Glaze For Sheffield Pottery Cone 10 Porcelain That Does Not Crawl

[oakhillstudio]

Hi Folks, 

 

I have started to use cone 10 porcelain from sheffield pottery.  Since it is full of impurities, I decided to fire my porcelain pots in oxidation. My pieces are pretty thin and I get crawling just along the rim of each piece....like clockwork. Crawling is also a problem along sharp ridges where curves of the form jut out and then recede. Does anyone use Sheffield's porcelain cone 10?  If so, would you be kind enough to share a clear porcelain glaze recipe that does not crawl, with me?

 

FYI,

I wear gloves to handle the ware, glaze immediately after bisque so dust and grease are not a problem. I have added 1.5% bentonite to my glaze to add some small platelets to my glaze mix which has made little to no difference. 

 

The recipe which I use (from Mary Risley (RIP) at Wesleyan University 40 years ago....):

 

Custer spar                33

Flint (silcosil)              32

Whiting (snocal 40)    20

Grolleg Kaolin            15

 

Help?!!!  Does anyone have any recommendations (bedsides switching to EPK in the glaze. I am in the process of testing it)?  

 

Sharon Nahill

Oak Hill Pottery

Meredith, NH

 

[Mark C.]

usually porcelains are clean clays -I know nothing of Sheffield porcelains out wet here as they are to far away to try.

Now as to your glaze is this base a white that you add colorants to or something else?Is this a liner glaze?clear or white?

I only work with porcelains and crawling is never an issue  with most glazes-what color glaze are looking for.

Mark

[neilestrick]

Porcelain should not have any impurities- that's the nature of porcelain.  The problem with crawling is probably because the thin rims get too saturated when dipping the glaze, which prevents good adhesion. Try glazing the inside first, clean off any overrun on the outside, then letting it dry overnight before glazing the outside.

[Guest JBaymore]

Since it is full of impurities, I decided to fire my porcelain pots in oxidation.

 

Can you explain this comment?  What type of impurities?  Are you sure they are from the clay... and not the local studio environment?

 

This is NOT my experience with their clays.  I use a small amount of their porcelain along with mainly stoneware.

 

If you are having problems, have you contacted them about it?

 

best,

 

....................john

[oakhillstudio]

Porcelain should not have any impurities- that's the nature of porcelain.  The problem with crawling is probably because the thin rims get too saturated when dipping the glaze, which prevents good adhesion. Try glazing the inside first, clean off any overrun on the outside, then letting it dry overnight before glazing the outside.

I adore the clays from Sheffield pottery, especially their white stoneware. I have been using it for years. I have only bought one batch of their porceIain, as I am relatively new to the medium. I misspoke when I said their porcelain has impurities. Most likely it is just that one batch. When I brought the problem to Sheffield, they were marvelous as always and replaced my porcelain batch, free of charge, with a new batch that is clean in reduction.  The old batch was bought 5 years ago and had what looked to be clumps of iron oxide dispersed though out the clay. When fired in reduction these left large black metallic blisters on the clay. My first cuts into one of the new bags look good. Since Sheffield did not want my leftovers of the old batch and I have a large body of work made with the contaminated batch, I am trying to find a use for this porcelain. I hope to avoid the black spots by firing in oxidation (they still show but are much lighter, also I wire cut each bag into sheets and hand pick out the powdery chunks of contaminant).

 

Thanks so much for the advice about glazing the inside first.  I will try that!

[oakhillstudio]

usually porcelains are clean clays -I know nothing of Sheffield porcelains out wet here as they are to far away to try.

Now as to your glaze is this base a white that you add colorants to or something else?Is this a liner glaze?clear or white?

I only work with porcelains and crawling is never an issue  with most glazes-what color glaze are looking for.

Mark

Thanks so much for your response. My glaze is a clear glaze.  I sometimes add mason color to it to give the ware a blue or blue green tint.  I don't have much crawling at cone ten in reduction, either,but then again I have not used this clear glaze much in reduction conditions. I always use celadons and these behave very well on Sheffield cone ten porcelain.

[oakhillstudio]

 

Since it is full of impurities, I decided to fire my porcelain pots in oxidation.

 

Can you explain this comment?  What type of impurities?  Are you sure they are from the clay... and not the local studio environment?

 

This is NOT my experience with their clays.  I use a small amount of their porcelain along with mainly stoneware.

 

If you are having problems, have you contacted them about it?

 

best,

 

....................john

 

Hi!  Thanks so much for responding to my plea for help.  I just wish I had reread my post before I sent it off last night. I did not mean to imply that all of Sheffield's porcelain is dirty!  Just my unfortunate batch! Please read the post I wrote to nellestrick, above.  As I said, the Sheffield folks have been wonderful and I love their clay.  The porcelain is an absolute dream to throw with. They have replaced the problem batch with a new one, free of charge, and I am eager to see how it does in my big reduction firing. When I cut the wet clay with my wire I do not see the clumps of brown stuff that the other one had running through it. My problem is that I made a lot of pottery with the old batch and I am trying to make the best of the situation.  I also would like to find a use for the old batch.

 

I guess I will just switch to the celadon glaze that I use in reduction, but leave out the iron and copper. Also, I will try to dry the pots after they are lined with glaze.

 

 

Sharon

[rayaldridge]

When I was a young potter, I used a stoneware that was sold by an old traditional pottery near Tuscaloosa AL.  It was dug from the ground with a backhoe and immediately prepared in a drum mixer.  It had all sorts of crap in it, from roots and stones to big chunks of hematite.  In order to use it, I built an extruder with a stainless mesh, and forced all of it through the extruder.  That took out the big chunks, but left enough iron bits in it to make for a very interesting surface.  If you have an extruder, you might try running your contaminated clay through a mesh.

 

Just glancing at your glaze formula, it seems to me that it would have a lot of surface tension.  You might try adding a bit of extra flux, to make it wet the porcelain a little more-- maybe a bit of frit.

[oakhillstudio]

Thank you, Mark C. And Ray for your kind advice. I am sorry to have been so long to reply but I have been away this month without access to my computer. Mark, I use the glaze I described which is clear (cone 10).  I add mason stain of different blue and green shades (1%) to give it color in oxidation and copper and iron in reduction.  It crawls along thin edges.  This seems worse in oxidation firing. Ray, I will definitely try to add more flux. Thanks for the suggestion. Nielestrick, the porcelain pieces are very thin but carved so the whole piece is dipped at once. I guess i will try to apply the glaze more thickly, so it will not run off the thin parts as much.  Thanks for the advice.

Happy New Year to you all!