The advantage of this technique is that the slip does not run during firing. So you can make nice, clean lines.
The important thing to remember when working with clay slip is that it must be made from the same type of clay as your piece. This way you can be sure that they will have the same shrinkage. You can also add oxides or pigments to the clay slip.
Sometimes other substances are also added to make the slip stick better: then it is called an engobe.
