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Carbon trapping shino glazes
The following article was published in Ceramic Review issue 220
"Carbon footprint ".

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

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