How to Mix and Apply Dendritic Slip Jeff Zamek

Why Dendritic Slip Works

The unique dendritic slip pattern is caused by a reaction between the wet underlying clay slip and the overlying wet application of dendritic slip. The instability where the two slips meet produces a surface tension gradient creating the tree pattern. This impressive dynamic reaction takes place within seconds. The contact line between the alkaline clay slip with a higher pH and the acidic dendritic slip with a lower pH becomes unstable as the surface tension of the dendritic slip is less than the underlying slip.¹ The immiscibility of both liquids yields the distinctive tree or branch-like pattern. This reaction is also known as the Marangoni effect.² Another way of stating the effect is a mass transfer along an interface between two fluids due to surface tension gradient.

The reaction is further enhanced when using a slip with a high clay content (alkaline) and a dendritic slip containing high levels of acidic materials such as tobacco, stale urine, hop extract, boiled leaves, lemon juice, coffee, apple cider vinegar, or any acidic material. While the dendritic slip technique has traditionally been employed at low earthenware temperatures, it can also work at the higher stoneware temperature ranges.

In the past, one variation used chewed tobacco and urine, which formed the acidic component of the dendritic slip. The brown, blue, and green colors were produced by adding manganese dioxide, ground iron scale, cobalt oxide, or chrome oxide.³ Current research shows that the tree-like patterns can be colored by the use of the carbonate form of metallic coloring oxides. For example, cobalt carbonate is less dense and has a smaller particle size than cobalt oxide. Cobalt carbonate, having a greater surface area and lighter density, allows for an enhanced reaction in the dendritic slip as opposed to cobalt oxide, which is denser and does not remain in suspension.⁴ One noted exception is where red iron oxide is used as a colorant in the mocha tea.

Mixing and Applying Dendritic Slip

The dendritic slip technique can be applied to any clay body, fired at any temperature range, and fired in any kiln atmosphere. However, three main elements should be in place to ensure optimum results.

• A reliable clay body formula that fits the base slip and dendritic slip in the drying, bisque firing, and glaze firing stages.
• A base wet slip formula that fires to a contrasting color with the overlying dendritic slip.
• A suitable, acidic dendritic slip formula that will react with the underlying wet slip and be compatible with a covering glaze.

Applying Dendritic Slip

After the pot is thrown, handbuilt, or otherwise formed, apply the base slip to the leather-hard clay surface as soon as possible to ensure a stronger bond between the slip and clay body. While the base slip is still wet, immediately dip a soft bristle brush in the dendritic slip mixture (dendritic slip settles very fast, so constantly stir it) and fill the brush with a small amount of the watercolor consistency dendritic slip. Then, barely touch the surface of the wet base slip with the brush. The dendritic slip will flow off the brush onto the base slip, leaving a pattern. A tree-like tentacle decoration can be developed by holding the pot vertically. Concentric ring patterns can be obtained by applying dendritic slip to the horizontal pot surfaces. It is important to note that the wetter the base slip, the greater the growth of the dendritic pattern. Timing is critical. If the base slip is dry when the dendritic slip is applied, the pattern will not develop. The reactive qualities of the dendritic slip can dissipate over time in storage. Always test both base slip and dendritic slip so they can interact successfully during the application process.

After the application of base slip and dendritic slip, let the pots dry thoroughly, after which they are ready for bisque firing.

the author Jeff Zamek obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980 he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.

1. The Observer’s Book of Pottery & Porcelain, by Mary & Geoffrey Payton, P. 114 Frederick Warne & Co. Ltd. Pub.
2. Morris, Stephen, www.physics.utoronto.ca/~smorris/edl/mohaware/mochaware.html.
3. Cited in an article by Jonathan Rickard, “Slip Decorated Refined Earthenware” p. 187, The Magazine Antiques, Aug. 1993.
4. Hopper Robin, Ceramics Monthly, April 2008, Answer Section page 16.

Sources:

Mocha and related dipped wares, 1770-1939 by Jonathan Rickard, Published by University Press of New England 2006.
Robin Hopper has an excellent video demonstrating the mocha diffusion technique at; ceramicartsdaily.org/ pottery-making-techniques/ceramic-decorating-techniques/mocha-diffusion/?floater=99.
Catherine Riedel, House & Garden Magazine, Jan/Feb. 2009.

Acknowledgements:

Wisconsin Pottery, 1082 Park Avenue, Columbus, WI 53925, 1 800-669-5196, www.wisconsinpottery.com, supplied images of dendritic pottery.
Frances Gubler, Collections Management Fellow, and Leslie Wright, Public Relations and Marketing Director, supplied images of Pearlware from the Shelburne Museum, www.shelburnemuseum.org.
Richard Lehman, Professor & Chair, Department of Materials & Science Engineering, Rutgers University, New Jersey, supplied technical information on lead glazes.
Jim Fineman, professional potter and technical editor.