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Big glaze shivering problems


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Hello everyone, I've been testing some new cone 10 recipes and this happened just twice with two completely different glazes in different firings, so I'm starting to worry a little.

I made cone 10 Honey Luster glaze and already tested it on tiles, so I decided to go for a bigger test and glazed some pots with it.  Apparently, in the middle of firing, the glaze went off the pot and splashed all over my kiln shelf. However all the areas where the glaze dettached from the pot are glossy and transparent so some glaze sticked to the pot and some glaze came off. I measured specific gravity before applying it and it was around 145 which is the recommended for this glaze. Also the glaze was sieved twice before aplication.. Inside glaze is perfect, only outside presents problems. I used pouring technique both inside and outside. Any ideas why this happened? 

EDIT: I looked what materials were in common between these two problematic glazes and in both I used colemanite as a substitute for gerstley borate. According to digitalfire:

Higher percentages of colemanite in a glaze can result in wrinkling of the fired surface, likely due to to a phenomenon called 'decrepitation' (very active decomposition) that occurs when colemanite is heated. The glaze layer can actually delayer, even disintegrate if sufficient colemanite is present (pieces of glaze can be spit off the ware onto other ware or the kiln shelf. For many applications Ulexite is thus a better choice. If you must use Colemanite, be sure to screen out any materials coarser than 200 mesh, or ball mill the glaze. Gum or other binders also help.
 

This is more likely the problem. What do you think?

 

img2.jpeg

img1.jpeg

Edited by thiamant
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Sounds like you likely have self diagnosed actually. What percentage of colemanite did you use,  what is the original recipe and what is your recipe with the colemanite?  Cone 10 glazes don’t really require boron to melt so it seems a bit odd that the original recipe included Gerstley.

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Posted (edited)
15 minutes ago, Bill Kielb said:

Sounds like you likely have self diagnosed actually. What percentage of colemanite did you use,  what is the original recipe and what is your recipe with the colemanite?  Cone 10 glazes don’t really require boron to melt so it seems a bit odd that the original recipe included Gerstley.

Indeed. I looked out one of my test tiles had the same problem. Also had colemanite in it. Can't be a coincidence.

The recipe has a high percentage of gerstley in it, 11.7%:

https://glazy.org/recipes/6371

This glaze is also in john britt's book high fire glazes.

I subbed 1:1 so, yeah... What other substitutions would be possible?

Edited by thiamant
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2 hours ago, thiamant said:

I subbed 1:1 so, yeah... What other substitutions would be possible?

Yeah, colemanite is boron and calcium so 1:1 won’t do. You can look each material up on Glazy and see the difference which Is significant in boron and of course lots of calcium. That recipe likely melts at cone six with its original recipe,  with the colemanite likely much lower. To try and substitute you will need to drop this into a glaze calculator  and modify to match as close as practical.

As a simple guess to make this more equivalent in boron to Gerstley your recipe would include about 4.25g/100 of colemanite instead of the 11.5G/100 Gerstley which then makes you increase the dolomite to about 9.5G/100 instead of the original 7.3g/100 all better for the decrepitation thing btw.

someone here will likely plug into a glaze calc for you, if not I will try later when home and post a possible workout.

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Here's a revision that uses Frit 3134 instead of Gerstley, which also increases the clay content for better suspension and application. Your other option is to use Gillespie Borate, which I've found to be much less problematic. It's technically a 1:1 sub for Gerstley, however I've found Gillespie to be a bit stronger, so I usually decrease it by 3% for starters.

 

Honey Luster Revised.pdf

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Going off topic from your glaze question but may I ask why you chose to fire cone 10 in an electric kiln? I noticed you are in Barcelona, is there the option to get cone 6 (midfire) clay bodies in Spain? Main reason I ask is firing to cone 10 versus cone 6 is going to significantly reduce the life of your kiln elements.

image.png.e61490403af173104d997d1e732770ab.png

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Thank you for all your comments and help. I Will try your suggestions... About going down to cone 6... I already planned to do some cone 6 tests when I have time, around 1220C. For now im doing slow firing to 1260 which almost achieves cone 10. (Maybe 9.5 or a bit higher) is it worth changing to 1220? My stoneware could be fired to 1220 but the clay manufacturer recommends 1240 minimum... some people told me that this clay can be fired to 1220 too without too much of a difference so maybe I will give it a try. But then i have to repeat all my glaze tests...

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Just about anything you can do at cone 10 in an electric you can also do at cone 6.

23 minutes ago, thiamant said:

Because gerstley borate is so common in north america maybe it would be nice to know a good substitute. I need to try these glaze calculators.

Do you know which boron frits are available to you?

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46 minutes ago, thiamant said:

My stoneware could be fired to 1220 but the clay manufacturer recommends 1240 minimum... some people told me that this clay can be fired to 1220 too without too much of a difference so maybe I will give it a try.

It might be possible to fire to 1220 but in all likelihood the absorption will be too high which means functional pots like cups etc would weep. If firing to 1220 use a clay that has that at the top of it's firing range.

Can you get Ferro Frit 3134 or Gerstley Borate? If not, which boron frits can you get? If colemanite is your only option then to have the same formula as the original recipe you linked to adjusted to use colemanite below, it still might have the decrepitation issue, don't know for sure, you would have to test it and see. Just round off the numbers to the tenth decimal point for a 100 gram test if you try it. I would add 2 bentonite to it also.

187034074_ScreenShot2021-03-10at9_51_33AM.png.f50e08aa867a9eeb1254202ea2e23e79.png

 

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38 minutes ago, Min said:

It might be possible to fire to 1220 but in all likelihood the absorption will be too high which means functional pots like cups etc would weep. If firing to 1220 use a clay that has that at the top of it's firing range.

Can you get Ferro Frit 3134 or Gerstley Borate? If not, which boron frits can you get? If colemanite is your only option then to have the same formula as the original recipe you linked to adjusted to use colemanite below, it still might have the decrepitation issue, don't know for sure, you would have to test it and see. Just round off the numbers to the tenth decimal point for a 100 gram test if you try it. I would add 2 bentonite to it also.

187034074_ScreenShot2021-03-10at9_51_33AM.png.f50e08aa867a9eeb1254202ea2e23e79.png

 

Well after some bad experiences I don't think I'll ever use colemanite again? haha

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Using your Frit PR-1000 is an easy fix. My version of the recipe using this frit below, totals 100, round off the numbers. I'ld add 2 bentonite (plus your manganese dioxide). Just to double check that this isn't a shivering issue, do you have a gloss glaze recipe that fits this claybody without shivering that can be compared to this one? COE for this (using Insight data) is 6.4 without the manganese, it would be good to compare just in case the glaze problem being  decrepitation is a red herring. It would be interesting to see if the non colemanite glaze shivers or not, not many people on this forum using colemanite, it would be good to get some info from this.

1722828944_ScreenShot2021-03-10at11_01_12AM.png.3eebf6d91e8d6fffcc8d4c5b4b6b419a.png 

Edited by Min
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Posted (edited)
34 minutes ago, Min said:

Using your Frit PR-1000 is an easy fix. My version of the recipe using this frit below, totals 100, round off the numbers. I'ld add 2 bentonite (plus your manganese dioxide). Just to double check that this isn't a shivering issue, do you have a gloss glaze recipe that fits this claybody without shivering that can be compared to this one? COE for this (using Insight data) is 6.4 without the manganese, it would be good to compare just in case the glaze problem being  decrepitation is a red herring. It would be interesting to see if the non colemanite glaze shivers or not, not many people on this forum using colemanite, it would be good to get some info from this.

1722828944_ScreenShot2021-03-10at11_01_12AM.png.3eebf6d91e8d6fffcc8d4c5b4b6b419a.png 

I forgot to mention I also have frit 3134 that i ordered from an UK store. (although it's not available in local shops :( )

Well considering I did a lot of test tiles on this firing and ONLY one of the tiles that had colemanite in it (the others didn't) presented the same problem... I think it's pretty clear. :) BTW May I ask what software you are using for recalculating the glazes? I'm kind of new to experimenting with substitutions etc.

 

Just so you guys can check, this picture is buttermilk cone 10 glaze where i subbed GB for colemanite 1:1.. same problem.

 

buttermilk.jpeg

Edited by thiamant
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1 hour ago, thiamant said:

Well considering I did a lot of test tiles on this firing and ONLY one of the tiles that had colemanite in it (the others didn't) presented the same problem... I think it's pretty clear. :) BTW May I ask what software you are using for recalculating the glazes? I'm kind of new to experimenting with substitutions etc.

 

Just so you guys can check, this picture is buttermilk cone 10 glaze where i subbed GB for colemanite 1:1.. same problem.

I agree that it probably is decrepitation, I was just trying to rule out any other possibilities. Given that if you do a 1:1 substitution of colemanite for gerstley borate there's nothing to show that a lower amount of colemanite (to balance the boron as in the original recipe) in a formula will also cause this problem. 

I use Insight glaze calc. There is a learning curve in using any of the glaze calc programs, knowing what oxides your materials supply helps speed up the learning curve. Other good thing about them is you should be able to enter your specific chemistry for the materials you use. There is a difference in kaolins, ball clays, feldspars etc, using the most accurate data available to you is helpful rather than theoretical analysis. Example would be the glaze revisions done for this recipe, we don't have the actual specific analysis for your colemanite or ball clay etc.

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4 hours ago, thiamant said:

I think it's pretty clear. :) BTW May I ask what software you are using for recalculating the glazes? I'm kind of new to experimenting with substitutions etc.

 

Just so you guys can check, this picture is buttermilk cone 10 glaze where i subbed GB for colemanite 1:1.. same problem.

Lots use insight, glazemaster is free now I believe, Glazy.org will let you input recipes and do the umf calcs.  for you as well as a stull map and their material database is very broad  for the price of creating an account (Free). Głazy will not substitute on material for another though.

obviously  there is a bunch to learn about głazes and not everything is known actually. Your defects are known to be related to apportionment wherein some amount (% of recipe)  of colemanite or even Gerstley can have certain adverse affects. Often  related to drying characteristics of the mixture.

Anyway, it’s the fun and interesting yet intricate part about learning more and more about glazes and as time goes on folks will come up with new ways to help examine and continue to build on existing knowledge. A calculator helps, but experience and time spent learning is the thing that allows one to solve issues or design their own along with. ........... lots of orderly testing.

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honey luster-1971

I used this as my go to liner and lip dip glaze when I made my living with cone 10 reduction stoneware-never had an issue with it

I know potter who made his living for 40 years with this recipe -no issues on stoneware reduction  at cone 10-he also used it as a liner and top dip glaze 

other cones I have no idea as you will need to adjust it to flux at lower temps.

Kingman or any potash spar

Modern sub would be Custar-1285

Kentucky ball clay (om4)  225

Colemanite /grestly borate 361

Dolomite                                     230

Talc                                          441

silica                                            599

Mag diox                                  250

Edited by Mark C.
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Posted (edited)
22 hours ago, Min said:

Going off topic from your glaze question but may I ask why you chose to fire cone 10 in an electric kiln? I noticed you are in Barcelona, is there the option to get cone 6 (midfire) clay bodies in Spain? Main reason I ask is firing to cone 10 versus cone 6 is going to significantly reduce the life of your kiln elements.

image.png.e61490403af173104d997d1e732770ab.png

About this diagram, does the duration of firing affect the kiln elements life also? My main problem is I only get to choose between low fire (900-1100) and high fire (1250+), so really 1220 is kind of land of nowhere? If there's not going to be that much difference between 1220 and my long 1260 firing I'd rather stick to cone 10.

This is the firing curve that I'm using: 

20 - 200ºC  @ 100º/h        ||       392 ºF @ 212 ºF / h

200-1050ºC @ 142 º/h    ||       392-1922ºF @ 288ºF/h

1050-1175ºC @ 100 º/h   ||     1922-2147ºF @ 212ºF/h

1175-1260ºC @ 60º/h      ||      2147-2300ºF @ 140ºF/h

1260ºC  soak 30 min         || 2300 ºF soak 30 min

With this I almost achieve cone 10 (9.5+)

 

Edited by thiamant
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@Mark C.,  I entered your recipe into Insight, look at the formulae for your recipe and the one I redid using the EU frit, they are more or less identical. (which is a sub for the original Honey Lustre with g.b.) Looks like your recipe is now being called Honey Lustre.

Re colemanite, I remember copying out some glaze recipes years ago from one of my first instructors. In her glaze recipe book she wrote colemanite but I remember her saying that she now uses gerstley borate 1:1 as a sub. I have to think the colemanite that was available in North America years ago was fairly different chemistry wise compared to the colemanite available in Europe today (which I believe is mined in Turkey). @thiamant's results would back this theory up as the colemanite looks to have caused the problem yet it sounds like you and your friend used this recipe with North American colemanite with no issues. Getting an analysis for the colemanite the op is using would be interesting.

"...does the duration of firing affect the kiln elements life also?" Yes it does (and the relays etc life). Good article on improving element life here.

1341922092_ScreenShot2021-03-11at9_32_44AM.png.42409dc2fab0387e43cc424ccbc319ad.png

 

 

Edited by Min
grammar
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I wonder if roasting the colemanite to drive off some of the chemical water would help with the decrepitation. If it helps with ingredients like Alberta slip or Ravenscrag which are also prone to trying to crawl off the pot.

I also seem to recall being told that Gerstley Borate could be subbed 1:1 in recipes, although I’ve never worked with the former.

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Min just got in from 12 hour doctor trip from hand surgeon in SF-long day and i'm spaced

i forgot to add the name honey luster

i have that glaze from 1971

I used colemanite in the 70's and gerstly borate in the 80s or whenever the other ran out-bought ton of gerstly borate when laguna said that was the end before they bought the mine and have an unlimited supply like now-i'm down to 20-30 bags now-i handeded them out as party favors to my potter friends for years-trouble is i do not use much over a year.

worked well ether way whatever i used

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1 hour ago, Callie Beller Diesel said:

If we’re looking at a different geological source for the material though (US vs Europe), there’s going to be working differences between the 2.

Absolutely. Even within the same mine there are going to be differences. Getting an analysis would help. 

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Honey Luster was a class glaze from JC school for us.

50 years later I do not use it but for at least 30 -40 years we had no trouble with it

I do not use it on porcelain. Most likely gave my bucket away long ago.

I use another liner on stoneware in salt kiln fires

I have seen issues around it but they where all clay issues with stoneware.

Back then If the clay shrank to much you would just use another body-cone 10 stoneware bodies are a dime a dozen so making changes is easy

This was a liner glaze as well mostly for us

I still use small amounts of grestly Borate  in a few glazes with zero issues.Cone 10 seems to flux out any issues

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