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Making a Blue Bottle

Making the Slip
Slip is liquid clay, which we use to pour into a mold. The plaster mold sucks up the moisture in the slip and the wet clay sticks to the sides of the plaster. The longer you leave the slip in the mold, the thicker the ceramic object's walls will be. Slips usually don't have have much plasticity, which means that if you dried it out and wedged it, it would be hopeless to throw with, except if you were throwing it against a wall... The following is a cone 4 slip, which fits well with our glaze:

Nephelene Syenite 36.0
SGP Ball Clay 30.0
EPK Kaolin 18.0
G-200 Feldspar (Potash) 9.0
Silica (Flint) 7.0
Sodium Silicate 0.6
1 large cup of water per kilo dry powder

Wear a dust mask when mixing dry ingredients. Add the sodium silicate to a first cup of water, then add the rest in increments. Keep stirring slowly. When the mixture appears to get fluid, but still is lumpy, stop adding water. Work the mixture with a drill attachment for mixing (available in ceramic supply stores). This will get rid of most of the limps. (Caution, DO NOT try to clean the drill attachment of sticking lumps, while it is still plugged in!) Strain the slip through a fine mesh.
Now you are ready to pour the slip in the mold.

Pouring the Slip
Tape up your mold with some large rubber bands, e.g. cut car tire tubes, or bicycle rack straps -- anything that will hold the mold together and will withstand the pressure of the slip, trying to force it apart. Stand the mold upright on a suitable surface. Place a funnel in the hole, and a strainer over the funnel (a kitchen strainer will do this time) -- this is just to avoid getting larger lumps into the mold. Carefully pour the slip into the mold (through the strainer). Stop pouring when the slip reaches the top of the mold. Don't worry if a little bit goes over the edge (except if you happen to be doing this on your ten thousand dollar Persian carpet, which I hope you're not!) Keep a bit of slip in a plastic cup. As the plaster mold soaks up the moisture from the slip, the level will drop. You should replenish this. Watch the thickness of the wall, as more and more clay adheres to the mold. When you have reached a sufficient wall thickness (should be around a sixteenth of an inch or 1 1/2 - 2 mm), pick up the mold and slowly tip out the slip, back into your bucket. Leave the mold to dry out a little. This should take about half a day to a day. You may need to cover the hole, if you live in a hot climate, otherwise the top of the bottle may dry much more quickly, causing it to split. When you think the bottle is dry enough (you can gauge this with a little experience by looking at the opening and seeing how much the clay form has separated from the mold), you can separate the mold. The bottle will stick in one side, Let it dry out a little more -- it will come out easily.

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