How to Decorate Pottery Surfaces with Slips in the Bisque State

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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.)

 

gessslips_largeI 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.

 


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**First published in 2013
Comments
  • Donna K.

    Beautiful! Funny, I think of Albany Slip as a glaze but only at ^9 or higher. I would think any clay body that does not contain flux ingredients naturally would have to have the Soda (soda or salt) or wood ash from that kind of firing to become a glaze (or what would normally happen with just the clay body itself. I see I have too ridged an idea of glaze.

  • Thanks for the useful information. Should be very useful for thin slip applications. I can add that back in the early 90s I studied traditional pottery in Korea and learned that in some cases they would apply slips to bisque ware meant for high fire reduction in a wood kiln. After returning to the states, I starting experimenting with engobes for bisque ware, using guidelines from D. Rhodes Clay and Glazes for the Potter. After lots of experimenting, I developed engobes that could be applied very thickly to bisque ware, fired in cone 10 reduction, and showed no post-fire defects whatsoever. The nice thing was that by adjusting the recipe, one could introduce defects, such as shrinking and peeling, that could be somewhat “randomly controlled” to produce very nice and earthy surfaces.

  • Shana S.

    Are these cone 10 or cone 6? Thanks!

  • Judi M.

    Beautiful work! I like the subtle differences of the base slips on the canister set. Really nice pieces.

    I am curious, however, about your source for Helmar Kaolin. Our source became unavailable many years ago. I know of no real substitute for it. Is there some still around?

    Also, what do you use in your black slip? It has a very nice tone.

  • Scott M.

    What results have you found with using slip made from the clay body you are using? I use this slip mixed with mason stains and/or oxides on my greenware. It seems to work much better than other underglaze formulas I’ve found.

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