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Dendritic slip: Jeff Zamek shares research and formulas.

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DENDRITIC SLIP DECORATION (WHOSE NAME IS derived from the Greek word 'tree') is a delicate fine-lined slip pattern that looks much like a complex series of tree branches or veins of seaweed. Interestingly, this pattern also develops in nature in instances such as lightning forking when it hits the ground or rivers branching into tributaries.

Mocha ware, banded cream ware and dipped ware, are all terms used at various times to describe this type of pottery. One origin of the name Mocha ware is from the Yemenese port city of Mukha. The city itself was called Mocha by the English-speaking world in the 18th and 19th centuries. (1) Another derivation of the name comes from mocha stone, a moss-like patterned quartz found in Arabia and imported to London in the last half of the 18th century. In the past, the generic term 'mocha ware' was also used to indicate an extensive array of coloured horizontal slip designs ranging from marbling (different coloured slips mixed together on the clay surface) to precise checkered patterns of slip trailing on pots. Some ware had a decorative textured design impressed in the moist clay or a variation of coloured slips applied in exact patterns on the pot's surface. (2)

Mocha ware also includes the sub category of dendritic slip ware. Today, however (especially in Britain) the term mocha ware refers to pottery made using the dendritic slip technique. While this distinction was not made in the past, the terminology reflects current usage. The distinctive mocha pattern was an innovative decoration on simply shaped utilitarian forms such as cups, teapots, salt containers, jugs, bowls and plates. In its initial period of production, it was the least expensive factory-made decorated pottery available for common use. The most popular items produced were mugs and tankards. Oddly, flat pieces such as plates, platters or serving trays do not occur when investigating mocha ware. The pottery was used daily in English kitchens and taverns and not regarded as exceptional or of ultra high quality. Each piece was thrown on a potter's wheel, after which it was lathe-turned over its entire surface, producing refined utilitarian red or white earthenware whose principal decoration was manipulated slip. One potter did not complete all of the steps in the production process in the British factory system which allowed for relatively fast efficient production of pottery with few losses.

The technique was developed in the 1780s in Staffordshire England. Another source attributes the first dendritic slip to William Adams of the Greengates Factory, Tunstall, England, 1745-1805. (3) An early reference to mocha ware, specifically dendritic slip, was published in Old China, a monthly magazine published in Syracuse, New York. (4) While the ware was developed in England, it also achieved widespread popularity in the US as early as the 1790s. The Uriah Kendall Pottery in Cincinnati and the Lewis Pottery in Louisville, Kentucky produced many functional items including mixing bowls and cups. Pottery was also made in Canada into the 20th century. Mocha ware was produced in France in the 18th century and followed the English technical qualities of manufacture; some of the French factories had hired English potters in the 1800s to transfer their techniques to their pottery production. French mocha ware was eventually exported to Louisiana in the US. (5) It is often difficult to establish the specific date of manufacture due to the lack of makers' marks on most mocha pottery. Because the ware was intended for daily use and was fired at low earthenware temperatures, causing less vitrification and subsequently less strength in the clay, the pottery was not durable, resulting in few intact early examples. Additionally, at the time of its manufacture and use it was not regarded as an object for display and collection.

At some point, depending on availability and the costs of clay transportation, both red and cream-coloured clays were used to produce the ware. Experimentation with clay body formulas was often tried to lighten the existing cream colour clay body that was in production. This aesthetic trend was initiated by market demands from merchants and customers wanting whiter refined pottery. To meet these requirements small amounts of cobalt were added to both the clay and glaze which had a bluing or whitening effect, much like a laundering agent. In an effort to increase glaze transparency and enhance lustre, red lead was replaced in glaze formulas with white lead. (6) The subsequent loss of C[O.sub.2] and water would make the lead compound more reactive on firing and might have lowered the firing temperature of the glaze; both economic advantages.

Aside from its highly toxic nature to workers who mixed the glaze and possibly customers who used the pottery, lead was an ideal glaze ingredient. It produced a smooth glass-like surface, covering many defects while keeping moisture from penetrating the underlying porous clay body. Lead also acts as a flux, bringing other glaze materials into a consistent melt over a wide temperature range. Lead has a high refractory index which contributes to brighter slip colours. It also offered glossy surfaced glazes of brown, black, yellow, green and blue which were used in specific areas of the design such as horizontal banding stripes or marbleizing patterns. When a lead based glaze was used without metallic colouring oxides it produced clear, transparent, glossy, glaze.

The pottery was constantly improved in both clay body and glaze adjustments so that the fit and finish of the fired form would evolve into a more refined object. There was a steady development of tools to use on the forms. Turning lathes were used to place horizontal decorative bands on the thrown ware. Many types of containers holding different coloured slips were invented to impart swirls of coloured clay to the pottery surface. On some forms, coloured slips were inlaid in the recessed areas. To improve the slips, they were run wet through sieves called lawns to remove contaminants such as organic material or large nodules that would mar the fired surface of the pottery. Other tools were developed to cut into the coloured slip revealing the contrasting clay body colour underneath. The surface of the clay was also manipulated with sprigging (clay figures or decorations attached to the form with slip in the leather hard stage) and other relief forms, giving a three dimensional quality to the pottery.

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There are two pattern variations of dendritic slip. In the first (the most recognizable) called seaweed, a brush was dipped into the 'mocha tea' which was made by boiling tobacco into a thick mud, thinning with water and adding a metallic colouring oxide. It was then loaded on to a brush and barely touched to the wet slip causing the distinctive pattern as the pot was spun on the wheel. The technique caused a noticeable horizontal spine of dark colour on the lighter coloured slip. The distinctive darker tree-like branches radiate from where the brush was dragged through the slip. Gravity and chance play a somewhat accidental factor in the dispersal of the pattern with many 'tree'-like variations possible. The overall glazing techniques guaranteed that no two patterns were exactly alike. There was an aesthetic contradiction between the straight conservative pottery forms of bowls and cups and the random tree-like patterns created by the dendritic slip application.

The second and most identifiable type of dendritic pattern was formed when the pot was held upside down and the 'mocha tea' filled brush touched to the underlying wet slip, causing the tea to run down and spread the distinctive branch pattern. (7) In nature, the tree pattern is present in moss agate gemstones. The same alkaline/acid reactions are at work in conjunction with iron and manganese metallic oxides found in the mineral. (8) Additionally, from approximately 1790 to 1810, a combination of slip marbling (different coloured slips in the wet state are dragged through each other, causing a marbled pattern on the ware) and dendritic slip tree patterns appear on the same pottery. This type of pottery was believed to be a minor sub category in the production run. (9)

PEARLWARE

The mug (p 39) and bowl pictured are examples of Pearlware, which was defined as earthenware pottery that contained small percentages of cobalt in the glaze and clay body to heighten the whitening effect in the fired pottery. The coloured bands of slip and darker dendritic patterns were also enhanced by the contrasting underlying white clay body. Pearlware illustrated the ongoing refining of technical improvements in clay body and glaze development to suit the changing market demands.

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. (10) The immiscibility of both liquids yields the distinctive tree or branch-like pattern. This reaction is also known as the Marangoni effect (the disturbance that the liquid-liquid interface, due to interfacial tension, has on mass transfer in a liquid-liquid extraction system). (11) 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.

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In the past, one variation used chewed tobacco and urine, which formed the acidic component of the dendritic slip. The brown, blue, and green colours were produced by adding manganese dioxide, ground iron scale, cobalt oxide or chrome oxide. (12) Due to current research, the tree-like patterns can be coloured primarily but not exclusively by the use of the carbonate form of metallic colouring 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. (13) One noted exception is where red iron oxide is used as a colorant in the 'mocha tea'.

MIXING AND APPLYING DENDRITIC SLIP--(ALKALINE/ACID REACTION)

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

1. A reliable clay body formula that fits the base slip and dendritic slip in the drying, bisque firing and glaze firing stages.

2. A base wet slip formula that fires to a contrasting colour with the overlying dendritic slip. (A wet slip is classified as composed of water, clay(s) and/or other ceramic materials that can be applied to wet or leather-hard ware, resulting in a covering surface fitting the underlying clay body.)

3. A suitable 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 moist 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 fast so constantly stir the brush in the slip jar) and fill the brush with a small amount of the watercolour consistency dendritic slip, then barely touch the surface of the wet base slip with the brush. The dendritic slip will flow off of the brush on to the base slip, leaving a pattern. A tree-like tentacle decoration can be developed by holding the pot on the vertical. 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. Obviously, 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.

BISQUE FIRING AND GLAZING

After the application of base slip and dendritic slip, let the pots dry thoroughly, after which they are ready for bisque firing. Most low and high fired ware can be bisque fired at cone 06 (1828[degrees]F). Clays, glaze and slip work best when fired over longer periods of time to their recommended temperatures, allowing for mechanical and chemical water to be driven off safely. For most functional pottery (three to 16 inches high, 1/4 to 1/2 inch thick) the bisque firing should take 12 to 14 hours to reach cone 06. Longer firing times are recommended for tiles, flat pieces and thick pottery or sculpture.

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GLAZE FIRING

A transparent glaze application will reveal the colours of the base and dendritic slip pattern. The glaze can be dipped, brushed or sprayed on the bisque ware. It is also possible to once-fire greenware if the clay body, base slip and dendritic slip fit compatibly with the covering glaze. Longer firing times are required than comparable bisque firing due to the clay body's slow release of organic material and mechanical and chemical water in the first stages of the firing. (14) When choosing a covering glaze, it is important to test for transparency and stability in the firing process. Some glazes may run or move on vertical surfaces during the firing, resulting in a 'bleed' edge to the fine-lined dendritic slip pattern.

Finished ware once-fired to cone 04 (1945[degrees]F) using red earthenware clay, yellow base slip with green copper design and brown dendritic tree patterns.

CURRENT USE OF DENDRITIC SLIP

Joel and Debra Huntley, owners of The Wisconsin Pottery in Columbus, Wisconsin, are the foremost practitioners of this almost-lost decorative technique. As with most ceramics techniques, it is the subtle procedures at each level that make the significant difference in the final pottery. Their entire line of pottery is produced by hand, capturing the skill and feeling of the original 17th, 18th and 19th century pots.

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Clay Body, Glaze, and
Slip Formulas. (15)

Zam #1 Low-Fire Cone
06 (1828[degrees]F.) Red Clay
Body Formula

Redart 55
Cedar Heights Bonding clay
 50 mesh 9
Thomas ball clay 17
M44 clay 7
Custer feldspar 6
Goldart stoneware clay 3
Flint 200 mesh 3
Grog 48/f (optional) 7%

Zam #2 Low-Fire Cone
06 (1828[degrees]F.) Red Clay
Body Formula

Redart 70
Texas talc 15
Thomas ball clay 10
Newman red stoneware
 clay 5
Grog 48/f (optional) 5%

Directions: Mix dry materials with
water until the proper forming
consistency is achieved. Cover the
moist clay completely with a plastic
sheet for one or two days to increase
its plastic properties. Wedge the
clay thoroughly before using.

Tom's White's Base White
Slip Formula (wet application)
Cone 06 (1828[degrees]F)--
Cone 10 (2345[degrees]F)

Helmer kaolin 60
Grolleg kaolin 25
Nepheline syenite (270 mesh) 15

Blue Slip

Add 2% cobalt carbonate

Base Slip Formula

(wet application)
White Slip cone 06 (1828[degrees]F)

E.P.K 30
Thomas ball clay 25
M44 clay 10
Goldart stoneware clay 5
Flint 325 mesh 20
Superpax 10
Bentonite 2%

Yellow Slip

Add 10% Mason stain
Titanium Yellow #6485

Directions: Weigh the dry
materials. Add water to achieve
a specific gravity of 1.350, then
place the wet slip through an
80 mesh sieve three times. The
slip is now ready to apply to a
wet to leather-hard clay body.

Dendritic Slip Formula
Black Slip

Manganese dioxide
 powder 20 grams
Water 29 grams
Apple cider vinegar 29 grams
Tobacco 1 cigarette (king size)

Blue Slip

Cobalt carbonate 5 grams
Water 29 grams
Apple cider vinegar 29 grams
Tobacco 1 cigarette (king size)

Green Slip

Copper carbonate 5 grams
Water 29 grams
Apple cider vinegar 29 grams
Tobacco 1 cigarette (king size)

Brown Slip

Red iron oxide 5 grams
Water 29 grams
Apple cider vinegar 29 grams
Tobacco 1 cigarette (king size)

Directions: Mix all dry ingredients.
Add enough water for
a 'watercolour' consistency.
Break open and add the contents
of one cigarette (don't use the
cigarette filter). Age the slip for
24 hours, then place the liquid
through a 100 mesh sieve three
times before using. Discard any
material left on the 100 mesh
screen. The shelf life of dendritic
slip is two to three weeks. After
that the properties of the growing
tree patterns rapidly decline.

Glaze Formulas

Gillespie Glaze
Formula Transparent
Cone 06 (1828[degrees]F)--
Cone 04 (1945[degrees]F)

Gillespie borate 90
E.P.K. 10

Transparent Glossy
Cone 04 (1945[degrees]F)

Ferro frit #3269 90
E.P.K. 8
Flint 325 mesh 2
Bentonite 2%
Red iron oxide 1%

Transparent Glossy
Cone 04 (1945[degrees]F)

Ferro frit # 3195 78
E.P.K. 9
Nepheline syenite (270 mesh) 13
Bentonite 2%

Directions: Mix the dry
materials and add water to
achieve a specific gravity of 1.350.
Place the wet glaze through an
80 mesh sieve three times. The
glaze is formulated for dipping,
pouring or spraying. Additions
of 2 percent CMC (based on the
dry weight of the glaze formula)
allow brushing applications.


ACKNOWLEDGMENTS

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, NJ supplied technical information on lead glazes. Jim Fineman, professional potter and technical editor.

OTHER REFERENCES

Robin Hopper has an excellent video demonstrating mocha diffusion technique at http:// ceramicartsdaily.org/pottery-making-techniques/ceramic-decorating-techniques/mochadiffusion/?floater=99

Catherine Riedel, House & Garden Magazine, Jan/Feb. 2009.

FOOTNOTES

(1.) A definition supplied by Jonathan Rickard in his book, Mocha and Related Dipped Wares, 1770-1939. Published by University Press of New England.

(2.) Cited in an article by Jonathan Rickard, "Slip Decorated Refined Earthenware" p 183 Van Nostrand Reinhold Pub.

(3.) Source: Brian Wright, Mochaware: The Hidden Gem.

(4.) Edwin Atlee Barber, Mocha Ware in Old China, vol. 2, Syracuse, New York, Keramic Studio Publishing Company, January 1903, pp. 71-73.

(5.) Source: Bryon Wright, Mochaware The Hidden Gem.

(6.) Rickard, Jonathan, Mocha and Related Dipped Ware, 1770-1939 published by University Press of New England, p xi.

(7.) Rickard, Jonathan, Mocha and Related Dipped Wares, 1770-1939 published by University Press of New England, pp 48-49.

(8.) Hopper, Robin, Ceramics Monthly, April 2008, Answer Section, p 16.

(9.) Rickard, Jonathan, Mocha and Related Dipped Ware, 1770-1939. Published by University Press of New England, p 51.

(10.) The Observer's Book of Pottery & Porcelain, by Mary & Geoffrey Payton, p 114 Frederick Warne & Co. Ltd. Pub.

(11.) Morris, Stephen, www.physics.utoronto.ca/~smorris/edl/mochaware/mochaware.html

(12.) Cited in an article by Jonathan Rickard, "Slip Decorated Refined Earthenware" p 187, The Magazine Antiques, Aug. 1993

(13.) Hopper, Robin, Ceramics Monthly, April 2008, Answer Section p 16,

(14.) Zamek, Jeff, What Every Potter Should Know. Iola, WI: Krause Publications, 1999 pp 179-180.

(15.) Zamek, Jeff, What Every Potter Should Know. Iola, WI: Krause Publications, 1999 pp 178-182.

Jeff Zamek walked into a pottery studio 36 years ago and started his career as an amateur potter. After completing a degree in business from Monmouth University, New Jersey, US, he earned BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. He developed the soda firing system at the college and went on to teach at Simon's Rock College and Keane College while he earned his living as a professional potter. In 1980 he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. He works with individual potters and industry, offering technical advice on clays, glazes, kilns, raw materials, ceramic toxicology and product development. He is a regular contributor to Ceramics Monthly, Pottery Making Illustrated, Pottery Production Practices, Clay Times, Studio Potter, Ceramics TECHNICAL and New Ceramics. Zamek's 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. Zamek is currently working on several ceramics research projects and is making pots as an amateur potter. (www.jeffzamek.com)
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Date:Nov 1, 2011
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