Which deflocculant should I use when making my own casting slip?

Sodium Silicate Na2SiO2 is the most popular deflocculant used in casting slips and has been so for many years. It's nearly always used with soda ash as when it is used alone it can make a slip stringy and thixotropic. It is effective, reliable and inexpensive but attacks the plaster in moulds much more than more modern deflocculants and it is easier to over-deflocculate a slip with sodium silicate. Sodium dispex is a possible alternative which avoids problems of brittle and soft casts associated with sodium silicate and soda ash.

Which Twaddle? We supply Sodium Silicate in both 75Tw and 120Tw. 140Tw was discontinued by the supplier some time ago. A degree Twaddle (°Tw) is a unit measuring the specific gravity of liquids denser than water.  1 degree Twaddle represents a difference in specific gravity of 0.005 or 1/200.  The difference between the grades is the concentration of sodium silicate:  75Tw is like thin water and 120Tw is similar consistency to treacle.  Your recipe should specify which Twaddle to use but if it doesn't a general rule of thumb is 75Tw with Bone China and Terracotta, 120Tw with everything else.  120Tw differs in chemical composition to 75Tw so it cannot be 'watered down' and used as a direct replacement.