Alternate Names: ZnO, Zincite
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|DENS - Density (Specific Gravity)||5.6|
Zinc oxide is a fluffy white to yellow white powder having a very fine physical particle size (99.9% should pass a 325 mesh screen). It is made using one of two processes that produce different densities. The French process vaporizes and oxidizes zinc metal, the American process smelts a coal/zinc sulfide mix and oxidizes the zinc fumes.
Ceramic grades are often calcined to remove any physical water (so they do not clump in the bag). But these grades also have a larger particle size and lower surface area (e.g. 3 square meters per gram vs. less than 1; however 99.9% still passes 325 mesh). While calcined grades are said to produce less glaze surface defect problems, many ceramists have used the raw grades for many years without serious issues. You can calcine (or re-calcine) zinc on your own in a bisque kiln at around 815C. However there are issues: First, because the calcined zinc wants to rehydrate (and get lumpy in the process) you need to store it in an airtight container (some people calcine a mix of zinc and kaolin to prevent this, but theoretically such a mix should not even need to be calcined). Second, the calcined zinc may produce slurry thickening issues over time.
Alot of zinc is used in crystalline glazes (typically 25%), because these have no clay content, they bring out the best and worst of both the calcined and raw materials. The raw zinc suspends glazes better (the calcined settles out significantly more). The raw zinc takes more water, but since the glaze can thin out over time it is better to add less than needed at mixing time and mix thoroughly. The raw zinc screens better (although it can be a challenge to get either slurry through an 80 mesh screen).
Zinc oxide is soluble in strong alkalies and acids.
It can be an active flux in smaller amounts. While boron dominates as the key flux in middle temperature glazes, for example, zinc is employed in some base glazes to augment the B2O3 or even replace it entirely. No combination of the common raw materials feldspar, kaolin, silica, feldspar, calcium carbonate, dolomite and talc will melt properly at cone 6, however, a 5% addition of zinc can transform the mix into a glossy glaze. 5% more and it will be a very fluid glossy glaze. The zinc can also significantly reduce the thermal expansion of the glaze it is fluxing.
Zinc generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion).
Zinc oxide is thermally stable on its own to high temperatures, however in glazes it readily dissolves and acts as a flux. Zinc oxide sublimes at 1800C but it reduces to Zn metal in reduction firing and then boils at around 900C (either causing glaze defects or volatilizing into the atmosphere; note that electric kilns with poor ventilation can have local reduction). While it might seem that zinc would not be useful in reduction glazes, when zincless and zinc containing glazes are compared it is often clear that there is an effect (e.g. earlier melting, more crystallization and variegation). Thus some zinc has either remained or it has acted as a catalyst.
The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance).
Zinc oxide is used in glass, frits, enamels and ferrites. Zinc oxide is also used in large quantities in the rubber and paint industries; in insulated wire, lubricants, and advanced ceramics.
Glaze Opacifier - White
Zinc oxide will produce opacity or whiteness, especially at low temperatures, if the calcium content is low. It does not opacify as well in boron glazes. It works well in combination with tin.
Closeup of a crystalline glaze by Fara Shimbo. Crystals of this type can grow very large (centimeters) in size. They grow because the chemistry of the glaze and the firing have been tuned to encourage them. This involves melts that are highly fluid (lots of fluxes) with added metal oxides and a catalyst. The fluxes are normally B2O3, K2O and Na2O (from frits), the catalyst is zinc oxide (alot of it). Because Al2O3 stiffens glaze melts preventing crystal growth, it is very low in these glazes (clays and feldspars supply Al2O3, so these glazes have almost none). The firing has a highly controlled cooling cycle involving rapid descents and holds (sometimes multiple cycles of these). Between the cycles there are sometimes slight rises. Each discontinuity in the cooling curve creates specific effects in the crystal growth. Thousands of potters worldwide have investigated the complexities of the chemistry, the firing and the infinite range of metal oxides additions.
We are comparing the degree of melt fluidity (10 gram balls melted down onto a tile) between two base clear glazes fired to cone 6 (top) and cone 1 (bottom). Left: G2926B clear boron-fluxed (0.33 molar) clear base glaze sold by Plainsman Clays. Right: G3814 zinc-fluxed (0.19 molar) clear base. Two things are clear: Zinc is a powerful flux (it only takes 5% in the recipe to yield the 0.19 molar). Zinc melts late: Notice that the boron-fluxed glaze is already flowing well at cone 1, whereas the zinc one has not even started. This is very good for fast fire because the unmelted glaze will pass more gases of decomposition from the body before it melts, producing fewer glaze defects.
The top base glaze has just enough melt fluidity to produce a brilliant transparent (without colorant additions). However it does not have enough fluidity to pass the bubbles and heal over from the decomposition of this added copper carbonate! Why is lower glaze passing the bubbles? How can it melt better yet have 65% less boron? How can it not be crazing when the COE calculates to 7.7 (vs. 6.4)? First, it has 40% less Al2O3 and SiO2 (which normally stiffen the melt). Second, it has higher flux content that is more diversified (it adds two new ones: SrO, ZnO). That zinc is a key to why it is melting so well and why it starts melting later (enabling unimpeded gas escape until then). It also benefits from the mixed-oxide-effect, the diversity itself improves the melt. And the crazing? The ZnO obviously pushes the COE down disproportionately to its percentage.
Out Bound Links
The hazards of these materials in the ceramic industry and process
The main ore of zinc.
Crystals can form during cooling and solidification in many kinds of glazes and they can be microscopic or very large, widely scattered or completely covering. Matte glazes (e.g. high CaO) are often such because of a dense mesh of micro-crystals growing on the surface. Unwanted crystallization is ca...
In Bound Links
Smithsonite, zinc spar
A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. I...
Between the melting and boiling points (and, of course, especially while boiling is proceeding) all glaze compounds vaporize to some extent. The amount of vaporization is related to the time and temperature and atmosphere of the firing. Obvious examples of cases where vaporization must be considered...