Eggshell Glaze Cone 6

[Babs]

this glaze, one of the recommended ones I have trouble with

.Eggshell Glaze
(Cone 6, oxidation)
Whiting. 9.5%
Zinc Oxide. 5.5
Ferro Frit 3124. 44.5
Custer Feldspar . 20.0
Bentonite. 7.5
EPK Kaolin. 5.0
Silica . 8.0
100.0 %
Add: Tin Oxide. 9.0 %
Red Iron Oxide. 3.0 %
From Central Carolina Community College,
Ceramics Monthly, October 2004.

It flakes off the bisqued pots prior to firing.

I have wiped the pots with damp sponge as I always do,  I have even bisqued higher than normal. It is an attractive glaze when the glaze actually stays on the pot even when cracked and fragile.

i dip the pots.

I get the deep toasty colour where thin and the whiter colour where thicker.

.

It has a high bentonite content.

Yes test, but the brilliant chemists out there, will dropping the Bentonite content help, and would one replace that with more EPK?

I've used MGSO4, glaze mix is thin but not wanting to add more water what is your advice?

[Marcia Selsor]

what is your firing schedule? It is very shiny for Eggshell. The recipes have similar chemistry but the amounts are very different for the Frit 3124  44.5 compared to  6, Custer spar 

Maybe you should run it though a Glaze calc program . At a glaze it seems high in fluxes. The Bentonite may be helping to stabilize it. Can't really say why it is flaking and too shiny. What clay are you using?

 

I  have used this eggshell recipe

Egg shell from Michael Bailey

 

 

Custer feldspar  46 

FF 3124              6 

EPK                  16 

Dolomite           15.5 

Whiting               9 

Zinc Ox.              0 .5 

Quartz ( silica)    7

[Babs]

Firing schedule

In centigrade

50/hr:200

100/hr:600

150/hr:1080

80/hr:1190

Hold 15mins

Off

C 6 is well down.

Clay. Aussie stuff..

Bisque usually 1000 15min. Hold

But have taken to c05

The recipe I used came from ca c6 recipe booklet

[Min]

Hi Babs, 

Can you get 3134 where you live, can't remember if you have it or not.  I've redone the recipe to get more clay into it, should behave a bit better. Also, has your zinc gone clumpy? If it has then re-calcine that also. If you don't have 3134 then you could try adding some CMC solution to a tiny test amount of the glaze, would avoid that if possible as adding it will slow down the drying time and make more drips.

 

The silica:alumina ratio is in the area that this could be less glossy. If you want less gloss try firing down to slow the cooling of the kiln. 

 

Whiting 9.90
Zinc Oxide 5.60
Custer Feldspar 24.70
EP Kaolin 18.50
Silica 14.20
Ferro Frit 3134 27.20
100.10

RIO   3

Tin    9

[Babs]

Thanks Min and Marcia.

I'll test your sub. glazes and let you know.

The shininess is exaggerated here but I don't mind the amount I actually get.

I've got a bucket of this other mixed

Perhaps I'll add more bentonite to it.

[glazenerd]

Babs:

Looking at the recipe: I do not see anything that would be causing that kind of flaking. The one thing that does come to mind is the bentonite content @ 7.5%. Bentonite absorbs three times its weight in water: and if it is a sodium based bentonite (most is) it will wick moisture. This would be a rare circumstance, but not out of the realm of possible: perhaps drop bentonite down to 3% and replace the remaining 4.5% with EPK.

 

OR

 

You might have a hot batch: hot meaning much higher PH level in the feldspar, silica, frit, or whiting. High PH levels cause the mix to become hydrophobic (sheds water quickly). Most glazes run in the 8.2 to 8.8 PH levels (fluxes are alkali), so if you have a hot batch it could run over 9: which is approaching a hydrophobic level. Ever notice if you handle dry feldspar or allow glaze to dry on your skin: you skin dries out?... that is hydrophobic....alkali dries out skin: reason salt is used to cure meats. Whiting is 8.4, feldspars 8.8 to 9.2PH, lithium carb 11.0% and pearl ash 13.45PH.

 

blah..blah..& blah.. anyway.. Mix a test batch as shown, but use 50/50 water and vinegar as the fluid. Yes it will bubble a bit, but fret not that is only hydrogen....Lower PH means less/no hydrophobic properties.

 

Nerd

[neilestrick]

Pretty sure it's the bentonite. It's just shrinking way too much. I've never seen a recipe with that much. Drop it to 2% and put the rest into the kaolin. You'll probably need some epson salts to keep it suspended well.

[Chilly]

When I had a commercial glaze flaking off before firing, the company sent me a bottle of "stuff".  Away from home at the mo, but I think it was a form of CMC.  Added a tiny percent to glaze and no more flaking.

[Babs]

Thanks all.

I'll add subbing the bentonite with epk to my testing.

Today I added more Epsom s to glaze bucket, flaking

Then I watered the glaze and added more Epsom. Better but await the firing

Will make up tests tomorrow.

I'm a bit flummoxed why this glaze was included as a C M cone 6 glaze without a bit of a hint of the prob.

Never seen a glaze with that amount of bentonite either...

[oldlady]

maybe a typo that nobody caught?

[Babs]

Yes but they did the mths to come up with 100%....

[glazenerd]

Babs:

 

A thought occurred to me about this glaze and your pic above.

Bentonite can have 2-4% magnesium and 3-6% iron: and given the colors in your pic, this recipe is using bentonite as a colorant in part? I cannot see the pic well enough to tell if it has a very light grey cast to it: but I am curious about it. If my hunch is correct: the cutting bentonite back to 3%; the addition should be dolomite or talc as a replacement for the 4.5%

 

Nerd

[Babs]

Thanks Nerd, no grey colouring happening, The bentonite I use is the grey stuff.

I'll have time to test later this week I hope or early next week.

Just wonder if anyone else used the recipe. If was a Ceramic Art daily inclusion in a cone 6 recipe booklet

[neilestrick]

Babs:

 

A thought occurred to me about this glaze and your pic above.

Bentonite can have 2-4% magnesium and 3-6% iron: and given the colors in your pic, this recipe is using bentonite as a colorant in part? I cannot see the pic well enough to tell if it has a very light grey cast to it: but I am curious about it. If my hunch is correct: the cutting bentonite back to 3%; the addition should be dolomite or talc as a replacement for the 4.5%

 

Nerd

 

If the bentonite has 2-4% magnesium, that's just 2-4% of the 7.5%, which is a negligible amount. Why add 4.5% talc to make up for it? I think you're over thinking it.

[glazenerd]

Neil:

Only reasonable/possible explanation I could come up with, for explaining the unreasonable amount of bentonite. Not over thinking anything, just trying to figure out why someone felt it necessary to throw that much bentonite in a recipe.

Nerd

[Babs]

Wonder if it was a typo and it was meant to be Ball clayðŸ˜

How would that amount of BC sit with the EPK

[Babs]

Ok it's listed in a publication of,Ceramics Arts Daily 33 tried and true Glazes and top,10 cone 6 glazes on Ceramics AD freebies.

Seems to be stayed on pot in pic. Do I guess it must be something I'm doing...

[neilestrick]

Neil:

Only reasonable/possible explanation I could come up with, for explaining the unreasonable amount of bentonite. Not over thinking anything, just trying to figure out why someone felt it necessary to throw that much bentonite in a recipe.

Nerd

 

I get that, but why add 4.5% talc to make up for 0.002% magnesium in the bentonite?

[glazenerd]

Neil:

 

I asked Babs if there was a grey cast to the glaze: and no there is not. Low levels of magnesium, combined with low levels of iron can produce a greyish color. Some sort of chemical interaction that I have yet to comprehend: but have seen many times. So I suggested talc to replace/add to the magnesium level to replicate any possible chemical reaction. Again, it has become a non-issue. Was trying to wrap my Nerd brain around a 7.5% bentonite addition: apparently there is nothing to unwrap. So my final conclusion about this is: " hey, I have 20lbs of bentonite I have to get rid of."

 

Nerd

[Min]

Was trying to wrap my Nerd brain around a 7.5% bentonite addition: apparently there is nothing to unwrap. So my final conclusion about this is: " hey, I have 20lbs of bentonite I have to get rid of."

 

Nerd

 

My hypothesis is the creator of that glaze had a boatload of 3124, used that to supply most of the alumina instead of the epk. Needed 7.5 bentonite to keep it from sinking like a stone in the bucket. Using 3134 instead the epk is high enough (@18.5) to probably make the bentonite totally redundant.