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Callie Beller Diesel

In Search Of A Cone 6 Clear...long-Winded.

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The story thus far...

 

Is kind of long.

I understand some glaze chem at cone ten.  This cone 6 business is a different animal.

I need a cone 6 clear glaze that is rock hard, food safe (no crazing after a freeze test), compatible with underglazes and Mason stain washes, and  is crystal clear over red clay.  After begging a few recipes from friends and off the internet and doing some initial tests, I've found that the main barriers to hitting all of these points are microbubbles that seem to be present mostly over the red clay indicating the glazes are melting before all the carbon has burned off, boron clouding, and some crawling off of the stain-washed areas. I am cutting my mason stains 50% with gerstley borate, as this has been giving me better brushability than straight frit.  I am willing to alter the stain vehicle as needed.

I'm not having too much trouble with glaze fit (yay!). My clay body is Plainsman m390, with some use of m370 as a white slip for contrast/texture/better canvas for colours.

 

So far, the recipe that seems to have the fewest problems is from digitalfire, the Ravenscrag clear mix of 80% Ravenscrag slip, and 20% Ferro frit 3134. This iteration had some microbubbles over the red clay areas (not the white) and it was suggested to me to flux the glaze out a bit more, after an initial test showed firing with a soak that brought the kiln to cone 7 did clear some of them.  Higher percentages of the frit did get rid of the microbubbles, but brought in boron clouding instead.

 

At this point, I'm heading to Insight, because enough with the testing of random recipes. I need to narrow the field before I put brush to test sieve again.

 

 

The only information I could dig up on eliminating boron clouding was from various points all over digitalfire, and pertained to low-fire temperatures. I can only assume that the soloution offered of raising the alumina will apply just as well at cone 6. (unless someone knows of a reason otherwise?)  So boron + not enough alumina= boron clouding.  If I'm sourcing flux from frit 3134 (high boron, no alumina), and the only other material in the glaze is Ravenscrag (a complete glaze at cone ten that needs flux to melt it at cone 6), how to get more alumina?  I already have frit 3110 (high boron, some alumina), but just swapping that out straight across the board actually takes the silica:alumina ratio lower (the boron/silica:alumina ratio stays the same).  So that doesn't fix the problem. So how about adding lithium to replace some of the boron? I have in my posession petalite and spodumene, sources of both Lithium and Alumina.

 

 Here's where my understanding breaks down. Just as a starting point, if I change the 20% frit from the original recipe from 3134 to 10% 3110 and 10% Spodumene (see screen shot), things look pretty good on paper. The Alumina:Silica ratio is way better, there's almost no boron to cause clouding, the expansion rate seems pretty good, and both frit 3110 and Spodumene begin to fuse at a higher temperature relative to frit 3134, giving the clay body time to off-gas and fix any potential bubble issue.  But have I taken my fluxes too low? How badly is the lithium going to mess with colour response? I don't totally know how to interpret these in relation to the limit formula (pink column on the right). Is anything else seriously out of whack that I'm not looking at?

 

Help!

 

 

post-63667-0-54455200-1463864226_thumb.jpg

post-63667-0-54455200-1463864226_thumb.jpg

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Is it possible to use 2 clear glazes on your work? 1 for the 390 then a different one over the 370?

 

If you can then I think I would go in the direction that Plainsman suggests for a clear with the 390 clay, the Alberta Slip recipe that is slightly amber over the red clay (with 1/2 to 1% tin added to stop any crystal growth if your kiln cools slowly). Then a different clear over the 370 without the iron from the Alberta Slip. (the Alberta Slip one looks yucky over white clay)

 

I use 370 a fair bit, G1215U (from Insight) fits it and has zero boron clouding, works over every stain I have tried, expansion is very low though so I would probably sub the 3249 frit for the 3134 that is in the original alberta slip recipe so the 2 glazes are closer in coe numbers.  I kinda think that the spodumene / 3110 version would craze on 370 from the amount of potassium in it.

 

There is a picture of the Alberta Slip recipe about half way down this page showing it on red clay. http://plainsmanclays.com/albertaslip/index.php?recipes6=1

 

edit: frit 3249 is hard to find, if you want to try the G1215U recipe I can mail you some to try

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The commercial clear I've been working with is one of the worst offenders for micro bubbles, and it's got a good pedigree too. (Mr Tony Hanson may sound like the teacher from Ferris Bueler's Day Off in the videos, but he knows his chem, and he works mostly with materials that are local to me. He also formulated my clay.) My pre-mix clear makes a fantastic white, but with no opacifier it's absolutely wretched over the red clay I'm using. If I was working with just the white clay, I'd use the store-bought and have done.

Hence the testing.

 

 

"Oh I'll just switch to cone six for the summer while the gas kiln is down," she said this time last year. The next time I suggest a "claycation" to myself, someone smack me with a rolled up newspaper!

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Min, I actually tested G3806C (bhttps://digitalfire.com/4sight/recipes/cone_6_clear_fluid-melt_clear_base_125.html)ecause the second picture down shows kind of what I'm after. It's also supposed to solve the problems with some of the other clears, and it won't wreck Chrome-tin pinks with the magnesium. (I've got a chrome tin stain I like.). My version still went cloudy, even with a super thin application. I'm trying to glaze work like the photo below. I'm only using the M370 over the 390 in a light layer so I have something to decorate on. I like the contrast between the two.

post-63667-0-29675400-1463889920_thumb.jpeg

post-63667-0-29675400-1463889920_thumb.jpeg

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I have to say the only real difference I can see between cone10 and cone6 is .2 boron and slightly lower on everything else.

 

A test I have run is to put a blob of glaze on some already glaze-fired bare clay and see if the bubbles are still there. In theory the already fired clay shouldn't be releasing any gas. From my search I found it is nothing to do with clay body but this was cone10 buff stoneware. Are you sure there's no bubbles in the white?

 

In all my bubble test there was only one that had no visible bubbles but still some bubbles at 20x microscope. I feel the bubbles come from dissolving silica into the melt and it sucking in gasses. Right now I think you have too much si/al compared to flux and it will still bubble, maybe worse as the boron has gone. Worth running it anyway and seeing. Try one with all the lower limit si/al so 2.4 and 0.25 and run some tests to see if it stack up in durability. Might be hard as the recipe is 80% clay but I think worth a test.

I want to work out firing my kiln at home so I can run more test with different silica. Plenty of interesting things happening there in glazes.

 

I also find a little lithium goes a long way so .1 might be a little too much. Maybe try the 10 but also 5 and under with more boron and different silica/alumina values. 

 

Can't see the chemistry for 3806C as my subscription to insightlive ran out. Interesting to know what si/al that one has.

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High Bridge said:  "A test I have run is to put a blob of glaze on some already glaze-fired bare clay and see if the bubbles are still there. In theory the already fired clay shouldn't be releasing any gas."

 

good idea. 

 

Assuming that you would get bubbles if you ran the HB's test,

the most likely source of bubbles would be the clay component of the glaze recipe. 

 

Try calcining the clay and use something like CMC to keep the ingredients in suspension. 

 

Or just try another source of clay such as a clean kaolin.

 

LT

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I have used several commercial clear glazes and ran into similar issues with my red clay. Microbubbles with one glaze and a slightly opaque crackle with the other clear (reminded me of the look of doughnut glaze). Grrrr!! 

 

If you do decide to use a commercial glaze, you might try purchasing Laguna Clear Bright in dry form. I've found that when I use the Clear Bright that I've mixed myself to an extremely thin consistency, no more than one or two coats painted on quickly with a mop brush rather than dipped, it does NOT microbubble around texture. If this is a road you're interested in taking, please PM me your email address and I will happily send you photos of all my clear on red results with details. 

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Sorry I've been a bit absent guys. Victoria Day weekend+family=less socially acceptable computer time.

 

Min, the bisque is done to cone 06, the length of the firing is 8 hours (I believe). The glaze is done at a "slow" rate, and this is the best information I can give you on that until I can get ahold of my kiln tech tomorrow morning. I fire at a local arts centre, and if I fill the kiln with all my own work, I can request firing specifics. If I'm sharing kiln space at all, or topping it up with member's work, I have to use their schedules. I think you're right, the firing schedule needs examining.

 

I agree that a higher bisque, or a short hold below the point where the glaze begins to fuze would likely solve a number of issues.

 

Given that the bubbles seem to be present regardless of the glaze recipe, I think the clay body beneath may be to blame for those. There is almost a universal presence on all my pieces to date of a very faint orange peel texture in the glaze over the red clay that seems to be absent lessened in the areas over the white decorative slip. The texture lessens considerably when I've made efforts to raise the flux, or add more heat work to help smooth things out. The best results I've gotten so far were from the Ravenscrag clay recipe, but it's got boron clouding instead. Hence the efforts to raise the alumina in relation to the boron.

 

Joel, I've been operating under the assumption that more silica and alumina help with strength and durability, and as long as you have enough flux and/or heat to make it melt when and where you want it to, more is better. This is admittedly cone ten logic, and may or may not apply here.

 

Is anyone bothered by the lack of calcium in the original post, or is just having enough boron/KNa/lithium to melt everything sufficient?

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Sorry Lou, I didn't mean to forget you. This red clay seems to be remarkably forgiving in terms of glazes fitting it. Crazing has blessedly been absent. I've got some cups in the freezer now to do a shock test in the AM just to make sure. That glaze link still looks cloudy over the red clay sample.

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Callie:

NZ kaolin is the purest source of alumina and silica: 50% silica and 36% alumina. I prefer it over all other clays for use in glaze recipes.It should not cloud a clear glaze if used under 10%. Admittedly I look at glazes differently than most: flux unity is not my top priority. Silica/alumina ratios are #1, then formula limits; then flux unity. I am not willing to sacrifice the first two in order to achieve unity.

Nerd

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Nerd, I know. NZ kaolin is also not available at my supplier without a special order, which I'm not willing to place for the sake of 100g of test material. My studio is a former 12 sq ft bedroom in my basement, I don't have space to store the other 49 3/4 lbs of material indefinitely. I also don't have access to various mesh sizes of silica because the nearest supplier of those have to send their materials over the Rockies and an international border. It's physically doable, but it's cost prohibitive. I need to work with what I have.

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I think it is always best to try and remove any assumptions you have about the process and melt as many different mixtures of rock as possible. If I can nudge you in the direction of a curry tile as it would make your life much easier for testing a range of different silica/alumina values to see what happens. It is going to be a little difficult with the 80% one thing but could help the bubbles.

 

In regards to limit formula I believe taking the lower limits can produce as durable a glaze as taking the higher limits. As Nerd says silica alumina ratio seems to be the key with getting the best glaze dance happening. I am still making a few assumptions as never done a leach test with anything, only soaked in vinegar for 5 days and attacked with a knife and fork. If you look at the limits they give you a ratio between 7.5 and 8.5.

 

I ran tests with double the amounts of silica/alumina to flux but can't remember the specifics. I do know there was little difference in the final outcomes. There will be an old post around somewhere.

 

I think dropping the silica and alumina is what you want to do. Calcium level doesn't seem low for me.

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The story thus far...

 

Is kind of long.

I understand some glaze chem at cone ten.  This cone 6 business is a different animal.

I need a cone 6 clear glaze that is rock hard, food safe (no crazing after a freeze test), compatible with underglazes and Mason stain washes, and  is crystal clear over red clay.  After begging a few recipes from friends and off the internet and doing some initial tests, I've found that the main barriers to hitting all of these points are microbubbles that seem to be present mostly over the red clay indicating the glazes are melting before all the carbon has burned off, boron clouding, and some crawling off of the stain-washed areas. I am cutting my mason stains 50% with gerstley borate, as this has been giving me better brushability than straight frit.  I am willing to alter the stain vehicle as needed.

I'm not having too much trouble with glaze fit (yay!). My clay body is Plainsman m390, with some use of m370 as a white slip for contrast/texture/better canvas for colours.

 

So far, the recipe that seems to have the fewest problems is from digitalfire, the Ravenscrag clear mix of 80% Ravenscrag slip, and 20% Ferro frit 3134. This iteration had some microbubbles over the red clay areas (not the white) and it was suggested to me to flux the glaze out a bit more, after an initial test showed firing with a soak that brought the kiln to cone 7 did clear some of them.  Higher percentages of the frit did get rid of the microbubbles, but brought in boron clouding instead.

 

At this point, I'm heading to Insight, because enough with the testing of random recipes. I need to narrow the field before I put brush to test sieve again.

 

 

The only information I could dig up on eliminating boron clouding was from various points all over digitalfire, and pertained to low-fire temperatures. I can only assume that the soloution offered of raising the alumina will apply just as well at cone 6. (unless someone knows of a reason otherwise?)  So boron + not enough alumina= boron clouding.  If I'm sourcing flux from frit 3134 (high boron, no alumina), and the only other material in the glaze is Ravenscrag (a complete glaze at cone ten that needs flux to melt it at cone 6), how to get more alumina?  I already have frit 3110 (high boron, some alumina), but just swapping that out straight across the board actually takes the silica:alumina ratio lower (the boron/silica:alumina ratio stays the same).  So that doesn't fix the problem. So how about adding lithium to replace some of the boron? I have in my posession petalite and spodumene, sources of both Lithium and Alumina.

 

 Here's where my understanding breaks down. Just as a starting point, if I change the 20% frit from the original recipe from 3134 to 10% 3110 and 10% Spodumene (see screen shot), things look pretty good on paper. The Alumina:Silica ratio is way better, there's almost no boron to cause clouding, the expansion rate seems pretty good, and both frit 3110 and Spodumene begin to fuse at a higher temperature relative to frit 3134, giving the clay body time to off-gas and fix any potential bubble issue.  But have I taken my fluxes too low? How badly is the lithium going to mess with colour response? I don't totally know how to interpret these in relation to the limit formula (pink column on the right). Is anything else seriously out of whack that I'm not looking at?

 

Help!

There is very good information on clear glazes in John Britt's "Mid range Glazes"  The glaze you are thinking of working with is not in the food safe limits.

David

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Books are now on order from Amazon, and I've got some test cups going into a cone 04 bisque this weekend. I'll retest my existing mixes on those and see what they do.

 

David, does the John Britt books have those limit numbers, or are they something you can email me? What reference numbers are you using?

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Books are now on order from Amazon, and I've got some test cups going into a cone 04 bisque this weekend. I'll retest my existing mixes on those and see what they do.

 

David, does the John Britt books have those limit numbers, or are they something you can email me? What reference numbers are you using?

I sent an email with the food safe limits, Hesselberth & Ron Roy.  I am using Matrix glaze program.   There is also another book by Michael Bailey "Glazes Cone 6" which has a section on transparent glazes.  I am assuming you have a glaze program, if not I could put in the recipes in Matrix and email you a formula of the glaze for cone 6 .  This could save you time before firing a lot of unknown glazes.

David

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Awesome. Thanks David!i

 

I have a subscription to online Insight, and they include a couple of different limit formulas to compare your glaze formulas to, but not a lot of info on how to interpret the comparisons.

 

I do indeed need to narrow the formula down a bit before making another big pile of plastic cups with 87g of leftover glaze.

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David, which unity values are you taking not to be food safe?

The alumina value is on the border line and the Al/SI ratio of 7.3 isn't going to give a good translucent glaze. According to Michael Bailey "glazes cone 6"  the best clear glazes are around a Al/SI ratio of 9.1 to 14.05.  A Currie grid might reveal some glazes at cone 6.  But it would be easier to take a cone 6 clear glaze that is close to AL/SI 9 and run a Currie grid on it.

David

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