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ZnO (Zinc Oxide)
Notes-Together with PbO it is considered one of the metallic oxide fluxes. In smalls amounts zinc helps in the development of glossy and brilliant surfaces the way lead did.
-ZnO starts its fluxing action around 1000C (i.e. bristol glazes) whereas by itself ZnO does not melt until 1975C. It is a late and vigorous melter for low fire glazes and thus useful in fast fire applications.
-ZnO is easily changed to Zn metal by the action of CO and H2 in the reduction phase of a gas-fired kiln (and possibly poorly ventilated electric kilns). Pure Zn metal melts at 419C and then boils and vaporizes at 907C.
-It does take time for zinc to volatilize and meanwhile it does encourage the melting process to begin earlier in stoneware applications, making it more vigorous. However zinc metal in a more molten glaze is also more reduceable.
-ZnO is a low expansion secondary flux which is handy to prevent crazing if used for, or instead of, high expansion fluxes. It improves elasticity so that glazes which might otherwise craze or shiver will fit.
-ZnO can extend firing range.
-In moderate to high amounts it acts to produce mattes and crystalline surfaces, especially if supersaturated (up to 0.8 molar) and cooled slowly (produces crystal phases like Zn2SiO4, that is, willemite). However, these crystalline surfaces can be rough enough to cause cutlery marking.
-Zinc can improve durability in some glazes. In others it can reduce resistance to acid attack.
-At low temperatures small amounts can have a marked effect on gloss and melting, although at temperatures below Orton cone 03-02 it is not normally an active flux.
-At middle temperatures, zinc can be used as a major flux in amounts to 5%.
-At higher oxidation temperatures it is valuable to provide a smooth transition from sintered to melted stage. Zinc is common in fast fire glazes.
-In certain mixtures it is very powerful, even in small amounts. The melting power per unit added drops quickly as the amount used exceeds 5%.
-Zinc can have amphoteric qualities if it is used with boron.
-Zinc has a complicated color response. It can have harmful and helpful effects on blues, browns, greens, pinks and is not recommended with copper, iron, or chrome.
Yikes. Cutlery marking this bad on a popular glaze!
An example of how a spoon can cutlery mark a glaze. This is LA Matte (G2000), a popular middle temperature recipe used by potters. The mechanism of its matteness is a high percentage of zinc oxide which creates a well melted glaze that fosters the growth of a mesh of surface micro-crystals. However these crystals create tiny angular protrusions that abrade metal, leaving a mark. Notice the other matte (G2934) has a much better surface yet melts much less (its mechanism is high MgO in a boron fluxed base).
Zinc oxide calcined (left) and raw (right) in G2902B crystalline glaze (contains 25% zinc) on cone 6 L3659 high expansion (low crazing) body. This has been normal cooled to prevent crystal development. Melt is identical, no crazing.
Zinc oxide calcined (left) and raw (right) in typical glaze (G2902B has 25% zinc) on typical cone 6 porcelain body. This has been normal cooled to prevent crystal development. Melt is identical, note how bad the crazing is.
A good matte. A bad matte!
Two cone 6 matte glaze mechanisms. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is LA Matte, a much trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily and crazes!
Two stains. 4 colors. Why? The chemistry of the base glazes.
We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Each pair shows the same stain on two different base glazes (G2934 cone 6 matte and PGF1 cone 6 glossy). Why does the maroon not develop in the left pair, why is the purple stain firing blue on the right? The Mason Colorworks color chart and reference guide specifies that the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it actually is expressing a minimum, the more CaO the better). But the colorless one has 11% CaO, it should work (the maroon one has only 9% and it is working)! Likewise the purple color develops correctly in the 9% CaO but wrong in the 11% CaO base. Both stains have the same caution on the reference guide. What is going on? It is an undocumented issue: MgO. The 11% CaO base glaze is high in MgO (that is what makes it matte), that impedes the development of both colors. When you talk to tech support at Mason (or any stain company), they need to know the chemistry of your glaze to help, not the recipe.
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
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