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Interesting Dilemma


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#1 Pots by Char

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Posted 15 August 2014 - 08:02 PM

I have two recipes for a clear glossy glaze..one is for low fire 04 and one is for mid range 5/7.  The interesting part is that they are practically identical recipes..only difference is the order of ingredients..How can this be?  I have actually made these up and tested them and they do fire to the cone they are supposed to.. The low fire recipe is Gerstley Borate 55, EPK 30, Flint 15.  The mid range is Gerstley Borate 55, Silica (or flint) 15, EPK 30.  Can the order you put things together have that much effect? And can a glaze have such a broad firing range?  I have made my own glazes for a while now but always using someone else's recipe...I don't claim to really know anything about glaze calculation, I just follow the recipe like cooking a stew. But this does mystify me...So if you glaze calculating experts out there could weigh in on this with some answers I would be most appreciative.

 



#2 neilestrick

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Posted 16 August 2014 - 08:44 AM

One is underfired or one is overfired, or both.  With the exception of the silica, it's close to being within limits for both temperatures. The silica is quite low in both cases, though.

 

Personally, I tend to avoid glazes that rely so heavily on one ingredient.


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#3 Dick White

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Posted 16 August 2014 - 07:54 PM

Try reversing the amounts of silica and EPK, to Gerstley 55, EPK 15, Silica 30. That is close to the cone 6 clear we have been using at the community studio forever.



#4 Babs

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Posted 16 August 2014 - 09:02 PM

One is underfired or one is overfired, or both.  With the exception of the silica, it's close to being within limits for both temperatures. The silica is quite low in both cases, though.

 

Personally, I tend to avoid glazes that rely so heavily on one ingredient.

Why is this Neil?



#5 neilestrick

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Posted 19 August 2014 - 08:43 AM

 

One is underfired or one is overfired, or both.  With the exception of the silica, it's close to being within limits for both temperatures. The silica is quite low in both cases, though.

 

Personally, I tend to avoid glazes that rely so heavily on one ingredient.

Why is this Neil?

 

 

If something goes wrong with that ingredient, your glaze is ruined.  When you have more ingredients in smaller amounts, an issue with one ingredient will have a smaller effect. I also find that glazes with more ingredients are easier to tweak, and offer more possibilities for substitutions should the need arise.


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#6 Babs

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Posted 21 August 2014 - 09:16 PM

Thanks, logical one!



#7 jammy43

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Posted 22 August 2014 - 12:29 PM

here is interesting info on Gerstley Borate from Digitalfire:

 

No common natural material comes anywhere close to melting like Gerstley Borate (GB). It begins to melt between 1550F and 1600F and is a clear amber glass by 1750F and ultraclear and glossy by cone 06 (Ulexite melts better but it is not commonly in use in ceramics). It has thus been a staple among potters for many years. 50% can be found in many cone 06-02 glazes, up to 30% in cone 6 glazes.



#8 neilestrick

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Posted 23 August 2014 - 08:51 AM

here is interesting info on Gerstley Borate from Digitalfire:

 

No common natural material comes anywhere close to melting like Gerstley Borate (GB). It begins to melt between 1550F and 1600F and is a clear amber glass by 1750F and ultraclear and glossy by cone 06 (Ulexite melts better but it is not commonly in use in ceramics). It has thus been a staple among potters for many years. 50% can be found in many cone 06-02 glazes, up to 30% in cone 6 glazes.

 

Yep. And whenever there was a problem with the consistency of Gerstley, which was all the time, everyone's glazes went bad and they panicked. That's why I go with a substitute like Gillespie Borate. Always consistent.


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#9 Dick White

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Posted 23 August 2014 - 10:59 AM

Yes, there was an issue with the consistency of Gerstley while the mine was still open. That is the nature of natural mined materials. As you get to different parts of the geologic deposit, the content changes. But the mine closed in approximately 2000, and Gerstley went off the market. A variety of manufactured fritted substitutes were developed, such as Gillespie Borate, Laguna Borate, and others. However, a year or two later, Laguna purchased all the remaining stock of unprocessed material from the entire mining site, and processed it as one HUGE homogeneous batch. That was 3,000 tons of it (6 million pounds)! So what you buy today will be the same as you will buy in the future until the Laguna warehouse is empty (probably decades...). And at that point, Gerstley will be no more.



#10 Mark C.

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Posted 23 August 2014 - 11:15 AM

Back in the day when Grestley mine was headed to close I bought a ton. I have had no issues whith this ton. Its more than I will ever use in my life. I sold 300# of it in early 2000's. If anyone needs some I have it.

I did the same with Kingman spar back in the day(3,000#s)-I'm down to the last 800#s now.That was in 100# bags

Having a solid steady supply of materials keeps your glazes consitant.

none of these cost much-the kingman was 300$ but I had to drive down and pick it up. from Kingman Az.

Mark


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