Flaking burnished surface

[tanjag]

I have just tried burnishing (with a smooth stone, not Terra Sig) and firing a pot (in a wood stove), using a an aluminum foil saggar filled with sawdust (to get a black surface). 

When I pulled it out I found that the surface of the clay was flaking off where it was burnished. The inside, unburnished, was fine. 

The clay is found clay and I know it contains a lot of organics, and shrinks significantly when fired. I assume this shrinkage caused the flaking.

Is there any way to avoid this, or should I just give up trying to burnish this particular clay?

Should also mention that I am *very* new to both ceramics and burnishing (first time!). Any and all advice extremely appreciated!

Tanja

 

[Rae Reich]

I know nothing about burnishing, so no advice on that, but those surfaces are very interesting. I like them. 

If these were mine, I'd say, "I meant to do that!"

[liambesaw]

It might just be because it was fired in a wood stove, I have a clay ramp in my mini kiln and it flakes like that because it takes the full brunt of the torch when I fire.  Maybe you can figure out a way to heat it a little slower?  I could also be totally wrong, I haven't fired in a wood stove before

[Babs]

At what stage of drying are you burnishing?

[tanjag]

2 hours ago, Rae Reich said:

I know nothing about burnishing, so no advice on that, but those surfaces are very interesting. I like them. 

If these were mine, I'd say, "I meant to do that!"

 They are kind of neat -- but I'd love to get to the stage when I *really* meant to do that!

[tanjag]

2 hours ago, Babs said:

At what stage of drying are you burnishing?

I am burnishing when its almost completely dry.

I also used some olive oil to burnish, which was suggested in several blog posts. Could that have something to do with it?

[tanjag]

2 hours ago, liambesaw said:

It might just be because it was fired in a wood stove, I have a clay ramp in my mini kiln and it flakes like that because it takes the full brunt of the torch when I fire.  Maybe you can figure out a way to heat it a little slower?  I could also be totally wrong, I haven't fired in a wood stove before

Thanks, this is helpful! Has your flaking happen with burnished surfaces, or unburnished (or both?)

I will have to see what I can do to slow the heating...

I am playing around with a stove right now because that's all I have at the moment! I have fired the same clay without burnishing with no flaking problem.

[liambesaw]

3 minutes ago, tanjag said:

Thanks, this is helpful! Has your flaking happen with burnished surfaces, or unburnished (or both?)

I will have to see what I can do to slow the heating...

I am playing around with a stove right now because that's all I have at the moment! I have fired the same clay without burnishing with no flaking problem.

Unburnished.  Have you tried burnishing without oil?

[tanjag]

15 minutes ago, liambesaw said:

Unburnished.  Have you tried burnishing without oil?

Trying that today -- will report back

[JohnnyK]

I've burnished pots in the stage just beyond leather hard, while they are still damp, with great results. I've also tried burnishing with oil when the pot is close to bone dry with less than good results. I've used polished polishing stones as well as soft plastic bags with different results. You might check out Summi von Dassow's books and videos. 

Reading, watching, testing and practicing are the best ways to learn your craft!

JohnnyK

[LeeU]

I'd scrape off everything loose enought to come off and pronounce them gorgeous.

[tanjag]

On 9/13/2018 at 6:40 AM, JohnnyK said:

I've burnished pots in the stage just beyond leather hard, while they are still damp, with great results. I've also tried burnishing with oil when the pot is close to bone dry with less than good results. I've used polished polishing stones as well as soft plastic bags with different results. You might check out Summi von Dassow's books and videos. 

Reading, watching, testing and practicing are the best ways to learn your craft!

JohnnyK

Thanks for sharing your experience! I tried it just past leather hard without oil (or water) and everything went great with the firing -- no flaking! And I will definitely check out the videos and books you suggested.

Tanja

[Babs]

Yeh I'd drop the oil . Oil would burn out and produce the flakiness as it would be mixed with the clay on the surface.

Folk polish pots with oil or beeswax  on a sheepskin pad after firing.

[neilestrick]

I had a friend in college who did large coil built vessels that he covered in terra sig and burnished bone dry using lard, sometimes oil, and it never caused a problem. Could be an issue specific to your clay body, though.

[Babs]

And quantity  of oil used. Furnishing   lot of elbow grease and a bit of spit:-)))

[Marcia Selsor]

I use quick terra sig. Apply when bone dry and burnish with a soft makeup sponge or foam. 

recipe  

250 grams of ball clay or any fine clay including red art  into a liter of water in a plastic bottle. If digging clay, weigh 200 grams and ask for a few hours before mixing with the water. Shake well. Add a few drops of Darvon 7 or Darvon 833. Shake well. Let it settle for about 2 hours. Punch or slice a hole in the bottle above the line where the heavy materials settled. Drain into a container. 

Brush or spray onto the pot. Burnish with a soft sponge. Your clay should be smooth, no grog or very fine grog. ..fine enough that burnishing won't scratch the surface. 

Bisque to ^08 hold for an hour. If you over bisque terra sig, the smoke will not be absorbed. You might try smoking in a barrel or covered brick box. Use  sawdust, charcoal briquets, and wood. There a is great video by Piepenburg on this method. The photo is Apache red clay terra sig, with feather burned onto surface.

 

Marcia Selsor.

 

[Babs]

Ooo aaaa beautiful

[Benzine]

Tanjag, are you bisquing the pots first, as Marcia mentioned, or just single firing them, in the wood stove?

I tried single firing some wares, for a pit firing.  It didn't go well...