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GreyBird

Pinholes & Rabbit holes

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So I brought my favorite glaze into class to show my teacher what I was going to do color tests on. Keep in mind she is not a glaze chemistry teacher but she is a very good at teaching throwing & hand building. She quickly pointed out that my favorite tile is full of pinholes. I was astonished. I could not see them at all. I actually thought she was pulling my chain but she was insistent and kept pointing and looking real close and saying "yup, they're there." I kind of wanted to poke her in the eye. I did not. After class, I went home and got my magnifying glass and a flashlight and did a close inspection and there are pinholes. This, of course, made me look at the rest of my glaze tiles more closely and low and behold my very favorite satin color, Cobalt Green, has a bunch of pinholes! Yikes! How did I miss that? and why... why, why, why.

I went online and started looking at ways to fix. I'm guessing it's a glaze issue since it only happens with some glazes, not all. If it were a clay body issue it would happen to all. I thought I might try a Satin Matte glaze such as Pikes Purple and switch out the colorants but we all know that will never go as planned.

Curious to how Cobalt could be producing such a beautiful Green,  I was reading on DigitalFire about Cobalt it mentions that it sometimes causes pinholes. So now I am thinking if I switch out the colorants, not only will that not work, but I'll just have the same issue. Anyway just using the colorants in another recipe would be yet another rabbit hole and I 'm currently thoroughly ensconced in my Hudson River Clay rabbit hole for now.

Any Ideas Anyone? Would maybe a 5 minute soak fix this? Here is the Cobalt Green Recipe & Photo. Also attaching a photo f the tile that started all the pinhole hoopla:

Neph Sy:   64.8
Spodumene:     13.9
Gerstley Borate:     4.6
OM-4:     7.4
Whiting:     4.6
Flint:     4.7
Cobalt Carb:     1
Rutile:     2
Bentonite:     2

 

tile1.JPG

tile2.JPG

Edited by GreyBird
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Gonna take a stab and say that both cobalt carbonate and whiting gonna make some gas, and then lithium from spodumene and the yellow from rutile help the cobalt go green.

Dark clay bodies always have some gas issues, so I wouldn't write that off, the glaze melt viscosity has a lot to do with how well it heals after gas comes through.  That glaze might have a high viscosity so it's not healing after the body gasses off.  Your other glazes might heal better due to a lower viscosity, never know.

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Cobalt greens are from really high levels of alumina in the glaze plus some titania and cobalt. As alumina is refractory the melt isn't going to be super fluid therefore the pins. It does look like you have less pins on the white clay test tile, if that's a porcelain then the body will be slightly helping to flux the glaze so less pins. You could try a soak at the top to see if it helps, glaze thickness will play a part too. Thick glaze layer will give you the solid green colour and, thinner and you should get the green breaking to blue. I use a fair bit of rutile, I've found a drop and hold firing schedule really helps clean them up. After the soak at my top temperature I drop at 9999 to 100F below top temp and hold for 15-20 minutes. Doesn't add much heatwork to the cones.

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5 hours ago, GreyBird said:

So I brought my favorite glaze into class to show my teacher what I was going to do color tests on. Keep in mind she is not a glaze chemistry teacher but she is a very good at teaching throwing & hand building. She quickly pointed out that my favorite tile is full of pinholes. I was astonished. I could not see them at all. I actually thought she was pulling my chain but she was insistent and kept pointing and looking real close and saying "yup, they're there." I kind of wanted to poke her in the eye. I did not. After class, I went home and got my magnifying glass and a flashlight and did a close inspection and there are pinholes. This, of course, made me look at the rest of my glaze tiles more closely and low and behold my very favorite satin color, Cobalt Green, has a bunch of pinholes! Yikes! How did I miss that? and why... why, why, why.

I went online and started looking at ways to fix. I'm guessing it's a glaze issue since it only happens with some glazes, not all. If it were a clay body issue it would happen to all. I thought I might try a Satin Matte glaze such as Pikes Purple and switch out the colorants but we all know that will never go as planned.

Curious to how Cobalt could be producing such a beautiful Green,  I was reading on DigitalFire about Cobalt it mentions that it sometimes causes pinholes. So now I am thinking if I switch out the colorants, not only will that not work, but I'll just have the same issue. Anyway just using the colorants in another recipe would be yet another rabbit hole and I 'm currently thoroughly ensconced in my Hudson River Clay rabbit hole for now.

Any Ideas Anyone? Would maybe a 5 minute soak fix this? Here is the Cobalt Green Recipe & Photo. Also attaching a photo f the tile that started all the pinhole hoopla:

Neph Sy:   64.8
Spodumene:     13.9
Gerstley Borate:     4.6
OM-4:     7.4
Whiting:     4.6
Flint:     4.7
Cobalt Carb:     1
Rutile:     2
Bentonite:     2

 

tile1.JPG

tile2.JPG

I will give you something to try:

your original recipe

0.80 alumina way too much to melt at cone 6

.07 boron too low to help this melt at cone 6

flux ratio .71 :.29 Very non durable

The chemistry says this is extremely unlikely to fully melt at cone 6, maybe not even cone 10. As a result it should appear matte and the flux ratio is inverted in a very non durable ratio. Your firing seems to bear this out. If this was an Orton cone it would soften at cone 8 and fully melt about cone 14 so I think some tweaking could help.

So tweak a little bit and try for a more standard cone 6 glaze

using your ingredients try

neph sy 28.09

spodumene 16.86

gerstley borate13.11

om4 19.47

whiting 14.98

flint 7.49

no bentonite should be needed as you have 19% clay to keep things suspended. Chemistry says this will melt at cone 6, and be a true matte. If you want to make it more glossy 30 g of flint total takes it to a full gloss. No guarantees but the chemistry is way superior to the original, the question is will it work on your claybody and will you like the color.

hope that helps - at .80 alumina I would never expect this to melt  a bunch especially at cone 6 let alone heal Over much.

good luck!

233CC149-1291-493D-8E5F-D76DC072FE65.png

84A1A132-FDC3-4969-BCF4-603A186BBD59.png

Edited by Bill Kielb

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1 hour ago, Bill Kielb said:

I will give you something to try:

your original recipe

0.80 alumina way too much to melt at cone 6

.07 boron too low to help this melt at cone 6

flux ratio .71 :.29 Very non durable

The chemistry says this is extremely unlikely to fully melt at cone 6, maybe not even cone 10. As a result it should appear matte and the flux ratio is inverted in a very non durable ratio. Your firing seems to bear this out. If this was an Orton cone it would soften at cone 8 and fully melt about cone 14 so I think some tweaking could help.

So tweak a little bit and try for a more standard cone 6 glaze

using your ingredients try

neph sy 28.09

spodumene 16.86

gerstley borate13.11

om4 19.47

whiting 14.98

flint 7.49

no bentonite should be needed as you have 19% clay to keep things suspended. Chemistry says this will melt at cone 6, and be a true matte. If you want to make it more glossy 30 g of flint total takes it to a full gloss. No guarantees but the chemistry is way superior to the original, the question is will it work on your claybody and will you like the color.

hope that helps - at .80 alumina I would never expect this to melt  a bunch especially at cone 6 let alone heal Over much.

good luck!

233CC149-1291-493D-8E5F-D76DC072FE65.png

84A1A132-FDC3-4969-BCF4-603A186BBD59.png

Hmmm, very interesting Bill, I never even thought to look at it using the glaze software. And here I just learned so much about glaze chemistry, I should have. I just assumed, since it was in a book, it would be correct.

I'm looking at it now... doesn't the ratio: Si:Al: 4.7:1 put it firmly in the Stull chart as melting at cone 6 as a Semi Matt Glaze?

Stull.jpg

Edited by GreyBird
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3 hours ago, neilestrick said:

That green looks pretty stiff, so  little more flux might help. A hold could help, too, but I'm not a fan of altering my firing schedule for one or two glazes, because then it affects all my other glazes, too. Easier to tweak the glaze than the firing.

Thank you Neil, good point. I agree. :)

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3 minutes ago, GreyBird said:

Hmmm, very interesting Bill, I never even thought to look at it using the glaze software. And here I just learned so much about glaze chemistry, I should have. I just assumed, since it was in a book, it would be correct.

I almost look at every glaze these days and find a major portion have pretty bad chemistry actually. Lots back from the days of adding things and firing and adding some more. Just finished three studio glazes, cone 6 with enough boron to melt at 04. Guess what, they fire to 04 perfectly while they crawl and have crazy amounts of silica to compensate at cone 6.

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38 minutes ago, GreyBird said:

Hmmm, very interesting Bill, I never even thought to look at it using the glaze software. And here I just learned so much about glaze chemistry, I should have. I just assumed, since it was in a book, it would be correct.

I'm looking at it now... doesn't the ratio: Si:Al: 4.7:1 put it firmly in the Stull chart as melting at cone 6 as a Semi Matt Glaze?

Stull.jpg

No, stull  tells us trends so 4:1 should indicate a matte. Look at Seger and cone theory for a better example of where things melt. Most cone six compositions will not melt above  an alumina amount of .56 - .60 even with boron. Stull was cone 10 so the trends are valid.

I’ve been working on simplifying this for our students, take a look at this and see if it makes more sense. Trying to make enough of these to get folks through the straight forward parts.

 

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Don't the numbers along the top of the chart represent cones? so, 02, 01, 1, 2, 3 etc.? If this is true then simply adding more Silica to a Matt glaze not only makes it glossy but should also move it's firing temp up? Unless I misunderstood those numbers along the top of Matt Katz's chart. Your chart just says cone 10 along the top and does away with those numbers. 

Edited by GreyBird

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37 minutes ago, GreyBird said:

Don't the numbers along the top of the chart represent cones? so, 02, 01, 1, 2, 3 etc.? If this is true then simply adding more Silica to a Matt glaze not only makes it glossy but should also move it's firing temp up? Unless I misunderstood those numbers along the top of Matt Katz's chart. Your chart just says cone 10 along the top and does away with those numbers. 

Nope, they are not this chart is for cone 10 which was actually cone 12 back in the day I believe. Stull held his flux ratio at .3:.7 and then fired many test to make this chart. This chart was actually lost for quite some time and rediscovered in the last fifteen years or so.. remember in Stulls time cone 02, 01 did not exist. The chart you see and the spreadsheet you see is Matt Katz with permission.

Edited by Bill Kielb

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Just now, Bill Kielb said:

Nope, they are not this chart is for cone 10 which was actually cone 12 back in the day I believe. Stull held his flux ratio at .3:.7 and then fired many test to make this chart. This chart was actually lost for quite some time and rediscovered in the last fifteen years or so.. remember in Stulls time cone 02, 01 did not exist

Oh boy, good to know. Thanks! And thanks for the video!

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Your welcome, a bit rough around the edges but there is an adjustment video or glaze fix we did for an artist in London. It is better and goes over the essentials a bit better as well while fixing a low-fire glaze. Same Madison Pottery yountube site.

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