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Clay composition and crazing


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14 minutes ago, Hulk said:

I still don't buy that the slip is elastic enough to accommodate the body and glaze coe differences

This is a bit of a moot point, ideally the slip and claybody should be close in values. 

Newer version of that glaze of mine from your post above, slightly lower LOI, below. This one uses talc, no dolomite, original G1215U beside it for comparison. Do you have any soda spar, Kona F4 maybe, if not what feldspar(s) do you have? 

708780098_ScreenShot2019-08-10at10_23_12AM.png.2054b7436cbad62965260b518673c9fc.png

 

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Thanks Min!

I have neph sy and custer, also everything in the two recipes you posted except dolomite, minspar, FF3249 - the recipes I'm seeing that have lower coe include one or more of those three (and/or spodumene, special talc, other source(s) of Manganese, etc.)

...last firing, tried Tony Hansen's G2926B, which comes out 6.29 in GlazeMaster (his original recipe is 6.35 - I did tweak it down some), hence next attempt should be well below 6.0, given the crazing is regular and the squares are about 1/8" or less.

I like working with the bmix and Sedona red clays. Finding a clear that works with them is the goal.

I am still curious about any influence the clay body may have on the glaze - per an authority on the subject (who kindly sent me a detailed message on the subject), clay can alter glaze coe.

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2 hours ago, Hulk said:

I am still curious about any influence the clay body may have on the glaze - per an authority on the subject (who kindly sent me a detailed message on the subject), clay can alter glaze coe

Try mixing small amounts of the clay body into the glaze recipe and see what happens.   Use bone dry clay body and measure the amount added to the recipe.  

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13 hours ago, Hulk said:

(and/or spodumene, special talc, other source(s) of Manganese, etc.)

Umm, think you meant magnesium not manganese, talc doesn't need to be a special sort. 

13 hours ago, Hulk said:

given the crazing is regular and the squares are about 1/8" or less.

This is troublesome. That is quite fine crazing. Are you doing stress tests to see this crazing or is this right out the kiln crazing?

13 hours ago, Hulk said:

clay can alter glaze coe.

Could you expand on this? Interface layer on porcelain bodies, perhaps stoneware having an effect on crazing? Is this from Ron Roy? 

I think you have all the ingredients to do that altered G1215U body with the ingredients in the recipe below (version 4). Should be fine subbing your potash spar for the soda spar, I tweaked the recipe but really negligible differences to the version 3 above. 

957447460_ScreenShot2019-08-12at7_47_27AM.png.ced4ac9c8270c6fa9bf64bfd4ca0cbad.png

edit: this looks okay on paper but will need to be tried on test pots before production use.

Edited by Min
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A completely tangential question related to this comment. When I was working before....everyone bisqued to cone 010 and now it is hotter at 04. I noticed making these glaze samples that the bodies don't suck up the glaze as fast as they did when the bisque was 010 suggesting to me that it is more closed. I am wondering what effect that might have in terms of the glaze/body interface?  I remember looking at broken shards in the past and the interface was quite apparent and the merging of the body and glaze quite apparent. 

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Hi Min!

Magnesium oxide, aye

The crazing is right out the kiln, mostly. The clears I've tried seem fine on the buff and red clays.

Thank you very much for the recipe, will try it! ...I get reducing Potassium and Sodium, increase MgO and/or other low coe oxides,  just need to keep tweaking it down and be brave enough to try - I'd been looking for published low coe recipes.

Clay can alter glaze coe

       Per authority on ceramic chem, here's (some of my rather thin) understanding

               We've seen excess cilica - white powder - expelled from bisqued clay, which may continue in glaze firing;  if/when this is taken up by the glaze, conventional wiz says glaze coe lowered

              Clay and glaze are both fluxed, however, the level (percent) of flux is very different, "...clay can alter the COE of the glaze because of the flux level differences."

 

MFPs Tangential question

My (also thin) understanding of bisque level to glaze film relationship is that cooler bisque absorbs more water, hence a thicker film of dry-ish glaze sludge adheres to the bisque.

That said, as cooler bisque is more porous, hence there should be more clay surface area - purely physical, before firing - as well.

Speaking of thin, Tony Hansen's points regarding lowering glaze specific gravity and slowing the drips using vinegar or Epsom salts to get a reasonably thin and well behaved glaze layer - so helpful!

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7 minutes ago, Hulk said:

Hi Min!

Magnesium oxide, aye

The crazing is right out the kiln, mostly. The clears I've tried seem fine on the buff and red clays.

Thank you very much for the recipe, will try it! ...I get reducing Potassium and Sodium, increase MgO and/or other low coe oxides,  just need to keep tweaking it down and be brave enough to try - I'd been looking for published low coe recipes.

Clay can alter glaze coe

       Per authority on ceramic chem, here's (some of my rather thin) understanding

               We've seen excess cilica - white powder - expelled from bisqued clay, which may continue in glaze firing;  if/when this is taken up by the glaze, conventional wiz says glaze coe lowered

              Clay and glaze are both fluxed, however, the level (percent) of flux is very different, "...clay can alter the COE of the glaze because of the flux level differences."

 

MFPs Tangential question

My (also thin) understanding of bisque level to glaze film relationship is that cooler bisque absorbs more water, hence a thicker film of dry-ish glaze sludge adheres to the bisque.

That said, as cooler bisque is more porous, hence there should be more clay surface area - purely physical, before firing - as well.

Speaking of thin, Tony Hansen's points regarding lowering glaze specific gravity and slowing the drips using vinegar or Epsom salts to get a reasonably thin and well behaved glaze layer - so helpful!

Excess silica expelled by clay as a white powder? Really?  Isn't that just scumming?  From what I understand the scumming on pots is more akin to soluble salt fluxes, which would also have an effect on glaze but I've never heard of clay bodies shedding silica as a white powder...  I use a pretty scummy red clay body so am pretty familiar with the white powder on the surface.  I could just be way off base though, I've just never heard of clay shedding silica as a powder.

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On 8/12/2019 at 1:09 PM, Hulk said:

Hi Min!

Magnesium oxide, aye

The crazing is right out the kiln, mostly. The clears I've tried seem fine on the buff and red clays.

Thank you very much for the recipe, will try it! ...I get reducing Potassium and Sodium, increase MgO and/or other low coe oxides,  just need to keep tweaking it down and be brave enough to try - I'd been looking for published low coe recipes.

Clay can alter glaze coe

       Per authority on ceramic chem, here's (some of my rather thin) understanding

               We've seen excess cilica - white powder - expelled from bisqued clay, which may continue in glaze firing;  if/when this is taken up by the glaze, conventional wiz says glaze coe lowered

              Clay and glaze are both fluxed, however, the level (percent) of flux is very different, "...clay can alter the COE of the glaze because of the flux level differences."

 

MFPs Tangential question

My (also thin) understanding of bisque level to glaze film relationship is that cooler bisque absorbs more water, hence a thicker film of dry-ish glaze sludge adheres to the bisque.

That said, as cooler bisque is more porous, hence there should be more clay surface area - purely physical, before firing - as well.

Speaking of thin, Tony Hansen's points regarding lowering glaze specific gravity and slowing the drips using vinegar or Epsom salts to get a reasonably thin and well behaved glaze layer - so helpful!

I always use bentonite in all my glazes......so I have never experienced some of these problems. I am struck however by the notion that if the clay body is more open such as at 010 or 08....the ability of the body to suck in glaze is improved and I would suspect increases the glaze/body interface. On my pots in the past, it was at least 2mm. The argument I heard for going to 04 was the pots being less fragile (and I can see this with porcelain) and decreasing pin holes.....well....when I was glazing 010 bisqueware and saw a pin hole, it was rubbed in.  Other piinholes that arise during the firing are a different issue I suspect....but inadequate absorption of the glaze might play a part?. 

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2 minutes ago, MFP said:

I always use bentonite in all my glazes......so I have never experienced some of these problems. I am struck however by the notion that if the clay body is more open such as at 010 or 08....the ability of the body to suck in glaze is improved and I would suspect increases the glaze/body interface. On my pots in the past, it was at least 2mm. The argument I heard for going to 04 was the pots being less fragile (and I can see this with porcelain) and decreasing pin holes.....well....when I was glazing 010 bisqueware and saw a pin hole, it was rubbed in.  Other piinholes that arise during the firing are a different issue I suspect....but inadequate absorption of the glaze might play a part?. 

A hotter bisque is recommended for red clays because of the amount of sulfides and carbonates in them.  The red color is usually from iron disulfide (or fools gold!) And it's very gassy.  The idea is that going to cone 04 will keep those gasses burning out longer than if you stopped at 06 or lower.  I bisque to 05 because glazing is just too lengthy at 04 for me.  At 05 I can dip for 6 seconds and be done, at 04 a 6 second dip barely covers the piece.

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I didn't want to quote the entire message - here's what I can copy:

  980C. " Metakaolin changes to spinel with ejection of finely divided and highly reactive SiO2."

  W.G. Lawrence - PHD Alfred & MIT.

  Ougland & Brindley. PhD (British Ceramic Society 61-599) also covered this topic in "Quantitive Study of High Temperature Reactions of Kaolin-Silica-Feldspar Mixtures"

It still comes down to this/these glazes fit that/those clays as verified by test and test some more.

Looks to me that said fit is not entirely/only matching up clay and glaze coe; glaze elasticity can give a (very) little bit of wiggle room, and there may be significant chemical interaction(s) as well.

 

 

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21 minutes ago, Hulk said:

I didn't want to quote the entire message - here's what I can copy:

  980C. " Metakaolin changes to spinel with ejection of finely divided and highly reactive SiO2."

  W.G. Lawrence - PHD Alfred & MIT.

  Ougland & Brindley. PhD (British Ceramic Society 61-599) also covered this topic in "Quantitive Study of High Temperature Reactions of Kaolin-Silica-Feldspar Mixtures"

It still comes down to this/these glazes fit that/those clays as verified by test and test some more.

Looks to me that said fit is not entirely/only matching up clay and glaze coe; glaze elasticity can give a (very) little bit of wiggle room, and there may be significant chemical interaction(s) as well.

 

 

I think that refers to "free silica", which would be silica in excess (not enough Feldspar to convert) where the silica doesn't enter a glassy matrix.  It doesn't migrate to the surface though, it's throughout the clay body and forms cristabolite. 

Here's another rabbithole to dive into, Tony Hansen mentions here that cristabolite in iron clays will actually improve glaze fit. https://digitalfire.com/4sight/glossary/glossary_cristobalite_inversion.html

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Tom, I had some shards of ^5 B-mix so I included a test piece with the Version 4 glaze I posted above in this weeks glaze firing, ice water/ boiling water stress test X 3 cycles, no crazing, I brushed sumi ink on to double check. Nice and clear, no boron clouding, pinholes or bubbles. 80 grams water to 100 grams base glaze, it's a thick slurry since it has a fair amount of both gerstley and epk. I do use a drop and hold schedule plus a slow cool down for my firings so this was fired like the rest of this load but I'm sure it would be fine with a drop and hold and no slow cool. I didn't have time to do an oven to ice water test which I think is more rigorous than the ice water / boiling water test, might want to do that. ^6 tip touching shelf.

IMG_2894.jpg.93ea4d1ccba9da63342eab6c36e55a3e.jpg

 

 

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  • 4 months later...

Hi Min,

Thanks again for the recipe!

...have fired two glaze loads since, forgot to load the test tiles first time, heh.

Second load, several tiles and a few pieces glazed with g1215u sub v4, low coe clear, above. The Sedona, Café and bmix test tiles look good - clear, shiny, no crazing; over the redder clay there's some microbubble clouding (expected), and frothy over the black clay.

The pieces, however - a few Café and Sedona mugs - have crazed liner glaze, no crazing on the outside (where the clear was used; the colours all look good). The pattern is definitely bigger than that of other low coe clears I've tried. My guess is the interior is more prone to craze than the exterior, perhaps due to being "pushed" as opposed to stretched?

Any road, getting closer! Have yet to try v4 on a bmix cylinder; looking forward to that, as the bmix test tiles came out great - the other clears all crazed on it. Remedy from here for the other two clays, hmm, lower that coe number another .5, more boron and magnesium, add a few % zircon...

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Hi Tom,

That's good the sub v4 didn't craze on the B-Mix test tile, will be good to hear if it works on a test cylinder.

Any chance the glaze was just thicker on the inside of those other claybodies than the outside? If a glaze is on the edge of crazing just having it thicker can push it into crazing. Does the clear for the red clay need to be non coloured? Could you use an amber coloured glaze or would mess up slips or underglaze work on them?

I don't think I would add much, if any, more boron to the sub v4 glaze. It's topping out at 0.34 as it is. Too much boron can actually induce crazing, plus it can make a glaze "soft".

edit: I played with the recipe, got rid of the tiny bit of nepsy that was in it plus reduced the custer, both to reduce the KNaO. Added some spodumene (to supply some lithia) to replace the KnaO I took out. (all 3 being alkali metals so R2O:RO ratio is still in the same range). Silica and alumina levels rebalanced then recipe re-totalled to equal 100. This is just an idea of where I would go to reduce the coe even more if that's the direction you want to take. I like your suggestion of adding a couple % of zirconium, that does help with crazing. (even though the calculated coe will raise slightly)

1437893327_ScreenShot2020-01-14at12_36_07PM.png.0cd663af8aa87570b7ffe08e775880d0.png

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Hi Min!

Looking into ordering up some spodumene, thank you for the revision!

I like the potassium and sodium lower, however, might tweak the talc to get the manganese a bit higher.

I've a clear that's working well for the red clay, just tried the v4 on it for fun.

The glaze might be a bit thicker on the inside, true.

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The version 5 above is just showing what I would do to lower the calculated coe down from where it is in version 4. You are using glaze calc software right? What I would suggest doing is working on a version of this with an even lower coe then just doing a quick line blend of the original one and one with a super low coe. See where the crazing stops.

Yes, there is room for more magnesium in there so play with the talc amount also. BTW spodumene can have detergent reside on it which in turn causes a bit of frothing when the glaze is mixed up. I don't worry about this for glaze testing but for a full glaze batch I weigh out the spodumene then add a fair bit of water to it, swish it around quite aggressively then let it settle and pour out the frothy water. 

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  • 7 months later...

Past time for an update!

First, thanks all for the input, and special thanks for the recipes and encouragement Min.

Am on second round o' lower coe clear with lithium.

Below is first revision - a bit less sodium, potassium, calcium; touch more magnesium; addition of lithium via petalite - note the mole number, lithium is "small!"; addition of zirconium silicate via Zircopax. The glaze fit the white clays better, very little crazing on the bmix, good on the venus white. It crazes on the cafe clay, but less. Not sure if I'll pursue a clear solution for the light red clay, sigh - vases and other non-food pieces! It didn't clear bubbles well on the red and black clays - wasn't expecting it to. Looks good on the buff clay.

Second revision is also below - a bit less sodium, potassium, calcium; more yet o' magnesium, lithium; a bit more boron and zirconium silicate. This is looking good for both white clays and the cafe clay as well; from there, more testing required.

The glaze update wasn't the only change, however. Looking over my notes, finally caved to the evidence that cone 6 is very likely too hot for the buff and red clays*, hence, dropped target peak temp forty degrees, found no movement on any of the cone 6 cones, between slight movement to fully mushed cone 5 cones (packs on each level). I've included a few faux celedon** glazed pieces in every firing, which crazes over the white clays - little to no crazing this time! Hence, the changes to firing schedule may be important! In addition to staying below cone 6, a) held 100F below peak for a full hour this time (vs 30-45 minutes), b) left kiln on set to "low" until temp fell to 1850F (somewhat controlled cool) and also shut off kiln vent at that point, and c) slower bisque - longer holds at the critical temps (there was no bloating in the black clay pieces this time).

 

1293697646_lowexii.JPG.b42e23660ce4502563a41d124fdaa497.JPG

1370727003_lowexiii.JPG.4f7ae46cf507c3add6c86c5036170c12.JPG

* Looks like the little bumps that form at the surface - which, under magnification, appear to be small drops - are associated with firing a bit hotter. These bumps can be removed with a fingernail (from bare clay) if they are small-ish; when they are bigger, they rub off with a green scrubby or polishing stone. Given the glaze layer is thick enough, looks like these heat bumps are incorporated into the melt, however, in a thin layer, there's little bumps in the glaze. A recent poster had asked about little bumps, which may have been sand/grog pushed to the surface...

**Credit Selsor's Faux Celedon (edited to lower expansion); I've been reluctant to post any pics, due to the crazing. I love the look though! The tiny bit of black makes the difference.

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The lithia seems to have done the trick. If you do get crazing you could drop both the Nepsy plus the Custer and bump the Petalite up to around 25 (to keep the same R2O:RO ratio). I wouldn't go higher than that with the Petalite though, too much lithia isn't a good thing - shivering.

Do you need the clear for the red clay to be colourless? Adding a tiny amount of iron can clear bubbles but leaves the glaze with an amber colour, looks nice on red/brown claybodies and just comes across as clear.

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1 hour ago, Min said:

Adding a tiny amount of iron can clear bubbles but leaves the glaze with an amber colour, looks nice on red/brown claybodies and just comes across as clear.

I was reading on digital fire today that 1-2% zircopax helps this too, seems counter intuitive to me since it's a refractory material but I'm gonna try it in a glaze soon

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