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By E.J. McCormick
Introduction. What do you get when you marry the best parts
of plaster casting to the best parts of epoxy and fiberglass? An
ultra strong, very hard, weatherproof, odorless, and most
importantly a non toxic material casting system that one can
sculpt like plaster called the FORTON MG Casting System –
sometimes referred to as FMG.
FMG consists of a gypsum plaster modified (hence the
abbreviation M.G., for Modified Gypsum) by the addition of
plastics and fiberglass. The system involves mixing precise
amounts of several products, including Hydrocal FGR-95 from US
Gypsum or alternatively, Densite HS from Geogia Pacific (both are
gypsum-based cements); dry melamine resin powder; a hardener; a
liquid polymer called Forton VF-812, and; fiberglass. These
various ingredients are the base to which aggregates, colorants
and metallic powders may be added; in which the latter, though
somewhat pricey, simulates the look and feel of metal.
Hiram Ball of Pittsburgh, who is arguably one of the most
knowledgeable persons there is on FMG application, first
introduced me to the process. Then a short time later I was
privileged to take a lifecasting class with
Mark Prent at his
Vermont studio and discovered that he too had switched to FMG,
after years of working in epoxy and fiberglass, for most of his
lifecasting work. It was there that I became a believer and have
used FMG extensively ever since.
FMG is compromised of a glass-fiber reinforced gypsum
(hydrocal) system. It was initially designed for
outdoor-architectural work, but the material can be utilized for
interior or exterior sculpture. What convinced me was not only was
it simple to use after one mastered its mixing proportions, but
most importantly it was non-toxic.
Advantages of FMG. The FMG Casting System is odorless and
easy to use for solid and hollow castings. The casting method is
essentially the same as that used for casting in polyester resin
and fiberglass. But, Forton is easier to work with, so a casting
made with this material takes less time to complete. Repairs are a
snap as the material can be worked with your plaster tools so that
patching, sanding and finishing are all easier on a casting made
of Forton than on a casting made of polyester resin or epoxy.
An additional advantage is that, unlike polyester and epoxy
resins, Forton may be cast directly into an alginate mold to
produce extraordinarily fine sculptural detail. From the
standpoint of health and safety the Forton casting system is
considerably less hazardous when compared with polyester resin or
epoxy casting.
Those of us who use FMG believe it an ideal medium for
lifecasting as it is lightweight and thus a great choice for
sculptural wall hangings. It is also weatherproof so a casting can
be displayed outside. FMG has a rapid mold cycling time of 60 to
90 minutes –and drying is even faster by adding an accelerant.
Aggregates and pigments can be added to the wet mix to achieve a
natural stone look. The look and feel of foundry casting can also
be achieved through the cold casting method of adding metal
powders. Once FMG and metal powder mix has dried the surface will
take on the same patina of 100 percent metal. The surface can then
be patinaed using foundry etching acids, or be polished to a high
shine.
Another advantage that casters will like is that FMG can be
worked as if it were simply plaster using plaster rasps and wet
sandpaper. However, the material allows better control and more
flexibility than plaster.
Speeding the drying Time. The demolding time without a
drying accelerator ranges from 1 1/2 to 2 hours. With an
accelerator one can shorten the time to as little as fifteen
minutes. Try that with just plaster or epoxy. The accelerator can
be found at the local garden supply store in the form of aluminum
sulfate—a fertilizer. Just mix one pound of the substance in one
gallon of ordinary water. The amount of accelerator used depends
on both the age of the hydrocal gypsum and the ambient
temperature. Though a formula is offered below, I found that trial
and error worked best. That is adding a little to the wet mix and
seeing it beginning to harden as it was being stirred. With the
first few squirts of accelerator into your mixture nothing appears
to happen. Then another squirt or two and suddenly the mixture
quickly thickens. So be cautious in your application so you don’t
wind up with hardened FMG in the mixing bucket rather than in your
mold.
Release agents. FMG is very versatile in that it can be
used in a wide range of mold materials. Latex and urethane molds
need a release agent. But silicone molds do not. A note of caution
though, I learned this the hard way-- typical release agents are
fine, except that silicone releases should not be used when you
are going to be painting or applying a patina acid. When I cast
into alginate I use LeKlair Cholesteral Plus Cream (found
at the hair section in Walgreens) as a release agent. It has the
added benefit of filling in pinhole and those ever-present air
bubbles. Gently rub the cream on to the mold in a swirling motion
using your fingertips to feel for imperfections. Do it twice to be
sure you have completely covered the alginate—even the top edges
of the mold should be covered.
The Formula. Now to the mystery ingredients that make up
FMG. There are five of them. A liquid known as Forton VF-812; two
powdered ingredients, a dry melamine resin and a hardener; FGR-95
(known as hydrocal); and chopped or woven mats of fiberglass.
Forton VF-812. The liquid component, Forton VF-812 is a
specially formulated acrylic copolymer to cross link with the dry
resin and hardener to make the end product moisture resistant and
ultra violet stable.
Resin. The dry melamine resin powder is added to insure
moisture resistance. One word of warning—the resin is extremely
hydroscopic. It will form hard lumps on exposure to atmospheric
humidity, so don't leave your container open too long.
Hardener. The hardener is a chemical pH adjuster to insure
that the cross-linking with the acrylic copolymer and the dry
resin takes place.
Hydrocal. The hydrocal is the basic ingredient. It is
gypsum - available from US Gypsum as FGR-95 or Densite HS from
Georgia Pacific (see supplier list at the end of the article) or
their distributors. Both gypsum cement products are formulated for
hand lay-up but also can be sprayed.
The first time I used FMG I could not find a local supplier for
FGR-95. Instead, reassured by the folks at Johnson Atelier, I
substituted Densite HL. (HL designates hand lay-up there is an HS
that means hand spray-up.) According to Hiram Ball Densite can be
used interchangeably with FGR-95. I have found no perceivable
differences between gypsum products.
Fiberglass. Fiberglass provides FMG
with much of its strength. The table below shows the proportions
of chopped fiberglass to use. However, there are disadvantages to
using chopped fiberglass as some of the strands may pierce the
face coat and show on the surface if the face coat has not
sufficiently hardened. To avoid this problem some lifecasters,
including myself, use 6-10 inch strips by 4 inches wide of biaxial
fiberglass mat. Biaxial fiberglass is a superior woven fiberglass
that conforms much more closely to the contours of a mold than
does conventional woven fiberglass which tends to bunch into
folds, trapping air-bubbles. The biaxial fiberglass will
stretch-out much more effectively, and will be about 30% stronger
than an equivalent weight of ordinary woven fiberglass. The
fiberglass mat must be thoroughly wetted on both sides then
carefully smoothed on by hand (using rubber gloves) to prevent air
bubbles.
Preparing the Mold. Thoroughly clean the mold using the
original formula Amorall. Mist the mold with the cleaner
and blow it out. Use your air compressor if you have one. FMG
itself can also be used as a rigid mold to cast FMG back into.
Application. Three coats of FMG are applied to your mold.
The first is a face coat without fiberglass. I usually dilute the
Forton VF-812 ingredient with 50% water to assure that it is thin
enough to capture the detail. This diluted mixture "white
water." (It had an absolutely benign meaning until the last
president gave it a double entandre.) After it has dried to
the touch a second coat is applied this time using a mixture of
chopped fiber glass or fiberglass strips. The strips should be
thoroughly wetted out before applying with the mixture. Once the
second coat has dried to the touch an identical mixture is laid
down as the final coat. Here I do a lot of smoothing using my
hands dipped in water to keep them wet. The end result is that
third coat has been smoothed out. If your piece is to be a wall
hanging then once the third coat is dried to the touch you can
attach a wire cable by securing it to in two places with more FMG.
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