the plaster for the first part.
picture shows the wedges in place holding the mould walls firm.
plaster for ceramic slip casting should be mixed in the proportion
of 70 parts weight of water to 100 parts weight of dry plaster.
Even if the plaster is new, sieve it into the weighing container
with a coarse (flour) mesh. With experience, one can judge the volume
of water needed,(calculations at the bottom of this page) and sift
the weighed plaster onto the surface of the water. Allow it to sink
of its own weight and soak for one or two minutes before mixing.
Stir gently with a kitchen tool, preferably the sort
with 'fingers' or coils which breaks up any lumps, for two minutes.
Allow to stand and shake or jiggle the jug to persuade the air bubbles
to rise. When a finger trailed across the surface leaves a mark,
it is time to pour slowly from one corner of the mould so that the
creamy liquid flows across the model, leaving no air bubbles trapped
in corners. Jiggle the mould again to persuade the plaster to settle
level on the (level) bench.
can be engraved with the name of the model and the part number when
the surface gloss has gone but before the plaster sets hard. This
is the plastic stage. Do not attempt to trowel or otherwise modify
the plaster after crystallisation has begun. The plaster will heat
up as the chemical reaction continues and the larger the part, the
higher the temperature.
attempt to disturb the mould until the heat generated has gone.
When it is cold, remove the walls, peel off the clay bed, trying
not to disturb the model, wash the plaster and soak it well in clean
water. Then cut the natches and wash away the chips of plaster.
round blue marks are natches, or the dimples that provide the locating
recesses for the second part which will have pimples. It is sensible
to make a practice of putting the dimples assymet- rically so that
it is impossible for the mould to be assembled incorrectly. It is
obvious in the case of the banana which way round the second part
should go, but may not be so obvious on a round model. (See, later,
the apple mould.)
of a steel teaspoon makes a good 'chisel' for cutting natches, but
do not make them too deep. They are locating dimples for the next
part which will have the corresponding pimple, but they should not
be press-studs, locking the mould parts together.
MORE PLASTER CALCULATIONS: 65 parts of water will give a heavy,
dense plaster which will not absorb water very well. (I use the term
'suck', as in, "it has little suction.") 75 parts of water
to 100 parts of plaster will give a soft, absorbent mould which will
wear easily and may break, especially when damp and held unevenly.
If 70 parts of water happens to be 700 millilitres= 700 grammes, and
you put into the jug 100 parts by weight of plaster= 1000 grammes
(1Kilo), the volume will not increase to 1700 millilitres,
because a large part of the volume of the plaster will be air.
will be about three fifths larger than the original amount of water.
700/5 = 140. Then 140 times 3= 420. Then 420+ the orginal 700= 1120
millilitres or just over a litre.
Most people can estimate the volume of liquid in a plastic
drinks bottle. They come in cylinders, I know, of a half a litre,
75 cl (bottle of wine) and one litre. But we get milk today in square
bottles, and this makes it easier to judge the volume required to
fill the mould. I try to make a note of the volume of water needed
for a particular mould part. A simple table will give me the weight
of plaster required.
E Frith, in Mold Making for Ceramics, (ISBN 0-8019-7359-7), is the
expert writing on this subject. He
is also good on slip-making.
page is boxing up for the second part.