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What's Happpening With This Glaze Chemically?


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#1 timbo_heff

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Posted 11 September 2013 - 05:58 AM

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Hi,

I have fired this glaze on a variet of clays and woodfired it: (same glaze on pots in my profile pict (longer all locust firing)

The backs are brown/amber probably like it would look in an electric or gas reduction but the face that gets ash (and more heat) turns into a nifty blue/green sometimes with amazing hare's fur effects.

So what is happening chemically on the ash face?

Is the ash adding silica and ergo diluting the ratio of the RIO ( as in a temoku iron reduction moving towards a celadon?

or is it adding oxides that are reacting?

Is behaving differently because of the additional heat on that side ?

 

.. and most importantly: how can I change this glaze to  behave like this all around the things, all the time?!? Less iron, more gerstley /gillespie ?

 

It's from John Britt's high fire glazes:

Custer Feldspar: 44.00 Grams spacer2.gifSilica: 12.80 Grams spacer2.gifWhiting: 17.70 Grams spacer2.gifBall Clay (OM-4): 6.10 Grams spacer2.gifRed Art Clay: 11.10 Grams spacer2.gifBone Ash: 2.00 Grams spacer2.gifTalc: 3.80 Grams spacer2.gifGerstley Borate: 2.50 Grams spacer2.gifRed Iron Oxide: 4.10 Grams

 

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#2 JBaymore

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Posted 11 September 2013 - 09:01 AM

Tim,

 

A couple of things are getting added on the fire facing side.......

 

Wood ashes are mostly a source of calcium oxide in the final melt.  SO the vast preponderance of what is "added" is calcium oxide.

 

As SMALL amount of the volatiles from the wood ash that are getting vaporized out of the ash are also being depositied there.  Mainly this is potassium oxide with some sodium oxide.  But compared to the level of calcium oxide..... very small.

 

Also there is the factor of the TIMING of when / how this stuff is getting added.  The fire facing side is getting hotter before the away from the fire face.  THAT is a big factor also.

 

Then there is the atmosphereic differences...... the fire facing side is getting a different atmosphere at a different time from the not fire facing side. 

 

It is WAY complex what is going on in there.

 

best,

 

............................john


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#3 timbo_heff

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Posted 11 September 2013 - 09:40 AM

Thanks John,

 

In spite of being a complicated chem, is there a way I could easily tweak this glaze so it looks more like the fun side all the time?

Is calcium oxide something I could add right to the glaze (bumping up the whiting? adding wollastonite maybe?)

 

The glaze is a bit too refractory: it takes a full cone 10/11 to get glassy: I wouldn't mind trying to flux it a bit anyway. (is matte on the 9 side of 10)

A complicating factor is that it is a bit  too runny once it does get to glassy temp: so I would want to add a bit more clay to control that, yes?

 

 

The glaze seems to do this more uniformly when over porcelain v. stoneware: : is this becase the stoneware is sending more iron into the glaze making it retain the boring brown?

 

Sorry for the perpondeance of questions ... I know I need a glaze chem class: but any advice in a direction would be great and helpful.

I think this could be a great glaze with some tweaks: but so tough to test glazes for wood effects, ug!

 

My impulse would be to try a little less iron, a bit more gerstley, custer and clay.

 

thanks!

TjH



#4 JBaymore

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Posted 11 September 2013 - 10:17 AM

In spite of being a complicated chem, is there a way I could easily tweak this glaze so it looks more like the fun side all the time?

Is calcium oxide something I could add right to the glaze (bumping up the whiting? adding wollastonite maybe?)

 

fast answer.......... apply your glaze.  When still very damp.... take a screen type sifter of some sort and sprinkle seived to 80 m wood ash on the piece into the damp glaze layer.  Fire.  See what you get.

 

best,

 

......................john


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#5 TJR

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Posted 11 September 2013 - 07:27 PM

Tim;

You have a lot of Redart in your glaze which is providing iron. Also, you have iron oxide added on as a colourant which also makes the glaze darker. Try a test of just the base glaze without the iron. Another test would be to reduce the Red Art to 5%.

You are getting calcium from the bone ash which is calcium Phosphate. It is a flux and a glaze hardener. You could increase that, but beware that your glaze may run. Test. Test. Test.

You are also getting calcium from the whiting.

TJR.



#6 Biglou13

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Posted 12 September 2013 - 04:40 AM

Ahhhh. Blessed by the kiln. That's what makes wood fired so special.
No mater what you do flame side will always be different, like mr John b said.
I like the variation, and would pay more for it!!!! But that's not your question......

Prolly not what you want to hear but.......spraying on glazes. ala Stephen hill. I would most like likely be a whole new set of glazes.
It's different than woodfire but will give you effect you want. And consistently. (Insert offcenter comment here) and possible in electric firing.
Caution big brother is watching.
The beige is blinding!!!!!!
The middle of the road is boring

The true sign of intelligence is not knowledge but imagination.
-Albert Einstein

#7 JBaymore

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Posted 12 September 2013 - 09:22 AM

This recipe comes up low in both alumina and also in silica for cone 10, compared to typical limits.  Low limit for alimina is .45.....it is at .31.  Low limit for silica is 3.5 and it is at 2.65.  SO this is not a particularly "hard" or "durable" glaze when fired in "non-woodfire" conditions.  When you add in the components that the fly ash and volatiles will add to the glaze...... the composition in this regard further changes toward lower hardness/durability.  The silica/alumina ratio indicates this also.

 

The quite low alumina accounts for the runniness.

 

It is already a calcium dominated recipe.....and any wood ash that lands on the glaze is increasing that dominance... and also lowering the alumina and silica numbers further.  Heavy calcium like this with low silica will cause the glaze to tend to "attack" the clay body ...stealing silica (and other body components) into the melt.  Hence some of the character of the hot face side.  There is also the high calcium matting effect....... where an oversupply of calcium will cause the precipitation of microcrystaline materials on the surface during the cooling phase........ hence some of the character of the hot face side. 

 

The boron / phopsphorous pentoxide/ iron / titanium trace are likely responsible for the blueish tinge.  The calcium and the iron are likely responsible for the iron yellow flecks on the crystals on the ash side.

 

 

Custer Feldspar.........44.00 
Silica......................    12.80 
Calcium Carbonate....17.70 
OM #4 Ball Clay...........6.10 
Redart......................  11.10 
Bone Ash.................... 2.00 
Talc........................      3.80 
Gerstley Bor (99)......... 2.50 
Iron Oxide Red.............4.10 
                              =========
                                 104.10

          CaO       0.59*   
          MgO       0.10*    
          K2O       0.15*    
          Na2O      0.07*    
          P2O5      0.02*    
          TiO2      0.01     
          Al2O3     0.31    
          B2O3      0.03     
          SiO2      2.65    
          Fe2O3     0.08*    
                       
                       Si:Al:    8.68
                      SiB:Al:    8.76
           Thermal Expansion:    7.78
              Formula Weight:  263.91
         
best,

 

...........................john
                


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#8 timbo_heff

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Posted 13 September 2013 - 08:22 AM

Wow ! Thanks guys !

I guess since the glaze is so far out of whack, maybe here is a better question/ approach :

 

Does anyone have a recipe for a cone 10 reduction glaze that looks kind-of like this that they would be willing to share ?






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