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Longterm Glaze Issues | Sometimes Runny, Sometimes Breaks The Pots


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Hi Nerd,

 

Oh I see,

Yes, we have potash feldspar, I will try that, it will take some time, as I need to order that.

I will ask for the spec sheet...

 

Do you know what actually makes the crackle glaze work right?

I just thought I should try other recipes, not only to depend on this one.

 

I haven't seen nz kaolin here,

what makes it so special?

 

 

 

 

 

Hi Nat:

G-200 is actually potassium: the Nep Sy is high sodium and the whiting is calcium. I think the whiting/ calcium is at the right level. Sodium has a higher expansion rate than potassium: so I was going to replace some Nep Sy with a potassium flux to lower the overall COE of the glaze.

I was looking at the Limoge porcelain slip on the PotteryCrafts website in the UK: not much info. They do use potash as a flux; so that gives some indication of the COE of the clay. The fact they are very specific about the firing temp is also revealing: 1230-1270C. ( hard cone 6 to cone 7) In comparing your slip to porcelain slips in the USA, I get a very generalized sense of the body. However, most porcelain slips here have a 13% shrinkage rate, and Limoge is only 11. For the moment I am going to assume that is due to a higher level of clay content. If you have a spec sheet, more than happy to see it. For the moment, I am also going to assume the alumina ratio is lower due to the pieces cracking.

Peter Frohlich from Austria uses a stoneware body from the Fuchs company in Westerwald, Germany. He fires crystalline glaze on a stoneware body which is nearly impossible unless the stoneware has porcelain properties. If they have that same stoneware in a slip, it would work well with this glaze. Crystalline glaze has a COE of 8.00 and up: which is in line with the expansion ratios of your glaze. Do you have access to New Zealand kaolin? The other option is to add 8-10% of NZ kaolin to your current slip to increase the alumina batch weight. The other assumption I am making at this point: your slip body does not have enough alumina to silica ratios: thereby weakening it overall. Sorry for all of the assumptions, but I do not have the specs for products to know exactly.

If you have chemical analysis for the silica and nep sy that would also help: trying to figure out where the magnesium is coming from. As Bruce pointed out, at some point bentonite or a gum will have to be added back for glaze suspension. Again I do not know your bentonite source or purity levels. The good news, I can see noticeable improvements from your first pieces to this last test piece: so we are moving in the right direction.

Final question: are you sieving the glaze before application?

Nerd

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What makes your glaze beautiful is also what is making it broken: Too much sodium used as a flux. Flux is needed to make glaze, and sodium is easily available and commonly used. Sodium also gives very pretty colour response, especially with copper as a colourant. The problem is that it also has a high CoE. Crazing is common with its use. As soon as you start taking enough sodium out of your recipe to make it crackle a little bit, but not so much that it breaks apart your pots, you will also change the colour of the glaze. (This may not be a bad, or even a big change, but it will be different. Only testing will tell you the exact change.)

 

Because the sodium is supplied by a mineral that is composed of other ions as well (the neph sye), to adjust the sodium levels lower, you need to find ways to re-supply the other ions which are also needed. This is why it's a difficult fix without glaze software and a lot of material knowledge. It's why some are encouraging you look at other recipes.

 

NZ kaolin is from New Zeland, and is some of the purest in the world. It's not contaminated with iron and other things the way that bentonite is, but will do the same thing that the bentonite does. It's in there to help keep your glaze suspended while in the bucket, and make application easier by hardening the dry glaze.

If you used your bentonite in your second test as well as your first, I'd say that's where the yellow colour is from.

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The crackle is caused by a mismatch between the clay and glaze as they expand and contract during heating and cooling (CoE). You have a size 6 pot in a size 5 glaze. You are actually looking for this difference in the glaze and clay whereas a lot of people try to solve this 'problem' Any clay with a much lower expansion than glaze will make the glaze crackle.

 

https://digitalfire.com/4sight/education/understanding_thermal_expansion_in_ceramic_glazes_198.html

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Hi Nat:

Others have already answered, so I do not see any need to add to it: although I have other motives for adding NZ. You are much closer than you think.. do not give up the ship just yet.Study the cup from the original post, then the piece after adjustments were made. What do the crackle lines tell you?  Notice how much less severe the split in the pieces are? Adjustments in glaze will effect color: some very subtle, others can be dramatic. Good thing is, color is one of the easiest things to correct.

Nerd

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Thank you all, dear people!

I'm not giving up :)))

but I will also try couple of other recipes to compare

 

Nerd,

i understand that about crackle size

I can see that on the cups that survived the firing

crackle is very fine and nice, as if only on the surface.

 

well, I keep you posting with some new results as soon as I have them

 

thank you so much for your kind support!

I wonder if there are any online courses on glaze chemistry

I have been reading some articles about how glazes work, but I feel I need help answering all the questions to understand it properly

 

 

 

 

Hi Nat:

Others have already answered, so I do not see any need to add to it: although I have other motives for adding NZ. You are much closer than you think.. do not give up the ship just yet.Study the cup from the original post, then the piece after adjustments were made. What do the crackle lines tell you? Notice how much less severe the split in the pieces are? Adjustments in glaze will effect color: some very subtle, others can be dramatic. Good thing is, color is one of the easiest things to correct.

Nerd

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thank you so much for your kind support!

I wonder if there are any online courses on glaze chemistry

I have been reading some articles about how glazes work, but I feel I need help answering all the questions to understand it properly

http://art.alfred.edu/academics/glaze-formulation.cfm

 

Not sure when it is offered, but Matt Katz is among the experts.

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