|Monthly Tech-Tip |
|Co-efficient of Linear Expansion||0.050|
|Dry M.O.R. (50% Silica)||582C|
-Manganese dioxide exists only below 1080C, above which the dioxide form disassociates to release its oxygen (see MnO for more information).
-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.
-In glazes below 1080C, it can give coffee color browns when used with tin.
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).
An Overview of Ceramic Stains
Understanding the advantages of disadvantages of stains vs. oxide colors is the key to choosing the best approach
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.
|Glaze Color||Manganese and cobalt mixture produce black. Iron can also be used. For example, a mix of 8 iron, 4 manganese dioxide and 0.5 cobalt make a raw black stain.|
|Glaze Color||Purple colors can be produced in glazes of high alkali (KNaO) and low alumina, especially in combinations with cobalt (look for a frit with this profile for best results).|