Clay Vs Glaze Coe Figures?

[Min]

I’ve been running glaze tests on Laguna’s WC-429, I contacted Laguna and was told by Jonathan Pacini that the COE figure (using their methods of testing) is 4.60 x 10 -6. My question is what is a reasonable expectation of a COE maximum figure for a clear glaze to fit this? My current low COE glazes are in the 5.7 area. When I shock tested them they appeared to survive on this clay without crazing but I'm not 100% sure since it's so hard to see crazing on a dark body.

 

Wondering if anybody has run tests comparing clay COE figures with glaze COE and come up with any data on a range where the glaze still fits.  I know that glaze calc COE’s only get you so far and some glazes with a higher COE might fit whereas lower ones don’t always do due to the family of fluxes used but just asking in general terms here. Even glaze coe versus published clay coe figures would be helpful for me to determine a working range of figures.

 

(I asked Laguna for sample glaze recipes that would fit and their commercial MS 29 was suggested, I’m not using commercial glaze though so that was no help) 

 

Any thoughts or insight? Thanks in advance for any help.

 

edit: I know I should run a series of tests from lower to higher coe's but am trying to find out if anyone knows of a window range? Trying to come up with a coe under 5 would be fun.

[mugmkr]

Hi Min, I don't know much about whether your glaze will fit this clay body, but Ron Roy teaches that crazing on hard to see glazes can be made visible by carefully holding a piece in steam (like over a kettle spout when the water is boiling) for 20 seconds or so, the craze lines can then easily be seen when the steam evaporates off. Works a charm in trying to tell if a dark glaze on a white body is crazing so perhaps this would be helpful for you. 

Best wishes, Owen

[glazenerd]

 

is 4.60 x 10 -6.    the "tenth" power is usually dropped - COE is just 4.60

4.60 is down in the flame ware range: I have to question that, because most stoneware runs in the 5.50-5.75 range (typical). To get below 5.00, spodumene would have to be used, or a low SAS value ball clay. There are ball clays out there, that have shrinkage rates that run 6-7%, and if blended with a higher % of kaolin would get down that low. Very unusual, but not impossible.

 

You would have to blend a glaze using spodumene, silica, and epk to get down to that COE range. Lithium 1.50 to 1.90 molar or so. Avoid sodium, or higher expansion fluxes.      I use a black light to look for micro fissure on hard to see glazes.

 

Nerd

[Min]

Thanks Owen and Nerd.

 

@ Nerd, a few of Laguna's porcelains are about the same coe but yeah for a stoneware I agree this seems awfully low.  Do you know how reliable dilatometry analysis is?  

[curt]

General thinking is that you want the clay to shrink slightly more than the glaze, so that the glaze is under slight compression in the finished fired object.  This makes the fired object much stronger, than, say, an object where the glaze is crazed.  This suggests you want the glaze COE to be slightly lower than the clay COE.

 

There is another way that this clay could be so low COE, at least from a chemical/ingredients perspective.  As oxides, Silica, Alumina, Boron and Magnesium are very low COE ingredients in a clay or glaze.   If a relatively clean clay (ie a clay with just alumina and silica and little other contaminants or feldspars, for example Tile 6) was fluxed with, say a Boron heavy frit (say, Ferro 3249 or Fusion F-69) and some talc for magnesium, I think you could get a clay with an even lower COE than 4.6.  That would also explain the quite low porosity on this body.

 

If this is what they have done, a simplistic way to look at it is that you might want to lean heavily on boron and talc (magnesium) as your glaze fluxes.  And you would anyway want a pretty clean kaolin for the alumina and silica in the glaze (probably not a ball clay methinks).  This would keep your glaze chemistry sympathetic to your clay chemistry, increasing the chances of a good end result

 

This is all guesswork, but just throwing it out there.

[glazenerd]

Min:

My only frame of reference about dilatometer accuracy is from what Ron Roy told me: pending use, should be calibrated by Orton every year. I threw together a very quick recipe just to see what it would take to stay under 5.00 COE. ( sitting here drinking coffee anyway)

 

I have not checked unity, nor formula limits: just an idea of what types of ingredients you will have to use.

Talc 10%, Silica 25%, EPK 20%, spodumene 25%, and frit 3110 10%   COE 4.86  

 

The big problem will also be alumina; which you need a lot of for functional glazes: low expansion materials are usually alumina poor. You could use alumina rich-higher expansion materials: then reduce the COE back down with 10%  zirco. To get under 5.00 COE, choices get limited.

 

This COE for the clay still bugs me: just does not seem correct. It would be hard to justify cost using spodumene as a flux, and ....hmmm wait a minute. Plastic Vitrox.. hmmm  lower expansion, lower shrinkage; relatively cheap.. hmmm. Mined in California, Laguna is sole distributor.. hmm; supplies a third of the flux needed for a stoneware body.. hmmm  red orange  newman red / plastic vitrox?? - that would fit.  12% shrinkage, slightly course--20% 40m fireclay perhaps. @ 43 cents a lb (US), too low for spodumene use.   sorry.. fonting out loud. 0.50% absorption... would have to be very high SAS (28) type clays to close the body up that well. Going through my mind, trying to formulate this clay, with 12% shrinkage... I would put the glaze at 5.25% COE or so.

 

Nerd

[Guest JBaymore]

 It would be hard to justify cost using spodumene as a flux, and ....

 

Just a side note FYI..................

 

Sheffield Pottery Supply makes a non-flameware stoneware body with spodumene in it.  Designated as #16425.  I've used it.  It produces DRAMATIC color shifts with a number of my glazes.....particularly a chun that goes from its "normal" light bluish opalescence to a STUNNING bright almost turquoise blue. 

 

best,

 

......................john

[glazenerd]

 

Sheffield Pottery Supply makes a non-flameware stoneware body with spodumene in it.  Designated as #16425.  I've used it.  It produces DRAMATIC color shifts with a number of my glazes.....particularly a chun that goes from its "normal" light bluish opalescence to a STUNNING bright almost turquoise blue.

Congratulations; now you know the whole theory behind my clay and slip experiments. Every angle of glaze formulation has been tried to produce or control color: clay however is the great unexplored expanse. I am very familiar with spodumene in clay: you should see what it does to crystals

 

Nerd

[Min]

Okay, pulling together some thoughts here. From Laguna’s ^5-6 Western Clay info I did an average of the published clay coe figures, it comes out to 5.546  I excluded paper and sculpture clays but included porcelains as they ran from low to high coe figures as did the stonewares. This seems to be in line with Nerds range of 5.50 - 5.75

 

I then started looking for info on the average glaze coe, from this John Sankey page I found a mean figure of 6.2 that he came up with from the Clayart archives.  Just from my general observation I would put the average range of coe’s between 6.2 - 6.5 for base glazes. (from using Insight for years)

 

Given that in an ideal situation the glaze is putting the clay into slight compression the above figures don’t seem conducive for the average glaze to fit the average Laguna clay. And yet glazes with a coe of below around 5.5 are few and far between which leads me to believe one of a couple things. 1 - the range of coe’s for some of the clay figures is off from Laguna. 2 - There is far more wiggle room for a higher coe value for a glaze to fit a clay than logic would dictate.

 

Think I’ll go back to doing a line blend of a very high coe glaze with a very low one and see where the crazing stops.

 

Thanks everyone for your input.

[neilestrick]

We used to mix a cone 6 spodumene body at A.R.T. Clay that was a beautiful toasty orange-brown. No matter how well it was mixed we would get variations in color from box to box. I suspect it was also very picky about kiln location, temp, etc. Anyway, most glazes did not fit it very well. It was super tight with a very low shrinkage rate, similar to a flameware body but not to that degree.

 

Laguna WC-429 does not look like anything special in terms of having spodumene in it, though. It looks like your typical cone 6 brown clay that uses Redart or such for its color. So I, too, would question that low COE.

[glazenerd]

Min:

 

Your clay issue has been on my mind all day; mostly because it does not make sense to me. I ran my databases, formulas, expansion tables, etc: and now it makes even less sense. The lowest COE formula I have is 4.54, and the highest is 6.74 and I have the correlating shrinkage rates for each. That said, a COE of 4.60 should be around 10% shrinkage, and Laguna states 12%. Have you ran a test bar to check shrinkage? If spodumene was used, pieces are usually denser at bisq- have you noticed that? If spod was used; the shrinkage from bisq to maturity is less, if potassium was used: the shrinkage from bisq to maturity is higher, and if sodium- higher still Potassium will actually expand the clay at bisq, spod will be more compact.  Something is off here... and IT IS DRIVING ME NUTS!!! trying to figure it out.

 

Nerd

[glazenerd]

It bugs me when I cannot figure something out: and this morning the "course" COE 4.60 came to me.....Christy Red clay- not sold directly in the pottery world: a very high iron, high potassium, 80 mesh clay used in brick. (most fire clays are used for brick)- our trade buys less than 5% of mined clays.

 

They mixed Christy Red ( or equal), newman red, plastic vitrox, epk, and taylor ball clay ( or equal). Which means they added very little (less than 10%) feldspar to reach a 2.90-3.10 KnaO molar.  Explains the higher iron, explains the coarseness, explains the closed body. and explains the low COE.. I am happy now. You pick the right clays, and mix it in the right proportions: you do not need feldspars at all. Very tricky Laguna, but very smart as well. Which also means the claimed 4.60 COE may very well be correct: if you add little to no feldspar: getting below 5.00 COE gets real easy.

 

All of the above clays run 48-55 Co-EX on their own: which would make it easy to get under 5.00..hmmm... smart

 

Nerd

[Min]

Thanks Nerd, I know what you mean about having to figure things out.