Dunting problem

[Steve Ayers]

I have a glaze that I have been using for 30 plus years that has recently started dunting tall cylinder pots.  The problem only happens on one glaze but it is pretty profound. On a batch of 100 bird feeders  everyone with this purple glaze cracked around the bottom, about 50 of them. I lost a cannister set and a couple crocks. I have used this glaze extensively for years with no problems with other shapes cracking.  I am planning on making a small test batch with what I believe is the standard correction to this problem, reducing the flint by 5%.   This year I will have been making pots by hand for 50 years. My knowledge of glaze chemistry is that of most potters, I know enough to fool the tourists.  I am including the recipe and am looking for incite.  I am wondering if some chemical has changed enough to start this problem as nothing else has changed.

Purple    136  lb batch

EPK 15.2  ,Custer spar 52  ,  Silica 26.4  ,  Dolomite  12.8 ,  Gerstley  Borate  substitute 10.6,   Whiting  12.4  

Rutile 6.5

 

[Callie Beller Diesel]

Hi and welcome to the forum!

Is this a cone 10 or cone 6 recipe? 

If you’re working at cone 6 and you’re using a white firing stoneware that contains talc, it might be the clay that’s changed rather than the glaze. The talc supply that everyone was using for the last long while is no longer available, so manufacturers are finding substitutes. I’m hearing reports about some of those substitutes presenting new COE issues, and that will affect lots of white cone 6 stonewares and casting slip.

[Steve Ayers]

It's a cone 10 glaze  used on our custom blend of a standard stoneware.  I have been using this clay for about 40 years. I did change the recipe about 10-12 years ago adding  kyanite to solve pie plate cracking issue.  Hawthorn bond is the primary clay in the recipe.

[Min]

Any new batch numbers of glaze or claybody ingredients? Mesh size of the silica the same in the claybody as previous batches? Glaze firing / cooling schedule change? What gerstley borate sub are you using (just curious).

I changed your recipe in lbs over to grams and plunked it in Insight using gb for your gb sub just to get a rough ball park look at the unity. I'm not seeing anything to set off alarm bells.

 

 

[Mark C.]

I'm going to ask about the clay body and whats changed with that? since its custom blend whats in it? Any talc as that has changed now market wise.

[Babs]

You glazing only one side? Inside / outside?

Difference between thickness of wall v. Bottom?

Got an image?

[Steve Ayers]

I have found a potential solution to this problem.  I called my clay supplier Bracker's Good Earth clay and talked to Cindy about another problem.  When I mentioned the dunting  she told me that Pete Pinnell  had a solution for other people who have been having this problem.  If they are using Hawthorn Bond in their clay bodies , I am, switch from 35 mesh to 50 mesh and the problem goes away.  Hawthorn is full of junk I am always pulling out rocks and having lime pop outs.  I am changing my recipe on my next order, I will keep you posted.   I am taking a chance by having my  entire  order made in the new recipe,  10-12,000 lbs. but the problem is so  sporadic that I don't think I would get a good test without making tons of pots with it.  There is still a problem with the purple glaze but hopefully the new clay will keep it under cover.