leaving a decent amount in crevices. I'm going to put a clear glaze, or maybe a white or light blue glaze over it and fire. At least that's the plan. However, it seems that I read somewhere that the cobalt would run easily? I don't want a mess, don't want to ruin the bowl either.

Anyone familiar with the properties? Should I wipe a LOT of it off? Should I wash it all off? What's gonna happen you think if I do what I said? Need a photo to see how much I have on there?

I did an Internet search, no luck. I'll probably take a look through some of my pottery books.

Heading back out to the studio, it's New Years Day and I can't start the year without clay under my fingernails .

Hope you're all having the best ever beginning to a new year!


  1. HI! Happy new year! You probably need to wipe off as much cobalt carbonate (CoCO3) as you can if you want it to be mainly in the cracks. That way the effect will be nicer. Cobalt is the strongest colorant. 2% by weight in a glaze will be blue, iron needs 12% to be dark. The oxide (CoO) is 2x stronger than the carbonate. The carbonate changes to oxide in the kiln as the CO2 goes into the air. If you see any it is very strong blue so wipe and wipe, use it watered down and keep your hands out of it as they will look clean but leave finger prints and blotches of blue. If you see a lot of grey/purple powder, then it is really thick and you might want to wash the whole thing, let it dry a day and then glaze it. It will still have blue in cracks. It probably won't make a clear or white glaze run unless it is very thick. But I would need to know the glaze reciepe and firing temp to know more. I like white better but both are nice. Blue willow pottery was done this way with cobalt washes of differrent strengths under and over a white glaze. Sometimes I addd a bit of frit 3134 or other frit to the cobalt and water so it melts into the glaze better raather than causing it to pull away in that area, which it can do if it is thick, leaving a dry surface. I also addd the powered ceramic gum CMC to the water to make it more brushable. Those aren't alway necessary esp if you mainly have it in grooves, carving or cracks. Good luck and have fun experimenting with oxides in the new year. Erik Painter

  2. Thanks so much Eric! Hope you have a fantastic New Year.


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