Red-brown slip was used to color this ancient Roman bowl.
image by www.ancienttouch.com
Slip is a mix of clay, chemicals, and water to form a slurry which is painted over pottery before being fired. Typically slip can be used either for decorative purposes, or as an adhesive to hold two pieces of clay together to form one item. Here are instructions on how to prepare slip and apply it to your pottery.
Combine water, clay, and deflocculant in an old large mixing bowl. The amount of water, and clay will depend on the specific deflocculant used and will be indicated on the label.
Mix by handle, being sure to crush globs of clay until the slurry is of an homogenous texture.
For use as a decoration the slip should be thin and slightly watery, like applesauce. For use as a glue the slip should be much thicker, like oatmeal.
Using Slip As A Decoration
The pottery you intend to decorate should be dry and hard like leather, which takes roughly one to three days after being shaped.
Use an old paintbrush of an appropriate size to thoroughly coat the unfired piece of pottery, and set it aside to dry for several hours.
When dry enough to handle, take the item and use a Ribbon or Needle tool to scrape or cut away areas of the slip in any design you like. The original color of the clay item should show through and give the piece an interesting two-tone look.
Once finished, fire the item in a kiln as you would any other piece of pottery.
Using Slip As An Adhesive
For the purpose of this explanation we will take two pieces of clay, an unfired jug and an unfired shaped handle.
Use a fingernail or any available tool to score or lightly slash the two pieces along the points where they are to be glued.
Use a paintbrush to smear a thick dab of slick on one of the two pieces.
Gently press the two pieces together and hold them like that for roughly a minute or until they will stay together on their own.
Wipe away the excess slick around the joint and allow the pottery piece several hours to dry before firing as you would any other normal clay item.