Please Help! What Causes This?

[bluecreekpottery]

What causes this? I keep getting cracks in my glazes that are raised that makes you want to pick at them like in the picture. It happening with all my glazes but it's bottle glaze so I don't think that's the problem. I tried cleaning my poyyery befor glazing and that didn't work. I'm really lost on what is causing this but it's happening on nearly everything I fire now(like 60 percent of my stuff). I've haven't had this problem until recently and I'm getting really frustrated. I love pottery so much but this is breaking my heart. I think it's something with my clay. Could this be due to something like sponge bits in my clay?

[curt]

A bit hard to tell from the picture, but it almost looks like fibers or something in your glaze burning out in the firing and leaving a hole.  Have you tried sieving your glaze before applying it?

 

That fact that the cracks are small (?) but "raised" suggests that they are something in glaze, rather than the clay body splitting open underneath.  Seems like the glaze must be rather stiff and matte, because often with something like that the glaze would melt and heal over.   What kind of glaze is it and what cone do you fire to?

 

A better closeup picture, and perhaps from a couple different angles might make it clearer what is going on. 

[bluecreekpottery]

A bit hard to tell from the picture, but it almost looks like fibers or something in your glaze burning out in the firing and leaving a hole.  Have you tried sieving your glaze before applying it?

 

That fact that the cracks are small (?) but "raised" suggests that they are something in glaze, rather than the clay body splitting open underneath.  Seems like the glaze must be rather stiff and matte, because often with something like that the glaze would melt and heal over.   What kind of glaze is it and what cone do you fire to?

 

A better closeup picture, and perhaps from a couple different angles might make it clearer what is going on. 

I use mayco, duncan, amaco, and coyote and its been happening with all of them. I fire to cone 6. I have tired sieving but it didn't seem to work. Could it be from overfiring maybe in my kiln? It's cone 6 clay and I fire to cone 6 but it has been hot out. Here's a picture of another piece where it happened and hopefully you can see what I mean by raised. 

[Mark C.]

If you break open this raised part is it in the clay or in the glaze-This photo is to hard to tell-I can see some raised areas-If the clay is getting overfired it can start too come apart(meaning bubble up or crack or pimple up or start to deform)

I suggest you break it on a raised section to see if its the clay or glaze?

post the results

[bluecreekpottery]

I added another picture to the original post. It's very zoomed in, but I hope you can see what I mean by raised. 

[glazenerd]

Interesting.hhmmm.

 

I noticed discoloration around the immediate area of these cracks in all three pictures. The failure ratio is 60%, and it is happening with multiple glaze colors from multiple glaze makers. So the only common denominator between them would be the clay. Looking at the dark discoloration around the cracks, which is usually a symptom of potassium off- gassing. Dark stoneware bodies are typically the worse offenders. However, the discoloration also has the colorants of the glaze.

 

There is however one indicator that screams the loudest: open craters/ tears in the glaze that are raised upward from the glaze surface. Large particle feldspar in the clay is the most probable cause. Potassium is the worst offender, so probably a dark stoneware body. It is happening at the top end of the glaze firing, and not enough time or heat to heal: a chronic cone 6 issue.

 

The test and the cure:

Use your current glaze firing cycle up to 2050F.

Then 130F an hour ramp to 2230F, hold for 10 minutes.

 

If the cracks/ blisters are gone, then it is most certainly large particle feldspar minerals in the clay. They can occur in any clay body; but dark stoneware tops that list.

 

Now if I am wrong: please remember I suffer from CAD ( chronic assumption disorder), compounded by my bad habit of breathing lithium carbonate dust. Better yet, please include the clay body type and firing schedule when you post another question. Information narrows the windows down.

 

Nerd

[Pugaboo]

Another simple test would be to buy a bag of completely different clay and make some simple test pieces and see if it happens on them. If it doesn't you know it's your clay and can focus on correctly the firing issues of your original clay. Also you says it's a recent change? Have you purchased a new supply of your original clay? If so do you have any old bags of this clay around to test out and see if it happens to those pieces as well.

 

What you need to do is start eliminating the different variables. I would start with testing a completely different brand of clay and work out from there. You might even try a different color of clay, if you use a dark brown try a light clay instead. You can buy 4 pound sample bags of different clays from Clay King and it might be an easy, cheaper, way to test some clays than ending up with whole 25 pound bags of clay you don't want.

 

Do let us know how it goes and I hope you can quickly figure it out.

 

T

[curt]

Yes, with more pics at different angles I agree with others here that it looks more like the clay than the glaze. Almost looks like a lime blow that didn't quite blow off, but that seems unlikely.

 

And Nerd makes a good observation that the discolouration looks like off-gassing. If it is some large-particle stuff in the clay maybe try taking a sample of the clay and turning it into a slip, sieving it at say 120 or 200 mesh, and then forming a test bar or piece and re-firing to see if the problem is still occuring.

 

You aren't once firing are you? Seems like something burning out rather explosively...

[Marcia Selsor]

I would agree with Mark and Glazenerd. I have seen things like this that are from impurities in the clay. Glazenerd may have the solution by using his firing schedule. I have seen what look like larger bits of iron do this. What clay are you using? Try a test with different clay as pugaboo suggested.Curt's suggestion of serving to find larger impurities and removing them is a good idea. See if you can determine if it is a bloated chunk of something coming from the clay by cracking it open.

 

Marcia

[Roberta12]

Now if I am wrong: please remember I suffer from CAD ( chronic assumption disorder), compounded by my bad habit of breathing lithium carbonate dust. Better yet, please include the clay body type and firing schedule when you post another question. Information narrows the windows down.

Nerd          

 

 

 

hahahahahahaha....  I have that too!!!  CAD!!!  â€‹   Perhaps there is a support group for us.   hahahahahahaha

Roberta

 

And yes, I agree with the posters above.  The clay seems to be your common denominator! 

[glazenerd]

Roberta: I would assume there is.

[Callie Beller Diesel]

Roberta: I would assume there is.

Yeah. We all hang out here!

[Pres]

Know it sounds crazy, but it looks like to me that there is a serious contaminate in the clay. Possibly some metal even, but then from the pictures it is hard to tell. The reason I am thinking contaminate, is the edges of the blemish, with the discoloration that is dark brown or so in color.

 

 

best,

Pres

[Magnolia Mud Research]

Are you using a clean brush to apply the glaze - one that does not shed bristles?

 

 

LT

[Min]

I agree with Pres, think it's a contaminate in the clay. Any chance this is a clay from Laguna in California? Or, hate to think it but could someone be tampering with your clay or glaze if you use a group studio?

[Marko]

Break the pot and dig out what it is. That's if you aren't going to sell or keep it. But if it's not sellable break it and learn. It looks like a clay issue. Best wishes.

[Mark C.]

You will get more info from breaking open this pot than wondering whats going on-Its part of learning just bust it up and let us know what you see?Then we can fix the issue.Or at least we will know what it is.