In contemporary art and design, there is a tremendous desire to explore ever more elaborate forms -thinking “outside the box”. In the case of ceramics, this often means trying new forming methods.
But, hand building forms is often times too labor intensive. Plus, designers need to be able to make precise and exact duplicates of any object they design. This is especially true when it comes to duplicating forms in industry.
Using molds in design and ceramics addresses those needs and desires. But, many find mold making and slip casting difficult and even a distraction from their creative endeavors. It seems to be too technically difficult and to have too many steps involved.
Like any forming method, there are parts of the process that are easily learned and others that take more time to absorb.
Slip casting is simply a forming technique for producing ceramic objects. It is especially useful for making complex forms that are non-concentric, irregular, square, and can also produce concentric forms.
Slip casting is used to produce pottery, figures, sculpture, and sanitary ware such as toilets and sinks.
Liquefied clay (casting slip) is poured into a plaster mold. The plaster mold is absorbent and will begin to draw water from the casting slip. The longer the slip sits in the mold the more water is drawn from it by the mold and the thicker the casting becomes.
When the desired thickness is achieved, the excess slip is poured from the mold for reuse. The remaining “skin” left in the mold is the casting. It will continue to dry, both by the mold and ambient air, and eventually shrink away from the mold. When the casting is firm enough to be removed from the mold and stand on its own, take to out of the mold and let it dry, or assembles it with its corresponding parts. The mold can then be cast again with slip.
Simple casting can be left to dry and complex forms can be assembled at this stage. Attach handles, spouts, feet, knobs, and place lids on the body of the form. Seam lines (fettles) can be cleaned up at this stage or left until the casting is completely dry. Moving the casting during the leather hard stage can create stress cracks in the form. The can appear either in the bisque stage and sometimes not until the form is glaze fired.
Casting slip is vastly different than ordinary clay slip. With the addition of a deflocculant (sodium silicate or Darvan) the electrical charge of the clay is neutralized. The particles change from having both a positive and negative charge to an all negative charge. Like two magnets, the like charge poles of the magnet repel each other. The clay particles with negative charge will do the same thing. With just enough water to form an envelope around each particle, the casting slip will stay in suspension. Terra sigilata is made in a similar manner, but excessive water is added to the mix so that the particles repel each other and slowly settle in the bucket with the finest clay particles above the heavier particles.
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