I've noticed that some people are getting some very nice results by refiring ^ 6 stoneware at ^06. Often the colors are are richer and more variegated. John and Amy Oilspot is an example found in the insight live recipes.

Is there any logic to when this works and when it doesn't?

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I keep staring at this post and wondering how that can be.  Refiring 10 cones lower shouldn't do anything, but it would be very easy to put some pieces in with bisqueware to 06.

I am new to this site.  I don't know where to find the insight live recipes.  Can you post a link to what you found?  Exciting....

Glazes crystallize and freeze between 1,800 F (~cone 06) and 1,500 F (~014).  How slow the glaze cools during this temperature range, or sometimes with macro-crystalline glazes repeatedly reheated and cooled within this range, largely controls the finished look of the glaze.

Before and during this temperature range where the glaze sets up, rutile slowly dissolves trailing white tracks behind it as it rides the thermal currents in the glaze and falls down the side of ware by gravity.  Some have said red iron oxide loses an oxygen bringing the under layer of of an oil spot glaze to the surface, but I believe it merely thermal currents as this process happens primarily at temperature below which red iron oxide decomposes.

Earlier in the firing, most glazes are merely melting and off-gassing.  Including more frit in the recipe leads to earlier melting since the frit raw materials have been previously supplied with the activation energy needed to fuse. Their molten state helps disperse non-frit materials into the melt at much lower temperatures and in less time than would otherwise be the case. 

For this reason glazes sourced mostly from frits will be far more reacted at the end of a firing than a glaze sourced from materials which take more time to melt.  Hold any glaze long enough at a peak temperature and it will finish reacting fully and always look the same.

As an example this is a Butterscotch glaze.  Although chemically identical, the glaze on the left used mostly frit and is far more uniform and reacted fully.  The frit glaze on the left combined with gravity has had enough time to obscure the lines between one dip on the bottom, two in the middle, and three dips on the top.

The tile on the right used non-frit materials, so is only partially reacted by the time it cools - with the dip demarcations still visible to an extent.

An undesirable example would be Amaco Cone 5/6 PC-42 Seaweed which displays colored swirls within clear glass, especially when cooled quickly.  However, if you cool this glaze very slowly, or refire to Cone 06 the entire glaze will have fully reacted to the drab olive green you see on the rim of this cup.

http://www.bigceramicstore.com/media/catalog/product/cache/1/color_image/180x180/9df78eab33525d08d6e5fb8d27136e95/1/7/17.png

So a second firing at a lower cone is essentially a different way to achieve a slow-cool. The trick with each glaze is to figure out when to stop the mixing process of different density layer through thermal current and how much crystallization  to allow.  With a very slow cool, I control the crystallization process by adding additional or less flux and glass.

Sometimes a second firing around Cone 06 is for an entirely different purpose.  Tim Sullivan fires to this lower temperature i oxidation after firing his pieces in reduction to cone 10.   I frequently add additional cone 6 and cone 06 glazes or fluxes on top of previously fired Cone 6 glazes prior to the second cone 06 firing.

Most Cone 10 glazes require firing to Cone 10 to melt, just as most Cone 6 glazes need to be fired to Cone 6.

But once melted, just like man-made frits, these glazes become fluid again at a much lower temperature, and the mixing process and any remaining chemical reactions continue once again.

Ceramic fluxes are "catalysts" which lower the activation energy required.


Dawn Atkin said:

I keep staring at this post and wondering how that can be.  Refiring 10 cones lower shouldn't do anything, but it would be very easy to put some pieces in with bisqueware to 06.

I am new to this site.  I don't know where to find the insight live recipes.  Can you post a link to what you found?  Exciting....

Great info, Norm.  I do crystalline glaze firing.  Crystals form best in a certain area of the my kiln, so I fill the rest of the kiln with other cone 6 glazes and often get micro crystals and other great surprises.  I have a number of glaze recipes I use that get micro-crystals, but for our purposes here, Coyote Clay's Gun Metal Green, Blue Purple and Red Gold crystallize nicely when held at 1925.  I had a couple of pieces with the Coyote Clay glazes fail in my last firing (as is normal with crystalline fires.  Grumble.)  I will try refires at 06.  Before and after pictures next week.

I have a test piece with a macro-crystalline zinc nickel glaze.   I had held the temperature at 1,950 F for only 30 minutes then slow-cooled at 50 F per hour down to 1,500 F.   This resulted in a few 1/4 inch blue tone zinc silicate crystals on an amber transparent background.

I refired this tube to Cone 04 without a slow-cool the entire piece became blue crystals without any amber background showing.  We'd have a lot more direct control over what happens if we had a live feed of these pieces inside the kiln, but we don't.


Dawn Atkin said:

Great info, Norm.  I do crystalline glaze firing.  Crystals form best in a certain area of the my kiln, so I fill the rest of the kiln with other cone 6 glazes and often get micro crystals and other great surprises.  I have a number of glaze recipes I use that get micro-crystals, but for our purposes here, Coyote Clay's Gun Metal Green, Blue Purple and Red Gold crystallize nicely when held at 1925.  I had a couple of pieces with the Coyote Clay glazes fail in my last firing (as is normal with crystalline fires.  Grumble.)  I will try refires at 06.  Before and after pictures next week.

Well, now that we can check the kiln computer and make adjustments from the IPhone, I can't imagine an internal camera is far off.  : )


Norm Stuart said:

We'd have a lot more direct control over what happens if we had a live feed of these pieces inside the kiln, but we don't.

This is the Nickel Zinc macrocrystalline glaze, with 1% red iron oxide for nucleation.

On the Left, held for 30 minutes at 1,950 F, then slow-cooled for six hours between 1,800 F and 1,500 F.  Small blue tinged zinc silicate macro-crystals on upper edge with amber background  - the typical look of .

On the Right, the same test tile refired to 1,888 F then the kiln cooled far faster without any slow-cooling.  Macro-crystals apparently gone? Yet small crystallization over the entire tile.

109%  Macrocrystalline ^6
  52%  Ferro Frit 3110
  24%  Zinc Oxide
  24%  Silica
   2%   Lithium Carbonate
   6%   Nickel Carbonate
   1%   Red Iron Oxide

A similar glaze with less silica and zinc - so no zinc-silicate crystals.  Less nickel - and a calculated COE of  20.84!  Slow-cooled between 1,800 F and 1,500 F.

100%  COE 20.84 - Crackle ^6
  50%   Nepheline Syenite
  30%   Soda Ash
  20%   Pearl Ash
  6%     Zinc Oxide
  0.5%  Nickel Oxide

111%  Olive Speckle ^06 - different crystallization in a Fluorine Frit
100%  Ferro Frit 5301
    1%  Bentonite
  10%  Nickel Carbonate

Adapted to Cone 6 with added 10% Silica and 10% Kaolin.  The yellowish green of Nickel Fluoride.

That olive speckle is speckletacular. 

The first test is such a perfect example.  Clearly the crystals in the glaze are blue, so if you can get microcrystals, you get blue glaze.  I cant tell from the photo, does it have a stoney matte finish after the refire?  

The more crystallized surface on the re-fired tube on the right is darker and more opaque, but the actual surface is just as glossy - so the surface is not de-vitrified as you might see in some macro-crystalline glazes.

Dawn Atkin said:

That olive speckle is spectacular. 

The first test is such a perfect example.  Clearly the crystals in the glaze are blue, so if you can get microcrystals, you get blue glaze.  I cant tell from the photo, does it have a stoney matte finish after the refire?  

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