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Bob Coyle

Glaze Test Ga28-A Green Breaking Blue

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This glaze was taken from Glazes Cone 6 by M Bailey and modified. It breaks blue where thick on clays containing iron. Really pretty on Laguna speckled buff.

Glaze test on Laguna clay bodies. Fired up to cone 5 with kiln setter shut off. 125 F/hr from 1700F to shutoff.

Comp              %

Neph Sy            68
G.Borate           08
Ball Clay           08
Whiting             05
Silica                04
Rutile               02
Titanium           02
Cobalt Carb      01
 

post-45594-0-96408200-1394212347_thumb.jpg

post-45594-0-96408200-1394212347_thumb.jpg

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The color is great, but I would definitely check it for durability and leaching. The sodium is off the charts in the limit formula, the alumina is pretty high, and that's a lot of cobalt. If you look closely at your tiles you're getting a lot of rough, pitted areas which aren't great for functional wares and signal that something is wrong in the glaze. Of course, all of these things are probably what's contributing to the great color, so you'll have to decide what's important. It would be worth testing some variations on this glaze to see if you can get the numbers within limits and still retain the color.

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Neil 

 

The leeching is no problem. All of my glazes soak for 2- 4 days in a plating bath containing a strong solution of sulfuric acid. If they survive this they will probably be OK with tomato juice. Also none of this is used for dinner ware, although I don't think anyone would get poisoned by using it on a mug.  

 

The rough pitted areas are kind of a "feature". They have a tendency to level out with more of a soak. The glaze is right on the edge of being under fluxed. At cone 6 it is smoother and shinier but more blue with less break. I like the more rough matte surfaces and try for this look on most of my glazes.

 

The proportions are really not that far off from those in M Bailey's book. His examples were all functional potter.

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What charts are these? I have no charts that tell me things like this  :mellow:

 

Beautiful glaze, would love to see one of your mugs with it on.

Oh you are way too literal up there! And they are not the music charts!! Spring will come soon and all will be revealed!

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What charts are these? I have no charts that tell me things like this  :mellow:

 

Beautiful glaze, would love to see one of your mugs with it on.

 

Glaze formula limit charts. They show the acceptable limits for each part of a glaze in the unity formula. If the numbers are within the limits, then a glaze is more likely to perform well. There are, of course, perfectly good glazes that have numbers outside these limits, but they are a good place to start, and can show you where to make adjustments if the glaze is having problems.

 

http://digitalfire.com/4sight/education/limit_formulas_and_target_formulas_206.html

High Bridge Pottery likes this

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The cobalt green effect in that glaze is due to the titanium (from the rutile) and the high level of alumina. Same effect as in the ^10 Reitz Green glaze. Because the high alumina is necessary for developing the green color, you probably won't be able to smooth out the surface by adjusting the glaze to a higher Si:Al ratio.

 

Interesting adjustment you made there by substituting additional Gerstley for the spodumene. You effectively changed out the lithium flux (from the spod) for twice the boron (from the additional Gerstley).

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Interesting adjustment you made there by substituting additional Gerstley for the spodumene. You effectively changed out the lithium flux (from the spod) for twice the boron (from the additional Gerstley).

That is correct Dick. I didn't have any spod at the time and tried an adjustment with Li Carb and upping the ball clay. I had a ton of G borate so I tried that as a flux  instead and it worked out OK. I added extra TiO2 to opaque it more.

 

I am trying for a more antique look and the roughness is fine. Firing at cone 6 will smooth out the bumps but produces more gloss with some loss of the green break. The green break also depends on the clay body. Works on darker stoneware bodies.

Dick White likes this

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