Archive for the ‘kiln construction’ Tag

This week I have been mostly making …. fireboxes.   1 comment

Heavy firebricks are laid on the concrete floor to protect it

Heavy firebricks are laid on the concrete floor to protect it

The wood-fired kiln I am building has a lower and an upper half. The lower half is where the wood is burnt to generate the heat needed to fire the clay pottery. It is built from heavy fire-bricks which, although they need to be heat tolerant do not need to be particularly good insulators. They are cheaper and more robust than the High Thermal Insulation  (HTI) bricks which will be used for the upper half. The kiln has two fireboxes – each one the equivalent of a hearth in a domestic fire.  They are located at the front and back of the kiln with one of them being stoked from the left hand side of the kiln and one from the right.

The first course of bricks protects the concrete base from extremes of heat.

 

 

A hole is drilled into the concrete where each set of 4 bricks meet.

A hole is drilled into the concrete where each set of 4 bricks meet.

While laying the floor bricks holes are drilled in the foundation slab to allow steam to escape easily.

 

 

 

 

 

 

 

Firebox course 1. This is the trickiest course to lay as the bricks are not all exactly the same length and the side walls have gaps in them.

Firebox course 1. This is the trickiest course to lay as the bricks are not all exactly the same length and the side walls have gaps in them.

The first course of bricks must be carefully planned to make sure the corners are square. Lots of measuring and set square usage. It incorporates supports for the ash grates which hold the embers from the burning wood until they are thoroughly burnt.

 

 

 

 

 

 

The fire bars on the left support the long billets of wood used to fire the kiln. As they burn they break up into embers which fall onto the ash grates below.

The fire bars on the left support the long billets of wood used to fire the kiln. As they burn they break up into embers which fall onto the ash grates below.

The ash grates and the fire bars (brick course 4) are made from mild steel angle iron and weldmesh. I measured and cut the pieces of metal and a friend in the village welded them together.

 

 

 

 

 

 

Fire boxes second course. The outer bricks are laid flat whereas the inner bricks are laid on their sides.

Fire boxes second course. The outer bricks are laid flat whereas the inner bricks are laid on their sides.

The second and third courses are pretty straight forward except that a number of bricks must be cut. For the heavy bricks I used an angle grinder and for the HTI’s I used an old panel saw which was no longer sharp enough for wood.

 

 

 

 

 

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Fire boxes third course. Ash grates in position.

No mortar is used. The slight unevenness of size in the bricks is accommodated by adding a thin layer of fire-clay slip mixed with sand between courses.

 

 

 

 

 

The fourth course supports the fire bars

The fourth course supports the fire bars

The fourth course supports the fire bars. The inner wall, just below the fire bars is partly made from HTI brick as it is easier to cut and is not subject to much wear and tear in this location.

 

 

 

 

 

 

Fire box course 5. The fire bars are not in position as the weight of the sixth course bricks is needed to prevent the central support bricks being pushed downwards by the unevenweight of a single set of fire bars on one side.

Fire box course 5. The fire bars are not in position as the weight of the sixth course bricks is needed to prevent the central support bricks being pushed downwards by the uneven weight of a single set of fire bars on one side.

Fifth course.

 

 

 

 

 

 

 

 

Fire box course 6

Fire box course 6 with fire bars in position.

Sixth course.

 

 

 

 

 

 

 

Fire box course seven. This serves as the foundation for the ware chamber.

Fire box course seven. This serves as the foundation for the ware chamber.

Seventh and final firebox course. Great care is taken to make sure the brickwork is level from front to back and side to side.

 

 

 

 

 

 

The fire box roof. Kiln shelves laid on ceramic fibre paper. The gaps at each end are where the heat from the fires will rise into the ware chamber.

The fire box roof. Kiln shelves laid on ceramic fibre paper. The gaps at each end are where the heat from the fires will rise into the ware chamber.

The fire-box roof, which is also the ware chamber floor is made from 25mm thick kiln shelves laid on a couple of layers of ceramic fibre paper (which is more like felt than paper) This provides a good seal between the upper and lower parts of the kiln and evens out the minor unevenness in the final course of firebox bricks.

 

The next stage is to build the ware chamber – the place where the pots to be fired are placed – out of HTI bricks.

 

 

 

 

 

 

 

 

 

 

 

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.