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Are Zinc/tin Oxides Active At Cone 6?


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#1 jrgpots

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Posted 28 July 2014 - 08:06 PM

I found a nice recipe for a cone 6 chun red in reduction and pale green in oxidation.

Custer feldspar,..............428
Silica........................153
Whiting.......................133
Frit 3124.....................122
EPK.................. .........51
Zinc ox........................41
Talc,..........................31
Tin ox.........................21
Bentonite......................10
Copper carb....................10

I know zinc oxide is a great opacifier at low temp and pretty much sublimated at high temp, cone 10.

So here is my question. At cone 6 how much of the zinc is still around to influence the glaze? And, if it has minimal influence, do I need to use it?

#2 High Bridge Pottery

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Posted 28 July 2014 - 08:20 PM

I have zinc in a cone 10 recipe   :wacko: maybe it is doing nothing.



#3 Babs

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Posted 28 July 2014 - 08:40 PM

Your recipe has not been converted to percentages, totalling about 950 at the mo. , the tin oxides, bentonite and  copper are additives, if you were to leave zinc out you would have to reformulate the glaze, hmmm I'm stil not confident about this , so can't offer to help further. I would do it for myself but not for others at the mo. Line testing, I love giving this advice to others.... now take your vitamins!



#4 jrgpots

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Posted 28 July 2014 - 08:53 PM

I know the glaze recipe doesn't add up to 1000 without including the oxides. That is what made me examine it more closely. It needs to be adjusted. So I thought that if it needed work, I'd question if the zinc was needed.

#5 Babs

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Posted 28 July 2014 - 09:12 PM

It's ok just make each component a percentage ofthe total not including the three additives ie tin, copper and bentonite so in the above, the feldspar is listed at 428 /950 approx, times by 100 to get  it as a percentage, I just estimated total here you can do the math!, Do this to all the other ingredients apart from  additives and the total you get should be 100 ie percent.  Then you can adjust hte additves accordingly.

The zinc oxide would be included also because of its fluxing action, and also I think it may have an effect on the copper red chemistry but this again I do not know the chemisrty for but have observed this.



#6 jrgpots

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Posted 28 July 2014 - 09:25 PM

Thanks.

#7 Dick White

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Posted 28 July 2014 - 09:34 PM

Running the batch recipe through a glaze calculation program reveals a variety of things. There is almost no clay in the recipe, so it will likely settle out quickly in the bucket, possibly hardpan. Bentonite can offset this somewhat, but the recipe as given has only 1% bentonite. At least 2% would be better.

 

You ask specifically about the zinc. Generally, zinc volatizes early in a reduction firing, and so is useless as a flux. In this recipe, there is only 4% zinc, so it's presence is not intended as a flux, but rather as a color enhancer for the copper red. This is common, and not a problem.

 

You included tin oxide in your title but then didn't ask anything about it. The tin here is necessary to assist the copper changing to a colloidal state and then turning red in reduction. Typical ratios are 2-3:1 tin to copper, with the copper in the .5-1% range and tin in the 2-3% range. Your recipe is within this limits, and the copper should be a nice red.

 

Finally, this recipe is very high in Custer feldspar - almost 45%. Unfortunately, Custer feldspar has changed in the past few years, and contains significantly less potash flux than before, and more silica. Thus, any recipe with this much Custer will not perform as expected if you use new stock purchased in the last 2 years.Such a recipe can be recalculated to increase the amount of Custer until the level of potash is restored. I have done that for you. A revised recipe for you to test is:

 

New Custer feldspar - 56.6

Whiting - 14.0

Frit 3124 - 12.7

Silica - 6.1

Zinc oxide - 4.3

Talc - 3.3

EPK - 3.0

 

Add:

Bentonite - 2.0

Tin oxide - 2.0

Copper carb - 1.0



#8 jrgpots

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Posted 28 July 2014 - 09:56 PM

Now that is service! I knew the tin helped with the copper in reduction to get the red and that was why I didn't mention it.

Thank you very much. I need to bite the bullet and get insight.

Jed

#9 Babs

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Posted 29 July 2014 - 02:18 AM

Thanks Dick White for the explanation of the above. This site is so informative, not just "add silica type advice" but the reason behind these actions. Thanks again.



#10 Marcia Selsor

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Posted 29 July 2014 - 08:42 AM

At  ^6 and ^10 Zinc silicate Crystalline glazes use zinc.  Zinc can be used as a flux and opacifier at ^6 .

Tin is used in oxidation and reduction iron reds at ^10 and ^6 helping with the microcrystalline structure. 

So I would say yes.

 

Marcia



#11 Bob Coyle

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Posted 29 July 2014 - 10:03 AM

Besides being an opacifier, the zinc oxide could also effect the color and texture of the glaze. I'd leave it in and give it a test firing on a pot or two.



#12 Marcia Selsor

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Posted 29 July 2014 - 11:18 AM

Absolutely. To get a sense of how, look at the reference codes for mason stains and what will and will not work with calcium, or zonc.

 

Marcia



#13 jrgpots

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Posted 29 July 2014 - 03:22 PM

Thank you Marcia and Bob. I refilled my supply of tin oxide and zinc oxide this AM. I was suprised at the cost of the tin oxide, $40-$55/lb depending on amount ordered. I got 1/4 lb.

Jed

#14 Babs

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Posted 29 July 2014 - 10:14 PM

Yes  price of Tin has escalated. Good reason to replace it by part or fully in some glazes if it is only the opacifying effect you're after.






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