The recipe we generally recommend for making up a casting slip from powdered clay is:-
25kg powdered clay -- 35-87.5g* sodium silicate 120tw -- 30g soda ash -- 10lt water
*sodium silicate (deflocculant) addition will need to be adjusted accordingly to achieve correct fluidity. Too much deflocculant can have the reverse effect on a casting slip, causing it to thicken, so it is best to add it gradually, mixing all the while. Fine-tuning the fluidity is best achieved with sodium dispex.
If you can find a copy (it is currently out of print), we would highly recommend the book Ceramic Faults & Their Remedies which gives lots of useful information on slipcasting and the issues you may come up against.