Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University

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About Colored Clays and the Techniques Used in My Work
 

If you have questions about my work or my techniques, please contact me.

A Brief History of Colored Clays
Colored clay techniques involving colorants deliberately mixed into the clay were first used approximately 2000 BC in Egyptian amulets and figurines, where copper oxide was combined with clay materials to get a range of green and blue colors.   About 1200 years ago, Chinese potters of the Tang Dynasty incorporated marbleized and patterned clays into wheel-thrown and handbuilt forms that were low-fired with a lead glaze.  Within several hundred years, Sung Dynasty potters were coloring porcelain clay with iron oxide and creating a variety of patterns in platters and other vessels.

In mid-18th century Baroque England, the Josiah Wedgwood Company and various smaller pottery workshops used colored clays very effectively in basalt ware, Jasper ware, and marbleized ware.  Wedgwood’s elegant black and white Jasper ware “Portland Vase,” a reinterpretation of a Roman glass original, is one of the masterpieces of English Baroque pottery. 

In early modern Japan , experiments in colored clays evolved into the neriage and nerikomi techniques, which were brought to Europe and America during the Mingei 20th century Japanese craft renaissance.  English potters were and are especially creative in use of these techniques, and England is where one most often encounters quality colored clay work today.  The techniques appear occasionally in other cultures, and yet remain relatively rare in contemporary ceramics.  It may be that there simply are not that many artists/craftspeople willing to go to the trouble.  Exceptional recent and contemporary colored artists include Thomas Hoadley, Curtis and Suzanne Benzle, Chris Campbell, Dorothy Fiebleman, Michael Haley and Susy Siegele. 


Mill Town Vessel #2
Vince Pitelka, 1991


Colored clay patterned loaves stored in 
Ziploc bags.  I am still using loaves
 that are over 20 years old.

My Introduction to Colored Clays
I began exploring colored clays while in graduate school at UMass-Amherst in 1985.  I had just come out of ten years as a studio potter, and my most interesting work featured the density and depth of colored slips.  In graduate school I was moving from wheel-thrown to handbuilt work, and it occurred to me that I could get the same quality of color and surface with colored clays.  Several decades earlier I had a number of opportunities to observe glassblowers Ro Purser and Dick Marquis using the Venetian murrini technique.  As part of that process, colored glass is formed in canes or other shapes and combined to make a "loaf" with a continuous pattern or image running through its length, with the objective of cutting off many layers of repeated pattern or imagery.  When the graduate faculty encouraged me to try this with colored clays, I had no idea that it was a well-established technique.  My experimentation proceeded quickly, and within six months I was getting intensely-colored pattern and imagery with a high degree of precision.  One of my professors suggested that I look at Jane Peiser’s work in the Penland School of Crafts Book of Pottery, and it was a revelation to discover her remarkable vessels.  Further research led me to the work of other American and English potters and the historic examples mentioned above.  My work has always been distinctly different from any other examples of colored clay work that I have found, and that is reassuring.  

Choice of Clays
Any claybody can be used for colored clay work, but existing color in the clay (as in a tan stoneware clay) will limit the range of color possibilities when colorants are added.  For this reason, most potters generally use a porcelain, porcelaineous stoneware, or white earthenware body.  Also, when laminating colored clays onto a slab or a leather-hard piece, it makes sense to use the same claybody as the substrate when possible.  However, porcelain offers special challenges for handbuilding, and I have found that it is not hard to find a stoneware body that works well as a substrate for colored porcelain lamination. 

Choice of Colorants
Clays can be colored with standard coloring oxides, or with commercial fritted stains like Mason stains.  The standard coloring oxides including red iron oxide, chrome oxide, manganese dioxide, rutile, and cobalt carbonate work well, but give a limited range of color, and the color can bleed badly in salt and soda firing or under any very mobile clear glaze.  For the sake of stability and the available range of colors, I have always used Mason stains, available from most ceramic suppliers.  A broad range of colors is available, but some are fugitive at higher temperatures.  The information available from Mason is not entirely reliable in this regard, in that some colors will survive to high-fire temperatures when the Mason literature indicates that they are only suitable for low-fire applications.  The best solution is to purchase small test packets of all the colors that interest you and test-test-test.  Mason has extensive color-sample charts at their website, and the colors are fairly accurate. 

For colors in the bright orange and red range, the more expensive zirconium-encapsulated inclusion-stains can give good results, although the specific fluxes in the claybody seem to affect color response, and fairly high percentages of inclusion stains are often needed to get intense colors. 

The percentage of oxide or stain added determines intensity of color, but different oxides or stains have different degrees of coloring strength in their raw form.  For example, 3% cobalt carbonate in a pure white porcelain body will give a very intense dark blue clay, while it would require 6% or 7% red iron oxide to get a rich dark brown.  Color achieved with oxides or stains is intensified at higher temperatures, so higher concentrations are needed for low-fired colored clay work.  When I was working at low-fire temperatures, I added 25% Mason 6600 Best Black stain in order to get a completely saturated jet-black.  Generally, good color can be achieved with stain additions of 3% to 10% for high-fire work, and 5% to 15% for low-fire.  Test, test, test.  If you want soft colors, don't waste your money on pastel-colored stains.  Get stronger colorants and just use less.

My Own Approach
For my colored clay work, I mix a porcelaineous stoneware claybody and add Mason stains to produce a broad palette of colored clays.  These clays are sometimes used as-is to inlay or laminate solid color, but more often I cut and combined multiple colors in complex ways to create loaves with a specific pattern running through the length of the loaf.  Patterns include stripes, checkerboard, brick wall, rock wall, herringbone, basketweave, polka-dot, “parquet,” wood grain, and a range of rock effects including simple marbleized effects and more complex effects resembling coarse-grained granite or conglomerate.  The techniques for making all of these variations of patterned colored clay loaves are very complex, and are covered in the colored clay workshop that I teach. 

Slicing a colored clay loaf - note the shims under the wire, controlling thickness of the slice.  
Note also that the loaf is resting on a strip of canvas as it is pulled through the wire.

Slicing Colored Clay Veneer
My work is laminated with very thin colored clay veneers.  These veneers are sliced from the patterned loaves using a special slicer of my design (see below) strung with .012” diameter multi-strand stainless steel aircraft cable, purchased from Sig Manufacturing, 800/247-5008, www.sigmfg.com, part #SIGSH455.  Shims (a mix of cardstock, matboard, and thin-sliced wood) placed under the slicing wire regulate the thickness of the veneer.  Detailed instructions for making this slicer can be found in my book Clay: A Studio Handbook.   The loaves are placed pattern-face-down on a strip of canvas, and pulled horizontally through the slicing wire.  With this slicer I can cut clay veneers as thin as 1/32", and thus each loaf produces a large supply of patterned veneer.  I rarely laminate veneers much thicker than 1/32”, because the likelihood of surface cracks increases with thicker laminations.  The veneers are laminated onto slabs to be used for soft- or stiff-slab construction, or onto already-constructed soft-leather-hard forms.  

Storing Colored Clay Loaves
I am still using colored clay loaves I made over 20 years ago. While doing a series of pieces, the loaves are kept moist in Ziploc bags, and I open the bags and spray the loaves with water regularly to keep them moist and plastic.  As another good option, simply keep the loaves in a plastic storage box with a snap lid, and drape a piece of moistened bed-sheet material over the loaves before putting on the lid.  I only pay such attention to loaves I am currently using.  Others are allowed to dry completely, which eliminates mold-growth on the surface and crystallization within the loaves.  To re-hydrate a bone-dry loaf, wrap in soft cloth (bed-sheet or tee-shirt material works well), dunk in water with a little vinegar or Epsom salts added, and store in a Ziploc bag, re-dunking the wrapped loaf once per day until it returns to plastic consistency.  Do not leave immersed in water or the loaf will slake down to slurry.  It generally takes a week or so to get a bone-dry loaf back to useable consistency. 

When the loaf feels soft once again, it must be "mobilized" by gently pounding each face against a sturdy table or bench surface.  Take care to lift the loaf and rotate to a new face in your hands, rather than rolling it from one side to another on the table, because that alone can seriously distort the pattern.  If this process is done carefully, pounding the face perfectly square on each face, there will be little or no disruption of the pattern.  Do not neglect this gentle pounding, because it is necessary to get all the particles in contact once again, redistributing moisture content and restoring the level of plasticity needed for slicing and laminating. 

When loaves are stored damp in Ziploc bags for some time, they may seem damp to the touch and yet feel very stiff.  Before attempting to re-hydrate, always gently and carefully pound each face against a sturdy table as mentioned above.  There may be enough water present in the loaf, and if so, the gentle pounding will "re-mobilize" the clay, restoring plasticity.  You don’t want to add additional water unless it is needed. 


Spraying with vinegar water while laminating
 colored clay veneer on a slab


Removing canvas sheet after rolling 
veneered slab to affix veneer slices

Laminating Veneer on Freshly-Rolled Slabs
For slab construction, the patterned veneer is usually laminated directly on a freshly-rolled slab.  Spray the slab with vinegar water, and lay the slices in place without scoring.  Once the slab is covered with veneer, lay a piece of canvas (12 oz. or 14 oz.) over the slab, carefully smooth it down with no wrinkles, roll lightly once or twice in all directions to ensure that the canvas is attached, and then roll vigorously in all directions with a heavy rolling pin.  As long as you don’t apply excessive pressure along the edges or at the corners of the slab, you can’t overdo this.  If the canvas sheets are stuck to the surface above and below the slab, there will be no distortion of the pattern no matter how much pressure you apply while rolling.

When forming curved surfaced with slabs laminated with colored clays, several simple accommodations will reduce the chance of surface cracking.  Always attach with vinegar water.  The vinegar will flocculate the clay, making it more plastic.  Many porcelain and whiteware bodies contain soda feldspars which release sodium ions into the clay over time, and this has a deflocculating effect making the clay short, causing it to crack easily when flexed.  Needless to say, this problem is aggravated when a colored clay loaf is stored damp for a prolonged period of time.  The vinegar will restore flocculation and plasticity.  In addition, when planning to bend laminated soft slabs to form cylinders, tubes, or cones, always smooth a piece of plastic wrap over the laminated colored clay surface so that it is well-stuck to the damp surface, and then form the curve.  This will force the slab to compress on the inside face, instead of stretching the outside face.  Problems with surface cracks decreased dramatically when I began using this method. 

Laminating Veneer on Leather-Hard Forms
On larger slab-built pieces and on coil-built pieces, I apply the veneer when the piece is at the soft-leather-hard stage.  The clay surface is worked with a toothed metal rib to produce a cross-hatch score pattern and any surface crumbs are removed with a soft brush.  After the veneer slices are cut to shape I spray a small area of the vessel with vinegar water, lay a piece of veneer in place, cover with a square of canvas, and gently roll with a small printmaking brayer or wallpaper seam-sealer.   In some cases I apply additional pieces of veneer to the surface for three-dimensional relief. 

Removing Surface Contamination and Smearing
Some surface distortion and smearing is inevitable during the laminating and/or construction process.  At the hard-leather-hard stage I scrape the surfaces with a flexible metal scraper to remove all distortion.  The standard kidney-shaped stainless steel scraper-rib works great, and can be sharpened by rubbing the edge of the rib against a sharpening stone, holding the rib at exactly 90 degrees to the stone surface.  Keep in mind that the edge of the ridge will be razor-sharp, so never remove accumulated clay by running your fingers along the edge of the rib.  For getting into corners and tight places the standard rib can be cut to smaller shapes with a pair of sheet-metal shears. 

In some cases I sand the bone-dry surface lightly with a green Scotchbrite sanding pad or with mesh sanding cloth (used for sanding drywall).  Be sure to wear an appropriate respirator and do this process outdoors.  The Scotchbrite pad will uniformly remove surface material without leveling any irregularities, while the mesh sanding cloth will tend to remove surface irregularities, but with a greater chance of sanding right through the surface lamination. 


Scraping hard-leather-hard veneer

Surface Finish and Firing
Until about six years ago, all of my colored clay work was sprayed with Duncan GL-611 or some other reliable low-fire clear glaze and fired to cone 03 in electric kilns.  More recently, I have been firing my work in atmospheric salt or soda firings, usually to cone 6.  I like the earthier, variegated, textural surface this gives, and for functional pouring vessels I like the greater degree of durability and utility at cone 6.  In our 20-cubic-foot crossdraft soda kiln I use 1 ½ pounds of soda ash dissolved in several gallons of hot water, charged by spraying a saturated aqueous solution into the firebox at 15-minute intervals as the kiln approaches cone 6.

Much of my current work incorporates pre-textured slabs, impressed or rolled with bisque stamps, coggles, sections of rope, or other textured materials.  Some areas are left free of texture or colored clay pattern.  I like the contrast between these various surfaces.  After bisque-firing, I glaze the interiors with a dark midrange liner glaze.  No colored glazes are used on the colored clay surfaces, but I do often apply colored glaze or oxide stain to the textured areas to enhance relief pattern or texture

Mixed Media Additions   
I have always like mixed-media combinations in ceramics, and I especially appreciate the wire, wood, or rope handles and metal or wood lids found in historic utilitarian clay pouring and storage vessels.  Many of my current vessels feature fabricated iron wire bail handles with turned wood grips, and some have small iron loop handles on the lids.  I make the wire bails from mild steel welding rod, and spray with vinegar or muriatic acid to encourage rust.  Once the metal has developed a light surface patina of rust I rinse it, dry it, and seal the surface with beeswax.   

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