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Zinc
Sept 13, 2017 22:39:38 GMT -6
Post by evan cornish-keefe on Sept 13, 2017 22:39:38 GMT -6
Recently I had an issue when mixing up a crystalline glaze with materials at a community ceramics studio, the glaze turned out yellow and full of pinholes, and it had just a small amount of copper for color so I'm wondering if the yellow is caused by too much zinc?
Someone recently mentioned to me that there are differences between Zinc that was produced via different methods, French and American processes, that one process results in yellow powder and the other is white? and that it's produced in varying mesh sizes, but this info isn't typically provided by ceramic suppliers but may cause noticeable differences in high zinc content glazes....
I was hoping someone could elaborate on this, and maybe suggest what to be aware of when using zinc from different sources or suppliers, since I'm feeling pretty ignorant it would be much appreciated!
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Zinc
Sept 14, 2017 6:19:59 GMT -6
Post by tileman2 on Sept 14, 2017 6:19:59 GMT -6
Evan:
Three varieties of zinc actually: "yellow" zinc from sphalerite ore. White zinc from the French process of vaporizing pure zinc ingots, and metallic zinc ( zincite). Metallic zinc is rarely ( if ever used in ceramics). It has a very low melt temp, around 1750F as I recall from memory. Yellow zinc is the most common in the States because sphalerite ore is common in the States and Mexico. In its raw state, it does have a heavy yellow tint: being roughly 80% zinc! the other 20% mineral trash such as cadmium! selenium! sodium! and a few others. Grades used in ceramics has been washed, roasted, and better purity grades. They also sell calcined versions such as Denzox: which have a very light yellow cast. The yellow cast itself comes from the sulfur content: which is why the calcined varieties are nearly white because it burns off the sulfur (carbons). However, iron is also a contaminant in sphalerite, which if the levels were high enough would produce a pale yellow color. Yet, low purity silica can have higher iron content as well. The yellow color of the zinc is from sulfur, which would burn out completely in a normal glaze firing. Yellow zinc is the only zinc that will tolerate cone 7-8 and above now that Cerox has been taken off the market.
White zinc is from the French process of vaporizing zinc ingots or metallic zinc ore ( zincite). This process vaporizes the zinc at high temperatures and the vapors ( zinc dust) is collected in bags further up the rotary kiln.white zinc is typically always 99% plus pure: which is often used in food products after further purity processing. White zinc does have one serious drawback: because of its purity, it will begin to vaporize out of the glaze around 2260F. Vaporization will occur at lower temps if higher levels of lithium are present. Cerox was the only French process zinc that could tolerate cone ten because Cerox was highly refined French process sphalerite. The EPA shut down most zinc mines and mills back in 2010-2012.
If you suspect the yellow colorant is from your zinc: then you have a fairly low grade of sphalerite that is high in iron, cadmium, and selenium. Would suggest you find a different supply for it. The most common sphalerite so used in pottery is Maximo 473, 873, and 910. Denzox is also a calcined version of Maximo product. Perhaps talk to Paul Brown about his availability.
Tom
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Ana
New Member
Posts: 5
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Zinc
Sept 14, 2017 16:13:59 GMT -6
Post by Ana on Sept 14, 2017 16:13:59 GMT -6
Was it copper carbonate? I once had something similar happen (though my stuff was not yellow because my glaze had a lot of Titanium, it was brownish and full of pinholes), because the ceramics store mislabeled the bags and sold me green nickel oxide instead of copper carbonate. They are almost the same color, nickel is a bit paler and a lot lighter than copper (and quite dangerous, because it is very fine and can stay in the air for a long time and contaminate everything, besides being poisonous).
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Zinc
Sept 14, 2017 16:49:30 GMT -6
Post by tileman2 on Sept 14, 2017 16:49:30 GMT -6
Any product sold as a carbonate is 40% material and 60% carbon(ate) fluff. Federal law prohibits certain materials from being sold in a pure oxide form: and the maximum content is set at 40%.
Copper carbonate, lithium carbonate! etch.etc.
If you are getting any color besides some level of green out of copper: then it has been contaminated.
Tom
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Ana
New Member
Posts: 5
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Zinc
Sept 14, 2017 17:09:20 GMT -6
Post by Ana on Sept 14, 2017 17:09:20 GMT -6
I don't really understand your answer... Evan said he is using material from a community studio. Copper carbonate and green nickel oxide are easily mistaken, all that I am saying is that it could be a mislabeled bucket (and since he is getting yellow and pinholes, nickel makes perfect sense).
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Zinc
Sept 15, 2017 6:31:41 GMT -6
Post by tileman2 on Sept 15, 2017 6:31:41 GMT -6
Ana: If I had finished my morning cup of Joe; I would have caught that. Tom
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gczop
Full Member
Posts: 202
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Zinc
Sept 15, 2017 8:18:48 GMT -6
Post by gczop on Sept 15, 2017 8:18:48 GMT -6
Hi Evan, Likely not the case here, but over-firing a cone ten crystalline glaze, well past cone ten, will result in a clear yellowish glaze; pinholes? don't recall. Gordon
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Zinc
Sept 15, 2017 13:42:38 GMT -6
Post by evan cornish-keefe on Sept 15, 2017 13:42:38 GMT -6
So... the zinc I usually use and the zinc I got from the back of the supply room at this community studio is a white powder marked simply as zinc oxide, both bought through the same ceramic supply company. I found out that this ceramic supply company switched to buying Cerox 506 in 2012, from a finer particle Zinc, and since then has switched Zinc supplier twice. For the past year or so they've been selling #205 from US Zinc, which they advertise as calcined but apparently it isn't.
But then, is a different source of Zinc enough to cause such a drastic change in the glaze? The yellow pinholed results are nowhere near the green crystal glaze that was intended.
Would Nickel oxide cause pinholes? I've only used Nickel Carbonate for the past few years, also it was only half a percent Copper Carb that was added to the glaze.
Thank you all for the expertise!
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paul
Junior Member
Posts: 51
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Zinc
Sept 15, 2017 18:40:06 GMT -6
Post by paul on Sept 15, 2017 18:40:06 GMT -6
My experience using some plain "white" zinc oxide was very poor results, No Crystals, sorta a matte streaky glaze. It also had black bumps with sharp points which seemed to me to be some of the zinc.
I get the best results with "Denzox" brand which is yellow fine powder. Non calcined zinc gave lousy results.
That does not mean some other variable was not involved, but definitely indicates that you used the wrong kind of zinc. Pin Holes can be caused by a number of variables.
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Ana
New Member
Posts: 5
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Zinc
Sept 16, 2017 5:07:12 GMT -6
Post by Ana on Sept 16, 2017 5:07:12 GMT -6
Maybe the light green one is nickel carbonate (I have both the light green and the black labeled as oxides). From the digital fire entry digitalfire.com/4sight/material/nickel_carbonate_1073.html it says it causes blistering, but the quantity you used is very low. You should probably have a look at the container just to make sure it isn't nickel instead of copper. And, if you are using the light green stuff labelled as nickel, the only safe way to do it is with a P3 mask, under an extractor, or you risk contaminating everything around you for a very long time. Probably not worth the trouble.
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