‘Slip’ refers to clay particles that have been suspended in water to form liquid clay.  It is used for a number of purposes in pottery.  For example, it’s used to join pieces of unfired clay together, a little like glue.  But it is also used for decorating pottery.

Decorating slip is normally colored using ceramic stains or oxides.  You can buy bottles of decorating slip.  But equally, it’s quite easy to make your own colored slip. 

The advantage of making your own colored slip is that you can make it exactly the color that you want it to be.  And, it is much more cost-effective to make your own. 

If you’d like to know more about making colored slip, you can refer to thePotteryWheel.com at the following link - how to apply the slip process.

How to Apply Colored Slip 

Slip is applied to greenware clay, that is, clay that has not been bisque fired yet.  Clay shrinks when it dries out.  If you apply clay slip to bisque fired pottery, it will shrink and flake off as it dries.

For that reason, decorating slip is applied to unfired clay.  It is also best to apply slip to clay that is still reasonably moist.  It has been suggested when applying a slip to pottery that has been allowed to firm up a bit but is not quite leather hard yet.

If you apply slip to clay that is very dry, then there is the risk that it will shrink as it dries and flake away from your pottery. So, ideally, it’s applied to reasonably fresh, but firm pots.

If you want to know more about the different stages of leather hard clay, you can refer to thePotteryWheel.com at the following link - post on leather hard clay. The great thing about slip is that because it’s made of clay, you can apply it to the whole pot.  Even the base of your pot.

Slip doesn’t melt in the same way that glaze melts, so it won’t stick to your kiln shelf as it is fired. You can also mix different colored slips without worrying that they will interact badly in the same way that glazes can. 

If you make your different colored slips out of the same clay body, then you can blend, mix and layer your colors on your pottery in whatever way you like.

Once you have decorated your pottery and allowed it to dry out, it can be bisque fired.  A clear glaze can then be applied to the bisque-fired pottery. 

Clear glaze will bring out the colored slip and can create a really rich gently textured finish on your pottery.   

Reference -  thepotterywheel.com -