The legend is that a shepherd built a fire for his night/s vigil,
on an outcropping of soft white rock. It burned there throughout the
night, and in the morning the shepherd was quick to note that the rock on
which he had built the fire, had crumbled under its heat. When the
shepherd emptied his water jug into the embers of the fire to extinguish
it; a portion spilled into the white powder which turned from a liquid
into a mysterious stone-like slab.
This is what happened. The
heat of the fire evaporated, the water which is chemically combined in
gypsum rock, thus leaving a crumbled white residue. When the water was
dumped into this residue, a re-crystallization took place forming the
slab. Essentially, this is the process from the mining of gypsum to the
completion of a plaster mold.
The mineral gypsum is
found in large deposits throughout the world in different colors. It is a
rock which chemically, is calcium sulfate plus about 20% water. Some of
the best known deposits are found in Nova Scotia and in the western part
of the U.S. Although usually white, it may be found in shades of pink,
yellow, brown or even black.
To change gypsum rock into plaster, calcinations
of the gypsum rock must be effected. This is done by placing the rock,
usually crushed for ease in handling, under heat of 350?F, for a period of
time. The same effect may be achieved at a temperature of 150?F. for
a longer period of time. When calcinations takes place, approximately two
thirds of the water content of the rock is removed through evaporation.
The first known use of
plaster dates back to the building of the first pyramids of Egypt. Then
known as alabaster, it was used to decorate and seal the tombs of the
Great Pyramids, and was probably used long before the days of the
Pharaohs.
The process of making a plaster mold as we
know it today, was discovered about the 18th century. While plaster molds
of a different type were known and used for the forming of plastic clay,
there were also molds made of wood, sulfur and metal. Lisippos, was one of
the first Greek "realists," took plaster casts from faces of
living sitters about 300 B.C., sending copies as presents to his friends.
It might be Interesting to take a short
trip from a gypsum mine to the mold maker's shop. Large mountains of
gypsum rock are blasted out of these deposits, which are shipped to the
processing mill where the rock is ground and stored in large silos. From
here it is passed through giant rotary calcines which remove about
two-thirds of the water content of the rock. Special agents are added to
retard the setting action or expansion of the plaster. From the mill it is
shipped to the purchaser where it is mixed with water and made into molds.
Every mold maker has his own methods of
making molds. Present methods do not differ greatly from those used a
century ago. However, the quality of the plaster and, hence the molds have
improved tremendously. The amount of water or plaster used in a mix will
Increase or decrease the density of the set. Impurities in the water may
retard or accelerate the setting time as well as affect the hardness of
the mold. These also affect the life of a mold so great care must be
taken.
Serious trouble can result from using
plaster into which particles of set plaster or other contaminating
materials have been mixed. Usually the quality of the mold is determined
only through casting. However, the outward appearance can be used as a
guide. Whiteness, usually denoted by the purity of the plaster,
cleanliness of manufacturing, and the finished look of a mold are guides
for distinguishing each mold maker's skill and workmanship.
A mold made of good plaster can be used for
casting a ceramic body, a porcelain body or a beleek body. If a mold is
used for casting a colored body. then a white body, the color may show on
tile first six to eight castings of the white body. It is therefore,
better to use separate molds for casting colored slip.
The question may be asked, "Why is a
plaster mold used in the casting of ceramics?" When water is mixed
with plaster, millions of small crystals interlace and cause the plaster
to set. Approximately thirty per cent of the water evaporates when the
mold has dried, leaving it porous. This gives the mold room to absorb the
water from the slip.
The Use of Molds
It often happens that a mold is ruined before it is
used and likewise a mold can be ruined before it has lost its usefulness.
First of all, proper drying is a very important factor in the life of a
new mold. When possible, new mo'ids should be dried in the sunshine and
where air circulates freely. Molds should never be opened to dryas this
may result in warping which causes the seams of the mold to spread and
allows the slip to run through. Once properly dried and used with care,
the number of casts made from one mold will average one hundred. If
sharpness of detail of the cast is a requirement, some molds having' fine
details may not give quite as many. Others with a plain surface will cast
well over a hundred times before the mold will require replacement.
It is important that emphasis be made on proper drying of the
mold, No plaster mold. should' ever be exposed to temperatures above
125?F. since this will cause the mold to crack and crumble.
In normal casting, a mold should not be cast more
than three times a day. Certain molds can be cast only once a day. When
excessive casting is practiced, the mold becomes very wet and after a
substantial number of casts have been made from any mold, pitting will
generally appear. When this hap
pens there is nothing that can be done to correct it.
Patching a mold is not recommended. On occasion,
however, it may be necessary to replace a chip made in handling or
shipping. If it is a small chip, then slip will run into this space when
it is cast, but this can be trimmed when removed from the mold. If the
chip is so large that it would create a hole when the cast is trimmed, it
is recommended that the chip be stuck back on with the use of shellac.
First the mold should be dry. Apply a liberal amount of shellac (ready
mixed orange shellac is very good) to the broken surfaces of the mold and
the chip. Light with a match and allow to burn no longer than ten seconds
or the heat will harm the mold. Extinguish the flame and press the chip
into place, holding it firm for approximately one minute. Any excess
shellac that squeezes out should be cleaned off with a small tool and then
wiped with a cloth dipped in alcohol. After drying for a day or two, the
piece will stay firm.
Continuous pouring of slip on the same spot of a mold may cause the
spot to crystallize and become non-absorbent. Once a mold has a
crystallized spot there is no way to correct it. This spot is caused by
the silicate in the slip and indicates that the slip was not thoroughly
agitated before pouring. Proper agitation of the slip before casting is
very important and is mentioned here because it affects the casting
quality of the
mold.
Casting is somewhat an art of it's own. While there are
hundreds of basic tips and hint, most are discovered when casting each
mold for the first time... For some basic tips, refer to our Free
Classroom with photo step by steps on casting.
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