Potters & Sculptors - Making Rock from Mud
I recently mixed up a batch of Variegated Slate and Waterfall Brown from Mastering Cone 6 glazes. I did a slow glaze firing with a 15 minute soak. The cone 6 cones bent down to just touching as they should. However what I found was my really smooth Highwater Clay called Little Loafers did wonderfully with the glazes while all other clay bodies had blisters or pinholing. These were all from the same firing and I would have some that were with the Little Loafer's clay that were perfect on the same shelf as one that had a bunch of blisters. This is the first time I've ever used these glazes and I'm pretty new to the process of firing. I've read a couple of things that I could try but I'd love to have some help narrowing down what I should try here. Do I soak for longer or for less time? Do I custom program a fire schedule that does a controlled cooling? There was a pretty good mix of clay bodies in this firing. Most of the bodies were standard clay bodies other than the Little Loafer's. The attached picture shows the cup made with a standard white stoneware clay with grog and as you can see, it has a large number of spotty blisters all over. This is the result I got with both glazes on all the other clay bodies. Interestingly enough I did a combination of waterfall brown and variegated slate on a Little Loafer's bowl and that one was beautiful too. It seems like my glazes work great for that single clay body with this firing but not with any other clay body. Any suggestions on how to fix this would be much appreciated.
It's a little hard to make it out in this picture but there are raised bumps that you can feel all over the cup and I mean everywhere. It almost seems like the grog is causing it because of the sheer number of them. It doesn't look like pinholing to me but it also doesn't look exactly like blistering. The spots are white which I assume is because that is the color of the clay body underneath the glaze. It seems like bubbles formed but never broke the surface?
Not a clue here, and can't really tell from the photo, but you might contact John Hesselberth at Frog Pond Pottery, co-author of Mastering Cone 6 Glazes. He's pretty good about dispensing advice on glaze problems from the recipes in the book.
Are all the clays you used of a certain type - like porcelain or stoneware? It really is best to fix on one clay body that you really like and use glazes that work well with it. Obviously the two you used work with the Little Loafers (odd name for a clay!) - stick with those.
Good luck - please post when you figure it out.
can you crush the bumps and they break? when you said raised bumps, I wondered if the problem is in the clay body....years ago I had some pieces that were in a community kiln and they were bisqued too low
the result showed up in the in the glaze firing-there were hundreds of bumps all over the pot-- under the glaze, like air bubbles in the clay that raised up
the studio manager raised the bisque temp from 09 to 06 and it never happened again
you could break your mug and see if thebumps are in the clay bod or on top
I use both of those glazes regularly, 2 of my favorites. Never have those problems and use several different clays .
What were the other clays?
What temp were they bisqued to? Very important. Highwater recommends 04 for their clays, that can make a huge difference in blistering and also in better dipping and drying on the bisque.
That blue is WAY bluer than what I get with verigated blue. How thick was it on the pot?
Will help if I can, but need more info.
Thanks everyone for your thoughts and suggestions. That particular firing was a combination of a few different clay bodies. Specifically, Standard 266, Standard 112, and Laguna's B-Mix with Grog. Something to note about that firing was that I have used all the above clays without these problems with other glazes from the same book. What I changed this time however was I added a soak time because one of the glazes I had was almost perfect but had a few minor pinholes in it. So, I read that there should be a 15-20 minute soak. So that firing with the above results I did a 15 minute soak. For whatever reason I had problems with that however I just did another load and this time I only did a 10 minute soak and I had no problems at all. Granted I only had a couple of pieces from the standard 112 clay body in this load and all the rest were Highwater Clay's Little Loafer's and Brownstone. Everything came out great though, so I'm thinking that for whatever reason it did not do well with that extra 5 minutes of soak time. The B-mix clay even seems fine on test tiles in this load. So, hopefully I have worked through this problem.
Also, Martha, I have no idea why mine turned out so very blue although I think the fact that it is very glossy is due to the fact that it doesn't have a controlled cooling set on it. Also, that Little Loafer's clay is such a smooth body, just about every glaze I put on it looks better.
I bisque to 06 although I didn't know about the recommendation from Highwater to bisque to 04. I think however that my Highwater clays did great. I may try doing an 04 bisque with them to see if I can notice any difference.
My experience. Blisters are from overfiring the glaze. Pinholes, its the clay. I have using Highwaters Ellen buff and all of a sudden I started getting pinholes. Only with some glazes. Usually the glossy ones. Tried everything for two years fighting it, destroyed alot of pots. Changed to Red Stone and no more pinholes. I threw out about 300lbs of Ellen Buff. Tried to call Highwater, but they were no help. I tried all the suggestions and it did not work.
I think that is the case here as well. I do believe they were overfired just a little and for whatever reason that caused problems with certain clay bodies. The soak time dropped by 5 minutes seemed to solve the issue in this case.
It is happening to me too now. With both varigated blue and waterfall brown. I think I bisque fired to 06 but didn't write it down. ugh.