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natanata

Longterm Glaze Issues | Sometimes Runny, Sometimes Breaks The Pots

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They will break readily.

 

And eventually.  Maybe not today or tomorrow, but they're a ticking time bomb.

 

 

Classic love story. 

 

Girls meets bad boy glaze. 

Girl falls in love with bad boy.

Bad boy breaks her heart.

 

She tries to reform the bad boy.

Tries and tries to change him…. but it’s not in his nature, he’s a bad fit.

 

Girl needs to meet a nice suitable and well balanced boy.

Well suited boy won’t tear her apart and leave her in pieces.

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Yes, thank you, I know that

But I love the glaze and I want to understand how to make it fit. I think it's possible.

 

 

 

What's happening is that your clay and your glaze are shrinking at different rates. The glaze is pulling the pot apart because it shrinks so much more than the clay. If 7 out of 10 pots are coming out of the kiln broken, the other three are guaranteed to be structurally unsound. They will break readily.

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Hello Nerd,

No I don't add anything to the powder, as it says it has everything necessary to make a working slip.

Should I add? In which proportions? How will it help?

 

 

Hi Nat:

I know several crystalline glazers in France and England that use Limoge porcelains. From what I know about them, they seem to be a high quality clay maker.

You described the bisque color as "beautiful white", which I take to mean a bright white color. That would mean that the magnesium is under 0.30% of batch weight, and the iron under 0.50%: otherwise it would not be pure white. Titanium would also be under 1% as well. So for now I will assume that the porcelain slip is not leaching impurities that would cause a goldish/yellow color in the glaze. That also means that either your silica or Nep Sy, or whiting has magnesium/iron impurities at a level which is causing this additional color to appear. We lowered the whiting, so if that color continues: we know it is not the whiting.

After this test firing, post some close ups of the piece please. If the glaze is what you want, but it is still cracking pieces then we might attack this from a different angle. Your slip is a dry powder, so you can also goose it up a bit to give it additional strength.You adding sodium silicate when you mix?

Nerd

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Nerd could you expand on sodium, potassium and zinc being gaseous. From what I have read it is only zinc in reduction that likes to turn into a gas. Are they a gas but staying in solution?

 

Natanata I think you could probably get something close by just swapping out to a different feldspar. Isn't Neph quite sodium heavy so raising the COE.

 

The COE does seem way too high for porcelain, this glaze probably wouldn't break stoneware clay. Have you tried it on other clays?

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Joel: KO2, Na, and ZNO are gases at 2230F (cone 6), which is from chemistry- not the clay world. Potassium and sodium do stay in the body, but zinc can vaporize in some cases. Actually sodium and potassium will vaporize at cone 10: hence the salt glaze method. Zinc will vaporize according to the method it is processed. Metallic zinc (zincite) will begin to boil off (vaporize) beginning at 2190F, french process zinc will start at 2240F pending colorants used. Yellow zinc (sphalerite) is usually stable up to cone 10. French process is actually processed by super heating zinc ingots until they vaporize, and the dust is collected in bags. I cannot give you the exact chemistry term: but this method alters the molecular structure making it much more susceptible to vaporizing when heated. I ran a clay test using 5.77% batch weight of potassium and fired it to maturity without glaze: it looks like Mars- full of craters. Obviously the clay body was fluid, and off gassing potassium vigorously.

Oddly, cobalt and copper accelerate zinc vaporization in several tests I ran: while iron and manganese could go up to 30F higher without vaporizing. I have long believed that some pinholing issues at cone 6 are in fact the result of zinc and potassium off gassing. Sodium would be more of a culprit at cone 10.

 

I lowered the whiting in Nats' glaze, but I left the sodium level very high on purpose. I wanted to "soften" the glaze so it would crackle. I lowered the COE by 0.50, but it has to be a few points higher than the clay in order to achieve the crackle effect. I think the thin wall design is more of problem than the clay variety- we shall see soon enough.

 

Nat: there is no need to add sodium silicate if the dry powder already has a deflocculant in it. Adding more will only make it brittle and hard to fettle.

Nerd

 

K205.77

 

5.77% potassium test (porcelain clay body) no glaze.

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Hello Joel,

Yes, I have tried it on other glazes, it works alright

 

Nerd,

Do you think making the cup thicker helps?

I tried it, but again, sometimes it works sometimes it breaks :)

I'll show the result of the test today later during the day.

 

 

Nerd could you expand on sodium, potassium and zinc being gaseous. From what I have read it is only zinc in reduction that likes to turn into a gas. Are they a gas but staying in solution?

 

Natanata I think you could probably get something close by just swapping out to a different feldspar. Isn't Neph quite sodium heavy so raising the COE.

 

The COE does seem way too high for porcelain, this glaze probably wouldn't break stoneware clay. Have you tried it on other clays?

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Interesting stuff that I haven't really thought about before. Is that high potassium clay fired to cone6? It's strange that potassium is a better high temperature flux but it is vaporising earlier, or have I mixed that up. If a large amount was disappearing wouldn't it be useful to show that in glaze calculations? Going to have to do some more reading with KNa.

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Nerd,

 

So here are first results and unfortunately unsuccessful

Glaze still broke the cup, see the pictures.

Color changed slightly, it's more blue now, rather then greenish, do you know why?

 

I'm also usually testing glaze for its colour and surface on smaller cups, they actually almost never break when I applied that glazes

This time, they didn't too. I also tried it on a small road porcelain plate. Everything's is fine, so it's this cup, naughty cup :)

 

Would love to hear your opinion.

Nata

 

 

Hi Nat:

I know several crystalline glazers in France and England that use Limoge porcelains. From what I know about them, they seem to be a high quality clay maker.

You described the bisque color as "beautiful white", which I take to mean a bright white color. That would mean that the magnesium is under 0.30% of batch weight, and the iron under 0.50%: otherwise it would not be pure white. Titanium would also be under 1% as well. So for now I will assume that the porcelain slip is not leaching impurities that would cause a goldish/yellow color in the glaze. That also means that either your silica or Nep Sy, or whiting has magnesium/iron impurities at a level which is causing this additional color to appear. We lowered the whiting, so if that color continues: we know it is not the whiting.

After this test firing, post some close ups of the piece please. If the glaze is what you want, but it is still cracking pieces then we might attack this from a different angle. Your slip is a dry powder, so you can also goose it up a bit to give it additional strength.You adding sodium silicate when you mix?

Nerd

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I actually quite like this yellowish spot that you can see on the plate

The plate is unglazed on the outside, interesting that the glaze didn't brake it...

 

Nerd,

So here are first results and unfortunately unsuccessful

Glaze still broke the cup, see the pictures.

Color changed slightly, it's more blue now, rather then greenish, do you know why?

I'm also usually testing glaze for its colour and surface on smaller cups, they actually almost never break when I applied that glazes

This time, they didn't too. I also tried it on a small road porcelain plate. Everything's is fine, so it's this cup, naughty cup :)

Would love to hear your opinion.

Nata

Hi Nat:

I know several crystalline glazers in France and England that use Limoge porcelains. From what I know about them, they seem to be a high quality clay maker.

You described the bisque color as "beautiful white", which I take to mean a bright white color. That would mean that the magnesium is under 0.30% of batch weight, and the iron under 0.50%: otherwise it would not be pure white. Titanium would also be under 1% as well. So for now I will assume that the porcelain slip is not leaching impurities that would cause a goldish/yellow color in the glaze. That also means that either your silica or Nep Sy, or whiting has magnesium/iron impurities at a level which is causing this additional color to appear. We lowered the whiting, so if that color continues: we know it is not the whiting.

After this test firing, post some close ups of the piece please. If the glaze is what you want, but it is still cracking pieces then we might attack this from a different angle. Your slip is a dry powder, so you can also goose it up a bit to give it additional strength.You adding sodium silicate when you mix?

Nerd

post-63238-0-72994700-1464346453_thumb.jpegpost-63238-0-72994700-1464346453_thumb.jpeg

post-63238-0-76854400-1464346567_thumb.jpeg

post-63238-0-78514200-1464346601_thumb.jpeg

post-63238-0-72994700-1464346453_thumb.jpeg

post-63238-0-76854400-1464346567_thumb.jpeg

post-63238-0-78514200-1464346601_thumb.jpeg

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Nat:

I actually like the results of this first test: 1. the glaze run issue seems to be resolved. 2. the split is not nearly as severe as the first pics shown, 3. and we are dialing in on a crackle ( cracks lines are getting closer.) So I need to lower the COE by another 0.50, and run it again.

The color changed because the whiting was lowered: i lowered copper carb. from .54 to .50, need to go down to .45 now.

Let me run a new calculation this afternoon, will post it later.

The thin wall is certainly adding to the problem.

 

Educated guess: the test plate is flat, and I fire almost all flat pieces. The glaze is still extremely fluid: and heavier particles will pool on flat surfaces. So this adds to my belief that the magnesium is coming from one of your glaze ingredients: yellow/gold spot.

 

Nerd

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Joel:

We work with clay, but are still dealing with chemistry. Phase changes in this case: solid to liquid (calcium), and solid to gas (potassium and sodium). The term being kinetic energy: the atoms become further spaced and accelerate upon heating. I bought this up in some of my earlier posts. We are dealing with the most basic chemistry principle: the octet rule: noble gases on the right of the periodic table are the most stable: and then as you move left they become less and less stable. On the left is alkali earths: which are the most unstable- potassium and sodium being in that list. Lithium is the most unstable: and hydrogen gas is the weakest of all. So all you have to do is look at the first ionization energy for each element to determine how quickly it will lose its first orbital atom. We call it the melt temp in glaze, but in reality it is when it loses it first atom (donor/ reductant/oxidation). The reason lithium is the most powerful flux is because it loses it first orbital atom quicker, with less heat.--then sodium, then potassium..etc etc. Then you get into REDOX reactions.. blah blah blah... hey this coffee tastes good.

Nerd

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Why are you lowering the calcium? I am calcium bias as it is great but couldn't we up the calcium instead and drop some sodium. I haven't been able to take a good look at the chemistry as my insight account ran out but wouldn't that keep the green and solve expansion issues with a bit of alumina for less running.

 

Had a read of frank hamer on KNa and he talks about potassium not being volatile at cone10 but sodium is anywhere over 1200c (2192f). There is also mention that high sodium turns coppers blue.

 

Going to have a day watching youtube ionization videos  B) sometimes I wish I went to my advanced chemistry lessons instead of playing card games.

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Joel:

Sorry for the confusion: I was speaking about lowering the copper carb to 0.45. The whiting is where I want it and I will be lowering the sodium and increasing something: will let you know what that something is soon as I run a glaze calc.

Nerd

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Hi Nat:

Going to make some changes: hard to get the COE lowered with that much Nep Sy. So I am going to swap some of it out for G-200; which has a lower COE value than Nep SY.  Nep Sy has a COE of 93.45 and G-200 has 83.40. This is close to the lowest COE this particular glaze recipe can achieve without changing it a great degree or adding other ingredients.

Nep Sy        30.0

G-200          14.5

Whiting        16.0

Silica            25.0

Zinc                4.0

NZ kaolin      10.0

Try 0.45 copper carb, do a smaller test with 0.40 just to check color.  The COE has been lowered an additional 0.40. Calcium, sodium, zinc, silica, and alumina are all approaching the upper end of formula limits. I think ultimately you are going to have to give thought to adding one ingredient to your dry slip in order to strengthen it.

Nerd

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Glazenerd,

 

Any substitute for G-200 -- which is no longer mined/mixed?  I've heard there is a Spanish substitute called G-200EU.  Do you see a need for bentonite to keep the glaze in suspension once you move past test size batches?

 

Bruce

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Bruce:

I bought several hundred lbs of G-200 at an auction years back and forget it is no longer mined. I believe G-200 HP is the replacement, but has a higher potassium content. Ron talked about the G-200 EU in KC, but I do not have the specs. I have been trying to formulate while staying true to the original recipe, which Nat seems to like.Nat mentioned Limoge slip, so I have to assume they are sourcing outside of the USA. That would mean the Bentonite would come from a mine in South Africa. A potter in France recently told me he had been having problems with the iron content; so I have been reluctant to recommend it. I was thinking a gum, only because Nat already has a contamination problem with other ingredients. I still think it is the silica: German silica usually runs pure and silica from India can be fairly dirty.

Nerd

 

Seen your photos in the other thread a few weeks back> beautiful work.

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Joel:

Let me give you a head start or catch you up to my thinking- theory.. something.

When you look at the specs on elements: two temperatures are given in Kelvins:Melting temp and boiling temp.

Both of these temperatures (Kelvins) are relating phase changes. Melting temp is the change from solid to liquid, and boiling temp is the second phase change from liquid to gas.

 

                                   Melt Temp                          Boiling Temp

Potassium                     63.85C                                758.85C   [1398F]

Sodium                          97.85C                                882.85C  [1621F]

Lithium                         180.85C                              1341.85C

Calcium                        841.85C                              1483.85C

 

So at cone 6 potassium is rocking and rolling, and at cone 10 sodium is doing the same. Which goes back to your question: how much is lost in off gassing? You like calcium because it does not begin to work until you get to higher temps.  One reason you see a shift in clay recipes: using potassium at cone 6 and sodium in cone 10. Obviously mesh sizes play a role, and there are other factors as well. I having been charting PH levels for the last two years as a different method of determining flux levels in lieu of straight batch weights.It does work well with lithium fluxes, mildly so for calcium, potassium, and sodium. I will say it is one method for checking new material deliveries instead of testing. Example: my Nep Sy tests at 9.2, so every time I get a new shipment if it does not test to that level I automatically know something is up.

I am still working on a conversion table for ionization energies because it is measured in kilojoules. I think it is one of the most important pieces of information. Everyone looks at temperature in degrees Fahrenheit or Celsius, but that is misleading. Temperature is the measurement of molecular movement; the faster an atom moves the more heat is produced.

Nerd

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Hi Nerd,

Im not sure if I can get the G-200 here in Germany.

Our local supplier has only Soda Feldspar, calcium feldspar and some kind of combination of two....

do you think I could try it with one of those?

 

Hi Nat:

Going to make some changes: hard to get the COE lowered with that much Nep Sy. So I am going to swap some of it out for G-200; which has a lower COE value than Nep SY.  Nep Sy has a COE of 93.45 and G-200 has 83.40. This is close to the lowest COE this particular glaze recipe can achieve without changing it a great degree or adding other ingredients.

Nep Sy        30.0

G-200          14.5

Whiting        16.0

Silica            25.0

Zinc                4.0

NZ kaolin      10.0

Try 0.45 copper carb, do a smaller test with 0.40 just to check color.  The COE has been lowered an additional 0.40. Calcium, sodium, zinc, silica, and alumina are all approaching the upper end of formula limits. I think ultimately you are going to have to give thought to adding one ingredient to your dry slip in order to strengthen it.

Nerd

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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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This recipe also lowers the COE an additional 0.40 points.

Nep Sy        42.0

Whiting        16.0

Zinc                4.0

silica             27.5

NZ kaolin       10.0

Copper Carb   0.45%

 

You can also use the same recipe and do a test tile with 0.40 copper carb. to check color.

 

Nerd

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I am sure your supplier will have a potash feldspar or be easily able to find it. Will the 10% kaolin not be enough for application onto bisque?

 

I am missing a step in the jump from mixtures of molecules to elements and their ionisation energies. We are not working with K but K2O or K2O.Al2O3.... and so on. If I did melt and boil a feldspar is it giving me elemental gas or molecular gas, if that question makes sense. We seem to have mixtures that work together to find the lowest melting point and nowhere near the boiling point. I understand maybe a tiny fraction may get enough energy to break bonds with the mix but it seems strange that I could lose a lot. Isn't my alumina and silica holding onto everything. That is why potash melts at 700 but feldspar 1200.

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All this seems to be drifting well away from the interests of the original poster, and might be better placed in a separate thread.

IMHO it's very speculative silicate-melt theory. 

 

>So all you have to do is look at the first ionization energy for each element to determine how quickly it will lose its first orbital atom.

[i assume that is a typo for 'how easily an atom loses its first electron'.]

 

Absolutely true, but to take an example: the sodium atoms in the nepheline syenite became sodium ions before they

were incorporated into the nepheline syenite crystal (after all it is an ionic crystal). That's probably millions of years ago.

 

>We call it the melt temp in glaze, but in reality it is when it loses it first atom (donor/ reductant/oxidation).

[i assume that is a typo for 'when the atom loses its first electron'.]

 

Err, I don't think so. The sodium in the nepheline syenite starts out as Na+ and stays like that through any melting process.

I would suggest that most atoms in a glaze don't change their level of ionisation from that in the raw ingredients.

[There are obviously a few exceptions, mostly involving transition metals.]

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Sorry, not going to apologize for drifting away. I have been faithful to respond when Nat poses questions.

Many typos, reason I try not to think out loud. Everyone has areas of interest and curiosity: this happens to be mine.

 

Joel: you have a liken for calcium, mine is NZ kaolin:

NZ Kaolin    50.40    35.50    0.25      0.05    0.00    0.00    0.00    0.00    13.80   
                   SiO2     AIO3     Fe2O3  TiO2    CaO    MgO   K2O    Na20    LOI

 

Great source of silica and alumina, without all of the pollution normally found in clay.

Nerd
 
 

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