Slip is liquid clay. Used for casting ceramic forms or for surface decorations. Slip usually has a ratio of 40% water to 60% clay except for casting slip which is no more than 32% water and 68% clay.
The simplest way to make slip is to dry out your clay, crush it up and add water to it and mix till it becomes a liquid. The thickness of the slip will vary depending on what you plan to use the slip for.
Sometimes materials are added to slip. These can be colorants for making a colored slip or a deflocculant, like Sodium Silicate or Darvan (Darvan 7 has a high molecular weight and is better for glazes and underglazes and Darvan 811has a low molecular weight and is better for slips. Deflocculants are electrolytes that cause the ball clay particles to become electrically charged and repel each other, they are added to slip to make it more fluid for slip casting.

Some ways to use slip:
Slip Trailing- is done with thicker slip and is applied with a slip trailer (similar to a bulb syringe) a precision applicator, a squeeze bottle with a needle tip or an air pen.

Brushing- applying a solid layer on the clay to change the color of the pot or to carve through.
Dipping- the pot into the slip for decoration or the change the color of the entire pot

Slip Casting and Casting Slip- is used to pour into plaster molds. Casting slip should not have more than 32% water in it, if there is too much water it can cause the slip to waterlog the molds and take too long to cast. Also, the slip will gel to in the mold and crack and be difficult to remove. This is why we add a deflocculant to make the slip more fluid without adding more water. You will want to start by mixing .5 oz of sodium silicate (or other deflocculant) with .5 oz of water and adding that to a gallon of slip, mix well. You want a specific gravity of 1.75-1.80 for casting slip.

Using Slip with Underglaze Transfers or Decals -brush on the clay before or after applying Underglaze Transfers. Also used to make your own mono print underglaze transfers and to “charge’ the dried transfers before applying to the clay, If using slip for this you may want to add a drop or two of Deflocculant to help make the slip brush out more evenly on your transfer.

Brush Decoration-brushed on the surface, like painting on clay with underglazes. You can also brush over stencils, paper resists and latex resists.

Stamping- stamps can either be pressed into wet slip on the pot to create a pattern or you can stamp on the clay using the slip like ink. You can also apply slip to a Gelli plate then stamp designs on the Gelli plate then roll or press your leather hard or bisque pot into the Gelli plate.
There are a few ways to make your own slip.
Making slip from your clay- an easy way to make slip is to let your clay dry out, break it up then add water to it till it is the desired thickness for your use. This works well for everything except casting slip which need other material added to it than just what is in your clay body. While you can make your own casting slip, I have found it is much easier to just buy it premade.