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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 an 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.)

trim-line
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

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