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Ceramics 101
Check your slip ~ or ~ How to use a Viscosity Cup
Before pouring your mold or dipping your bisque in
glaze you must be sure your slip or glaze is at the proper viscosity (not too
thin or too thick). Get your clock or watch (with second hand) and follow these
simple steps:
1. Be sure slip (or glaze) is smooth and well mixed.
2.
Dip viscosity cup into slip or glaze and lift straight up.
3. Begin timing,
counting the seconds, the instant the bottom of the cup breaks the surface of
the liquid.
4. Note when the solid stream of slip or glaze breaks into drips
for the first time. That is your time.
Slip
~
Generally for slip the number should be 25-35 seconds. If your
slip is too thick you can add water, mix well and retest.
Dipping Glaze ~
Generally for glaze the number
should be about 29 seconds. Check the back of your glaze bottle for exact
number.
How to pour a mold
Pouring molds is a time consuming and meticulous job.
Molds are heavy plaster objects, usually made in two parts sometimes more. The
molds are held together with tight, heavy bands or straps.
Your mold
should be clean and dry before pouring. So, the first thing to do is open the
mold to be sure it is clean inside. Use an airgun or soft brush to gently
remove any loose debris. Do not touch the inside of the mold as this can cause
of build-up of oils from the fingers which could eventually cause the mold not
to absorb liquid. Be careful not to scratch the inside of the mold as this will
make the greenware less than perfect. Note: Clean the outside of the mold after
every use - scrapping off the dried slip while it is still fresh enough to be
easily removed. One suggestion from Dan M. of the Duncan Sharing board is to
use a green Scrubby to clean dried clay from the outside of your
mold.
Always be sure the mold bands are tight enough to hold the mold
together so that slip doesn't ooze out along the seam lines and ruin the piece.
To check if the bands are tight enough, try pulling the mold apart with two
fingers and using moderate pressure. If the mold opens the band is not tight
enough. Choose a smaller size. Do this check carefully!
After
being sure the mold is tightly banded, it is then filled with liquid clay which
is called slip. Pour the slip into the mold in a smooth continuous manner. Some
people advocate pouring the slip over the handle of a wooden spoon to break up
the flow and prevent hard spots. Another method is to pour the slip over your
fingers or, if there is a large opening in the mold, continuously move the
angle and direction of the flow. It is actually the force of the slip hitting
one spot in the mold which can cause a hard spot (sometimes called a hot spot).
Pour the mold smoothly and do not stop until the mold is filled completely to
the top of the pour hole. Stopping and starting the flow of slip will leave
unsightly lines in the finished piece. Now just wait for the mold to absorb the
liquid in the slip and build up on the inside of the mold. While waiting for
the piece to set up it may be necessary to add additional slip to keep the pour
hole filled.
The plaster of the mold is dry and it extracts the water
from the slip - forming a layer of clay on the inside surface of the plaster
mold. The thickness of the piece is determined by how long the slip is allowed
to build up in the mold. The longer the slip is in the mold, the thicker the
object will be. When the piece is deemed thick enough (see below) the excess
slip is then poured out of the mold. Carefully tilt the mold to the side and
let the slip flow outward. If the slip makes a glugging noise it may pull the
soft clay away from the sides of the mold and ruin the piece. So, tilt it
slowly so that all the slip drains out of the mold smoothly and silently. After
draining, when all the drips have stopped, return the mold to its original
upright position and allow it to dry. If the mold is for a mug, teapot, etc.,
you may want to place it with the handle side down so that the handle fills
with slip (before the drips have stopped), not so that the handle drains. A
solid-pour handle is desirable over a hollow handle, which could harbor
bacteria. If slip does not fill the handle when you turn the mold handle side
down, simply use a spoon or other utensil to fill the handle with slip while
the piece is still wet. Leave it in this position until the slip is leather
hard.
Most open-pour molds so not require draining and may be cast
solid, depending on size and thickness of the piece. Keep filling with slip so
that the back is flat and piece is solid.
Many people are confused about
when to drain a multi-piece mold. Some people advocate cutting out a vee shaped
wedge from the pour-throat to determine thickness. Most items are thick enough
when they reach 1/8 to 1/4 of an inch, depending on your personal preference.
Plates, bowls, platters, etc. may be poured solid, which means you must keep
topping off the slip in the mold until the item is solid. (Some plate molds
need a thinner slip to fill completely and will require a longer time to
dry.)
Many mold trimmer tools have been made with a
built-in measure. The drawn-in white line on this image indicates where the
measure is on this style of mold trimmer tool. To measure the thickness of your
piece simply hold the trimmer with the narrow end pointing straight down. Slide
it into the slip and over to the side of the mold and into the thickening clay
of the piece. Then carefully pull the trimmer straight up and out of the slip.
The thick part of the clay will be clean and only the liquid slip will stick to
the trimmer. If the clean area goes from the edge of the tool to the measuring
line then your piece is thick enough. Dump it!
Page 4 - Ceramics 101
Simple, Secure, Convenient - Creative Glazes.
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