What it is
Mica is widely distributed throughout the world and occurs in igneous, metamorphic and sedimentary rocks. Mica
is similar to granite in its crystalline composition. The nearly perfect cleavage, which is the
most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.
Mica can be composed of a variety of minerals giving various
colours and transparency. Purple,
rosy, silver and grey colours come from the mineral called lepidolite.
Dark green, brown and black come from biotite.
Yellowish-brown, green and
white come from phlogopite.
Colourless and transparent micas are called muscovite.
All these
have a pearly vitreous lustre.
The melting point of mica depends on its exact composition, but ranges
from 700⁰C to 1000⁰C.
Glass has a specific gravity of about 2.5, and mica ranges
from 2.8-3.1, so it is slightly heavier than glass.
Tips on uses of mica
powder and flakes
The naturally occurring colours are largely impervious to kiln forming
temperatures. Other added colours have
various resistances to the heat of fusing. This is determined by the
temperatures used to apply the colour to the mica. Cosmetic mica is coloured at low temperatures and will not survive kiln forming with their colour in tact.
Mica does not combine with glass, but is encased by glass as
it sinks into the glass surface. You can
use various fluxes to soften the surface of the glass. Borax is one of those. The cleaving of the mica results in only the
layer in contact with the glass sticking.
The upper layers brush off. This
applies to both powder and flakes. One solution is to fire with mica on top in
the initial firing and then cap for the final one.
When encasing mica exercise caution. Micas flakes must be
applied thinly, as air is easily trapped between layers which leads to large
bubbles from between layers of glass.
This is the result of the shearing of layers of the flakes allowing air
between layers. Although powdered mica
is less likely to create large bubbles, air bubbles are often created for the
same reason. This is the reason it is
most often recommended to fire the mica on top.
Of course, one use of the mica to make complicated designs
is to cover the whole area and fuse.
Then sandblast a design removing the mica from areas of the glass. You
can then fire polish, or cap and re-fire to seal the mica.
Mica safety
MSDS for mica only mentions the
inhalation of the dust as a risk. Mica is resistant to acid attack and is
largely inert. Inhalation of the dust is a (low level) risk. Any significant health and safety problems
relate to the coloured coatings.
