Slip has many uses in the pottery studio. Most often, slip is used on clay in the green state, but potter Terry Gess does things a little differently. He uses slip to decorate his pottery surfaces when they are in the bisqueware state. He likes the freedom that comes with knowing he can experiment and if he doesn’t like the results he can just wash it off and start over. But there are technical challenges to this method. Today, Terry explains the challenges and the method he has developed to overcome those challenges. He also shares three of his slip recipes. – Jennifer Poellot Harnetty, editor.
The challenge and subtle beauty of slips are of great interest to me. Potters firing with salt, soda or wood kilns often employ slips on the exterior surfaces of their work to achieve thin, skin-like surfaces. Slips are related to glazes, but they are comprised primarily of kaolin and will record the nuances of the kiln’s flame, combustion and atmosphere. The result can be similar to the blush on a peach or the subtle patina of a weathered wall. It is, however, a most fickle process, and results can be difficult to reproduce.
On any given piece, I use up to four different kaolin clays in individual slip recipes in order to achieve subtle variations in the surface treatment. There are a number of different kaolin clays available from around the world, and they all have different qualities and benefits. I apply the slips by overlapping, dipping, layering, pouring, using wax resist, and other basic glaze application methods.
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I apply my slips onto bisqueware. Bisque gives me the opportunity to experiment, make mistakes, and to change my mind and wash everything off without destroying the pot. The greatest technical challenge to this approach is shrinkage. The slips need to be watery-thin. (If the slip looks wonderfully thick and creamy in the bucket, then it’s much too thick for bisque.)
I standardize the process by using a gram scale and graduated cylinder in order to carefully measure and record the specific gravity, or weight-to-volume (density) comparison of each of my slips. To measure the specific gravity, divide the weight of a given volume of your slip by the weight of the same volume of water. Most of my slips are 1.2 specific gravity (meaning they are 1.2 times as dense as water), while most glazes are in the 1.6 – 1.7 range. Adding approximately 2% Veegum Cer (which is a mixture of synthetic and natural gums) and 1% bentonite to the slip helps with glaze suspension and also binds each layer of slip to preceding layers on the surface of the pot.