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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.
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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.
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Mill Town Vessel #2
Vince Pitelka, 1991
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Colored clay patterned
loaves stored in
Ziploc bags. I am still using loaves
that are over 20 years old.
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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.
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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.
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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.
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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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