This is a step by step tutorial on how to slip cast porcelain Espresso cups. Slip casting can be useful for making multiples of one object. For a detailed tutorial on how to mix plaster please refer to the "Mixing Plaster" tutorial for more information.

Some basics copied from the following instructable:
https://www.instructables.com/id/Slipcasting-aka-Ma...

Slip. What the hell is it?
Well, there are a couple of kinds of slip. If you've ever taken a ceramics class you've undoubtedly used slip to glue pieces of clay together. This slip is typically made by adding water to clay until it liquefies. The slip used for slipcasting is somewhat different in that a deflocculant (typically a mixture of Darvan and bentonite) is added to ensure maximum flowability. I purchased pre-made slip in gallon containers from my local clay retail shop, but you can also make your own.

How the hell does this process work?
Plaster and clay resist one another. In a nutshell, that is the key to understanding this process. A dry plaster mold will absorb the water in slip at a fairly uniform rate. You fill a plaster mold with slip, let it sit for 15-20 minutes, pour off the excess, and what remains is a .25" thick shell of clay in the form of your mold. Many, many objects are made using this process, including cups, plates and even toilets.

also detailed tutorial about slip casting

https://www.instructables.com/id/SlipCasting/