Jump to content
Hulk

Clay composition and crazing

Recommended Posts

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

 

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
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.  

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites

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. 

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites
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?. 

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites

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.

 

 

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites

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

 

 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.