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Frequently Asked Questions about Ceramics

Here is where we explain common questions about Mayco Products and Ceramics in general.

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How to pour a plaster mold for ceramics

TIPS ON MOLD CARE 

IF A BRAND-NEW MOLD STICKS TOGETHER NEVER PRY OR FORCE APART, as this can cause the mold keys to break off. Proceed with your casting as usual and the mold should separate easily by the time the piece of greenware is ready to be removed. 

PRODUCTION CASTING: Do not over cast molds. When casting in succession, you will find it necessary to leave the slip in the mold a little longer each time, in order to obtain the proper thickness. Depending upon the size of the mold and the thickness of the mold walls, you can usually get from four to five casts before the mold has absorbed so much water that it must be allowed to dry. Excessive casting will weaken detail and considerably shorten the life expectancy of a mold. 

DRYING DAMP MOLDS: ALWAYS PLACE MOLD PARTS TOGETHER TO DRY.  

This will prevent warping of the mold. Do not force dry in an oven, or place damp 

Molds can be sun-dried. It is extremely important that a damp mold dries evenly from all sides. Drying a mold too close to a kiln or any other direct heat source can cause it to powder, chip or warp.

HOW TO CAST A MOLD 

When casting molds, there's no better place than the kitchen, where plenty of water is available. Casting slip and dried clay are easily cleaned up with a damp sponge. Cover the work area with a plastic sheet, oilcloth or newspaper. Assemble the other materials needed: casting slip, a large measuring cup or pitcher for pouring, trimming knife, draining pan and cleanup sponge. Before casting, always open the mold to check the design and to be certain there is no dust or foreign particles inside. To clean the casting area, dust with a soft dry brush or go over it with a cotton ball saturated in rubbing alcohol. Never use water to clean the inside of a mold this will damage the design. Note that cleaning the inside of any mold is especially important between casting different clay bodies, such as changing from regular white slip to stoneware or porcelain slip.

Place rubber bands tightly around the mold to hold it together. This will prevent the casting slip from leaking out of the mold.

The liquid clay is called casting slip and should the consistency of thick cream. It should be stirred well just before casting. 

If the greenware is hard to remove from the mold lightly judo chop with the side of the hand around the edge of the mold. This will usually release it. If it doesn't, the may contain oil or other residue and should be cleaned­ with rubbing alcohol, in the manner stated above. Follow the step-by-step directions below and discover that casting your own molds is half the fun and thrill of making ceramics.

the-mold

1. Separate the mold, check for dust and foreign materials and ­then put mold together, matching mold keys, and securing with rubber bands.

mold-pouring

2. Mold is ready to cast. Use pouring container that will hold more than enough slip to fill the mold. Pour slip into mold slowly, steadily and evenly, filling the mold slightly above the pour hole.

fettle

3. The plaster mold absorbs the water in the slip, thus forming a hard clay shell next to the mold. Keep the mold pourhole  full until desired thickness is obtained. Cut a  notch (as shown) to determine the thickness of the clay shell, which should be approximately 1/8" thick.

dump-mold-out

4. When desired thickness is obtained, slowly drain excess slip from the mold. When all slip has drained completely from one pour hole, follow the same procedure with any additional pour holes the mold may have. After draining, let the mold set until the slip becomes leather-hard clay. The length of time required to reach this stage will vary depending upon the size of the mold.

fettle-the-cottle

5. When clay is leather-hard, remove any excess from the outside of the mold and from the mold pour hole, as shown. Take trimming knife and insert the blade between the mold wall and the clay.

remove-the-spare

6. Trim away and remove this excess clay. Place the mold on its side, remove the bands carefully and allow it to stand a few minutes longer.

open-the-mold

7. To open mold, slowly and very carefully lift the top half of the mold straight up. Do not move it from side to side, as this will damage the cast piece. If the top does not release easily, let it stand for a few more minutes.

mold-opened

8. Do not remove the piece from the mold until it is firm enough to support its own weight. Handle the piece with care when removing it from the mold. It will be soft and its shape can be distorted if not handled gently. If the piece does not release easily, wait a few minutes until it has pulled away from the plaster mold. (With large, flat pieces such as plates or ashtrays, loosen the casting and allow the greenware to dry in the bottom half of the mold for four or five hours longer to prevent warping.)

remove-from-the-mold

9. When the piece is loose, gently tilt the mold forward and allow the piece to fall into your hand. If the piece has several parts to be attached, do this shortly after removing them from the mold. Attach pieces with casting slip when the clay is leather-hard. Carefully place the cast piece in a safe, dry place and allow to dry before handling again. For delicate and footed items, place greenware in a soft bed of shredded paper to dry.

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ANALYZING SLIP DEFECTS

1. Short, crumbly or brittle greenware -caused by too much sodium silicate in the slip mixture.

2. High shrinkage or cracking in mold -too much water in the slip.

3.      Gummy ware after the normal set-up time-too much soda ash or too little sodium silicate in the slip.

4.      Clay settles to the bottom of slip container, leaving a large waterhead; (sometimes floating black particles are visible on the surface) -can be caused either by too much sodium silicate and/or too much water in the slip. 

5.      Thick slip after normal aging period -high evaporation of water.


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