|Monthly Tech-Tip |
Alternate Names: ZnO, Zincite, Zinc 99
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 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 must 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. Many zinc-bearing frits are available (e.g. Fusion FZ-16 has 15% ZnO) and incorporating one of them to source it instead of raw zinc oxide will produce glazes that are more fusible, have better clarity and fewer defects (a classic job for glaze chemistry calculations).
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.
These materials have many issues. They can create problems in your glaze slurries (like precipitates, higher drying shrinkage), cause issues with laydown and dried surface and cause fired surface defects (like pinholes, blisters, orange peeling, crystallization). And lithium and barium have toxicity issues (as raw materials). And the lithium, barium and strontium are carbonates, that means carbon burns off during firing (with lithium, for example, 60% of its weight is lost). Yet the oxides that these materials source to the glaze melt, ZnO, Li2O, BaO and SrO can be sourced from frits. In doing that you can solve almost all the problems and get better glaze melting. Fusion Frit F 493 has 11% LI2O, F 403 has 35% BaO, F 581 has 39% SrO and FZ 16 has 15% ZnO. Of course, these frits source other oxides (but these are common in most glazes). Using glaze chemistry you can often duplicate the chemistry of a glaze while sourcing these oxides from frits.
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.
We are comparing the degree of melt fluidity (10 gram GBMF test 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.
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.
Metal oxide powders are used in ceramics to produce color. But a life time is not enough to study the complexities of their use and potential in glazes, engobes, bodies and enamels.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling
|Materials||Zinc Oxide Raw|
|Oxides||ZnO - Zinc Oxide|
|Temperatures||Zinc oxide boils and volatilizes (850-950)|
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Zinc Compounds Toxicology
Zinc Oxide at Wikipedia
|Density (Specific Gravity)||5.6|
|Glaze Opacifier||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.|