Archive for the ‘kiln build’ Category

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 week I have been mostly making….Concrete   Leave a comment

Shuttering for the concrete kiln base. The uneven stable floor meant there were gaps at the bottom of the shuttering in some places.

Shuttering for the concrete kiln base. The uneven stable floor meant there were gaps at the bottom of the shuttering in some places.

The wood fired kiln build has started. The first thing needed is a good level foundation. The design I’m basing my kiln on had a 4 inch (10cm) thick concrete slab. I happened to have some 14cm wide plywood strips which I exchanged with the lady who rents our paddock for a weeks rental of the paddock so my foundation was going to be 14cm deep. I created a 2 metre by 1.5 metre rectangle using the plywood as shuttering  and filled it about one third full with the hardcore which we find in the flower and vegetable beds. (The local soil is very stony and the house has been here for over 400 years and during that time it seems anything unwanted or broken was dumped somewhere in the garden.) The top edges of the shuttering were adjusted, using a spirit level, to make sure the concrete slab would be perfectly level.

After the first concreting session the gaps between the shuttering and the stable floor are sealed.

After the first concreting session the gaps between the shuttering and the stable floor are sealed.

The location of the stable means it is not feasible to get a concrete lorry to it so I had to mix my own concrete having wheel-barrowed the sand and cement from the road at the front of the house. I did it in two stages. The first session I mixed the concrete quite stiff and concentrated on filling the gaps between the bottom of the shuttering and the ground. The second session the mix was much wetter which allows the concrete to be levelled easily using a plank resting on the top edge of the shuttering.

The shuttering is full of concrete and the mixer is taking a rest

The shuttering is full of concrete and the mixer is taking a rest

 

 

 

 

 

 

 

small round lumps which appeared overnight in patches on the surface of the foundation slab

small round lumps which appeared overnight in patches on the surface of the foundation slab

Its a rule of DIY that nothing ever goes 100 percent to plan. When I came to look at the set concrete the following morning there were a few patches where the surface was lumpy. It certainly wasn’t lumpy when I left it the day before. On close examination it occurred to me that the lumps were all about the same size and shape. I wondered if bubbles had somehow formed in the wet cement, but a bit of prodding with a knife indicated they were solid. Fortunately concrete hardens slowly over a few days and I was able to investigate further by digging some of the lumps out. They were all approximately spherical and of very low density although quite hard and clearly mineral (rather than organic). I have no idea what they are but presume they were some sort of contaminant in the ballast I used to make the concrete with. Clearly they were light enough to float to the top of the liquid cement and protrude a couple of millimetres above the surface before the concrete began to set. Lumps on the surface are the last thing you need if you are going to lay bricks without mortar, which was the plan, so I investigated ways of removing the lumps.

Although not what you would call a smooth surface, the concrete was either a the right level or indented which meant my bricks would sit stably and levelly on it and not wobble

Although not what you would call a smooth surface, the concrete was either a the right level or indented which meant my bricks would sit stably and levelly on it and not wobble

Digging them out one by one was too slow and tedious but a trip to the tool shed reminded me I had a plasterer’s float that was designed for making large areas perfectly flat. It wasn’t intended to be used in the way I was going to use it, but since I last attempted to use it 30 years ago (unsuccessfully – plastering large areas is best left to the professionals unless you want a very rustic look) I felt it wouldn’t matter if it got a bit roughed up. It worked surprisingly well. As I moved it across the surface from a completely flat area to one of the bumpy areas its sharp metal edge would either knock the little spheres right out of the surface or it would cut them off level with the surface. It also levelled any raised areas around the sets of cat and bird footprints which too had appeared overnight. An hour later I had a foundation on which I could build the kiln – although it needed to harden for a few more days.

 

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.