| Plaster Isn't
So Hard to Use
by Brian Crocker How much plaster should you mix? This requires
only a little mathematics. Having set up the model on a base with
the cottle around it, take all the outside dimensions, length, width
and height, preferably in centimeters, multiply these to get cubic
centimeters and this will give you the volume of mixed plaster you
require and from that the percentage [%] of water. Then using the
above list , the kg's of plaster.
 This
is not an accurate method but near enough, there should be plenty
for the Mold and some left over, it is very frustrating to pour
a Mold and find that you do not have enough mixed plaster. An accurate
calculation is quite possible but a lot more involved but not really
necessary in a studio.
Weigh out the water (%) into the water bucket pour
it into the mixing bucket, weigh out the plaster into the dry plaster
bucket and fluff it up with your hands or use a sieve and pick out
any lumpy pieces. There shouldn't be any lumps if the plaster is
fresh.
Make sure "all" the equipment is ready and
easy to reach [ don't forget to plug in the electric drill ] . Danger
you are using water near electricity.
From this time on, the mixing, plaster debubbling,
pouring of the plaster and final release of the Mold is controlled
by time.
The Sequence is as Follows,
1. Set the timer to
6 minutes, ( this is the time you have to mix the plaster, debubble
and scoop out the bubbles ) slowly sprinkle the plaster into
the water, N.B. plaster into water,, don't just dump it in, then
with the mixer, a paddle or your hand, cream the plaster into the
water try to avoid making bubbles. When nice and smooth pick up
the bucket by the handle and rattle it on the floor and as the bubbles
rise, "watch your timer", stop and scoop them off and
discard them into a paper lined rubbish bin... Not
down the sink, it causes "solid drain pipes".
2. Set the timer to 3 minutes, ( this is
the time you have to pour the mixed plaster into the Mold and debubble
(vibrate) the Mold ). Pour in the plaster and try not to pour in
any bubbles, if you can, have an assistant to scoop them out. Pour
slowly and down as close to the surface as possible and onto a position
away from the model surface so that the plaster flows gently up
to and around the model.
As soon as the model is covered by about 10 mm of
plaster stop and give the cottles a number of taps (vibrate) on
all sides,, gently to release more bubbles, if too violent
you might loosen the cottle, now scoop out the bubbles.
A loose cottle is an experience you can do without.
Having plaster leaking out all over the bench tends to foster a
feeling of helpless panic.
Carefully finish pouring the plaster, still in a position
away from the model.
3. Set the timer to 2 minutes, and finish
debubbling, this will bring you up to a total of 11 minutes from
start to finish, during this time the plaster is thickening and
it is difficult to release more bubbles and there is a possibility
of spoiling the new Mold.
You may have noticed that I keep mentioning bubbles,
they are the enemy of the Mold maker. a few on the surface or in
the body of the Mold are OK but if they stick to the model surface
your work can be spoiled. Don't try to fill holes with plaster later,
throw the Mold away and start again from the start, soaping, polishing
etc.
4. If the model is convex in shape ( a bottle,
vase, sprig or similar ) set the timer to 30 minutes and leave the
plaster to set. After this time the plaster will have reached 40°C
or more [ it feels warm ].
For the next 4 minutes it will be at it's maximum
expansion and will have very very slightly moved away from the model
by about 1 to 5 thousandths of an inch.
Now you can remove the cottles and gently tap
the top plaster of the Mold with a soft rubber hammer but be very
careful the plaster is only as firm as a matured cheese, also tap
the sides of the Mold and the bottom of the model then try to shake
the model free, ‘as a last resort use very thin soft timber wedges
coated in soap, place them around the joint line and push them with
your finger. Or use compressed air at the joint line.
5. In the case of a concave model ( a bowl
or similar ) leave the plaster Mold with the model inside in a
warm dry place for 24 hours or more . You can blow air over it with
an electric fan but make sure the air does not blow on the one surface
all the time, move the Mold often to present a new area to the
fan.
It is important to ensure that the air is not blasted
onto the Mold it should be gentle and not warmer than 35 to 40°C.
If only the outer surface of the Mold becomes dry through ‘forced
drying' the inside will take longer to dry, it needs a capillary
action to draw off the inside moisture. When you feel the Mold
and it is not cool but at ambient temperature [ if ambient is not
too cold ] it is time to release the model as in 4. .
If the plaster will not release, the model may be
undercut at some part,, or there is some area of the model that
was not soaped enough,, or the wet plaster may have been poured
in so forcefully that the soaped surface has been washed off and
the plaster has stuck to the model.
In cases where there is a "hang up" gentle
persuasion must be used then increasing to bruit - force if necessary
(you may ruin the Mold). But never forget the power of compressed
air, at the joint line.
At first this all may sound complicated, but the above
methods are some of the basics used by many Professional Mold Makers
every working day in production Potteries.
I must emphasize that these are basics, and not the
complete answer to Mold making, it is a profession that takes many
years to learn. There are several good books on the subject, enquire
at a College where it is taught as a Professional Subject.
Not just part of another course.
Give it a try, you may be like many Potters who have
success, if not, you will be sure to learn by your mistakes.
Happy Mold Making.
Brian Crocker
Retired Tutor, Adelaide College, Underdale South Australia
Email:
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