|Monthly Tech-Tip |
Alternate Names: Mno2, Pyrolusite, Manganese(IV) oxide, Manganese Oxide
This material is available as a pure material or as a ground ore (pyrolusite). Thus while generically it is pure MnO2 the actual name-brand materials may only be 75% MnO2.
Above 1080C, half of the oxygen disassociates to produce MnO, a flux that immediately reacts with silica to produce violet colors in the absence of alumina, browns in its presence. Thus if it is being used in glazes fired below 1080C it should be considered as MnO2, if above it should be taken as 81.5 MnO and 18.5 LOI.
In glazes it will behave in a refractory manner, stiffening the melt. Because of the expulsion of oxygen at 1080, glazes using manganese should avoid this temperature range to reduce the chance of blistering and ruining of the glaze surface.
Manganese dioxide is the key to Rockingham brown wares which are made by employing about 3% iron oxide and 7% manganese in a transparent lead glaze of a recipe such as: Feldspar 28, Kaolin 14, Flint 4, Lead bisilicate 40, Whiting 4.
Manganese browns have a different, often more pleasant character than iron browns.
Manganese oxides can occur in a number of less common forms: (i.e. Mn2O3, Mn3O4, Mn2O7).
Pure manganese dioxide is often added to clay bodies to darken their color (even to black), 10% or more might be employed. Of course, using it for this purpose at cone 6 would require thorough testing to guarantee that manganese fumes are not being released during firing. For lower temperatures (e.g. 1000C) some manufacturers consider it safe for use as a colorant.
The stains are still powders, showing their advantage over using raw metal oxides to color glazes. Pretty well all black glazes employ manganese along with other metal oxides, double or triple their combined percentage is needed to stain a glaze black. Much of the reason for this is that they dissolve in the melt.
This bag will give you a clue as to what manganese dioxide, MnO2, is mainly used for. Staining bricks.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
Notice how the bubbling from the off-gassing of the manganese has variegated the surface.
By Tony Hansen
This glaze is 49% Wood Ash, 24% Soda Feldspar and 27% Ball Clay. 10 copper carbonate and 10 manganese dioxide are added to that. This beautiful sculpture was made by Dan Ingersoll, aesthetically this glaze is perfect for it. But there are two red flags here. Significant manganese and copper metal fumes are certain to be generated at cone 10 (they are seriously not healthy) so anyone using this must be very careful. But there is something much more serious - this glaze is being used on functional ware. Copper is well known to destabilize other metals in the fired glass. This 10:10 combination is a perfect storm for leaching heavy metal into food and drink. This is not an argument for the use of commercial glazes, it is one for common sense application of the concept of limit recipes.
Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).
Manganese Toxicity by Elke Blodgett
A story of the struggle of one person to identify and deal with manganese toxicity
Manganese: Creativity and Illness by Dierdre O'Reilly
A story of one persons struggle with manganese toxicity
Manganese and Parkinsons by Jane Watkins
A story of one person and manganese poisoning.
Manganese Inorganic Compounds Toxicology
Manganese in Clay Bodies
Manganese is used to stain clays (using black) and to impart fired speckling (as a decorative effect). It is dangerous?
SDS Prince Minerals Manganese Dioxide
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.
Metallic based materials that impart fired color to glazes and 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.
|Temperatures||Manganese dioxide decomposes to MnO (535-)|
In ceramics, it is used primarily in clays and glazes to achieve fired speckle (including the brick industry).
|Oxides||MnO2 - Manganese Dioxide|
|Oxides||MnO - Manganous Oxide|
Ore of manganese.
A manganese mineral.
|Density (Specific Gravity)||4.9-5.0|
|Body Color||When added to terra cotta bodies in amounts around 5% manganese dioxide will produce dark gray to black firing bodies.|
|Glaze Color||Large amounts of manganese can produce metallic effects in a glaze. However, these glazes must not be used on food surfaces.|
|By Tony Hansen|
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