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Pres

mixing opacifiers

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Recently while mixing a glaze that used tin as an opacifier, I ran out of the tin and in need of the glaze immediately I used zinc oxide to make up the difference. Not a very scientific method, but I did it anyway. After testing on test tiles that turned out well with only slight difference in the white coloring, I fired a load of pottery two days later. End result was that the stains that I used over the glaze for inglaze technique were very vibrant and different from the glaze using just the tin oxide. There were more purples, fresher greens, and richer browns. I have seen a lot of glaze formulas out there, and never seen any that mixed the opacifiers. I was wondering if anyone had tried this? Is there a reason you shoudn't do it? How about mixing tin and zirconium type materials?

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Pres:

 

I am surprised you did not get any responses to this. I would like to start it back up again as gaining experience from others on this topic could very well let us all use less tin - which would be a nice thing. I have only done this once with a cone 10 glaze and I was less than thrilled with the results. I thought the mix I used create a glaze that was dull and did not show off the slips I tend to use under glazes.

 

Anyone else out there with experience mixing opacifers?

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I have reduced Tin the content and added Zircopax back in my stoneware days (70-80s) with good results on some very touchy matt glazes.All cone 10. Wamo Mamo was the name of glaze. I think it was from Alfred's originally?

Mark

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Pres,

 

Keep us informed about your discoveries. I sometimes sub 1.5 to 2 parts Zircopaz for 1 part tin to see the difference in colors the sub causes and sometimes to save a few bucks. Here is some very interesting info from Digita Fire's Reference Database:

 

 

"-Twice as much zircon is required to produce the same level of opacity.

-Like zircon, tin melts at very high temperatures and thus does not go into solution in typical glaze melts.

-Zircon will stiffen the glaze melt more than tin.

-Zircon will likely produce a harder glaze surface.

-Zircon will reduce the thermal expansion of the glaze more than tin.

-The quality of the white color is different (tin tends to be more of a blue white, zircon a yellowish white).

-Tin is very expensive, this is likely the main reason for its much more limited use as an opacifier today.

-Zircon tends to have less of an effect on the development of metal oxide colors (e.g. tin reacts with chrome to make pink).

-If gloss is an issue, silica might have to be reduced to compensate for the silica introduced by a zirconium silicate opacifier being substituted for tin.

-While there are other products that produce varying degrees of opacity, none are as neutral and non-reactive as tin and zircon. Other opacifiers also tend to variegate the glaze.

-Tin does not normally opacify in reduction firings.

 

Tin is also a player in the development of ceramic colors, for example chrome tin pinks and maroons. Tin with iron in oxidation makes a warmer shade of brown than zirconium does. Tin oxide can react with titanium and rutile to variegate the glaze or even complete transform its color."

 

Jim

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Personally, I do not think of Zinc Oxide as an opacifier, but rather a flux. If I wanted to opacify I would use a zircon-based material like Zircopax or Superpax, or possibly titanium dioxide. What probably happened with your glaze is that the zinc increased the melt of the glaze a little bit, and the stains melted in better and increased their vibrancy.

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Personally, I do not think of Zinc Oxide as an opacifier, but rather a flux. If I wanted to opacify I would use a zircon-based material like Zircopax or Superpax, or possibly titanium dioxide. What probably happened with your glaze is that the zinc increased the melt of the glaze a little bit, and the stains melted in better and increased their vibrancy.

 

 

Or it was simply a chemical reaction. The coloring chems reacting differently to zinc and tin. Or both.

 

Jim

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Pres;

I am agreeing with Neil on this one. You caused your glaze to flux more, and consequently the colours came out brighter. I wouldn't have thought to use zinc as an opacifier either. I have a great purple glaze that uses tin and copper carb., and I also use copper carb and tin as an over glaze colourant that goes red when all the stars line up.For my white glaze I use zircopax as the opacifier.

This is why it is great to make your own glazes-you get these fortuitous accidents.

Ain't ceramics great! TJR. :P

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Your posts have reminded me of a related problem.I am a little confused here. I know that when I have a glaze that is pin holing, I need to make it more fluid by adding more flux. However, I have been told that one way to correct pin holing is to decrease the amount of zinc. This seems counter-productive, and following this thread, I am now even more confused. Any voice of reason here?

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Zinc in too large a quantity can cause pinholing. Too much of a good thing. All fluxes have a threshold where too much is a bad thing, with many different consequences. Too much of any particular one could cause pinholing, crazing, shivering, crawling, etc. This is where limit formulas come into play. They give you a range of quantities where the material should be stable.

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Hope Im not too late in coming to this, Linda Arbuckle's majolica recipe splits it between zircopax and tin oxide (to bring down the cost, I'd assume). Ive seen varying degrees of this in similar glaze recipes. It seems the heavier one favors the zircopax, the hotter the white, the further the tin is favored, the cooler the white is...

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Your posts have reminded me of a related problem.I am a little confused here. I know that when I have a glaze that is pin holing, I need to make it more fluid by adding more flux. However, I have been told that one way to correct pin holing is to decrease the amount of zinc. This seems counter-productive, and following this thread, I am now even more confused. Any voice of reason here?

 

 

Chris;

I used to get pinholing when firing Majolica at Cone 04. The problem doesn't occur so much with my stoneware as it is a longer firing at a higher temperature.Your problem might be as simple as dust on your pots! Before I glaze, I dip my bisqueware in a clean five gallon bucket of water. You would be surprised at how much sand/grit collects in the bottom of the bucket. Of course, then you would let the pots completely dry over night.Works for me!

Since I sell a lot of my work to the public, I sand my bisque, then rinse it. It's one more extra step, but it adds to the craftsmanship of the finished product.

TJR.

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Guest JBaymore

Anyone with some insight here?

 

Note that in reduction firing (or in a supposed oxidation firing where reducing conditions might unintentionally exist) zinc oxide is incredibly easy to reduce, therby resulting in the production of both carbon dioxide and raw zinc metal. Zinc metal has a very low melting point and is quite violatile at typical ceramic firing temperatures. The out-gassing of CO2 as well as the fuming of zinc out of the glaze both have the potential to create pinholes / small bubbles in the glaze surface.

 

If the firing does not continue long enough for these holes to heal after the bubbles have been generated, or if the glaze viscosity and surface tension is such that the holes do not heal over easily... this can contribuite to "pinholing".

 

Note also here that the most common cause of pinhioling is coming from the clay body, and issues with the bisque process..... it just shows up in the glaze firing. Try firing longer less densly stacked bisques and make sure there is a good flow of oxygen through the bisque kiln.

 

best,

 

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

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Guest JBaymore
.........splits it between zircopax and tin oxide (to bring down the cost, I'd assume).

 

Unfortunately, zirconium products prices are rising rapidly.

 

best,

 

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

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