Archive for the ‘studio design’ Tag

This week I have been mostly making….a ware chamber.   Leave a comment

The first course of the ware chamber is bedded on ceramic fibre paper to ensure it has a perfectly level surface

The first course of the ware chamber is bedded on ceramic fibre paper to ensure it has a perfectly level surface

The kiln I am building has the ware chamber (the area – strictly speaking volume – where the pots are placed for firing) located above the fire boxes. The ware chamber floor is the fire box roofs. It has approximately the same length and depth dimensions as the base of the kiln and is just over a metre high , however this varies because the top of the chamber is arched. The internal volume is approximately 0.85 cubic metres (30 cubic feet) although not all of this can be used for stacking pots as shelf-free space must be left over  the flues which bring the heat, gasses and flames into the chamber from the fireboxes below.

 

 

 

Once the first ware chamber course is complete the corner supports can be put in place. These are held in place by threaded rods.

Once the first ware chamber course is complete the angle iron corner supports can be put in place. These are held in place by threaded rods.

The ware chamber is built with HTI (High Thermal Insulation) bricks. These are similar to, what in the UK, are called ‘breeze blocks’ or ‘Durox’, but they are very high temperature tolerant and even more thermally insulating. The material is like pumice but with a uniformly small bubble size. I read somewhere, many years ago, that it was developed as part of the US space programme for insulating the Space Shuttle during atmospheric re-entry and earned the craft the nickname ‘the flying brickyard’.

 

 

 

 

By the time the sixth course is finished the chimney outlet has been bridged using a piece of kiln shelf as a lintel. Note the thin slivers of HTI brick needed to make this wall the same length as the firebox walls and the way the bricks have been cut to allow a vertical panel of bricks to be removed

By the time the sixth course is finished the chimney outlet has been bridged using a piece of kiln shelf as a lintel. Note the thin slivers of HTI brick needed to make this wall the same length as the firebox walls and the way the bricks have been cut to allow a vertical panel of bricks to be removed

The HTI bricks are cut from a larger slab of material after it has been made and are therefore very uniform in size.  This is in contrast to the firebricks used for the base which are cut before they are fired and can vary in length by as much as 5mm with the majority being slightly longer than their nominal length of 230mm. This means that a row of 6 HTI bricks is almost always shorter than a row of 6 heavy fire bricks and necessitates cutting thin slivers of HTI brick to make up the length. Fortunately HTI bricks are easy to cut. The second, third, fourth and fifth ware chamber courses  have a gap in them at the back where the flue to the chimney will join and at the front of the chamber the bricks are laid such that they can be removed to allow the pots to be placed into the chamber. Although strictly speaking this panel of bricks is not a door, for simplicity’s  sake that is how I will refer to it from now on.

 

 

 

 

 

The ware chamber walls are complete, the door bricks have been cut so it can be laid in row-lock and two 'spy-hole' bricks have been put in.

Viewed from the front, the ware chamber walls are complete, the door bricks have been cut so the brickwork can be laid in row-lock and two ‘spy-hole’ bricks have been put in.

The remaining 7 wall courses are straightforward and similar and and when they were complete I took down the door bricks and cut one of each alternate course in half so that the door wall could be built more stably. In a couple of places I replaced a brick with two wedge shaped bricks one of which was slightly too long for the space available for it. This made it stick out from the wall which will allow me to pull it out during a firing to see what is happening in the kiln and pull out test pieces.

Finally, on each side of the kiln, a piece of angle iron is supported by brackets on the corner supports just above the top of the chamber wall. This is the ‘arch support’ which will prevent the arch, which forms the roof of the ware chamber, from pushing outwards and collapsing.

Next, the tricky bit, building an arch.

 

This MONTH I have been mostly making …. purchases.   Leave a comment

600 Fire bricks stacked by the house woodshed and covered with polythene to keep them largely dry.

600 Fire bricks stacked by the house woodshed and covered with polythene to keep them largely dry.

The wood fired, salt-glaze kiln project has been reactivated now that the essential DIY tasks have been completed. I’ve dug out the drawings, photos and materials-list of the kiln I’m basing mine on and attempted to get hold of what is needed. I was kindly given the kiln design by Bob Park of Greystoke Ghyll  Pottery nine or ten years ago. Although I’d applied to attend his kiln building course he didn’t have enough other applicants to make it worth his while. However, when I asked, he agreed to show me how to fire the kiln over a weekend and give me the kiln plans, for a very reduced course fee.  A lot of water has passed under the bridge since then and several of the suppliers on Bob’s list have ceased trading or stopped selling the items listed on the kiln materials list so getting hold of what’s needed is less than straight forward. I’m also modifying the design slightly as I have an existing building (stable) which I want to house the bulk of the kiln, but the stable roof is made of asbestos concrete which I am reluctant to cut a hole in (due to the health hazard of asbestos dust) for the chimney, so the chimney will be built outside the stable and I will need a horizontal flue linking the ware chamber to the chimney. The kiln is based on the Olsen ‘fast fire’ design but has clearly been modified either by Bob or whoever he got the design from. Fortunately  Fred Olsen published a book ( The Kiln Book: Materials, Specifications & Construction ) explaining the design principles used to design the original and I am applying those principles to modify the design.

On the left a small pile of standard HTI bricks and on the right a variety of kiln shelves being stored in the old chicken shed to keep them nice and dry

So far I have purchased

600 heavy fire bricks which will be used to build the bottom half of the kiln including the fireboxes, and the bottom half of the chimney.

400 lightweight HTI (High Thermal Insulation) bricks which will be used for the top half of the kiln ie the ware chamber.

80 HTI arch bricks which will form the arched roof of the ware chamber.

35 Kiln shelves and a selection of props to support them – for stacking the pots in the kiln.

An assortment of other pieces of kiln shelf which will be used in the construction of the kiln forming the ware chamber floor/firebox roof, chimney floor, various lintels, the firebox doors and the flue damper.

All of these materials were delivered on pallets by lorry and because the track to the stable where I am going to build the kiln is too narrow for lorries the pallets had to be delivered to the front of the house. The road has no pavement and our drive is too soft to support a pallet truck so the pallets had to be left on my neighbours drive. After each delivery I spent several hours moving the materials by wheelbarrow to their temporary storage locations where they would be out of the way and suitably protected from the elements.

 

This week I have been mostly making…Glazes   Leave a comment

At last the pottery is fully functioning. It seems to have taken forever to get to this point but I have now done my first successful glaze firing. The sources of delay have been many and varied. There have been the usual decorating and DIY jobs needed when you move to a new home. The very large garden was a wilderness and is now reasonably tamed and producing bumper harvests. We’ve had a continuous stream of visitors all keen to see what we have let ourselves in for. Several of them stayed for long weekends and some friends from New Zealand stayed for a fortnight. Various major remedial works have been completed such as re-roofing the summer house and moving the gas tank (and as a result also the garden tool shed). Major building work has also commenced and is still ongoing. Getting this started has involved much deliberation on how we wanted the internal layout of one end of the house to be changed followed by the process of getting planning permission, choosing a builder, specifying internal fittings, sanitary ware, kitchen units, floorings, wall decoration etc. etc. All in all it has been difficult to spend any consistent amount of time in the pottery.

pottery waterworks

Outside the clay room. The water comes from the roof and is stored in the green water butt. To the right are shelves for drying pots prior to bisque firing and a table on which glazes are made. In the far left foreground is the kiln lid. Behind this are shelves for kiln props and above these the energy regulator which caused the first glaze firing to go wrong. This is attached to the firing controller (the box with its red instruction manual tucked behind it).

Nevertheless, although intermittent, progress has been made on the potting front. After finishing off the clay room I started to throw pots. It was a bit of a shock to find that although some aspects of throwing, such as centring, hadn’t faded away with lack of practice, other skills such as ‘just knowing’ how close to the wheel head  I had pushed my thumbs down when opening out a ball of clay had deserted me. The other vital skill of ‘just knowing’ how thick the wall of a pot is when you are pulling it up had also faded. The first few throwing sessions produced pots with excessively thick or thin bases and with similarly unsatisfactory wall thicknesses. Initially the walls were too thin and the pots would collapse half an hour after they were removed from the wheel. I then had a phase of leaving the walls too thick. I resolved to make sure that every session in the pottery would begin with an hour of throwing practice. Fortunately, after just a few weeks, these skills returned but all the practice did leave me with many tubs of clay for recycling and very few pots which would be consigned to the kiln. Just as well really because the drying/glazing/kiln room was still under construction.

When in the pottery and  not practicing throwing I lined the remainder of the garage walls with battens and insulated plasterboard, shelves and tables. On the outside wall of the clay room I installed a worktop and second hand sink and on the worktop, against the outside wall I built a stand for a water butt. The water butt  is filled by run-off from the roof and with a capacity of 160 litres provides all the cold water the pottery needs. A hosepipe attached to the water butt tap allows me to direct the flow into the sink or to a glaze bucket sitting on the floor.  If I want hot water I just boil a kettle. Underneath the sink is a big water tank where clay and glaze ingredients can settle out before the water  goes down the drain. Finally I wired up the kiln, put it in its final position and put packing under some of the legs to make it level and stable. I then packed it rather loosely with the unfired pots I hadn’t felt needed throwing in the recycling bin, and did a biscuit (bisque) firing to 1000C. All went well. I had the essentials for pot making.

Before I left Cambridge I allowed all my glazes to dry out completely – for ease of transport – so the next job was to reconstitute these with water, which also includes the time consuming job of sieving them. Many an hour was spent stirring the glaze slop to push it through the sieve a pastime which still continues.

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The glazing and decorating area. An old kickwheel I use as a banding wheel, a low table to support the buckets of glaze at a convenient height and shelves holding bisque fired pots awaiting decoration.

As the stream of visitors slowed down and my throwing skills returned I got more time for making and had another kiln-full ready to bisque, However there was one last hiccough to delay full scale production. When I did my last glaze firing in Cambridge it was clear that the kiln elements were worn out. The temperature struggled up to 1240C and took 15 minutes longer than it should have done. One or two of the pots looked a bit  ‘overcooked’. Time for a new set of elements. Replacing the elements themselves is not too bad a job but the kiln’s instruction book said that after changing the elements the kiln should be fired once to 750C while empty and twice to 1000C with pots in before a glaze firing is attempted. Over a period of 3 weeks and with a lot of frantic throwing activity I duly did as instructed and finally was ready to produce a finished, glazed pot.

I glazed enough pots to fill the kiln, carefully selecting those pots which I hadn’t put too much effort into or which, after the biscuit firing were clearly destined to be seconds – bits of lime popping in the clay, warped, non-circular things etc. A first firing with new elements is prone to mishaps!  Caution, pessimism, realism, experience, I don’t know but BINGO! the mishap occurred. The kiln has a single cooker style energy regulator which limits the amount of power going to the kiln. It is also fitted with a programmable firing controller which switches power on and off to achieve the desired firing rate. This makes the energy regulator redundant – it is always left set to full power and consequently I have never needed to consider it since the first day I got the kiln. Unfortunately, at some point in the months since I left Cambridge the setting of the energy regulator got changed and I didn’t notice. Even when the firing controller was trying to give continuous full power the kiln was only getting about 80% power. This didn’t matter below 1000C as the kiln achieves this quite easily, which is why I didn’t notice the problem sooner. However as the temperature got above 1100C the rate of temperature increase fell below what the firing controller was trying to achieve. It was as if I hadn’t changed the elements and despite  frantic efforts to diagnose the problem I didn’t spot the incorrectly set regulator. Eventually the kiln reached the desired temperature but 50% of the pots went straight to the bin and the rest are substandard.

I eventually spotted the regulator problem a couple of days later while cleaning plaster dust off it and was actually relieved to know that the cause of the problem was nothing more serious. The next glaze firing was fine.

I now need to find outlets for my pottery and make a start with preparations for housing, building and fuelling the wood kiln.

Posted October 27, 2013 by damiankeefe in Uncategorized

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