Stephani Stephenson on sat 19 apr 03
This answer assumes you already have a cone 10 stoneware throwing or
sculpture clay
you can use that same general clay body to make some casting slip.
You can use existing slip , but a more precise way is to collect
scraps, let them dry out,
put dried scraps in bags then break up lumps by pounding them, etc.
(Rolling over them with a steam roller or truck works nicely too.....)
for every 100 lbs. dry weight of clay, you will need
1 ounce of soda ash
2 1/2 to 3 1/2 fluid ounces of sodium silicate
1/2 ounce of barium carbonate, ( if scumming is a problem or hard
water is used, i don't usually use this)
5 to 5 1/2 gallons of water
put sodium silicate into a small container. Dilute it with a like amount
of water. set aside.
put water into slip tank
measure Ba Carb and Soda Ash and dissolve into warm water. Set aside.
Add Ba Carb/soda ash warm water mixture to water in slip tank, mix well
while still mixing, begin to add about 1/3 of the bone dry clay
scraps. Let them absorb the water.
add 1/3 of the diluted sodium silicate solution to the batch.
mix for awhile, allowing the Sodium silicate to thin the batch
Repeat with the next 1/3 of clay, then the next 1/3 of Sodium
Silicate/water solution
mix about 10 minutes, then add the final 1/3 of the clay.
add the final 1/3 of the clay. Suggested mixing time after this is 2
hours for 100 pounds.
(I don't think I have ever mixed it that long)
One thing I always do is let the slip age as much as possible (cover it
tightly so that it doesn't dry out, and mix it periodically)
also I leave the grog in the slip, this helps with cracking, shrinkage ,
etc., as with regular clay...
I have also added paper pulp to casting slip, which you might consider
if you are doing large slab castings.
Also , I don't add the Barium, but you can decide if you want to .
These instructions are taken from Laguna Clay and have worked well for
me.
Yes, I eyeball the slip to judge it, also I pour some in a small mold
and see how it responds... but you can test for specific gravity by
using a hydrometer or by weighing some of the slip (minus the weight of
the container. )
Specific gravity for your casting slip should be 1.75-1.82
100 cc or ml of water weighs 100 g,
100 cc or ml of your slip should weigh 175-182 g.
Even Laguna notes that if you are getting a different reading but the
slip works great, go with the slip...
Also recommended is that if you need to further disperse or
deflocculate the slip, use Darvan rather than more sodium silicate or
water.
You can also use a viscometer to test viscosity. Another way to do it is
to dip finger in and observe how it coats/runs off of the finger. Add
this to mental clipboard of observations!!!!!
best wishes
Stephani Stephenson
steph@alchemiestudio.com
http://www.alchemiestudio.com
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