Saturday, September 18, 2010
Doll Composition Slip
Today I cast some CompoBell CS-1000 doll composition slip into the Plaster Mold I made back in mid July. This casting is a shrinkage test, so the first thing I did was scribe a couple of marks into the inside of the mold, one inch apart, using a needle tool.
The slip needs to be stirred before use. I used a paint stirrer attached to my electric drill. This slip is too thick! There are two properties that can be adjusted with slip: specific gravity, and viscosity. Specific gravity is adjusted by adding water. Viscosity is adjusted by adding Slip Thinner, or a deflocculant, such as sodium silicate, in very small amounts.
Specific gravity means, how much more than water does the slip weigh? If water weighs 8.3 pounds at 70 degrees Fahrenheit, and a gallon of slip weighs 14 pounds, then the specific gravity is the ratio of slip divided by water, (14/8.3), which is 1.69. This is a ratio, it is generally accepted that 1.75 is good for slip casting. So the slip would be a little light at 14 pounds per gallon. Water weighs less than clay so there is too much water in the slip. I can’t take it out so I would have to add more dry materials, if I had them. (if the slip was too heavy I could add water.) Okay, now I weigh the slip again and it is 14.5 pounds plus a little. Doing the arithmetic, I can tell that the slip is right at 1.75.
The Viscosity refers to how thick a liquid is. With slip I can change how thick it is by adding more defloculant (Sodium Silicate) NOT by adding water. How thin do I want the slip to be? It depends on what I'm making. Hollow beads or tiny miniatures require thin slip, large items require thicker slip. The slip must be thin enough so I can pour off the excess slip through the thinnest part of my doll, which is the wrist.
I measure viscosity by how long the slip takes (in seconds) to drain out of a container with a small exit hole. What I need to do is time the slip using a homemade viscosity meter.
A plastic bottle or jar, with a lid can be used. I drill a hole in the bottom of the jar, and another hole in the lid of the jar. Then I put one cup of slip into the jar, while holding my finger over the hole in the bottom. I place the cap on the jar and hold my finger on the finger hole in the lid, then let go of the bottom and the slip will not come out until I let air in the top. I time how long it takes until the flow changes from constant to drips..
I should try to do this viscosity test exactly the same way each time I do the test.
If the slip is too thick and the specific gravity is right, then I can add some defloculant, or some special slip thinner. These are very powerful in their action only 2-3 drops per gallon need to be added.
Since the specific gravity is good for this slip, I need to buy some slip thinner, and add a couple of drops to my gallon of slip.
I have not baked the casting yet. I will do that tomorrow. However, so far it has shrunk 1/32nd of an inch. That 1/32nd of an inch will translate to 3/4ths of an inch for the whole doll because 24 X 1/32 is 24/32nds, or 3/4ths of an inch.. So my 24 inch wax doll will be 23.25 inches tall when cast in slip, if there is no more shrinkage. I'll measure my marks again after I've baked the casting.
Shrinkage is complicated to control, but it is useful to know so that I know how to size the molds. More water in the slip gives increased shrinkage and increases cracking in the mold.
If I put too much slip thinner in the slip, it casts slow and the inside of the cast appears sharp and greenware is hard, brittle.
Not enough deflocculant and the slip casts fast, seems soft and flabby, the molds get wet sooner. and the inside of casts look smooth.
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