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Mid Fire Glaze Changes When Refiring At Lower Temperature

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I'm not sure why some cone 6 glazes change color rather drastically when refired to a lower temperature, but maybe someone can explain.

 

Specifically, Coyote Ice Blue glaze fired to cone 6 turned out the usual beautiful multi-hued blue, brownish at the breaks.  Then I decided to put some low fire clear glaze on the bottom and refired the piece at cone 06.  The result was an awful, mottled green and brown camouflage-like color.  Yeechhh!!  So I refired back up to cone 6 again hoping to recover the blues.  The piece now looks much better - the greens are gone - but the subtler blues are also gone and there is more and deeper brown.

 

As it happens, I did the same thing to several other pieces with different commercial glazes on them.  All of them suffered significant color degradation after the 06 low fire, and recovered only a portion of their original color in the second cone 6 fire.  With one exception: a bowl with a combination of Amaco Textured Turquoise and Amaco Iron Lustre looked pretty bad after the low fire but almost completely recovered its original color after being refired to cone 6.

 

I thought it was safe to refire pieces at a lower temperature, but I am obviously mistaken. 

 

Any insights on this phenomenon would be appreciated.

 

 

 

 

 

 

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I too have noticed this phenomenon. I use a lot of transfers and such so fire to cone 6 then refire to cone 04. White there is no issues with at all. Amacos Temmoku changes some but not in a bad way, coyotes Really Red, Sedona Sunset, and Sapphire do really well. The one with the most change is Coyotes Sunset pink there is quite a dramatic color shift. The Shinos from both companies change but I use them with an eye to exploit these changes so it's ok. I can't remember the names of some of the others but I have written on the bottles which ones not to use for a transfer piece.

 

I too would be interested in exactly why these changes occur chemically speaking. To me it looks like portions of the glazes chemical makeup soften at lower temps and then rise to the surface of the glaze or something.

 

T

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I too have noticed this phenomenon. I use a lot of transfers and such so fire to cone 6 then refire to cone 04. White there is no issues with at all. Amacos Temmoku changes some but not in a bad way, coyotes Really Red, Sedona Sunset, and Sapphire do really well. The one with the most change is Coyotes Sunset pink there is quite a dramatic color shift. The Shinos from both companies change but I use them with an eye to exploit these changes so it's ok. I can't remember the names of some of the others but I have written on the bottles which ones not to use for a transfer piece.

 

I too would be interested in exactly why these changes occur chemically speaking. To me it looks like portions of the glazes chemical makeup soften at lower temps and then rise to the surface of the glaze or something.

 

T

I'm sure the glaze chemistry changes  - for whatever reason - as the glaze goes from ambient temperature to 2200+° but it would be nice to know why so one could figure out in advance which glazes will react adversely to down firing.  Since you mention marked changes in Coyote Sunset Pink methinks I better be careful with Archie's Base, which I use a lot.  Hopefully someone here can provide more insight into the chemical goings on of these glazes under fire.

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My guess is some of the problem might be due to additional soak in of the glaze on refiring, and possible further reaction with the clay body . Also whatever fine crystal structure that  occurred at cone 6 was undone at the lower temperature. A lot of the "breaking" fine structure of glazes has to do with crystal formation.

 

I'm not sure you could use a rule of thumb to predict haw a glaze would react to refireing. Just have to try it on a test piece before you commit to a kiln load.

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I've noticed this with non commercial colours, iron seems to Brighten and the so reds become brighter, I suppose if there's any iron in the commercial glazes you are using it may be more dominant and thus muddy the waters?? In my case I was happier with the bright irons, make other glazes "softer" in looks.

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It  has to do with crystal growth. If you soaked the kiln at low temps you could get a similar result. Slow cooling an iron red will increase its redness, for instance. When you refire to the same temp, things change because the melt is different than the first time. Refires will tend to melt at a lower temperature, and things may get melted better than the first time with the additional heat work.

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Refiring a C6 glaze to low fire temperature is not always bad news years ago CM magazine ran a story and some glaze formulas  that came alive when refired and quite ugly just being fired at C6.  Denice

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It has to do with crystal growth. Iron is the most notable visually. It is a major factor for other ingredients as well. Sadly, the number of variable potters have with glaze chemistry (no to mention the use of natural ingredients) is far too many to give scientific analysis to.

 

 

How about a bad analogy. 100 people are in a room for a meeting. They go have lunch and come back to the same room. Will they all sit in the same seats?

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Mathew, They'll try to!!! :)

Neil. I know this has been posted before but can you give the cooling ramps from C 6 to accentuate this.  I think you posted something about when you do a soak on the way down but I can't find it with my searching.

Now firing the kiln with controller and this is may come more easily to  my repertoire now I hope!

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It has to do with crystal growth. Iron is the most notable visually. It is a major factor for other ingredients as well. Sadly, the number of variable potters have with glaze chemistry (no to mention the use of natural ingredients) is far too many to give scientific analysis to.

 

 

How about a bad analogy. 100 people are in a room for a meeting. They go have lunch and come back to the same room. Will they all sit in the same seats?

 

I like your analogy.

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Mathew, They'll try to!!! :)

Neil. I know this has been posted before but can you give the cooling ramps from C 6 to accentuate this.  I think you posted something about when you do a soak on the way down but I can't find it with my searching.

Now firing the kiln with controller and this is may come more easily to  my repertoire now I hope!

 

I cool at a rate of 175F/hr down from peak temp to 1550F. Many people let it crash cool down to about 1900 before starting the slow cool, but I do it from the peak so that I get the same results in all 3 of my kilns, which will crash at radically different rates. Some folks also go as low as 1400F, but I have not seen any difference in my glazes below 1550F. I've tried cooling at rates below 175F/hr, but some of my glaze come out too matte for my liking. I would start at 175F/hr and go from there, reducing the cooling rate in 25 degree increments. You could also try adding a hold at around 1900 and see how that affects things.

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Thanks Neil.

When you write "some folks... below 175degsF". do you mean  like 200degF, or are you meaning the other way 100Fdeg for example. I think it is going the 200F way.

Have a weird brain here, sorry.

AH Farhenheit  Glad I noticed that.!Now what was that conversion rate, I'll get that.

Can I give the USA metric measurements for Christmas?, easier maths...

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