What Causes Glaze/clay 'tide Mark'?

[oly 23]

The interesting browny-orange coloured line that appears like a tide mark between glaze and clay body, does it have a name, does anyone know what causes it – is it mostly clay or a specific glaze glaze or combination, and can it be controlled?

 

I am going to do some tests but It would be good if someone can give me some clues what to look for.

 

The clay was a basic buff/grey stoneware electric fired to cone 6 and glaze is:

 

EC semi crystal glaze:

Neph Sy 40

Dolomite 15

Whting 8

Barium Carb 7

Flint 27

Bentonite 3

Copper Ox 1%

[High Bridge Pottery 681]

I'm pretty sure it is to do with sodium

[Marcia Selsor 1,786]

I think it is where the soluble chemicals travel through the clay from the glaze.

Marcia

[TJR 361]

I am looking at Barium Carb as the culprit. You can switch your barium to strontium carb by multiplying the barium by 0.75. You will have less stront. in the glaze than barium.

TJR.

[alabama 154]

If you glazed, and then took a damp sponge and lightly feathered the edge all around does the

line go away?  Does this show up on all the pottery using the same clay and glaze?

What happens if you change clay body or glaze?

Alabama

[oly 23]

If you glazed, and then took a damp sponge and lightly feathered the edge all around does the

line go away?  Does this show up on all the pottery using the same clay and glaze?

What happens if you change clay body or glaze?

Alabama

Hi, yes it does appear always with this combination of clay and glaze, I don't know about feathering the glaze, one to try, likewise I have not yet tried the glaze on another clay. Thanks for the ideas!

[Pres 1,914]

Almost looks like wet partly settled glaze dipped and then the second glaze dip went up to it, and because of the wet area resisted partly.

[Chris Campbell 1,224]

I absolutely love that toasty line! It is also in the thin area around the lip and handle. Yummy!

 

I have a clay I sometimes use when I want to carve into a surface ... When I use a turquoise glaze it breaks on the edges with that wonderful color. I was told it had to do with the iron in the clay body but I can't swear to that.

I can't get to it right now but when I can I will take a picture and add it.

[oly 23]

I absolutely love that toasty line! It is also in the thin area around the lip and handle. Yummy!

 

I have a clay I sometimes use when I want to carve into a surface ... When I use a turquoise glaze it breaks on the edges with that wonderful color. I was told it had to do with the iron in the clay body but I can't swear to that.

I can't get to it right now but when I can I will take a picture and add it.

I so agree, it is the kind of character/effect I am trying to achieve in my firings. I'd like to be able to replicate it with other glazes/clays, if I can only work out what causes it... 

[Chris Campbell 1,224]

Here is the image showing the turquoise glaze and the toasty lines. This was made very early in my clay career ... maybe the '90s so I now cringe at the crazing.

[BlackandWhitedogpottery 2]

Ive also have this happen, I've noticed this with just about every stoneware I've used. Maybe it has to do with the heat? Ive never seen it happen on a foot ring but it will always happen on either an unglazed rim or glaze part on the side of the vessel.

[oly 23]

Here is the image showing the turquoise glaze and the toasty lines. This was made very early in my clay career ... maybe the '90s so I now cringe at the crazing.

That toasty effect is very similar isn't it!  So, maybe it is the copper oxide – if that is what you used too?

[oly 23]

Ive also have this happen, I've noticed this with just about every stoneware I've used. Maybe it has to do with the heat? Ive never seen it happen on a foot ring but it will always happen on either an unglazed rim or glaze part on the side of the vessel.

Yes, I've seen it on other stoneware too, I need to try and track down the exact cause if I can. Thanks for the ideas!

[Chilly 701]

I've had that happen when the kiln was over-firing.  Assumed the clay was "burnt".

 

Love that glaze, I have all the ingredients except the Barium Carb.  What will happen?   Yeah, yeah, put it into DigitalFire and find out.  Hummmm, I tried using that lovely software and couldn't understand the results.  Maybe I'll try it and test it and find out the manual way!

[rayaldridge 279]

My guess is that something in the glaze is volatizing and those fumes are altering the clay near the glaze edge.

[Marcia Selsor 1,786]

Here is a link to the soluability of things with a list of many common ceramic materials. I still think it is the soluble salts from the glaze that seep into the clay further than the solids. In this case, the barium carbonate. The salts fume during the firing and create that blush along the edge.

http://www.solubilityofthings.com/water/ions_solubility/ksp_chart.php

 

Marcia

[Joseph Fireborn 1,286]

I personally love that line. It finishes the glaze application between the raw clay and the glaze, it makes the piece look finished. "This is the end, enjoy this beautiful clay surface now." That's how I feel about that line.

[TJR 361]

I had a turquoise glaze that I was testing. It had that toasty blush, which I loved. I took it and used it as a positive decorating motif.[see my gallery images for those turquoise mugs] I used a resist technique, and SHABAM!.

The glaze used barium- switched to Strontium. It also has copper.

Soda ash would also cause blushing.

One solution for you, if you don't like the effect, is to glaze all the waty to the bottom of the piece.

TJR.

[High Bridge Pottery 681]

I remembered Chilly posting this image. As you can see the neph and wood ash have the sodium flashing round the edges. This happens to my glazes with no barium or copper in.

 

[oly 23]

I remembered Chilly posting this image. As you can see the neph and wood ash have the sodium flashing round the edges. This happens to my glazes with no barium or copper in.

 

That's interesting!

 

Maybe one of the brainy guys on here will be able to tell us what it is that Neph Sy and Wood Ash have in common that they both give the 'toasty' tide line?

[rakukuku 122]

I get this too and don't know why but i think its cool.  another distinguishing characteristic of handmade ceramics . don't see this in commercial stuff.  could it have something to do with reduction and glaze thickness?  rakuku

[PSC 79]

Isn't it just fuming like adding salt or soda to a salt or soda kiln, the salts fume creating a self making glaze? In this case its the salts in the glaze fuming to make a self glaze.

[Amy Eberhardt 56]

Whatever is causing it, I love it!! I wouldn't change a thing!

[neilestrick 3,344]

It's solubles seeping out at the edge and/or fuming. Lots of glazes do this, it's not necessarily tied to the barium. I would point to the neph sye as the culprit rather than the barium.

[quangvinhdd 0]

I think clay