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I've not been able to find a decent clear glaze that doesn't craze.  I'm firing to cone 5, and I'm using a ^5 porcelain.   I've done a few of the recipes from the book the Complete Guide to Mid-Range glazes, as well as used a few commercial glazes, and have had zero success.  I've done numerous tests with different coats and I either get a clear that is too milky or crazes.  

 

It is most frustrating because I do a lot of sgraffito, and need to be able to produce pieces that I can sell, and not toss.

 

It only happens on the bare clay surface, not when it's applied to a black underglaze. 

 

Any recipes, or thoughts would be very helpful... I'm not very good with glaze chemistry.  

 

Thank you!!!

 

Laurène

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I have a very lovely clear that was reworked by Ron Roy for Sue Hintz. This is his Version #2. I have use this on Frost, Bray ^6 porcelain. Armadillo ^6 porcelain and Nara ^6 porcelain.No crazing.

 

Cornwall Stone   33.5

G200                   22

Whiting                18

Gerstley Borate   10

EP Kaolin               5.5

Silica 325 mesh   11

add bentonite        2

total                     102

 

works well with pink and Satin mason stains. It is a nice  satin to gloss at a strong 6 more satin at a medium 6. Does not run.

Marcia

Pres and Magnolia Mud Research like this

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Laurene,

 

Which of the clear recipes from that book have you tried? And which had the crazing lines the furthest apart? (the John Britt book is what you are referring to?) If you could post that info I'll see if my clear for porcelain has a lower coe, might do the job.

 

M.

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Thank you so much Marcia, I'll try it!

 

 

Laurene,

 

Which of the clear recipes from that book have you tried? And which had the crazing lines the furthest apart? (the John Britt book is what you are referring to?) If you could post that info I'll see if my clear for porcelain has a lower coe, might do the job.

 

M.

 

 

Yes it is from John Britt's book.  I don't have the recipes on me but I tried Kitten's clear and Easy E, both of which had a higher amount of Silica.   The Easy E didn't seem to craze but was bubbly and milky in spots, and the Kitten's clear crazed.

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Okay, so if you go to the clear recipes on page 72, have a look at the bottom row of figures on the chart of glazes. See where it says "Insight CTE"? Follow that along the bottom. The Kitten clear has a coe of 6.9 and the Easy E has a coe of 6.88 Those are both fairly high coe's. What this means is that generally speaking the higher the coe the greater the chance of crazing. I would mix up Marcia's recipe which has a coe of 7.71 and also the one called NDVC. If both of those craze then try one of the low coe glazes from page 77  Just a heads up though, if you use a glaze that has too low a coe you can get shivering which is quite a dangerous thing so once you find the glaze that doesn't craze I would test it for shivering to make sure you haven't gone too low.

 

BTW I think the Easy E clear might have been underfired at ^5 to look bubbly.

 

Edit: CoE of Marcia's recipe changed from 6.36 to 7.71 due to my error entering formula

S. Dean likes this

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Okay, so if you go to the clear recipes on page 72, have a look at the bottom row of figures on the chart of glazes. See where it says "Insight CTE"? Follow that along the bottom. The Kitten clear has a coe of 6.9 and the Easy E has a coe of 6.88 Those are both fairly high coe's. What this means is that generally speaking the higher the coe the greater the chance of crazing. I would mix up Marcia's recipe which has a coe of 6.36 and also the one called NDVC. If both of those craze then try one of the low coe glazes from page 77  Just a heads up though, if you use a glaze that has too low a coe you can get shivering which is quite a dangerous thing so once you find the glaze that doesn't craze I would test it for shivering to make sure you haven't gone too low.

 

BTW I think the Easy E clear might have been underfired at ^5 to look bubbly.

 

Oh, I see.  Thank you so much for explaining that for me.  I'm still new to gaze chemistry!!  :)

 

I'll do some tests this weekend and let you know how they come out!

 

Thanks again!

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I would mix up Marcia's recipe which has a coe of 6.36   corrected to 7.71

 

I'm getting 7.71, I checked all my formula % and they are spot on?

 

post-80153-0-71429100-1501331530_thumb.jpg

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 I would mix up Marcia's recipe which has a coe of 6.36

 

I'm getting 7.71, I checked all my formula % and they are spot on?

 

attachicon.gifJul. 29, 2017.jpg

 

 

Yeah, you're right. I didn't enter the recipe properly. 

thanks for catching that!

(I'll go back and correct that)

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What clay body are you using? That can have a major influence on crazing or shivering.

Marcia

 

+1 

 

It makes sense that the clear Marcia posted doesn't craze on Frost. Frost has a  coe of 6.99 so the glaze having a coe of 7.71 doesn't sound too high.  Wiggle room in how high the coe can go, I've found it doesn't have to be super close to the clay coe to fit. If the clay you use has a published coe then it saves a bit of work sorting out what coe range will likely fit. Also, not sure how accurate the published coe's will be, firing conditions being variable. 

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 I would mix up Marcia's recipe which has a coe of 6.36

 

I'm getting 7.71, I checked all my formula % and they are spot on?

 

attachicon.gifJul. 29, 2017.jpg

i trust Ron Roy's adjustment. This is Sue Hintz clear adjusted by Ron Roy. I just use it. Ron Roy was the tech at Digitalfire and co author of Mastering Cone 6 glazes. I take his word on it compared to people playing with COE without testing.

I have used this over several years wth no crazing....as recently as two weeks ago on a commission using Bray ^6 porcelain

 

 

just sayin'

Marcia

 

 

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Good to see Ron's COE figure is the same as yours. 

(I did  correct mine in my original post but can't change where quoted)

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I feel I'm having a problem with the obsession about the precision of the calculated COE of this glaze. Cornwall stone, when the real stuff was available, was variable. The blended substitutes that are available now are variable from one to another. The recipe provided by Marcia has G200, while the picture from RonSa has G200HP, and neither of these have been available for years. Gerstley back in the day 20 years ago was variable from bag to bag, while the current product distributed by Laguna is consistent but a different analysis from the "average" content of the old stuff. If you have expanded your materials list in your glaze software for all the different historical materials, you will find the calculated COE jumping all over the place depending on what combination or permutation you select. Does anybody know how long ago Ron reformulated this recipe for Sue Hintz so that we can guess at what version of the materials were in play at the time, and thus can work forward to recreate it with current materials?

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while the picture from RonSa has G200HP, and neither of these have been available for years.

 

I purchased 20# of G200HP from US Pigments 2 months ago and it looks like they still have some left..

 

https://uspigment.com/product/feldspar-g200-hp/

 

For now. Advertised as limited quantity. Mix it 70% HP with 30% Minspar and you'll have the same thing they were selling as original G200. Until it runs out. Then you'll have to buy either Laguna's new Mahavir spar or try to work around CusterLP. (G200HP stood for High Potash as that mine was higher K than from the earlier exhausted G200 mine. Similarly, we should refer to the current Custer product as Low Potash.)

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The materials changes are a royal pain, aren't they?

 

Sigh...........................

 

I feel like I spend my life updating materials data files.  And coming up with codes for old materials with the same names as new materials.....so that you can TRY to keep things straight.

 

best,

 

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

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My technique for keeping them straight in the data file is to use a consistent main name but add a date or producer indicator to the data title. Talc (generic), Talc - NYTAL, Talc - AMTAL; Gerstley 1989, Gerstley 1999, Gerstley 2012; Feldspar - Potash (generic), Feldspar - Custer 2008, Feldspar - Custer 2011; etc. And in GlazeMaster, I can have two recipes on the screen at the same time, one with the old material and the other with the new, and the UMF differences are immediately apparent. Still a PITA to dither with the new until it is reasonably close to the old, but better that than recipes that don't melt because Custer isn't Custer anymore.

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Thanks.  I do the date thing also.  It just makes recipe listing look weird.  And then there is the "select the correct year" aspect.

 

And Insight puts two side by side also.

 

best,

 

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

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Ok, I calculated the recipe using the original materials and then revised it to a nearly equivalent UMF with current materials. I'll test it when I do my next batch. Maybe we have the holy grail/unicorn of clear glazes? And lucky you to have some stocks of the originals.

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I use a lot of clear and have been on the hunt for clay/clear glaze fit for some time.  I started out using the clear from MC6, but found that on some white clays both porcelain and stoneware, I would get crazing.  I thought I had a good fit with Nara 5 porcelain and that clear recipe but discovered that crazing would appear later, not immediately.  I have switched to Laguna's #16 porcelain, and at this point, I am not getting crazing, but......I am going to do that boiling/freezing test to see what happens.  Laguna doesn't seem to list the COE.

In my quest for a better clear, I discovered that MC6 had another recipe using G200HP (which is what I have) so I mixed that and noticed no difference whatsoever. 

After all that, here is my question, I met a potter in Denver who works exclusively with Laguna #16 and uses one of the Low Expansion clears in John Britt's book on page 77.  She said it was so she would NOT have crazing.  If the clear has a low COE 5.54 or 5.71 doesn't that mean it would not expand as much and be prone to crazing?? 

Just sign me,

Seriously Confused

 

aka Roberta

 

Clear Recipe from MC6

G200 feldspar       20

FF 3134                20

Wollastonite          15

EPK                       20

talc                         6

Silica                      19

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