Lowering Coe?

[Guest]

A couple of recipes I'm looking at have Nepheline Syenite north of 40%. I'm guessing that in order to lower the COE I need to reduce this.

 

I've thought about Frit 3195 or a Feldspar but obviously they are not a perfect match.

 

Any suggestions would be helpful.

 

Thanks

[Min]

I would start with replacing some of the sodium and potassium from the nepsy with lithia and magnesia. Spodumene is a lithia feldspar and what I usually use to supply it, I would swap out some of the nepsy for that. If you need magnesia plus boron then frit 3249 is a good choice. If you don’t need the boron then dolomite is a good choice. If you don’t need the extra calcium the dolomite supplies then you could use talc. I’ld suggest working on the fluxes first with your calc program then balance the alumina and silica. Keep the silica to alumina ratio close to the original glaze to maintain the same gloss to the glaze. Adding silica and alumina (for silica and clay) helps reduce coe also, if there is room for it in the formula.

 

Some glazes are tricky trying to reduce coe and maintain the look / qualities of the original glaze. Do you know what coe fits your clay? Having a target to aim for makes testing more relevant. Fluxes don't always work in a linear fashion either. What throws a screw (or wrench and a spanner too) in the works is sometimes a higher coe glaze will fit your body but a glaze with a lower coe but made with different fluxes won’t.  This theory does’t have much written about it, a brief explanation is about 1/3 way down this page under the Using Calculation to Fit a Glaze section. (one last thing, coe figures don't work with matte glazes)

 

If you can find a similar glaze that doesn't use upwards of 40% nepsy it would make your job easier. 

[Guest]

Most excellent reply, thank you.

 

 

What throws a screw (or wrench and a spanner too) in the works is sometimes a higher coe glaze will fit your body but a glaze with a lower coe but made with different fluxes won’t. 

 

I was afraid of that. (I like saying a monkey wrench)

 

 

I would start with replacing some of the sodium and potassium from the nepsy with lithia and magnesia. Spodumene is a lithia feldspar and what I usually use to supply it, I would swap out some of the nepsy for that. If you need magnesia plus boron then frit 3249 is a good choice. If you don’t need the boron then dolomite is a good choice. If you don’t need the extra calcium the dolomite supplies then you could use talc. I’ld suggest working on the fluxes first with your calc program then balance the alumina and silica. Keep the silica to alumina ratio close to the original glaze to maintain the same gloss to the glaze. Adding silica and alumina (for silica and clay) helps reduce coe also, if there is room for it in the formula.

 

I guess its not a simple swap. It looks like I need to look at the analysis and try to get close as possible to what is there to begin.

 

 

Do you know what coe fits your clay?

 

Standard doesn't have this info on their website so I'm still in the guessing stage.

 

 

If you can find a similar glaze that doesn't use upwards of 40% nepsy it would make your job easier. 

 

This is what I'm looking for,

Coyote Cedar

 

Not my pot

 

Recipe that I'm hoping is close

 

Selsor Red - John Britt pg103 - COE 7.96
Nepheline Syenite 56.3
Silica 20.8
Whiting 10.4
Gerstley Borate 12.5
Bentonite 2
Tin Oxide 1.5
Copper Carbonate 0.8

 

[dhPotter]

Ron wouldn't the Selsor Red be a reduction Copper Red?

 

Email Standard and ask them for the characteristics of the clay in question. 

[Magnolia Mud Research]

Ron
 
I suggest you have a close look at the expansion pages on the glassproperties.com website:
http://glassproperties.com/expansion/
 
It will give you some insight into how various components influence the expansion.  The rest of the website is also informative.  The only difference between glass and a glaze is the spelling.

 
LT

[Marcia Selsor]

Yes it is a reduction glaze. Mostly used on ^6 Porcelain. No crazing on the porcelain we were using. Bray porcelain.

This has been adjusted by John

 

 

Selsor Red - John Britt pg103 - COE 7.96
Nepheline Syenite 56.3
Silica 20.8
Whiting 10.4
Gerstley Borate 12.5
Bentonite 2
Tin Oxide 1.5
Copper Carbonate 0.8

 

 Here is the one that Michael Bailey used in his Oriental Glazes book in 2004. The mug is ^6 porcelain  with this glaze fired in Reduction to solid ^6.

Selsor Copr7  ^6-7     The higher silica and slightly lower Neph Sye helps against crazing. I'd say up your silica a percentage point or 2.

 

Neph Sye             55.3

Silica                    21.4

Gerstley Borate    12.6

Whiting                 10.7

                            100.0

Tin Ox                      1.5

Copper Carb             .3

Red iron                    .3

[Min]

"I was afraid of that. (I like saying a monkey wrench)"

yeah, but I tend to screw things up    

 

"It looks like I need to look at the analysis and try to get close as possible to what is there to begin."

Noooooo, silica and alumina levels you would try and keep close, fluxes need to change. Sodium and potassium levels need to come down and magnesium and lithium levels need to go up. (using those ingredients I mentioned)

 

Coyote Cedar used cadmium red stain to get the red. 

 

If you have a base glaze that fits your clay I think you could get the effect of the pot you pictured using dabs of an iron wash applied with a coarse sponge underneath  a base glaze coloured with Cd stain. (Usual caveats about the safety of using Cd stains)

 

There are some tutorials online on how to adjust glazes with Insight have you come across those?

[Marcia Selsor]

When I had crazing on Woo yellow on my Frost, I upped the silica 3% and added 2-% kaolin. 

That worked for me. Always test, especially for a specific clay body. 

My training emphasized the ratio between silica RO2  and fluxes RO for melting temperature and the ratio for surface was between silica RO2 and clay R2O3

Marcia

[Marcia Selsor]

Ron Sa 

Are you having crazing problems?

Or are you reacting to the COE?

Marcia

[Guest]

Or are you reacting to the COE?

 

Yes, I guess I am

 

 

There are some tutorials online on how to adjust glazes with Insight have you come across those?

 

I'll have to take a good look through these, thanks.

 

 

"It looks like I need to look at the analysis and try to get close as possible to what is there to begin."

Noooooo, silica and alumina levels you would try and keep close, fluxes need to change. Sodium and potassium levels need to come down and magnesium and lithium levels need to go up. (using those ingredients I mentioned)

 

Ok, Got it.

 

Ron wouldn't the Selsor Red be a reduction Copper Red?

 

Sigh, just when I thought I was on the right track. I see John Britt adds 0.8% Silicon Carbide to to the glaze for oxidation.

 

 

 Here is the one that Michael Bailey used in his Oriental Glazes book in 2004

Selsor Copr7  ^6-7     The higher silica and slightly lower Neph Sye helps against crazing. I'd say up your silica a percentage point or 2.

 

Neph Sye             55.3

Silica                    21.4

Gerstley Borate    12.6

Whiting                 10.7

                            100.0

Tin Ox                      1.5

Copper Carb             .3

Red iron                    .3

 

I'll give this a try, and I see this is your recipe in Bailey's book. Cool

 

I think I'm going to have to do something easy for the rest of the day, like make pasta for tonight's dinner.

 

Thanks everyone for your replies.

[High Bridge Pottery]

A dash of lithium feldspar goes a long way.

[glazenerd]

Ron:

My clay is 6.12 COE, with 10.5 fired shrinkage. Sending it up to Ron next month to have the COE formally tested.

[Marcia Selsor]

 

Or are you reacting to the COE?

 

Yes, I guess I am

 

 

There are some tutorials online on how to adjust glazes with Insight have you come across those?

 

I'll have to take a good look through these, thanks.

 

 

"It looks like I need to look at the analysis and try to get close as possible to what is there to begin."

Noooooo, silica and alumina levels you would try and keep close, fluxes need to change. Sodium and potassium levels need to come down and magnesium and lithium levels need to go up. (using those ingredients I mentioned)

 

Ok, Got it.

 

Ron wouldn't the Selsor Red be a reduction Copper Red?

 

Sigh, just when I thought I was on the right track. I see John Britt adds 0.8% Silicon Carbide to to the glaze for oxidation.

 

 

 Here is the one that Michael Bailey used in his Oriental Glazes book in 2004

Selsor Copr7  ^6-7     The higher silica and slightly lower Neph Sye helps against crazing. I'd say up your silica a percentage point or 2.

 

Neph Sye             55.3

Silica                    21.4

Gerstley Borate    12.6

Whiting                 10.7

                            100.0

Tin Ox                      1.5

Copper Carb             .3

Red iron                    .3

 

I'll give this a try, and I see this is your recipe in Bailey's book. Cool

 

I think I'm going to have to do something easy for the rest of the day, like make pasta for tonight's dinner.

 

Thanks everyone for your replies.

 

it's my recipe in the John Britt book too but John tweaked with it. I sent several to him and he adjusted them as he saw fit. he's the author uniformly went through each glaze and tested them. He made an Oribe out of this one too.! He's a wizard!

[Marcia Selsor]

Ron Sa,

I think you should  first do some test firings to see IF there is a crazing problem. THEN try to fix it. 

Marcia

[Min]

 

What the MC6G book plus Britt’s book have are a series of clear glazes with coe’s ranging from low to high. If you have a ballpark idea of what fits your clay I think it narrows down the field of glazes that are likely to fit without crazing (if that’s your goal) or shivering . Granted, different fluxes are going to give different results but if you are going to compare say a high sodium and potassium glaze like ones high in nepsy to another one which is also high in the same fluxes you will know beforehand if it’s likely going to need some tinkering with.

[Guest]

If you have a base glaze that fits your clay I think you could get the effect of the pot you pictured using dabs of an iron wash applied with a coarse sponge underneath  a base glaze coloured with Cd stain. (Usual caveats about the safety of using Cd stains)

 

When I use a clay that has granular manganese such as Standard 112 the fired glaze looks like this.

 

it's my recipe in the John Britt book too but John tweaked with it. I sent several to him and he adjusted them as he saw fit. he's the author uniformly went through each glaze and tested them. He made an Oribe out of this one too.! He's a wizard!

 

I should have figured that one out by the name.

[JohnnyK]

Please enlighten me. I love to see these somewhat technical dissertaions BUT...

What is COE?

[Fred Sweet]

JohnnyK-

Coefficient of thermal expansion. The reversible expansion of a material as one heats it. Not to be confused with shrinkage, which is a "one time" event.

[Guest]

Johnny

 

I'm still working my way through this article, seems very informative 

[Guest]

Standard clay got back to me with their COE

 

112-5.33
211-5.56
225-5.34
730-this clay is a special I do not have COE
630-5.5

 

So what happens when a glaze has too low a COE?

[Joseph Fireborn]

My understanding is that you want to get around the same COE to reduce crazing. If its to far off it will shiver or dunt if I remember correctly. It seems to be pretty ballpark to me and my testing. If your within a 1-2 of the COE I don't notice any crazing, if your starting to get 3-4 off the COE you start getting crazing. At least this has been my limited experience, I only worry about my inside glazes nowadays.

 

Min what have you noticed as far as this goes? 

 

Edit: I should say that I don't really worry about crazing. I drink out of a lot of cups that I have bought from other potters that have crazing on the inside and I still seem alive. But we already had this discussion last week. COE is important to understand and use if you want to have wares that don't degrade as badly over time. 

[Min]

Standard clay got back to me with their COE

 

112-5.33

211-5.56

225-5.34

730-this clay is a special I do not have COE

630-5.5

 

So what happens when a glaze has too low a COE?

 

Shivering. Far less common than crazing. Slivers of glass (glaze) coming of pots, usually on rims and edges. Also, if you have a low coe glaze on one side of a pot and a high on the other you can get dunting. Pot literally tears inself in half.

A little more light summer reading. https://digitalfire.com/4sight/troubleshooting/ceramic_troubleshooting_glaze_shivering.html

 

oops, sorry Joseph was writing this as you posted. I use clay from Plainsman (370 mostly), they don't test for coe  All my non crazing glazes are under 6.12 for the coe. My clear is 5.82 and I've got pots I've been using for years with this glaze and they haven't crazed. I stress test the heck out of my glazes, anytime I've gone over 6.2 I get crazing. (This is for bases only, not with colourants or opacifiers added.)

[glazenerd]

JohnnyK:

 

This is one of my favorite chemistry facts; because we have been taught the effect, not the cause.

 

" temperature is the measurement of how fast molecules are vibrating ( moving)."

 

Nerd

[Marcia Selsor]

recently fired a gas kiln with a friend. Here are the copper red results June 2020. I used my original Selsor Copper Red. The deeper red on a few pieces were a little hotter with cone 6  almost flat.