Last Friday I fired the little yellow gas kiln at CSAD for my first time. Starting from about 8.40am the kiln climbed fast to begin with (up to 220C by 9am) and then rose steadily by about 100C per hour, a little slower than in previous weeks because of fluctuations in gas pressure (probably because gas was being used in the foundry). At 1000C, just before 1pm, the flue at the top back of the kiln was covered over in order to create a reduction atmosphere and was left this way for most of the final part of firing. As you can see in the image below two large pyrometric 1280 (09) cones were placed in the spyholes in the front top and back. By 3pm the top cone had completely melted while the bottom one was still only bending a little so in order to reach an even kiln temperature Gemma opened up the flue at the back which had been covered for reduction, encouraging air flow in the kiln. The firing was finished by 3.20pm.
In order to get a better impression of how the gas kiln behaves it could be an idea to place cones at the back on the right side too to see if there are hotter or cooler spots. Since the pyrometer was placed in the right side of the kiln and only read 1237C when the top cone was gone, it suggests to me that the right side might be a little cooler. It might also be valuable to place a 1300C cone at the top too to get a more accurate reading.
The little yellow gas kiln
The packed kiln
I was a little disappointed upon opening the kiln on Sunday since lots of the glazes hadn’t behaved as I hoped, although there were a handful of beautiful bowls and jars – my own nephyline syenite matte pink glaze worked particularly well. The main problem was that lots of the glazes were applied too thinly. My glaze application has improved since first year when I was painting them on and had lots of patchy results. Now I make a big enough batch of glaze to dip the pots in and this results in a much more even coverage. The chun and celadon which I had tested in Sweden turned out ugly patchy browns but perhaps they were just not thick enough. The best pieces seemed to be the ones most enclosed in the centre of the kiln shelves. More reduction could have been encouraged by packing the kiln tighter or even putting work in saggars. The insides of the lidded jars had a lot more brighter colours than the outsides because they’d reduced better.
While I only used one layer of glaze on the vessels, I tried layering different glazes on the test tiles above and the results turned out to be a lot more exciting this way. The shino over matte pink results in a matte lavender while the pink over chun creates a purple/blue crystalline – like glaze breaking to pale yellow where thin. The pastel colours in blues, greens, pinks and lavender have a quality of delicacy, lightness and quietness much like the glazes of Katherine Pleydell-Bouvarie. I’m drawn much more to the matter surfaces and they way they soak up the light in a soft, introverted manner. Somehow these surfaces feel more organic than the glistening, glassy ones which have an almost sticky, plastic texture. Shinyness distracts from the form too. My next step will be to experiment with overlaying these glazes on vessels in a gas firing hopefully later this week.
Over the past couple of days a group of us current ceramics students at HDK along with some alumni, have come together to pack the 3.5m square anagama kiln at Nääs along with our smaller wood firing kiln ‘Elinor’. We’ll begin firing early Monday morning, working in shifts of four hours per pair until Wednesday night when the anagama reaches about 1300C. Elinor can be fired to temperature in one day so on the Monday we’ll be feeding two kilns simultaneously.
We began by emptying all our boxes and unwrapping our glazed pots. The anagama needed to be swept out before the kiln could be packed and wadding had to be prepared. The recipes for wadding vary but we used approximately 7 parts Alumina to 3 parts Kaolin (China clay) alongside rye, water and a generous shake of sawdust. This makes up a kind of off-white putty which is rolled into small cocktail sausage shapes that stick all the kiln shelves and supports together. Since the anagama slopes inside, the supports don’t lie flat but are angled on the base and need to be stuck down. Small balls of wadding are spat on and stuck to the base of pots before they’re placed in the kiln. It can be knocked off after the firing but without it the coating of ash through the kiln could stick everything together.
The chun type glazes were placed closer to the back of the kiln because they’re liable to run while glazes that required higher temperatures to melt were put close to the opening where the flame can hit them. Unbisqued work was also put to the back to avoid cracking. This is one of the differences between an anagama and an ordinary wood kiln – in a wood kiln the pots are shielded from the flames while in an anagama they are exposed to them. The temperature is also encouraged to fluctuate in this firing e.g after 1260C we will drop down to 1220, then up to 1270, down to 1240 and so forth to climb steadily, building up lots of layers of ash. We’ll have a pyrometer in the kiln but have also placed eight lots of seven pyrometric cones ranging from 1000C to 1325C in order to measure the temperature difference throughout. These cones are visible in the peep holes down either side of the anagama’s length.
Pyrometric cones on broken kiln shelf ready for the kiln
The temperature gauges
Firing temperature schedule
Today we also prepared the daub to seal in the bricks at the kiln’s entrance – 50/50 sand and clay which could be dug up just a few meters away from the kiln, with added water to make a paste. A space for the firebox was kept at the front – here we can push in wood to feed the fire. The base of the entrance is built with alternating columns of soft and hard bricks so that the soft ones can be removed if needed to take out fuel from inside. Around the door too are hard bricks as supports. Some of the bricks at the front had to be sawed and sanded down to make them fit as tightly together as possible.
Bricks in place at entrance
When packing it’s important to think of how the flames will flow. Like water they will always take the easiest route so it’s good to have a range of heights on each shelf and nothing too large close to the back which could block off flames to the smaller pots in the narrower end. Since we didn’t have quite enough work to fill Elinor, soft bricks were places in the gaps which will keep the air flowing evenly throughout. Generally the packing hasn’t been an especially difficult process, it just takes a lot of time and shuffling things around. Fingers crossed for the next step!
I felt my heart sink when I went to open the kiln this morning. Instead of a rainbow of bright colours – lime greens, turquoises, salmon pinks and cobalt blue, I found my series of white earthenware thrown plates had all turned a yellowish off-white. Checking back over the glazes I’d used I realised I’d made some mistakes with the calculations when I tried to double the ingredients. I’d added 1% of coloured stain to the new glazes instead of 10% to the base glaze.
I should have realised something was off by the pale colour of the glazes in liquid form. I was hoping to display these colour experiments on the wall for next week’s corridor exhibition but I’ll have to think of something else instead. The firing itself didn’t really go to plan either. The first time I though I’d put the kiln on, I came back in the morning to find the kiln still on 50C. I hadn’t pressed and held the start button down to begin the program!
Hopefully I’ve learnt a lesson to keep neater notebooks so I’m not cramming illegible glaze recipes into every area of free blank space.
Last week I tried to make my own press mould for the first time, not very successfully. My plan is to create press moulds from composite thrown forms so I can build them together into large sculptures. I find it easier to hand build on a large scale with grogged clay, but it’s painful and not very effective to throw with heavily grogged clay, so I will create press moulds of the thrown objects instead. These forms will be for my final individual project . I’ve narrowed the brief down to explore the imagery of in Italo Calvino’s ‘Invisible Cities’, Ch. Thin Cities 3, especially the idea of a network of pipes as underground veins…
“Whether Armilla is like this because it is unfinished or because it has been demolished, whether the cause is some enchantment or only a whim, I do not know. The fact remains that it has no walls, no ceilings, no floors: it has nothing that makes it seem a city except the water pipes that rise vertically where the houses should be and spread out horizontally where the floors should be: a forest of pipes that end in taps, showers, spouts, overflows. “
I began by bisque firing a form I’d constructed from thrown sections then made a two piece plaster mould of this. The ceramic got stuck in the plaster when I tried to release it, either because I hadn’t used enough soft soap or because I hadn’t placed the middle line in the exact centre.
Since for press moulding it doesn’t matter that the plaster is completely sealed together (unlike slip casting), I used glue to stick the broken plaster pieces together.
This afternoon I’ve been making press moulded shapes ready to start sticking them together tomorrow to build large lattice structures.
Alongside these I’ve been throwing functional ware for the anagama firing we’ll do at Naas in a couple of weeks. These 500g bowls are a little on the heavy side, I’m still a little afraid I’ll turn off too much clay and end up with a hole. I’m been experimenting with the angle and depth of the footrings to see what looks best…
With two weeks left to go of the Room project I think it’s time for reflection on how the project has developed so far and how I intend to bring my exploration of ideas to culminate in a final installation.
I chose the HDK’s black grogged stoneware clay to begin making with, the graininess makes it ideal to hand build with because it keeps its form well. At first I worked quite strictly from the collages I made from the earlier tram drawings but discovered quickly that this ‘steampunk’ aesthetic wasn’t what I wanted. I don’t like the way the clay is manipulated to look like metal or rivets, instead of celebrating the qualities of this material I am hiding it. I realise that since this bothers me perhaps the tenet of ‘truth to material’ is somewhat important in my work.
After a tutorial and discussing with others I decided to focus on simplified forms instead of details. I still preferred my collages to the clay models, so this week I took the approach of collaging clay to create more two-dimensional ‘illustrations’ of my illustrations. These were made by rolling thin slabs and assembling them roughly and quickly together when in a leather hard state. The rough edges and unfinished, breaking apart look is an attempt to capture the fuzziness of how the memory of a place appears in our mind.
I added 10% coloured stains in different proportions of colour to this to try and match the colours found in tram interiors in Gothenburg. The orange, yellow and light blue are prefect although the pink was supposed to be red and the blue is too purple. Unfortunately on the black stoneware these glazes bubble but I still intend to use these glazes to decorate my original ‘sketches’ in clay – the haziness of the colour might work to reflect the blurriness of memory and the patchiness might reference the dirtiness of the trams.
Over the next two weeks I’m going to continue working with this collage technique but in a white low firing clay, hopeful the juxtaposition of these ‘sketched’ sculptures and a smooth, uniform and neatly coloured glaze will create impact. I’m going to try working on a slightly bigger scale so that there is some different in height levels in the final staircase exhibition. I have tried placing some objects on the stairs already to see how they look in this different context but the dark colour of the clay means they are lost against the surroundings. I hope the bright colours will change this and create a sense of playfulness and intrigue. I also plan to create more accurate blue and red glazes, a grey and a lemon yellow.
I wanted food-safe glazes to decorate a series of white stoneware mugs I made last month and decided to use a celadon base glaze then adjust the percentages of iron oxide and tin oxide to get a line blend. Celadons originated in Japan and the jade colour is caused by iron oxide being starved of oxygen in a reduction atmosphere during firing.
The original A60 base celadon glaze (1) uses the recipe below. It fits the body well and is a grey-turquoise satin colour.
CORNISH STONE 56
BALL CLAY 20
(RED IRON OXIDE 1.5)
(2) 1.5 IRON OX. & 1 TIN OX.
I thought the addition of tin would make the glaze more matte, but the effect here is the opposite, the colour becomes thinner, more watery. Brown speckles.
(3) 2 IRON OX.
My favourite mug, the addition of 0.5g more Iron ox. to the original recipe gives a slightly darker more yellow-green than blue-green.
(4) 3 IRON OX.
With double the iron oxide in the original recipe, the glaze becomes less smooth and the surface begins to break apart a little. This is a much darker, more ‘forest’ green than the others, breaking to dark brown where thin.
(5) 1.5 IRON OX. & 2 TIN OX.
Again the addition of tin has made the glaze less transparent but the speckling is attractive. Bluer.
(6) 2 IRON OX & 2 TIN OX.
Strangely, despite being the most different mixture to the original recipe, this one looks most similar to (1). The more iron in proportion to tin, the fewer speckles.
Renata held a fun glaze workshop with us in which we made a glaze based on our date of birth. We randomly chose glaze materials to mix in proportion to the year, month and day we were born. By comparing each other’s glazes we could get a good idea of how each material behaves when fired.
My glaze was the result of mixing a high proportion of whiting with some dolomite and ball clay and a small amount of Wood ash. Fired to stoneware temperature the result is a very dry, matte and slightly flaky off -white/pale green glaze blending into pink lower down in a gradient effect. The ash has pooled in a shiny green line along the base where it has melted. It’s not suitable for functional ware because it hasn’t fully melted, but I’m excited to use it to decorate sculptural forms.
Whiting (calcium carbonate) acts as a flux and also creates matte effects (as does dolomite). Ball clay is a source of alumina and enhances glaze suspension. I want to make a series of glaze tests using the same four ingredients but changing their percentages so that I can get a better glaze fit.
UPDATE: After a couple of months the glaze has flaked off completely