Question:
What is Slip casting?
Mixing Time
Mixing Time
Mix the slip for several hours for best results. It is only by putting energy into the slurry that you can thoroughly wet every particle and extract the best performance. Less deflocculant may be necessary if the slip is mixed for a longer period. This is an advantage because some deflocculants, like sodium silicate, attack your molds and the less that you use, the better. After the slip stands overnight and is mixed for a few minutes the next day, it will usually cast better.
Specific Gravity
Specific Gravity
This is the weight per unit of volume of the slip. Specific gravity is defined as the comparison of a liquid's weight with the weight of an equal volume of water. In metric it is even simpler: water weighs one gram per cc (ccs and milliliters are the same). If a liquid weighs 1.8 grams per cc, then it has a specific gravity of 1.8; it is 1.8 times heavier than water. A slip with a specific gravity that is too high is said to be “heavy”. If the viscosity is too high, it is said to be “thick”.
The more water in a slip, the lower its specific gravity will be. The more solids, the higher it will be. As mentioned, slip with too much water will soak the molds more quickly, give slow casts, and result in excessive shrinkage that cracks the ware.
Viscosity
Viscosity
Viscosity refers to the mobility of the slip; its “thickness” or “runniness”. A slip that has high viscosity is thick like syrup and one that has low viscosity is fluid. A deflocculant is used to lower the viscosity of a slip and make it more fluid. This lower viscosity, in turn, makes it possible to add more dry material, which again raises the viscosity.
Slip casting: description
Slip-casting. In this case you buy or make plaster moulds, into which you pour liquid clay (slip.) The slip coats the inside of the mould, so when it hardens and you pull the mould away, a cast piece remains ready to decorate and fire. It would seem like if you poured slip into a mould, you would get a solid chunk of clay. But the slip only sticks to the inside surface of the mold at a certain thickness. You pour out the rest of the slip. When the piece dries it shrinks and separates from the plaster leaving you with a hollow piece.
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| Patricia Walters-Fischer |
