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What the heck is plaster anyway?
Plaster is
a relatively harmless dry powder made from Gypsum Rock (calcium sulphate) CaSO4 2H2O.
The mineral Gypsum is commonly found
throughout the world. There are vast deposits in the United States and Canada .Gypsum is
white or pale grey in color and occurs in sedimentary beds or veins. Veins of Gypsum range
in thickness from a few feet to 30 feet thick. Gypsum rock is 2 in the hardness scale, it
can be scratched with the fingernail. It has a wide range of uses. As a product
called Snow White ® filler it is approved for food and drug use. It is used in plastics where
finesse/whiteness are most critical. Gypsum as Terra Alba is used in flour, baking
powder, pills and breweries. It 's uses as a building material and as a craft material are
well known.
The Story of
Plaster Production
Long ago people figured out that if they
heated some kinds of rock they would break down (the rocks not the people). They found
that gypsum rocks broke down at a much lower temperature than the other rocks. If they
crushed those crumbled rocks into a powder and added water to make a paste or
slurry, that paste could be used to cover their walls because it hardened up rock
hard again. This nifty idea caught on and soon people were making plaster cows and
whatever other ideas they could come up with.
This plaster stuff has really caught on
hasn't it. Yes, it's one of the safest and
easiest materials to work with.
A minimal health risk from using this product could arise from nuisance dust
possibly inhaled during the mixing stage. An exhaust fan or a simple filter mask
will resolve this concern. As far as mixing the common industrial plasters with your
hands or applying them with your hands the risk is low here also. The worst they may do is
dry your skin through constant use. The gypsum cements that contain additives such as
polymers should be handled judiciously just as you would craft paints or any other craft
products. (As with all products there will be a minute percentage of the population that
has a reaction. You may wear rubber gloves if you think this may be a concern for you.)
Because of the different stages plaster goes through in curing, it can be manipulated in
many different ways. It can be poured into molds as it is in the plaster
statuary business, applied with a spatula as in direct sculpting, carved as in plaster
model making or polished as it is in the application of Venetian plaster walls.
How do they make this stuff ? We're not still sitting
around the campfire heating up rocks are we?
Well the basics are still the same. Gypsum rock still has to be removed
from the ground and it still has to be heated to make plaster. (Different
temperatures have different effects on the gypsum by the way)
 While
the basics are still the same we do not use sticks and stone knives to scrape up the raw
gypsum anymore. Nope! In the open quarrying process called strip mining, massive earth
moving equipment and scrapers first remove the clay and shale overburden. The bed of
gypsum rock is then scraped clean, drilled and blasted loose. 
This produces rock of suitable size for loading into large haulage
trucks.
The gypsum rubble is then transported to the crushers where it goes
through various pulverizing processes. Grinding and grading of the rough mineral
rock takes place there. Small harder pieces of aggregate will be found in the
powder after the initial grinding stages. At the United states Gypsum Company these
are sorted out to to be made into Hydrocal cement. When the crushing processes are
complete the gypsum rock leaves the crusher as a smooth fine powder.
Calcining. "Is it soup yet mom ?"
This gypsum powder is transferred into huge kettles which hold 30 tons of material. The
calcining now takes place in the kettle as the material is heated and stirred. After
a carefully monitored length of time in the kettle at an exact temperature,(120° to 130°
for the kinds of plaster we use) the material which is now plaster is removed.
Controlling the temperature has different effects. A temperature of 194° produces
a material used for flooring. To long a time in the kettle at to high a heat will produce
a material called "dead plaster". The properly calcined plaster is ground once
more to a very fine consistency and packed into bags.
The special gypsum to be made into HYDROCAL is heated under pressure. This
gives it a different cellular structure which is the reason for it's greater
strength.
O.K. Boil this all down
for me, what do I really need to know?
The
important characteristic about Gypsum rock as far as we are
concerned is that when it is heated over 120° to 130° it's structure breaks down. (It
has to be heated long enough to drive out 3/4 of the chemically bound moisture to do this.
) The gypsum rock is ground into a powder. It has now become plaster. Mix the
proper amount of water with the powder and it turns back into gypsum. Heat the
gypsum to 120° again and presto change-o we have plaster again. Anyone who has ever
dried a plaster statue at too high temperature for too long a time can attest to this
unique characteristic. The statue will be full of cracks and will fall apart if handled.
The short and skinny of it all is, when gypsum rock is calcined, ground into a smooth
powder it then becomes plaster. It is packaged in bags, shipped to your local supplier and
is ready for you to use. It is a safe, inexpensive product, open to worlds of creativity.
When you pour a plaster statue or mix a large batch of plaster a chemical reaction
takes place during the setting process and the cast plaster heats up as it hardens.
The larger the volume you cast, the more it will heat up. Temperatures may reach
150° deep within the mass of a large casting. .Do not encase any of your body parts
or anyone else's in plaster! There is the definite danger of burns, not to mention the
embarrassment of having to go to the emergency room to have a block of plaster
chiseled off a part of your anatomy. Believe it or not people have written to
me about this very experience.
Can I make my own plaster?
If you take your cast plaster piece which is technically now gypsum once again, heat it
up in an oven over 120° until 3/4 of the chemically bound moisture is driven out of it,
it will calcine. With a lot of time and effort you could then grind it into a
plaster powder. The whole process could be repeated over and over. (If you had nothing
else to do with your life.)
There are two basic categories of plaster
that we use today:
- Alpha Hemihydrate (alpha gypsum) and
- Beta Hemihydrate (beta gypsum)
The Alpha Gypsums or Gypsum Cements
- Alpha Gypsums calcined under pressure
are called gypsum cements - These Alpha Gypsum casts are harder and
stronger with limited absorptive power. They require 22 to 45 lbs of water per 100 lbs of
gypsum cement (this is called the Use Consistency Ratio). They are used primarily when
greater strength is required. Because of their extreme hardness some Alpha gypsums can not
be carved or scraped after hardening. Alpha Hemihydrate is produced in many
different formulations. You can mix an alpha gypsum with a beta gypsum such as moulding
plaster to increase the plaster's strength or hardness. I use 3 scoops of
plaster to one scoop of HYDRO-STONE in my sculpted model work. To help you choose the ones
that best suits your needs, check out the list below.
- Beta Hemihydrates are known as industrial plasters, plaster
of paris, kettle plaster and kettle stucco. Beta Gypsums are made by calcining in a kettle
at atmospheric pressure. They require more water to make a workable slurry because of
their irregular crystalline structure. They require 65 to 160 lbs. of water per 100 lbs of
plaster ( consistency ratio). Beta Gypsums are not as hard as the alpha gypsums. That's
why they are easier to carve and scrape. Because of their high water absorptive power,
they make excellent pottery molds. There are many beta Gypsums from which to choose. Use
the list below to help you in making your decision.
How Many
Kinds of Plaster are There?
or How do I love thee? Let me count the ways
click on any product below for
more info
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