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Callie Beller Diesel

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

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I think it is closer to 8-9 for what you could call the best gloss, or slap bang in the middle.I have noticed perfectly transparent and glossy glazes down at 7:1. Can't say how food safe it is but certainly seem as durable from my tests. I have done a few leach tests with alumina 0.25-0.4 and silica 2.2-3.5 and seen little difference in the results keeping the flux consistent~ None seemed to leach but maybe my tests were bogus.

 

The bottom row on this tile is gloss all the way from 6.5:1 to 15:1 where is starts to silica white out. It is not cone6 but I think the results would be similar. I should be able to get more si/al in there at cone10 but anything over 0.5 alumina looks detrimental to the glazes gloss and clarity.

 

Most limit formula are wide enough to be little use other than a starting point to test from.

 

med_gallery_23281_912_39694.jpg

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I have used insight for clear, specifically the variations of 1214 Q series. This was adjusted by Ron Roy for Sue Hintz back in the 90s and works well with red/pink stains. Does not craze on Frost ^6. Unfortunately if you have new chemicals these may not work as expected.

^6 Ox. Clear  Sue Hintz   Nice satin matt, not a harsh shiny glaze

Cornwallstone  33.5

G200                22

Whiting             18

Gerstley Borate  10

EPK                    5.5

Silica                 11 

add Bentonite     2

                        102

 

If using ^6 limits which are sometimes hard to find, you can plug in a number of ^6 glazes to programs and work out an average for references. I use Michael Bailey's book ^6 Glazes as well as Mastering Cone 6 glazes when I began working in Oxidation about 15 years ago after firing reduction for over 30 years.

I taught glaze chemistry and had an army of students who created test tiles for 25 years.  Always test. Chemicals change.

Marcia

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With respect, love and gratitude to everyone who is trying to help, I don't need any more new clear recipes. People have shared most generously in this area. I need help figuring out how to test one that I have already chosen because it meets most of my criteria. I just need to get it so it meets ALL of my criteria.

 

Maria, Insight Live includes three different sets of limit formulas to compare cone 6 glazes to. My clay body is red, and seems to contain sufficient silica that crazing isn't a big issue. I want a clear that looks good on red clay. I can accept some yellowing because it won't be obvious over dark clay, but clouding of one kind or another is what I'm trying to solve.

 

Joel, I like how the Currie tiles give a lot of information in a small space and use materials economically. Like David suggests, I'll run one on the glaze when I get home from my family reunion next week

 

David, thank you again for your materials substitutions.

 

I will keep everyone updated when the next batch of tests are done.

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With respect, love and gratitude to everyone who is trying to help, I don't need any more new clear recipes. People have shared most generously in this area. I need help figuring out how to test one that I have already chosen because it meets most of my criteria. I just need to get it so it meets ALL of my criteria.

 

Maria, Insight Live includes three different sets of limit formulas to compare cone 6 glazes to. My clay body is red, and seems to contain sufficient silica that crazing isn't a big issue. I want a clear that looks good on red clay. I can accept some yellowing because it won't be obvious over dark clay, but clouding of one kind or another is what I'm trying to solve.

 

Joel, I like how the Currie tiles give a lot of information in a small space and use materials economically. Like David suggests, I'll run one on the glaze when I get home from my family reunion next week

 

David, thank you again for your materials substitutions.

 

I will keep everyone updated when the next batch of tests are done.

I agree that the Currie method is a great teaching and testing tool.  Unfortunately  if a glaze is not within the cone 6 food safe limits for alumina and silica then the 35 Currie grid also won't have any glazes within the limits but some may look good.  You can customize the Currie grid or can get great results with a custom 20 grid.  This involves keeping the corners within the limits.

David

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I don't think you understand what is happening with the currie method. It gives you a whole range of silica and alumina values so some of them have to be within the limits. Except it would not work very well with this glaze at all given the recipe.

 

I still can't see anything outside the limits in the original glaze recipe just the alumina looking too high. Are you taking the lower limits as values you can use in a glaze or that you need to be somewhere in the middle/top to have a food safe glaze? What is so bad about the lower end of the limits?

 

If it's not within the limits that doesn't mean it unsafe. Only a clue that it might be. Same the other way, being in the limits is only a clue that it is food safe glaze or a durable glass.

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Just arrived at this thread so sorry late to the party. Rest assured you are in good company in the endless search for a good clear glaze.

 

Search "bubbles" on this forum as some of us have spent plenty of time researching/discussing this issue. Plenty of good pics as well as theories about what may be driving them. Some of those test suggest that the success of a clear glaze is far more sensitive to the Si:Al ratio than other glaze types, due to all the maladies you are experiencing and then some.

 

Also look at the Currie tile thread in this forum which has lots of useful info and examples of this kind of glaze testing and what it can do. There is also some discussion of "limits" (a misnomer in my view, they should be called "guidelines"). I think you should do a Currie test once you have settled on what you think is the most likely list of raw materials. Get some successful glaze candidates before stressing too much over whether they fall inside or outside of these or those limits.

 

I would also say that Boron, the mainstay of many mid and low-fire glazes, is a likely culprit in these stories. It is a great flux at lower temps not least of all because it is an early and aggressive melter. This means it could be trapping gasses in the glaze (from both the body and other glaze materials, eg carbonates) that would otherwise be off-gassed. And potentially dirty sources of boron, eg gerstley borate or colemanite, may be introducing a host of other unidentified impurities which create new issues, off gassing and otherwise. At least using a Frit means the boron has been pre-cooked! And of curse there is the boron clouding....

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I may have just found the Unicorn, guys!

 

 

So I ran all my clear tests to date again (except for the ones that visibly had too much boron) over test tiles that had been bisqued to 04 instead of my usual cone 06. I varied the specific gravity, and I fired to cone 6 with a ten minute hold at the end. This, I know is is a shelf cone 7. The one that has turned out crystal clear?

 

The commercial dry mix.

 

The manufacturer's suggested specific gravity was 1.45, and that fired cloudy as all heck. I took it down to 1.28-1.3, flocculated with Epsom salt soloution, and here we have the results. The mug on the left is the thinner layer, the edge of the plate on the right is the denser mix from a previous firing.

post-63667-0-38022500-1465359417_thumb.jpeg

post-63667-0-38022500-1465359417_thumb.jpeg

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They did, but not to the same extent that the commercial mix did. It got wildly better than its previous tests of itself and the other recipes I was trying.

The commercial mix is actually an "open source" glaze listed on Digitalfire as G 2916 F that Plainsman sells pre-mixed. I tried for a good shot of the test tiles, but it's about an hour before sunset and the light is all wrong to capture it accurately. I'll post one tomorrow. I will also post the specific gravity that worked, as it's written down somewhere else right now.

https://digitalfire.com/4sight/recipes/plainsman_cone_6_m340_glossy_transparent_liner_115.html

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Very interesting recipe that got me having a good look at my best clear glazes. Nowhere near that level of silica in the glaze. I don't get it, how are they doing that. It has 70%+ mol silica in and when I go anywhere near that much my glaze are no longer clear or glossy even at cone10.

 

Luckily I have something I can get closer to an answer with if I could only get the homemade kiln controller running. Very confused now about glazes.

 

Glace C and A are my best. Glaze A I have added 32 extra silica to up it to 70%. I have run that test. By mistake I have also left in an 8 addition to glaze C on the screengrab.

 

gallery_23281_1027_167334.png

It is really not clear. Bottom right tile.

gallery_23281_912_153204.jpg

 

Sorry for going off on a tangent aboutmyself but silica is a tricky beast.

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They did, but not to the same extent that the commercial mix did. It got wildly better than its previous tests of itself and the other recipes I was trying.

The commercial mix is actually an "open source" glaze listed on Digitalfire as G 2916 F that Plainsman sells pre-mixed. I tried for a good shot of the test tiles, but it's about an hour before sunset and the light is all wrong to capture it accurately. I'll post one tomorrow. I will also post the specific gravity that worked, as it's written down somewhere else right now.

https://digitalfire.com/4sight/recipes/plainsman_cone_6_m340_glossy_transparent_liner_115.html

Hmmmm...some tantalising clues there, Diesel.

Was it the slightly higher bisque? Tightens up the clay body, enables thinner application, less glaze layer for bubbles to sit in...or, was it the specific gravity adjustment for much the same reasons?

 

Or Was it the ingredients? Comparing 1947U and M340 in Joel's post, silica and alumina combined are fairly comparable. However, M340 has a double helping of turbo-charged potassium and sodium, a massive dose of nice clean pre-baked Frit, much less of that relatively refractory calcium, double espresso of boron, pinch of barium....hmmm... A nicely balanced flux package chock full of stuff that will melt early and actively. Only thing missing if one wanted to quibble would be a bit of lithium, which is also quite active at lower temps.

 

Joel, extending this approach to your glazes C and A (which I think we have seen before?) are those cone 6 glazes, or cone 10, I forget? C seems to be trying to trade off very high calcium at one end of the barbell against very low alumina and silica and massive boron at the other end of the barbell - with not a lot in between. If this is a high-fire glaze it would be leaning way too heavily on boron in my view.

 

Glaze A seems more balanced if it is a cone 6 glaze. If it is a high fire glaze it almost seems over-fluxed compared to 1947?

 

How are these two glazes working for you?

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The recipe certainly got me thinking much more about the raw ingredients than the actual chemistry. My glaze are cone10 and I can get nowhere near that amount of silica to melt. If I do start adding large parts of silica they become underfired and nothing like a crystal clear. It is a nice little bit of chemistry with some good material properties I think.

 

Glaze A is working well for me but I don't use it with the 32 silica in the recipe above. They have been posted before, always good to be able to look back at my tests on the forum and remind myself. Why does my chemistry not match anything  :huh:

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Joel, why did you take the kaolin or the China clay out of your glaze A? If you're sourcing all your silica straight, it seems like it would sink like a rock in the bucket, and I'm not surprised you're having issues with additions being refractory. When some of it comes from clay, it's already partly combined into the glaze so the flux and the heat have less work to do.

 

Curt, I'm pretty sure my fix is coming from a) a more thoroughly off-gassed bisque making fewer bubbles to start with (red clay from my neck of the woods is notorious for having lots of carbon, organics and other fun stuff) and B) a thinner layer of glaze from the specific gravity adjustment. As you say, less room for bubbles to get caught in. Here are some images, and my observations.

 

The plate is the same clay body bisqued to 06 and a specific gravity in the 1.4 range, which I thought would be lowered enough from the recommended 1.45. Obviously not, though. It's quite bubbly and cloudy, there's some minor pitting, and the glaze crawled in one tiny spot on the front. I think the crawl was from dusty bisque, though. Over the white clay slip decoration, it's a perfectly acceptable clear, with no yellowing or alteration of the Mason stain colours I use. It just looks awful over the red base clay.

 

The test cup in my hand was bisqued to cone 04, has the same glaze with an SG of 1.3, is markedly less bubbly and looks a lot nicer over the red clay. Still crystal clear over the white. The glaze is markedly smoother. I had to flocculate the glaze to keep it suspended, but both pieces were dipped the same way: quickly, with no holds. Notably, once the glaze was flocculated, the same amount seemed to stay on the cups, regardless of the amount of time they were in the glaze.

 

That glaze was originated by the Fusion Frit folks, and was intended for industrial dinnerware, so I feel that the chemistry probably wasn't at fault, just my application over a clay body that has some particular requirements.

post-63667-0-90665700-1465683144_thumb.jpeg

post-63667-0-90665700-1465683144_thumb.jpeg

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These tests were made up to have no quartz or kaolin in to try and stop bubbles. I was trying to source as much alumina and silica without adding any, with the idea being it is all chemically combined with the flux so a much better mix for melting and no LOI in the recipe. C has a little whiting to really push the calcium.

 

I then ran test putting in silica additions which you can see above (also tests incrementing the frit). When the recipe gets to 32 silica added (which is the chemistry above) it really starts to not fully melt. Even adding 32 raw quartz only just gets up to 70%mole which is still lower than those other two glazes.

 

I think I may be confusing people with my ramblings with what is going on. The 32 silica in glaze A is not in the recipe but an extra 32 on top of the recipe. It is a little difficult for suspension.

 

There's probably still very small bubbles in your thinner glaze but if you can't see them that's the best I think you will get.

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There's probably still very small bubbles in your thinner glaze but if you can't see them that's the best I think you will get.

 

Most certainly it's this. They might not be really obvious in that photo, but the bubbles are certainly there. They're just a lot smaller, and less numerous.

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