|Co-efficient of Linear Expansion
-Manganese monoxide exists only above 1080C where the dioxide form disassociates to release its oxygen.
-Manganese is a colorant using in bodies and glazes, producing blacks, browns, and purples.
-Manganese is a constituent in many igneous rocks, and thus occurs in many clays weathered from these parent rocks. In most cases it is a very minor oxide, but does occur in much greater amounts in some slip and highly stained materials. It is thus a color contributor in many traditional and historic slip glazes.
-Smaller amounts are easily dissolved in most glaze melts; however, around the 5% threshold, the manganese will precipitate and crystallize. In large amounts in a glaze (i.e. 20%), metallic surfaces are likely.
-Above 1080C, half of the oxygen disassociates to produce MnO, a flux which immediately reacts with silica to produce violet colors in the absence of alumina, browns in its presence. Manganese browns have a different, often more pleasant character than iron browns.
-High temperature glazes well above 1080C can use large amounts of manganese to produce very metallic bronze-like surfaces. Manganese dioxide by itself can be used and will fuse well, even running down the ware.
-Manganous oxide is unaffected by reduction, but is normally considered, at its best, in oxidation slips and glazes above 1200C.
-Manganese fuses and dissolves very well above 1200C in oxidation. Like iron, it will dissolve to a greater extent in a hotter melt. This means that if more than about 4% MnO is used, the oversupply will precipitate on cooling leaving a network of crystals in a manner similar to iron in high fire reduction. Speed of cooling, glaze fluidity, and amount of manganese will all affect the results.
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
A source of MnO used in ceramic glazes and the production of ceramic stains. Commonly made by grinding pyrolusite rock.
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
|Very pleasing tan-brown reduction fired glazes can be achieved with 5% manganese dioxide in reduction.
|In glazes below 1080C, manganese can give coffee color browns when used with tin and dull browns in lead and low alkaline glazes. The presence of alumina assures brown (if it is lacking colors will be violet).
|By Tony Hansen
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