L6 Term 1 Technical Project: Reduction Glazes

Images and Key in the link:
Technical Project PDF

I began this term with the intention of developing a series of ash glazes with the hope of getting similarly expressive effects to those in the wood kiln at HDK. The project would also link in with my growing interest in ‘non-spaces’ through using plants from hedgerows and Cardiff’s invisible edgelands. Looking at the ash glazes of Bernard Leach and Katherine Pleydell Bouverie, I admired the quiet, subtle colours and how they work in a calming way on the viewer. Over the summer I collected together a few different ashes – from our log burning fireplace at home as well as from bracken and rushes I had sourced in the countryside in North Wales. I dry sieved these ashes without washing as I had read in the Phil Rogers glaze book that it wasn’t necessary to wash ashes.

Using the simple 60:40 ash to feldspar ratio recommended by Phil Rogers, I experimented with different feldspars, discovering that Potash crazed the most. I also discovered that increasing the proportion of China Clay made the glaze more matte (Test No1.1). Unfortunately the glaze application on the test tiles is patchy since the tiny amounts of ash I had to work with meant the mixtures ended up containing too much water. I left them to evaporate overnight but the small amount of ash meant I couldn’t get a very thick coverage. A second series of line blends (Test No1.2) was made to see what happened when I added increasing amounts of Potash Feldspar to different ashes. Ideally I would have added increasing amounts of ash instead since these results are too similar. The problems I had with obtaining enough ash led me to work with different glazes instead. All my ash tests were fired in reduction although I did test them in Oxidation too but they were colourless.

I liked one of my ash glaze tests very much because of its matte quality, strong iron speckling and mint green colour (Ash glaze A6 on the PDF) so as result decided to experiment with creating glazes that had a similar quality of subtlety and softness. I came across a glaze I had adapted from Jeremy Jernegan’s dry glaze handbook last year. The original glaze had been a matte white reduction glaze with Potash feldspar being the main ingredient. I had adapted the feldspars (as I did with the ash glazes) and discovered that adding Nepheline Syenite instead created a shiner, more viscous glaze (probably because it is higher in alumina than Potash). As I hoped to use these glazes on functional jars and bowls I didn’t want them to be too matte and flaky.

The original glaze is white however with Nepheline Syenite it becomes blue-white with patches of pink flushing depending on the reduction and application. I wanted a series of glazes with a similar satin quality but in different colours so added metal colourants to the base recipe in proportions as shown in the PDF (Test No2.1) and then, deciding these were too dark, created a Triaxial Blend with the Grey-blue, Turquoise and the lighter base glaze. I added 4 brushed on layers to each of the 16 tiles but unfortunately the results are a lot drier than I expected, not really suitable for functional vessels. The darkness of the glazes is probably a result of using Reduction St Thomas which is a darker clay body than the usual white St Thomas, which I chose because of the iron spotting it encourages. The dryness of the glazes in this test could be a result of them being on the lower level of the gas kiln where they perhaps didn’t quite all reach vitrification temperature.

Having never done a glaze technical before I felt a bit lost as to where to begin and how to alter glazes to get the results I wanted. Although the idea of using natural materials seemed attractive as it fitted with my philosophies of material vitality, finding the materials is such a dedication that it didn’t seem to be practical with the large quantities of glaze I needed for my functional vessels.  This project has been valuable to explore how colourants can impact glazes though and made me confident using the reduction kiln which I used for the first time this term.

 

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Shino Glazes

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One of the glazes that I’ve been firing in the gas kiln over the past couple of months seems to yield very different results each time it’s fired. Depending on the application, the thickness, the amount of reduction and placement in the kiln, the shino glaze (recipe here) I have sometimes turns out a bubbling bright orange, a thick opaque white, a smooth fiery red or a heavily crazed salmon pink (as it did in the wood kiln). Some beautiful results with it came on a mug from a recent reduction where the glaze was speckled with lustre-like iron spots on a shiny cream and orange glaze.

I’m interested to learn more about what gives a Shino its distinctive qualities. Researching in the library I discovered that Shino glazes probably originated in the Mino area of Japan around AD 1573-1615 and were named in honour of the shogun at the time, Shogun Shino Soshin. While Chinese ceramic aesthetics at the time were moving towards industrial perfection, shinos with their imperfections, strength of character and individuality were developed under the influence of the Japanese tea masters who had a very different ideal visual aesthetics. Historians suggest that shinos developed from Japan’s attempt to make a white ceramic to rival the pottery made in China and Korea at the time. Prior to this, Japanese ceramics had been glazed with a mixture of earthy ash and iron glazes.

Shinos can be loosely divided into three main categories – traditional, carbon trap and high alumina. Traditional shinos are around 60-80% feldspar and 20-40% clay. Since they are high in feldspar and clay they contain large quantities of alumina and silica whose natural impurities cause texture and imperfections in the glaze, part of their charm. Australian shino recipes developed from Japan adjusted the traditional recipes to contain Nephelyine Syenite (70-80%). The more nepheline syenite, the shinier the glaze .

Carbon Trap shino’s were developed by Virginia Wirt in America in the 1970s and are characterised by an addition of 3-17% soda ash to the recipe. The soluble soda ash leaves deposits of ash on the surface of the pot as the glaze evaporates which can result in grey or black flashes on the surface. With 8.1% soda ash my shino from HDK could be classified as a carbon trap shino recipe. The soda should be dissolved in hot water before adding the other glaze ingredients as I found out when the glaze started forming hard lumps and sticking to the bottom of the mixing bowl when I made it. Another chemical in the recipe is spodumene, a high lithium feldspar which could have been added to the recipe to help thermal shock resistance. I’ve read that a better carbon trap can be achieved if the work is dried for longer and also that putting lids on pots after glazing will encourage the puling of soda ash to the outer surface of the jar.

Ian Currie in his book ‘Stoneware glazes’ divides shino into three different subsections depending on the surface they are on: Firstly, ‘normal’ shinos which are a thick crackle white where applied thickly and a ‘fire colour’ where thin. Secondly he talks about gray shinos which are shinos over a traditional iron bearing slip. Gosu slip (also known as mouse gray) is a traditional slip coloured with iron and cobalt pigment. The third he calls marbled shino which is shino applied to marbled dark and light clay. The red/orange colour from shinos depends on the iron oxide being activated in the reduction firing. While most shinos don’t contain iron oxide in the recipe, iron can be introduced in a number of ways – either from the clay body itself, clay in the glaze recipe or an underlying slip. It’s interesting to note that the heavy iron spotting in my shino might be a result of using high iron reduction st Thomas clay. The high iron content is visible when the pots are bisque fired and turn a salmon pink.

Unfortunately the small gas kiln at CSAD is temporarily broken after last Friday’s firing. I hope to experiment further with shino recipes in the new year though, especially layering them over slips. I’ve read it’s possible to colour shinos effectively too with stains and oxides.

Variations on a Matt Glaze

Test num (800x279).jpgLast week I mixed together a few variations on the Nepheline Syenite matt white which was itself a variation on a white base glaze in Jeremy Jernegan’s Glaze Handbook . This glaze has been working really nicely in reduction so far with a smooth, soft satin finish, blushes of pink and lilac and minimal running. I wanted to see if variations in the colour were possible with the hope of pale mint greens and turquoises reminiscent of the hazy, sunny colours in a Wes Anderson film. The additions of colourant to the base glaze were:

  1. 1 Cobalt carbonate, 2 Chrome Oxide  =  Turquoise
  2. 1 Cobalt carbonate, 2 Nickel Oxide  =  Grey-blue
  3. 2 Ilmenite, 2 Rutile  = Brown
  4. 1 Cobalt Carbonate = Blue

The colours on the right indicate what colour the metals add however the glazes are all a little more brown and dark than I expected. The iron rich clay body I use is probably a factor in this, as is the fact that the glaze includes Nepheline Syenite which I already know adds a pinkish hue to this recipe. Swapping the feldspar back to the original potash or soda and using a porcelain slip underneath might remove the red tint from the colour and lighten the glaze. I’m hesitant to change the clay body itself. I’ve had problems over the past two years with jumping from one clay to another which results in lots of reclaim and the problem of keeping the different clays apart or the unpredictability if you mix them. I’ve decided this year to have fewer variables and hopefully learn something by working to get this clay to suit my artistic expressions.

Chrome oxide gives glazes a green colour (in percentages up to 2%) and mixed with the cobalt carbonate, a common blue colourant, gives turquoise. Cobalt carbonate is different to cobalt oxide in that it is a slightly less powerful colourant and since it’s a lighter powder, it can spread more evenly through the glaze.
Nickel oxide on its own produces green/brownish grey and in combination with chromium oxide creates more attractive shades of those colours, however here it serves to dilute the brightness of the cobalt.
Ilmenite is the name for the combination of iron and titanium oxide and as well as giving brown speckles in glazes is used in many crystalline glazes. Rutile is again titanium oxide but this time with up to 25% iron oxide.
Source: The Potter’s Dictionary of Materials and Techniques

Gas Firing Gallery

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Eight weeks into this first term I’ve succeeded in getting two loads of work fired in the little yellow gas kiln, although I haven’t quite been able to fill it myself yet and have relied on sharing space. I’ve been focusing so far on two simple forms – bowls for the Llantarnam Grange project, alongside jars which I began experimenting with over summer. Drawing from my experience of wood firing at HDK I am attempting to create a collection of classic/timeless and uncluttered shapes on which the activity of the glaze is brought to the foreground. Hopefully my exhibition statement for Llantarnam Grange can shed light on my thinking process:

Working with clay can teach us the value of patience, something which has become scarce and undervalued in our current society. The experience of time in relation to making is central to my practice. Through the stilling of clay as it slowly centres on the wheel-head, I enter into a different time zone where the material has control over my experience.
Throwing on the potter’s wheel becomes a method of quiet introspection where tensions in my subconscious manifest themselves in the finished vessels.  I find a place of refuge in this sphere of stillness, a meditative zone reflected in the soft curves of the forms and subtlety of glaze.
My practice is characterised by the balance between imposing my own design on the clay and surrendering control to the vitality of materials. Rather than painting on patterns, I prefer to leave the surfaces of my pots at the mercy of the kiln. Flames from the reduction firing leave traces of the action and movement of the firing process in the form of flushes of colour and fluidity of running glaze.
As a result the vessel surfaces become as American writer Harold Rosenberg said of Abstract Expressionism: ‘not carriers of images but [carriers] of events’.

My intention originally was to undertake a technical in researching ash glazes however after a series of unsuccessful line blends and a realisation that brushing on the glazes resulted in patchy, unattractive finishes, I have decided to focus more broadly on the effects I can achieve in reduction with other types of glazes. I realised that the large quantities of ash I would need to glaze the amount of pots I had would be difficult to get. The unpredictability of ashes from different sources meant my tests would be largely pointless unless I had a singular plant source. Upon visiting the Leach Pottery I discovered that an ash glaze made from one type of plant ash can vary wildly in colour depending on whether the plant comes from a heavily mined area or not since pollutants in the ground can affect the chemistry. Although I embrace unpredictability to some extent in the way the glaze varies over the form with iron spotting from the clay body, pooling in the centres of bowls and crystallisation on the glaze, I want some idea of what the glaze will look like.

All the glazes above result from layering one or a couple of the five glazes below:

Chun, Celadon and Shino – Tested at HDK
Matt pink Nephelyine Synetite Glaze 
Phil Rogers ‘Fake Ash’ Glaze :

Potash Feldspar      19
Whiting                    31
Talk                           2.4
Bone Ash                  2.4
Ball Clay                    6
Quartz                        9
Cornish Stone           15
China Clay                 15

I had a lot of trouble with a Derek Emms red reduction glaze recipe. The heaviness of the copper carbonate meant the suspension was terrible and the sediment fell to the bottom immediately no matter how much I mixed the liquid. I tried dipping, pouring and layering but the red was very patchy still. Perhaps this was due to the atmosphere of the kiln being not reduced enough though.
Although I’m happy with the subtle qualities of these glazes so far, without the right light the quiet shades of green, blue, pink, purple and red  can end up looking dull and grey. My next step will be to work with the same body (Reduction St Thomas) but applying a porcelain slip to the surface before bisque. Hopefully the colours will be a little more vibrant on a whiter surface. I’ve used a slip recipe from Jasper, adding 10% Potash Feldspar to porcelain to stop the slip cracking with shrinkage.

It might be useful next time I fire the measure how much the clay shrinks with each firing. – Measure Jars on the bisque shelf.