MICA POWDERS
by Grace Baggot
Mica powder is a pigment used in a binder like varnish or shellac to create metallic finishes. The
advantage mica has over metal flakes is it’s inability to tarnish. By creating your own metallic paints, you
are able to dictate the color and choose a binder that is suitable to the job you have on hand. By choosing
a binder that does not yellow with time, you can have a surface coating who’s life span exceeds today’s
standards. To create mica powders, the stone, mica, is ground. Mica cannot tarnish since it is not metal.
It grows in a layered structure, kind of like an onion, and has a mother of pearl translucency. This is what
gives mica powder it’s fantastic reflective quality.
When these mica platelets are ground, they create a particle that is many faceted. Bronze powder, the
metallic pigment of choice in the past, is basically a two faceted flake of metal. Being metal, bronze
powder has all of the oxidizing properties of bronze itself. These pigments are usually stable until the
protective coatings are worn off or the metal content of the coating chemically reacts with the binder. Since
mica is an inert rock, it does not chemically react with any binder during or after application.
When color matching is an issue, micas are helpful because they come in over 35 colors. Shades of gold,
copper, silver, pewter, mother of pearl with red, blue or violet hi-lites and hornet blue/green. To achieve this,
the mica powder is treated with titanium dioxide or ferris oxide. This treatment does not make the powders
chemically reactive in any way.
As with all pigment, the size of the particle of color is important and with some micas there is a choice
between regular and micro which is extra finely ground. An external mica is also available in some colors
and this particle is larger than regular.
Micas can be mixed together for creativity and precision when matching and creating coatings. In the
past, mixing your own metallic coatings was hazardous because airborne metallic powders are very toxic.
There is no such hazard with mica powder. The airborne particle is an inert rock instead of a flake of metal.
It is important to note that when touching up or color matching, there will always be a significant
difference between any gold pigment and leaves of metal. When light hits a piece of metal leaf, gold or
other, the light is reflected off of a single reflecting plane. When light hits a gold pigmented surface, the
light is reflected off of many different reflecting surfaces. This makes metallic paints grainy next to the way
light can travel on the surface of a metal leaf.
So why bother if it is not good enough? Well, because sometimes we have to and we can. Micas mix
easily by the brush stroke or by the batch for touchup. This could be more economical than releafing. The
effects of graininess are not always visible from a distance. Touch-up in pigment could be restricted to
shadowed or dark areas and releafing only the highlights. Matte gilding can sometimes be grainier than a
finely pigmented gold paint.
The advantage to mixing your own touch-up medium is that you can insure easy removal when the time
comes for a complete overhaul. You should choose your binder accordingly. Should you get the big
renovation job, and it is to be done with conservatorial considerations, you will be glad you were the one
caring for it previously. You will be able to remove old repairs with ease and without damaging the original
gilding.
All surface coatings are a mixture of a binder and pigment which means that mica is great for any type of
surface that has been coated with a binder instead of mixing the binder and pigment before application.
This makes for a more opaque and even surface. The pigment adheres to the surface of the binder. There is
no need to seal this surface though one could. The binders of choice for this technique are the traditional
linseed oil gilding size or the modern water based acrylic gilding sizes. These sizes can be pigmented with
tinting pastes exploding creative possibilities.
Mica powders are not new. They are in cosmetics, automotive paints and plastics to mention a few. It is
their use in decorative painting and gilding that is revolutionary.
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