Coastal Muffle Kiln

Having measured my kiln and the packing area I have to accommodate the new muffle (or saggar)  I then made a plan on paper of the box like structure I was going to make to install in it.
Leaving 2″ clear space all round for the flame path was a suggestion that Joe Finch made, so that became my basic measurement.
Internal space minus 2″ all round.
When I had visited Joe to discuss the idea we had made a small model of how we thought it should work. Using this as my template I scaled up to measurements taking into account the shrinkage of the clay at about 10%.
I made a wooden frame which was the size of the largest piece of the saggar. Using sheets of canvas and working on large pieces of plywood I bashed and rolled the stiff clay into the frame. I scraped it down to avoid any air gaps.
To turn the slabs over I covered them in another piece of cloth, and another sheet of plywood. I then clamped the 2 sheets of wood together and flipped the slab over. This allowed me to work on the reverse side, smoothing out any air gaps and filling any weak areas.
I made 2 large pieces for the lid and the base. 2 square pieces for the back and the door, and 2 pieces with added sections for the side walls.
I added handles to the front door in order that it was easily removable.
Drying all of the pieces slowly proved a big challenge. I had to find space for each slab to have air circulating and at the same time drying slowly over a number of weeks in order that they neither cracked nor warped.
Finally after 6 weeks all of the pieces were bone dry, mostly not warped and ready for a trip in the car to a friend who has a very big kiln to fire them all in.

I live by the sea in East Kent. Our coast faces North. The sea is Khaki green with clay and silt, and usually calm as it is protected by the entrance to the Thames estuary.

I had never heard of a coastal kiln until I started planning this project – but on arrival in West Wales to meet with Joe Finch (Master kiln builder) the first thing I encountered in the wild cove of Cwmtydu was an 18th century coastal Lime Kiln! Apparently they are 10 a penny in that part of the world. Nestled in beside the crashing pebbles and foaming surf of the   Atlantic ocean, they are designed to heat quarried limestone to turn it into Lime to provide fertiliser for the nearby fields.

With my head full of the strange symmetry of this encounter I visited Joe Finch for an afternoon of planning and trying things out. Joe had agreed to help me design the muffle and advise me on how to construct it.

I took with me the measurements of the internal dimensions of my gas kiln. Having thought about the problem in 2 dimensions and talked around the idea we went into this ceramics studio to think about it in 3 dimensions. We rolled out slabs using a re-purposed mangle, which saved time and energy.

His thoughts were that with a structure of this size we should need to make something that comes apart in sections, so we were effectively designing a box with extras. We decided on the steps to follow and the order in which the pieces should be made, dried and fired.

The individual sides of the muffle box would need to be made by paddling stiff saggar clay into a frame used as a former to create a dense slab. The lid, the base and the back piece would all be flat slabs of the correct dimensions. The two sides and the front would also have added elements or ‘fins’ to help the whole ‘key’ together. The whole muffle when fired will fit together with ‘seats’ of ceramic fibre in-between each piece to help it to bed down and to make a good airtight seal.

Joe sent me home with a car full of saggar clay which he had previously mixed in his old industrial dough mixer. He uses the following recipe. It will fire to a material with the density and toughness of kiln shelves.

50kgs Fireclay

25kgs Kyanite

25kgs Molochite 16/30mesh

3kgs Cellulose fibre

My next post will be on the logistics of making, drying and firing 6 large slabs of clay that each weigh 25kgs.