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Low Expansion Cone 10 Clear Glazes


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Hi all,

I'm working on some porcelain pieces with Clay Planet's glacia and pier porcelains. However, I'm having a lot of trouble with finding a glaze to fit.

So far I have tested a few starting recipes, all around 6.5 - 6.8 calculated COE. I was most happy with this one:

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Tony Hansen's ^10 Clear (G1947U)

27.0 Custer Feldspar

23.5 Wollastonite

20.5 EPK

26.5 Silica

2.5 Zinc Oxide

Calculated COE: 6.5

However, all the tests crazed. Predictably, the glazes closer to 6.7 COE were a bit worse than the 6.5 glaze.

From there, I tried reducing KNaO and CaO and increasing zinc, silica, and alumina:

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21.0 Custer Feldspar

17.5 Wollastonite

24.5 EPK

33.5 Silica

3.5 Zinc Oxide

Calculated COE: 6.0

However, this also crazed badly on the Pier and moderately on the Glacia, and is slightly undermelted (slightly matte, didn't flow well),  so it feels like a bit of a dead end.

I was hoping for some help with figuring out what to try next. I'm thinking of starting with a boron-fluxed glaze to get some more room to play with melting, something like:

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Tony Hansen's Matte Base Glaze (G2571A)

28.5 Custer Feldspar

5.5 Wollastonite

28.0 EPK

15.0 Silica

19.0 Dolomite

4.0 Gerstley Borate

I would drop much of the dolomite to reduce MgO matteness, and increase the boron (probably with a frit replacing the gerstley) to correct the melting from removing dolomite, and increase silica and alumina to reduce expansion.

Is that a good idea? Is it possible to just improve the one I already have?

If anyone has some low COE (5.0 - 6.0 range ideally) glaze recipes they're willing to share I would be grateful as well.

Edited by LeeS
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@LeeS,I’m just going to pile on a bunch of info that I think is pertinent to your questions. Simplified Periodic Table, I’m including this to show why I suggested lithia (using spodume) and why m

Stop crazing: Traditional way for gloss glaze is to increase Silica by 1.25 :1 clay so a progression of  1.25 sio: 1 clay 2.5 sio: 2 clay 3.75 sio: 3 clay and so on until crazing d

@Lee S,to lower the expansion of the G1947U glaze I would be looking at the high expansion fluxes, ie the sodium and potassium that is being supplied by the custer spar. I would change some of the cus

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Stop crazing: Traditional way for gloss glaze is to increase Silica by 1.25 :1 clay so a progression of 

1.25 sio: 1 clay

2.5 sio: 2 clay

3.75 sio: 3 clay

and so on until crazing disappears.

 

Don’t really like any of those glaze formulas actually all have R2O:RO in non durable range (R20 less than 2.0 can create issues). I don’t like to be less than 0.2:0.8 and gerstley not needed at cone 10. I am not a fan of calculated COE as well as the final melt will usually not be the calculated.

something you can try that has decent chemistry:

neph sy 28.99

silica 23.19

EPK 21.74

wollastonite 26.09

Flux ratio 0.23:0.77

Alumina = 0.51 

Good flux, wollastonite, should fire stiff and not run and can be tuned as above until crazing stops if necessary.

I wish you were looking for cone 6, have used a matte and high gloss on all my low expansion porcelain for quite some time so I have those perfected.

4,3,2,1 old standby pasted below from Glazy as well, again use crazing adjustment above as necessary

 

Edited by Bill Kielb
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@Lee S,to lower the expansion of the G1947U glaze I would be looking at the high expansion fluxes, ie the sodium and potassium that is being supplied by the custer spar. I would change some of the custer for spodumene to supply the low expansion flux lithia. There is room for more of both silica and alumina in the formula and will likely still get a good melt.

Original recipe on the left, my altered version in the center, spodumene replacing some of the custer and a slight bump in the silica and alumina, then re-totaled to 100. (just for fun I entered Bill's recipe from above also). If this still crazes then I would increase the spodumene, decrease the custer further and increase both silica and alumina. Do you use glaze calc? 

329304060_ScreenShot2019-06-26at12_08_34AM.png.d1a1f46c19714b1d541e16df2161576b.png

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A little lithium has always gone a long way to stoping crazing for me. I would try the spodumene addition. I would reduce the silica and alumina, but that's just me. 

 

@Bill Kielb What is the R2O/RO looking at? Is it just the alkali metal oxides ratio to alkaline earth metal oxides? 

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Thanks everyone for the help.

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Don’t really like any of those glaze formulas actually all have R2O:RO in non durable range (R20 less than 2.0 can create issues). I don’t like to be less than 0.2:0.8 and gerstley not needed at cone 10.

I see your point about the flux ratios, but I'm having a bit of trouble understanding how to get good flux ratios but low expansion. Low expansion seems to mean dropping as much of the KNaO as possible, but that also drops the flux ratio out of the 0.2:0.8 range if taken very far. Am I missing something here?

I know boron isn't needed at ^10, but cost isn't really important here so I was thinking a boron frit could provide a bit more flexibility for adjusting melting in my base recipe. One other thing I noticed is a lot of ^10 potters are firing in big gas kilns with long cycles and soaks, so lots of "^10" glazes don't seem to melt particularly well until the bottom of ^11.

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I am not a fan of calculated COE as well as the final melt will usually not be the calculated.

That's fair, but unfortunately can't fire that often, so I think it's a valuable starting point. I use it mostly as a method to guide my testing, not the end-all.

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Original recipe on the left, my altered version in the center, spodumene replacing some of the custer and a slight bump in the silica and alumina, then re-totaled to 100. (just for fun I entered Bill's recipe from above also). If this still crazes then I would increase the spodumene, decrease the custer further and increase both silica and alumina.

Thanks Min! Do you change KNaO for Li2O on a 1:1 molar basis? Or just aim for something close and adjust with testing? Would you prefer Li2O over boron?

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Do you use glaze calc? 

I use Insight. Your pictures are familiar haha :).

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19 minutes ago, LeeS said:

see your point about the flux ratios, but I'm having a bit of trouble understanding how to get good flux ratios but low expansion. Low expansion seems to mean dropping as much of the KNaO as possible, but that also drops the flux ratio out of the 0.2:0.8 range if taken very far. Am I missing something here?

Yep, lots of successful low expansion glazes that use sodium, calcium, and potassium. Interesting dilemma so my suggestion is find something that works with a durable flux ratio and adjust out the crazing. Pretty easy to do a clay silica progression in one test fire. If the glaze ends up in slight tension it strengthens your finished product (we know this from testing)  so adjusting till it’s gone is an easy way to sneak up on this point.

Not too difficult to take 100 or 200 grams of glaze and progress it through 5 -10 variety’s with simple Clay and silica all in one firing. If it works, super simple easy method. Now with respect to eliminating crazing in a true matte, a bit more difficult.

Seems you  have done a bit of testing  and firing already and have glazes with marginal R2O to begin with and still craze.

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I like using a boron frit at cone10. Much better than using zinc as a flux. 

Would you be willing to share an example recipe? No worries if you would prefer not to of course.

I'm struggling with a starting point because most people say you don't need boron at ^10, so there isn't too much info about it.

So far I think I'm going to give all three suggestions a shot. Adding alumina + silica to a basic ^10 recipe, adding Li2O in varying amounts, and using a boron frit. It's easy for me to make up a ton of tests, not easy to fire a bunch of times.

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The last thing I was working on is a bubble free transparent. It's a slightly odd recipe as I was trying to remove any silica or clay. I was also removing zinc because I hear it is bad with some colours, and I don't know how well it reacts in reduction firings. Fired to cone10 oxidation on the tile.

 

It is true that you don't need boron at cone10 but I see no negative reasons not to include it. I think your observation that a lot of the recipes are probably more a cone11 is a good one. 
gallery_23281_1027_143994.png.2db15a6e9a28dbe890137fa5ba53bc48.png

Edited by High Bridge Pottery
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2 hours ago, LeeS said:

So far I think I'm going to give all three suggestions a shot. Adding alumina + silica to a basic ^10 recipe, adding Li2O in varying amounts, and using a boron frit. It's easy for me to make up a ton of tests, not easy to fire a bunch of times.

Sounds like a good plan. I would also try a magnesium fluxed clear, this one from Matt Katz could be a starting place. Play around with the recipes in Insight to get the COE down to the low 5's while keeping the silica:alumina ratio the same (and keeping an eye on the limits so you don't have a crazy high amount of something). Run a progression line blend of the lowest calculated COE glaze and the same glaze with the highest COE you are interested in testing, somewhere along the line there will be one that should fit.  COE figures are the most useful when looking at a glaze then without adding or deleting one of the materials just changing the proportions of the materials.  

If you do want to try a boron frit and have access to Ferro 3249 that would be a good choice. It a low expansion frit that supplies boron and also magnesium (along with alumina, silica and a bit of calcium). Try using Insight and adding around 5% of it to any of the recipes above then rebalance them and see what it does to the COE figures.

Just have to ask, why ^10? Is that the only cone you have access to fire to in a shared firing?

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The last thing I was working on is a bubble free transparent. It's a slightly odd recipe as I was trying to remove any silica or clay. I was also removing zinc because I hear it is bad with some colours, and I don't know how well it reacts in reduction firings. Fired to cone10 oxidation on the tile.

Very cool, thanks!

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I think your observation that a lot of the recipes are probably more a cone11 is a good one. 

Haha yeah, glad I'm not crazy.

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Sounds like a good plan. I would also try a magnesium fluxed clear, this one from Matt Katz could be a starting place.

Ah great find. Somehow missed that in my glazy digging. Without modifications it calculates out to 6.2 COE, so it looks like a good start.

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 Run a progression line blend of the lowest calculated COE glaze and the same glaze with the highest COE you are interested in testing, somewhere along the line there will be one that should fit.  COE figures are the most useful when looking at a glaze then without adding or deleting one of the materials just changing the proportions of the materials.

Good advice. I'll try this with each of my 'flux' attempts.

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If you do want to try a boron frit and have access to Ferro 3249 that would be a good choice. It a low expansion frit that supplies boron and also magnesium (along with alumina, silica and a bit of calcium).

I don't have access to 3249, or any other MgO frits (I'm sure I could get it, but the local supplier doesn't stock it). I think I might formulate something along those lines with 3124 and talc though. If this all works out I might order a magnesium or lithium frit to experiment with.

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 Just have to ask, why ^10? Is that the only cone you have access to fire to in a shared firing?

Haha, I was afraid someone would ask. There isn't a good answer, and I'll probably switch to ^6 at some point. But for now the firing cost isn't significant, and my tiny kiln has no problem reaching ^10 with power to spare.

I do have a few reasons why I started there, the main one being I learned at ^10 because that's what the local community centre fires to, so it seemed like a comfortable starting point. Also, my work is slipcast translucent porcelain, and my local supplier has the most selection of ^10 porcelain clays. Last, I wanted to formulate my own glazes, and more raw materials melt at ^10 so it was a bit easier to learn the chemistry.

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On 6/26/2019 at 11:03 PM, LeeS said:

Haha, I was afraid someone would ask. There isn't a good answer, and I'll probably switch to ^6 at some point. But for now the firing cost isn't significant, and my tiny kiln has no problem reaching ^10 with power to spare.

I do have a few reasons why I started there, the main one being I learned at ^10 because that's what the local community centre fires to, so it seemed like a comfortable starting point. Also, my work is slipcast translucent porcelain, and my local supplier has the most selection of ^10 porcelain clays. Last, I wanted to formulate my own glazes, and more raw materials melt at ^10 so it was a bit easier to learn the chemistry.

Yep, the earth is cone ten.  Most all glazes started out as cone 10  actually, and methods such as Boron or Bristol or low amounts of silica and alumina were developed to lower their melting point. After all, silica and alumina (glass formers)  do not melt below 3000 degrees without flux.

Just a thought as you work with glazes it’s enticing  to seek the exotic answers but often easy answers are just that ............easy. It’s easy with clay to be taken down the rabbit hole as well.

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29 minutes ago, Bill Kielb said:

Just a thought as you work with glazes it’s enticing  to seek the exotic answers but often easy answers are just that ............easy.

If the crazing is slight then yes I agree adding some alumina and silica in the form of clay and silica is very easy and will likely fix the issue. I don't find it's the best solution for glazes with moderate to heavy crazing though. To quote Tony Hansen it's a bit like adding white paint to dilute the colour black. I realize you don't like COE figures but I find them useful for gloss glazes. An example of what I'm talking about in the screenshot below. Granted to the COE might not need to go as low as the random 5.8 that I used in my glaze at the top of this thread but I'm just using that as an example. In order to bring the glaze recipe you posted earlier down to the same COE of 5.8 and maintain the silica:alumina ratio you need to increase the epk and silica both by over double their original amounts. That will have an effect on glaze melt. It's not exotic it's just a case of swapping out high COE fluxes for ones with a lower COE value.

771590699_ScreenShot2019-06-28at3_43_26PM.png.22185dcca562cb81e223ce71c88078f8.png

 

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@Min

Seems like  pretty complicated substitutions to me . All three glazes started out sporting less than desirable Flux ratios so I would not be married to any of them.  That would bug me first and foremost. I am not sure why anyone would pick these to start? Maybe they had good calculated COE numbers.

Lithium, magnesium, spodumene, Boron all fun to play with. I think the Katz magnesium was probably the easiest thing I read here to start with.

My point is - and I have seen this in teaching - there is a tendency to gravitate to  what appears to be the most the most complicated solution  which often creates a lot of work and often wastes lots of natural resources. I am not a fan of the calculated COE because it does not reflect the COE of the finished product. It is not something I would base my initial design on.

Just my view though, I have  a ton of recipes with bad RO, too much Boron, all sort of chems added including the kitchen sink and of course bentonite, that work but basically waste natural resources. To each their own, my caution was to try and avoid going down the rabbit hole. It’s easy to do with clay.

I have two glazes attached that work extremely well over my lowest expansion porcelain for about two years now. Both have too much sodium in them to work. Should I scrap them and conjure up something more exotic?

 

 

Edited by Bill Kielb
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@Bill Kielb, by adding silica and alumina you are of course lowering the COE. Whether you calculate it or not this is what's happening. Do it this way and / or with fluxes.

Like I said before, if just adding silica and alumina works to eliminate mild crazing then great, if not then changing the fluxes is a very valid option for more significant crazing. I guess it all comes down to what our own definitions and comfort levels of what a complicated solution is.

I'm all for walking softly on the back of Mother Earth too.

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18 minutes ago, neilestrick said:

How are these too high in sodium?

They are not, in my view and they work extremely well on low expansion porcelain so that likely confirms they are just fine..  The OP picked  original glazes that were extremely low in sodium I believe,  and of course there are many suggestions to change to lithium. He is already making a lithium based glaze. ..... or something to that effect since sodium and potassium is a disaster of sorts or often thought to be and now calculated COE is sort of driving the design along with other low expansion oxides. Interesting in that the three original glazes were not redeeming with respect to durability as well.

 My point was don’t always look for the most interesting solution as in substituting lithium as this often ends up complicated. Lithium can change the durability, texture, color, and firing range. Lithium beyond about 0.2 is generally not great

The separate point of the two glazes above is they are proven successful yet many would consider the sodium a killer and change this to something lithium based. They work well so .17 - .18 sodium in this composition must be fine when fired.

Edited by Bill Kielb
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14 minutes ago, Bill Kielb said:

They are not, in my view and they work extremely well on low expansion porcelain so that likely confirms they are just fine..  The OP picked  original glazes that were extremely low in sodium I believe,  and of course there are many suggestions to change to lithium. He is already making a lithium based glaze. ..... or something to that effect since sodium and potassium is a disaster of sorts or often thought to be and now calculated COE is sort of driving the design along with other low expansion oxides. Interesting in that the three original glazes were not redeeming with respect to durability as well.

 My point was don’t always look for the most interesting solution as in substituting lithium as this often ends up complicated. Lithium can change the durability, texture, color, and firing range. Lithium beyond about 0.2 is generally not great

The separate point of the two glazes above is they are proven successful yet many would consider the sodium a killer and change this to something lithium based. They work well so .17 - .18 sodium in this composition must be fine when fired.

Now I see what you're saying. But I don't think anyone said that sodium should never be at .18. Just because it works in one recipe doesn't mean it works in all recipes. Sodium is hardly the only determining factor. Plus you're not using it on the same clay body as the OP. I also don't see why it's unreasonable to test a lower expansion flux. Adding 15% EPK and silica will also affect durability, texture, color and firing range.

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@neilestrick

 

In general, no. You will move up the gloss line and maintain roughly the same ratio As well as the flux ratio. So as fixes go, if that works it is pretty simple and least likely to change your look and durability for a gloss glaze. True mattes, different story entirely, much harder to fix.

Adding spodumene for neph sy is significantly different in composition..  All fixes can work, the silica and alumina is probably the simplest provided you have a glaze you like for a reason. If you are not attached to the composition then just try a lithium recipe and adjust it to fit.

 

Edited by Bill Kielb
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@Bill Kielb I disagree. You can only add so much EPK and silica before the melt changes, and the crazing isn't always fixed when you reach that point. I've been there. And yes, adding in a different flux can change things, but the OP was just looking for a clear and didn't like the original glaze anyway, so there's nothing to lose. I agree that if he can fix it with flint and EPK then that would be a simple solution, but that may or may not work. I think other methods for solving the problem are valid.

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