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the big pot firing

Posted jane -->on 27th Feb 2011

I packed seven of the big pots into the kiln on Thursday, trying to make plenty of space between them for the heat to move around. Perhaps there were too many in there – none of them were more than an inch short of the roof, and some just about touching. the firing certainly went differently. I had noticed that when I bisque fired them the kiln seemed a little slow, and the firing was quite uneven, as the black clay had started to go black near the top of the pot. the glaze firing was very awkward – with the flue as closed and the pressure as low as I had it on the last two firings at 1030 C the temperature stuck and even went down. in the end more pressure and a wider opening to the flue worked better, but I found that I needed to lose the visible signs of reduction to get the temperature to rise quite frequently. the firing took longer – about two hours longer.

in fact the amount of reduction was fine, and I was very pleased with all the pots, except that the black clay large bottle with wirebrush vertical scoring, on which I used the barium carbonate glaze, cracked right open across the base.

the crack ran right up the front of the pot and opened it up across the shoulder seam, which broke off half the neck as well. this is quite a disappointment. I think the clay of the base melted into the kiln shelf slightly, which prevented the pot shrinking as it cooled. I will have to put any big pots like this on the shelf with either clay supports or pieces of kiln brick under them in future.

you can see the dark rings made by the two black clay bottles, and a dust track from the snap right across where it broke.

the crawling shino worked really well on the other black clay piece.

the dolomite/tin copper glaze behaved as it has been doing for the past couple of  firings, where I have dipped in and out with no hanging about and got a thin coating. on the pale stoneware the coppery ginger is breaking where the surface of the clay is disturbed on this side of the pot, which was close to the front of the kiln, but on the other side it is all over rather than just on the scored lines.

I overlapped the two dolomite glazes, the green/orange, and the blue/grey in these two bottles, which were both at the front of the kiln. this is the st thomas reduction clay, which gets quite toasty and makes the glazes darker. I picked this section of the pot for a close-up to show off my scored lines.

on the photos of the whole pot it’s hard to see the blue of this glaze where it breaks, but in real life it is quite noticeable. I’m very pleased with it, a blue that is just green enough.

this is a reprise of what I did in the summer with porcelain; plastered gooey porcelain over the surface of a buff stoneware, sprinkled it with china clay powder and rolled it in, before making the pot, and then glazed with barium carbonate.

here is the dolomite/tin/copper glaze doing something completely different – it is over a very white stoneware full of molochite grog, on very thin, and in the middle of the kiln. a gorgeous pumpkin orange,  where over the edge of the buff underneath, it is just brown.

apart from the sad state of the big black bottle, a very pleasing firing. you can see the pots here

after the rain

Posted jane -->on 24th Feb 2011

after the rain, spring is here today. the sun on my back was almost hot.

in the garden the birds are busy; goldfinches whistling and zzzzing; the thrush muttering his subsong from the ivy in the sycamore; the bluetits are having a go at enlarging the entrance hole in the tit box – they do it every year, I don’t know why but I can hear the tap tap as one of them pecks away at it.

I am preparing pots and the kiln for glazing.

the hellebores seem happy with their mulching and exhibit masses of big juicy flowers. more crocuses keep appearing out of the mulch; satiny purple and sunshine orange.

the ditches at the bottom of Blood Hill are full of snowdrops too.

walking along the woodshore hazel catkins dangle like fuzzy pale green raindrops blurred by slow exposure.

yesterday’s rain still gathers on every twig. the ponds in the wood are full of water, the ditches slimy with decomposed leaves.

the beech tree, iron grey with all her limbs reaching up between the ash and the oak.

a thrush sings from deep in the wood. new buds are opening into leaf on the honeysuckle, and the primrose crowns are full of tight new leaves. buzzards call to each other overhead.

in the mud rucked up by the shepherd’s land-rover there are muntjac hoofprints.  larks sing above the big field that rises alongside the lane.

two early primroses in flower already where the hedge sides have been cleared; there will be a good show of them this year.

the big pots are quite awkward to glaze; I splash my feet and the floor filling them with the liner glaze and pouring it out again. the blue glaze is in too small a bin and it overflows – I continue to push down – too late to do anything else. glazing has to be done in one fell swoop, and outdoors.

getting better all the time

Posted jane -->on 17th Feb 2011

at last, a firing from which every piece came out well. besides getting the right glaze on the right pot at the right thickness, and placing them in the right position in the kiln, real attention to detail during the firing is needed to achieve a good result.

an amazing green flame through the 1200’s, as I tried to keep the reduction light. between 1030 C and 1100 I had a strong smoky reduction, a little flame peeping out of the spyhole brick, and a very strong sulphur stink.

the last cone which is only a little bent is cone 11, signifying if bent over fully, 1300 C, or the equivalent heat work – I turned off the gas when the pyrometer read 1262; the slowness of my firing and the reduction produce the same effect at the lower temperature. the floor of the kiln at the front is a bit of a dead area, I get a better result by raising the pots up a little, though I did slip a porcelain stem bowl underneath. the southern ice porcelain matures at a lower temp anyway.

at the back the pots on the floor fire well, as long as they are well spaced out. my crazy blue glaze toned itself down a bit this time, probably because there was a little less reduction, so the copper in it stayed green rather than pinky purple.

I had three refires on this top shelf, two were underfired in the cold spot at the front of the kiln in the last firing, the other was just a dull colour with a lot of grey.

this firing has a nice mixture of colours – orange, rust, chocolate with blue bits, a purer, veined blue on the second glazed porcelain piece, and grey green to celadon green.

this bottle was on the top shelf at the front, next to the flue; that’s probably the hottest place. this is the dolomite/tin/copper glaze, fairly thin – a wonderful glowing dapple orange/rust that sometimes happens on the super white stoneware.

the same glaze on porcelain when thinly applied almost disappears, and will turn pink where there’s enough reduction. this has had its second firing; the first left grey patches where the copper was a little thick. more copper burns off this glaze in the second firing and lightens it up.

the new shino I mixed up in January for a liner will come up white when it’s on thick enough, so the indented polka dots catch more glaze and are white. the clay is that “school clay” a buff stoneware, which turns this rich rust under the shino.

this porcelain wash has proved difficult to glaze satisfactorily. it is a cold white, so colour in the glaze tends to be less rich and warm and the surface can be rather uninteresting. it is better when the wash is thin and allows the buff clay underneath to show through. dipping the bottle sideways in the two different dolomite glazes, the green and the blue, gives the shape some help too. I call the vertical overlap a tidepath, referring to the tidemark I make by overlapping horizontally.

the blue dolomite glaze at the right thickness does very good things on the layering of  the superwhite stoneware over iron-bearing stoneware. these watering cans came when I was using images of a french collection of old watering cans to make an embroidery for the summer knits. the idea of making spouts with sprinklers was too tempting to resist. I think this is the one with the most presence.

this is the blue glaze behaving itself over porcelain layered with terracotta, and doing what I originally got on my porcelain tile tests; a speckled bird’s egg blue.

working outside, glazing and packing the kiln, has been a pleasure this week; my garden is owned by a thrush, and he has been singing for a week now, starting with a throat clearing, lung stretching series of spluttery croaky noises which developed to full blast jazz song by mid week. sometimes I could hear three, two far off in opposite directions, and “my” thrush low down on the ivy hedge next to the pottery workshop, very loud.

you can see all the pots from the firing here

another firing, a more successful one this time

Posted jane -->on 13th Feb 2011

spring has come on a lot since my last post, we have had a good three weeks of mild weather. along the edge of the wood hazel catkins dangle,  smudged dashes of pale green against the brown-grey winteriness of the trees, one of the first signs of spring.

crocuses and tiny irises are out in the garden, and all the snowdrops that fringe the wild garden are at their best.

mild weather has made working in the pottery, especially glazing, which I do outside, on the verandah, much more pleasant. on Friday I had another glaze firing, after two weeks of making, and a biscuit firing. now I have plenty of pieces to glaze, at least another kiln-full after this one. I made the pack looser, and applied some of the things I had learnt from my last firing – at the moment the kiln arch has some leaks and the gap made by the bricks on the top of the flue needs to be quite a lot smaller to get the right amount of reduction.

I started reduction a little early – at 1009 C, and kept it strong with real back pressure and a lot of soot for the next 100 degrees, then allowed less reduction, keeping the flame that emerges from the flue quite small and clear. it turns out I could really have had even less, and kept the copper glaze a bit clearer.

a lot of the pots are made with an iron bearing stoneware, or even crank, with a layer of a very white smooth stoneware rolled on top as a layer. the white clay gets marked with stains from the iron bearing one, and it results in an interesting mix, with which the dolomite/tin/ copper glaze works very well, producing oranges, rusts and an almost celadon green, although matt.

the blue version of this glaze has turned out to be quite problematic. I have had greys and blacks from it mostly, except when I use it for a second firing over the barium glaze. however this time the blues are quite bright over the porcelain layered pots. this was due to the fact that I put it on much thinner; in order to do that I did not mix up the whole of the bottom of the bin ( this was still in a solid lump after the big freeze) so possibly the composition was different.

the refiring of  the last firing’s disaster was even more of one, the glaze has bubbled so the poor pot has an advanced case of acne. I don’t know why and will have to ask advice on this one.

on the whole a good batch. I am not so happy with 2 more strident blue pots hiding at the back of this group, (one was at the front of the kiln) but the blue is better on this one, as the copper in the glaze has kept a slight greeny tone, instead of the purple of over reduction.

this group of little bottles has turned out nicely, glazes doing different things depending where the pot was on the shelf. centre left is the blue glaze again, but over stoneware clays which tone it down.

the best glaze for my rough white stoneware mixed clay is the crawling shino, and this is probably my favourite pot. its pointy shoulders are working nicely with the double dip down the middle, and the cracked wall echoes the crawling shapes.

I made this pot in the summer; it’s good to have it resolved at last – and the green/orange dolomite glaze is stunning on the iron-bearing stoneware on the inside. I have a web page of photos up here.

a new batch awaits firing in the workshop, mostly only just small enough to fit in the kiln

this is reduction st thomas clay with added grog – there are several made with the rough white mix, and some with black clay, both of which will look good with the crawling shino. probably the st thomas will get the dolomite glaze

and this, which is st thomas with wet porcelain and china clay rolled into it, will be best with the dry matt barium glaze.

after looking again at this firing, I want to have one of these in the stoneware layer, using crank as the base and the fine white stoneware which has molochite grog in it as the top layer. it has been producing some lovely results with the dolomite/tin/copper glazes. so more clay out and some more making this week. at least three more firings to go – much intense forward visualisation at this point.