Digitalfire Ceramic Oxides Directory
|Logged in as Level 2 access: Logout|
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.
Out Bound Links
In Bound Links
Copyright 2003, 2008 http://digitalfire.com, All Rights Reserved
INSIGHT is ceramic chemistry