Pottery Video of the Week: Slipware Decoration – Mocha Diffusion and Slip Dotting Pottery

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Though it might sound like a fancy coffee drink, mocha diffusion is a little-known technique of ceramic surface decoration developed and used in the southwest of England. Through a reaction between acid and alkali, intricate veined patterns reminiscent of trees or ferns are created in this process. Pottery with this decoration is referred to as mochaware.

A while back, we posted an excerpt from Ceramics Monthly, in which Canadian potter Robin Hopper provided an explanation of the technique and a slip recipe that works well with it. Since then, Robin has recorded a new DVD. Today I am sharing an excerpt from that video, in which he demonstrates how to do mocha diffusion and his slip dotting technique. I have also reposted the original article with the slip recipe below. –Jennifer Harnetty, editor.

 


 

How to do Mocha Diffusion

by Robin Hopper

 

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More on Mocha Diffusion

In this technique, thick slip is applied to wet or leather-hard clay (I prefer leather hard) that hasn’t started to change color in drying. Then an acid/color mixture (or “tea”) is applied. Mocha ware was traditionally made with both red and white earthenware, but may be done on almost any clay body at almost any temperature. From my experience, a smooth clay body with a high degree of ball clay or plastic kaolin, such as EPK, is the most ideal.

mocha-diffusion-slip-recipeVarious slip recipes are good, the most important ingredient being a high percentage of ball clay. A basic recipe, which will fit most bodies, and which can easily be colored with stains or various oxides, is shown at left. This slip is good on most clay bodies from Cone 04 to 12, in any atmosphere. The thickness should be like double cream, or room-temperature 10W30 motor oil.

Acid/Color Mixture

The mixture that is used to form the patterns is called “mocha tea.” It was originally made by boiling tobacco leaves and forming a thick sludge that was then thinned with water and mixed with colorant. However, nicotine solutions are only one form of mild acid; many others will work, such as citric acid, lemon juice, urine, coffee or vinegar, particularly natural apple-cider vinegar. One of these would be mixed with colorant. Most colorants work quite well, although carbonates and stains are usually better than oxides, since they are typically a physically lighter precipitate than oxides. Heavy materials such as black copper oxide, black cobalt oxide and black iron oxide do not work well, because the acid can’t adequately hold them in suspension. A ratio of about one heaping teaspoon of colorant to a quarter cup of mild acid is usually a good starting point. However, a good deal of individual testing has to be done to get the two liquids to work together to create significant dendritic formations or diffusions.

Mocha Diffusion Method

The leather-hard pot is dipped, brushed or poured with slip. While the surface is still wet, and before it has begun to lose its shine, the acid/color mix is dripped or trailed into it. It is best done using a well-loaded brush held just touching the slip. If the viscosity of the slip, and the acid/color mix is right then the feathering pattern will take place quite naturally, as the acid eats a fern-like pathway through the slip pulling the colorant with it. Traditionally, the surface is coated with a thin coat of clear glaze, or transparent colored glaze, but this might cause the color to bleed out or become absorbed into the glaze, particularly at temperatures above Cone 4. I prefer to use the technique on high-fired ware that does not need to be glazed. This is a technique that usually takes a while to get used to, but can give interesting results when used sensitively.

This clip was excerpted from Making Marks: Ceramic Surface Decoration with Robin Hopper, which is available in the Ceramic Arts Daily Bookstore!

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To learn more about Robin Hopper or to see more images of his work, please visit chosinpottery.ca.

For more great glazing techniques techniques, be sure to download your free copy of Four Great Ceramic Glazing Techniques: How to Formulate Successful Crystalline Glazes, Add Depth Through Carving and Layering, and Glaze in the Majolica (Maiolica) Style.

 


 

Comments
  • Robin Hopper is a great teacher and I love watching him explain different techniques. What is strange is that just this morning I was trying to explain mocha diffusion to two of my students who are potters(I teach biology, not ceramics)and as soon as class was over I opened e-mail and, poof, there is Robin Hopper explaining it. Excellent!

  • OH WOW!!!! I can’t wait to try this! This is awesome…I love it that no matter how long one works with clay there is ALWAYS something new to learn. It never ends!!!! Gee, now I gotta decide…clean house or try this Mocha Fusion method in clay….hmmm, guess which one wins?

  • Thank you Mr. Hopper, this is timely, I’ve been exploring with Mocha Diffusion in the last few weeks and I do have so much more to learn. I love all your work and “un gros merci” for your generosity of your expertise.

  • Robin Hopper is the most inspirational teacher, speaker who always amazes me how God really uses him to bless many others with ideas and thoughts.

    Thank you for making this available to others.

  • Thank you for giving us simple readily available materials to simplify the process and achieve wonderful results

  • Mocha effects can also be produced with tea or coffee – these will break down the surface tension of the various slips causing them to bleed and product the fine feather like tendrils of colour. Another decorative technique is using an actual feather then gently mix and swerl the different coloured slips – this will produce a marble effect simular to banging or tilting the form after applying the coloured liquid slips.

  • Does it really ruin the feathery tree-like effect if you put a clear glaze over it??

    Does this work over slip made from the clay body?

    I always learn from Robin Hopper’s books and videos, and they are entertaining, too. Thanks!

  • Wonderfully beautiful AND simple to do. Doesn’t get better than that. I also have his beginner’s throwing on the wheel DVD I highly recommend it. Thank you for the clips:)
    I HAVE to get this DVD!

  • Many thanks for your varied responses. GINNY – This decorative slipware process is normally done in earthenware at around cone 04 in temperature. At that temperature the glaze will most likely not obliterate the feathery qualities as the glaze hardly starts to absorb or fuse the colorant of the diffusion. However, I choose to do this work on porcelain – the object shown is a black colored porcelain – at cone 8 – 10, part of the colorant will be absorbed into the glaze so I prefer to do it unglazed from choice. Just because a traditional process is normally done at one temperature doesn’t mean that you can’t radically change the temperature, but you will most likely have to slightly change the recipe. Trial and error is normally the best way. Timing is one of the most important considerations with this process – if the clay object is too soft it will likely slump. Too hard and it will likely crack. The other important thing is to get the viscosity or thickness of the slip right. Some people like it fairly thin, others like it quite thick. Try it and see!
    It is best to use a slip like the one given as it is quite alkaline. The Mocha Mix of colorant and vinegar is mild acid. Together they have a reaction. Prepared clay bodies often don’t have the right degree of alkaline or plastic material so don’t work as well. When you work with Mocha diffusions it is good to have a small slab of clay to test on first. It sometimes takes a little time to get the acid/alkaline balance just right. Best wishes with it. Robin

  • Well it’s nice to know that I can use different sources of acid – I’ve been chewing nicotine patches in order to cut down on my baccy habit….

    Seriously, what a lovely technique – can’t wait to try this out – thank you Robin!

    Terry

  • Terry,
    I started researching Mocha Diffusions when I was a student when I found some old pieces. I found out that the process usually used a stinky sludge from boiled tobacco leaves. I figured that this was just a mild acid, so I tried every mild acid I could think of: Orange Juice, Lemon Juice, coffee, tea, urine, wine, vinegars etc.. But the one that worked best for me and I’m still using 50+ years later is Natural Apple Cider Vinegar, usually available at any grocery store. Try it and See!

  • what type of feldspar do you use. I went to order some feldspar and found 3 different kinds and I am too much of a newby to decide which is best (but I think the Custer may be the one if “refractory” means diffusion):
    Feldspar, Custer — A potash feldspar used in both clay bodies and glazes. It can function as a flux or as a refractory depending upon formula and firing temperature.

    Feldspar, Kona F-4 — A soda feldspar used in clay bodies and glazes where a sodium flux is required.

    Feldspar, G-200 — A potash feldspar used in glazes and clay bodies.

  • and just noticed there are about 9 different types of kaolin too, which do you use?

  • will the acid residues used in the mocha diffusion (lemon juice, vinegar, coffee, tobacco) effect the coils of the kiln or cause them to deteriorate more quickly?

    will the manganese used for the dendritic effect be “food safe” once it is covered with a clear glaze? Will the manganese liberate harmful fumes when firing the greenware such that the fumes might effect other pieces in the kiln or otherwise contaminate my kiln?

  • I experimented with this for my son’s (also a potter). Christmas gift…Thank you, Robin!!

  • Is manganese oxid (mocha tea technique) “food save”(f.e. on dishes) when covered with clear glaze?

  • I still have not heard an answer to the question. Is using manganese dioxide food safe when covered with a glaze? Would someone answer this question, please.

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