Much has been written about the origins of the 'original' shino glazes. In this article I would like to give a brief outline of these. Also looking at the glazes now known as carbon trapping or 'American' shino, and how these recipes may be formulated using UK materials. The mechanics of how this glaze feature may be developed through glaze composition and firing cycles. Using fuel burning kilns i.e. Gas, oil and wood, how carbon trapping may be encouraged as a decorative ceramic surface. Also a brief look at high flux content clays in protracted anagama wood firings and there ability to trap subtle carbon patterns and markings beneath the natural ash glaze and soda fluxed surfaces.
It has been speculated that the type of glazes known as shino were developed
in response to Japanese tea masters' demand for Chinese white porcelain wares
in the Momoyama period (1573 - 1615). This cream white glaze was highly prized
for Tea ceremony wares. Feldspathic materials from the Mino area of Japan
mixed with ash, and clay created a viscous white glaze which had a tendency
to crawl and pin hole, these qualities have been and predominantly still are
viewed as faults in Western eyes but viewed in the light of desires by tea
masters to use utensils which bore an organic, natural beauty they were admired
as another integral aesthetic facet of the object. Several classifications
of shino were identified: Plain shino (Muji shino), Picture shino (e-shino),
Crimson shino (beni shino), Red shino (aka shino), Grey shino (nezumi shino)
and marbled shino (neriage shino), each having a subtly different surface.
Some of these glazes were applied over iron bearing slips or delicate Iron
brush work. Due to the firing techniques used, iron bled through the glaze
creating exquisite surfaces.
The family of glazes now known as American shino were developed by Virginia
Wirt in the early seventies, as part of a project which was set at the University
of Minnesota, "to try and replicate early Japanese glazes". Wirt
is widely acknowledged as the first to add soda ash to a shino type glaze
in order to attempt to utilise carbon trapping. From this beginning many recipes
have been created which push forward this decorative technique notably by
Malcolm Davis, Dick Lehman, Jack Troy, Jeff Oestrich and Cris Gustin, to name
but a few, all from the USA. It is arguable as to how reminiscent these glazes
actually are to the original momoyama glazes however there are some very similar
molecular similarities in there composition. The colours which these glazes
produce are within the same pallet with the added dimension of areas shaded
by the entrapment of carbon.
UK materials offer some wonderful scope for this type of glaze. The main common
material is soda ash; a low temperature flux: this may be added to other higher
temperature fluxes such as, nepheline syenite or soda feldspar. Nepheline
syenite melts at a lower temperature than soda feldspar so may provide an
advantage in the trapping process. An addition of ball clay will give a rudimentary
starting point for further experimentation. High iron bearing ball clays such
as Hymod AT will give a correspondingly darker colouration to the glaze burning
to darker reds and oranges, lighter ball clays such as Hyplas 71 will give
lighter blushes of colour. Red earthenware powders or locally dug low firing
clays may be added to the glaze to introduce higher iron in addition to other
possibly beneficial impurities. I prefer to introduce the bulk of the iron
in this way using a lighter firing ball clay as a base. It is interesting
to test with several dug clays to see what differences become apparent in
the finished glaze. These clays should be dried crushed and turned into a
slip. The slip is then worked through an 80's sieve and re-dried and crushed
the dug clay can then be added as a dry material to the dry glaze mix. Several
ball clays have been used in tests some with higher carbon contents than others
(blue ball clays and black ball clays), I found no significant differences
in the carbon trapping qualities of the resulting glazes.
The addition of spodumene, petalite or lepidolite, all lithium feldspars which
contain soluble alkalies, can enhance the colour of some glazes as well as
producing a hard durable surface, they may also reduce crazing in the glaze
if this is desirable. I have carried out many tests using porcelain, this
can lead to some striking black and white contrast. A very small percentage
of iron bearing clay may be added either to the porcelain body itself or as
a very thin wash to encourage oranges, pinks and reds within the glaze but
still maintain the translucent bright quality of the body.
Composition
All the firings which have been carried out have been undertaken using kilns at The Ceramics program at Loughborough University School of Art and Design and the anagama kiln at Wysing Arts near Bourne in Cambridgeshire.
Carbon trapping in these shino type glazes can be elusive, the position of
pots in the kiln and cycle of reduction appear to be key ingenerating the
range of carbon spotting, haloing and lining which can appear. Also the orange,
red and pink colourations of the main body of the glaze also responds to these.
The clay body on which the glazes are applied greatly effects the glaze finish.
Most carbon trapping glazes contain a quantity of soda ash. This material
is soluble, and starts its melt as low as 800 degrees C. When a piece has
been glazed with a high soda ash glaze, crystals may be seen to appear on
the surface creating beautiful patterns in there own right during the drying
process. This crystal accumulation may be controlled through allowing areas
of the pots to dry at differing rates or applying resists toinhibit the crystal
growth. It is important that these crystals are not disturbed while handling
pots post glazing, as these areas are where the strongest potential is for
trapping carbon. Soda ash solution may also be painted over the surface of
the glaze to promote more intense localised crystal formation. Due to the
soluble nature of soda ash if a glaze is applied just as a liner glaze the
soda can migrate through the ceramic body and promote flame flashing and carbon
trapping on the exterior of the work. In order to facilitate this, the work
should be given a very low bisc to allow maximum porosity through which the
soda can migrate.
Firing
During the reduction firing, fine particles of carbon build up on the surface of the pot and settle between the soda crystals. When these fine soda crystals start to melt they trap the carbon against the body of the unmelted glaze. When the main body of the glaze starts to melt the carbon present becomes encased in the main glaze melt. It is for this reason that the reduction cycle for this type of glaze firing begins as low as orton cone 013 (850C) or even slightly earlier. I have started reducing as low as cone 015 (800) without any detrimental effects. A very heavy reduction at this stage gives the best chance of collecting carbon. It therefore follows that firing with a fuel that is capable of generating a lot of soot if inefficiently burned will give the largest carbon build up. Oil and wood produce the most carbon in early reduction.
A natural draught gas kiln needs to be closed up tight (dampers fully in)
to achieve sufficient reduction, I hold this reduction for about two hours.
If it is an internal kiln the room must be very well ventilated to avoid the
risk of exposure to carbon monoxide. In anagama, the fire box is stoked with
larger amounts of wood. A good bed of hard wood coals, allows more control
over the stoking of softwood to achieve a really smoky atmosphere. The under
grate air and lower stoke holes are also closed to produce a slow rolling
flame through the kiln. In the oil fired kiln just a small adjustment to the
damper can achieve a strong, smoky reduction. After this first period of heavy
reduction a constant heavy/ medium reduction is kept until close to the start
of the main melt of the glaze. In the case of the anagama the cycle is between
very heavy reduction on the stoke changing gradually to oxidation as the stoke
burns down giving off heat, and then back into reduction on the next stoke.
Most of the carbon trapping glazes which I use, benefit from a cone 10 to
12 firing although I have fired to lower temperatures these carbon trapping
shino glazes like a good soak at lower temperature (cone 9), to achieve a
good melt. In some instances the soda ash flux can appear as a rather unattractive
(to my eyes) green glass on the surface of the glaze I feel that firing higher
and for as long as possible gives the best results. A longer duration to the
firing and a period of very heavy reduction followed by a period of oxidation
(10 minutes or so), right at the end of the firing promotes orange flashes
in the glaze due to very fine iron particle suspended in the glaze matrix.
This system works well in gas or oil kilns, however with anagama I tend to
reduction cool on the last stoke i.e. fill the firebox with wood, cap the
chimney and clam up the stoke and spy holes, this produces a final heavy reduction
and causes the final stoke to burn down slowly. I have had equally good results
from all methods.
Undoubtedly the finest results have been achieved in wood firings due to the
quantities of smoke, ash and the overall duration of the various stages of
the firing (three to five days overall). Duration allows a thorough melt of
the glaze and the fluctuations in atmosphere generate wonderful colours due
to interactions of iron in the body. My shinos work very well if placed in
saggars in this type of firing. A perforated saggar which allows a small amount
of flame to impinge on the pot inside can result in black carbon spots which
correlate to the positions of the holes, satin matt surfaces on the glaze
rather than glassy due to high alumina to silica ratio and crystallization
in the cooling can introduce wonderful surface drama with a fluid natural
ash glaze settled on the top. Equally beautiful but different results have
been achieved in the gas and oil kilns. The overall glaze surface tends to
be less flawed with the glaze taking on a wonderful soft lustrous quality
with more even distributions of carbon markings responding to subtleties in
the forms.
High fluxed bodies.
All of the bodies which
I use contain a fairly high nepheline syenite flux content (up to 22%). This
can create a fluxed glassy surface on the finished pieces, in extended wood
firing. These clays also work well in all other firings and add to the fluxing
action of the glazes, creating a strong clay glaze interface.
The mechanics of these bodies ability to trap carbon under this final glassy
surface, in extended firings, relies also in part on alkalies carried by the
fire flames in the very early stages of a firing as well as the migration
of soluble soda in the body. In a similar way to that of carbon trap shino
glazes, the alkali content in fly ash and other gases emitted from the burning
fuel interact with the ceramic surface of the pots, combining with soda still
present from the nepheline syenite in the body. The migrated soda layer melts
early on in the firing and in this way traps the carbon when the body becomes
glassy. High flux porcelains have been seen to take on a shine in the very
early stages of a firing (cone 06).
Pieces which have been placed towards the rear of the kiln have shown some
subtle carbon trapping under the fluxed body surface due to a build up of
carbon when the front temperature indicates the start of reduction but the
back of the kiln could still be as low as cone 017 (700ish).
To conclude, in this article it has only been possible to give a brief outline
to the potential of these types of glaze and carbon trapping in general. As
always, the form of an object is its main strength, but the ability of this
type of glazing to emphasise marks and reflect the relationship to other work
in the kiln introduces a subtle element of serendipity to the finished object.
There is much scope left for further exploration into the use of soda ash
and other soluble materials to create surface finishes. Also with our wealth
of native Devon and Dorset ball clays as well as numerous local clay deposits,
new surfaces are waiting to be explored and utilised. The subtlety, warmth
and empathy to form which carbon trapping glazes can impart makes these glazes
a useful addition to the ceramic pallet. There is also the magic factor which
allows the maker to lose control and hand it over to the fire. Forcing preconceptions
of the objects we produce to be challenged.
Recipes
CT 1 Orton cone 10 - 12 reduction
Nepheline syenite - 42.5
Soda feldspar - 10.25
Hywite HK2 Ball clay - 14.32 (can be substituted for Hyplas)
China clay - 9.5
Soda ash - 17.9
Fires to a white/ orange glaze with grey to black carbon trapping.
CT 2 Orton cone 10 - 12 reduction.
Nepheline syenite - 50
Spodumene - 20
China clay - 12
AT Ball clay - 8
Soda ash - 12
Red clay powder - 3
Fires to a rich red to dark orange to cream with strong carbon trapping. Can give a beautiful lustrous gold.
CT 3 Orton cone 10 -12 reduction.
Nepheline syenite - 52
Prima ball clay - 14
China clay - 10
Soda ash - 13
Red clay - 2
Another rich orange carbon trapping glaze very sensitive to firing atmosphere.
Reference
"Classic stoneware
of Japan: shino and
oribe". Ryoji Kuroda & Takeshi Murayama.
Kodansha 2002
"Black shino". Mel Jacobson. Ceramics Monthly
"Revival Fires: Another face of shino". Jim Robinson. The studio Potter: Vol. 21 No. 1 December 1992
"Wood-fired Stoneware
and Porcelain". Jack Troy. Chilton. 1995
