Difference between Dry Clay and Dry Casting Slip?

[JF_Potter]

I'm new to the whole slip casting technique and trying to learn.  I'm trying to source slip for slip casting and see a lot of places offer dry slip casting bodies, but what separates those bodies from normal dry clay you can buy?  I understand you add chemicals, including a deflocculant, to the dry slip, but beyond that what's the difference and why couldn't you just do that to a  normal dry clay body?  Are there certain properties of the casting body that make it better suited to casting?  

[PeterH]

4 hours ago, JF_Potter said:

...
  Are there certain properties of the casting body that make it better suited to casting?  

I think that it's the other way round. There are properties of a throwing body which make it less suitable for casting.

This is most evident in porcelains, where whiteness, translucency, high plasticity, availability of ingredients, and cost need to be balanced.  Higher plasticity tends to result in poorer whiteness and translucency. Extreme throwing-body examples are using rarer and more expensive ingredients, or formulating a marginally plastic body (for accomplished throwers willing to trade-off ease of throwing for a better fired appearance).

A casting body doesn't need to be nearly as plastic, and may cast faster because of this (fewer microscopic particles).

PS Check the small print. Some dry casting bodies already contain the necessary  deflocculant.

PPS From  Formulating a Porcelain https://digitalfire.com/article/formulating+a+porcelain
For casting porcelains, an all-kaolin approach is quite feasible (using 50% kaolin rather than 25% kaolin and 25% ball clay) since these bodies benefit greatly from the reduced drying shrinkage and increased water permeability associated with the larger particle size of kaolins. However an all-kaolin casting porcelain may not have the strength to pull away from the plaster mold without cracking, add a little plasticizer if needed.

[Jeff Longtin]

Most casting bodies are 50% clay and 50% non clay. Most throwing bodies have a greater percentage of clay in them.  Throwing bodies are designed to be plastic. This increases their shrinkage. (Both before and after firing.) You want to have shrinkage, in a casting body, but not too much.  (Enough so the casting pulls away from the plaster mold but not so much that the casting cracks while setting up in the mold.) Some throwing bodies can be turned into casting slips but most are too plastic. 

Another factor that determines how well a clay body will cast is the actual size of the clay particles. (Some porcelains are made up of small particles and some are made of larger particles.) A well designed casting body will have a balance of both large, and small, clay particles.  

[Callie Beller Diesel]

Some clay bodies that are meant for throwing with are sold as dry materials, because it costs money to ship water in moist clay. Some of these recipes can be used for slip casting, but some contain extra plasticizers like bentonite that will cause a slipcasting body to gel.

[Min]

How easily a clay will deflocculate comes into play also. Kaolins have a lower cation exchange rates than ball clays which in turn have lower exchange rates than iron bearing clays. The lower the exchange rate the better they will defloc. Adding some ball clay is okay for casting slips but it would be best to use specific ones for casting, like SPG#1, which are larger particled than the typical ball clay used in plastic bodies.