Chic Lotz + keith Montgomery on mon 25 feb 08
> Could someone tell me the difference between magnesium and
> manganese? I
> have some of both. Thanks. Sally
Magnesia is an alkaline earth flux oxide with the chemical symbol MgO
after it has been fired. Magnesia, also known as Magnesium Oxide has
a melting point of 5072 F (2800 C) when used alone. However, used
with other oxides, its fluxing action begins at ^3 or 2138 F (1170
C) . It is found in magnesium carbonate (MgO after firing), dolomite
(CaO + MgO after firing) and talc (3MgO + 4 SiO2 after firing). Used
in small or moderate amounts in a glaze, it will help produce a
glossy glaze. When used as an auxiliary flux, it becomes fluid
later in the melt where some of the magnesia dissolves, but some of
it remains in suspension. Used in EXCESS amounts, it will produce a
opaque matte glaze. The excess magnesia crystallizes during a slow
cool, producing very tiny crystals that can be seen through a 30x
magnifier. The crystals are what make the glaze appear matte.
Excess amounts of the ingredient magnesium carbonate will produce the
crawling, lizard skin type glazes. High Magnesia glazes are one of
my favorites ....... especially when overlapping a high alkaline
glaze. The tiny crystals appear to float on the surface of the very
glossy alkaline glaze. Remember, too much magnesia can cause a glaze
to become unstable for food surfaces, but can be magnificent on
sculptures since it is known to soften the edges on textures and can
produce a fatty,buttery surface to the touch. In mid-fire and high
fired glazes with low silica, magnesia + cobalt can produce pinks
and purples. However, the low silica content can make them unstable
for food surfaces. As an auxiliary flux in high fire glazes,
magnesia increases the fluxing of the glaze melt, and adds to its
viscosity (the fluidity of the melt). In reduction firing, magnesia
encourages iron oxide from the clay body to flux and enter the
glaze. Using a thinner glaze application in oxidation will allow the
clay body coloring to burn through.
Manganese Dioxide... is a metal oxide that is black and is used to
achieve black, browns and purples. But, as with most colorants, it
is affected by the type of flux in the base glaze and the ratios of
the all the glaze components. It is thought to be quite toxic during
handling and application so be sure to wear gloves and a mask. It
can also be toxic in an unstable glaze base.......so avoid using it
on food surfaces unless you've put it though some vigorous testing.
Used as a wash or a slip, it can produce a metallic-like surface
which can be quite nice on sculptures.
Chic Lotz
chic@PoteryPoet.com
www.PotteryPoet.com
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