Building Portable Kilns out of Ceramic Fibre

Here is an information sheet on building a DIY raku kiln, written by Steve Mills of Bath Potters Supplies and uploaded by ukpotters.com. The info sheet is no longer on Bath Potters Supplies website, but Steve told ukpotters.com it was OK to publish it, so here is my edited version. ukpotters.com  adds: if the kiln is not intended to last very long, you could just make large "staples" from Nichrome wire, or suitable "buttons" can be carved from scrap bits of HTI firebrick.

Building Portable Kilns out of Ceramic Fibre 

(based on information sheet by Steve Mills)

Simple ceramic fibre kilns can be built on any framework, eg chicken wire, because its lightness means it doesn't need a heavy supporting structure. The two types of kiln outlined here are updraft kilns. They are the easiest for the budding kiln builder to start with. 

Warning: Ceramic fibre blanket is composed of compressed fibres of alumina and silica. During the making of the kiln, the blanket will release dust WHICH CAN BE HARMFUL especially if you have respiratory problems and/or sensitive skin, so use a good quality respirator or anti-toxic-dust mask, well-covering clothing and rubber gloves at all stages of construction.

For firing these kilns, use propane gas and a good quality blow-torch. For a single-burner kiln, use a BULLFINCH model 1260 burner allied to a short extension tube No1112, the standard handle No1340 with on/off valve, 3 metres of high pressure hose and the No1051 tiny reg 2 variable regulator. This will deliver up to 25psi of gas, far more than most of us will need. This burner is obviously very powerful, but it can be turned down to a candle flame. This gives you the option of speed when needed. Outside of the UK, use any good plumber's burner that will deliver at least 19KW (67000 btu) and will handle up to 4 cubic feet gross capacity.

Make the burner port approximately 3" diameter, which is 0.5" bigger than the burner.  Make the flue 60% larger, ie 4.8" diameter.  In practice you can make the flue larger and start off with two bits of brick over part of the flue to restrict the exit. The above seems to work well for kilns from 0.8cu ft to 4cu ft.

Before building your kiln make buttons to hold the fibre to the walls and the roof:

#1 Disc cut out of scrap stoneware clay (well grogged) with a 1.5" diameter cutter. A pinch of clay applied to the back and a hole cut through. (You can make them like ordinary buttons but then the holes create a heat path through the fibre which is ultimately destructive and corrodes the wire holding it.)

#3 A short length of nichrome wire is looped through the hole in the button and taken out through the fibre and the kiln wall and bent over to hold it in place.

You won’t need a great number: eg 18 for the example below. Space them about 8" or 9" apart and stagger them. The bottom half of the kiln will only need them in a circle just below the rim. The lid will obviously need more support.

CONSTRUCTION (A) THE OUTSIDE-IN KILN.

#1 Oil drum. If it has a separate lid drill at intervals around the rim and fix with self tapping screws or pop rivits. Cut a slice out of the drum so that the internal height is 2' (610mm) PLUS the thickness of the blanket you are going to put in the lid and base. Then subtract about 1.5" (37.5mm) to give you a compression joint between the lid and base. You should have a finished internal height of 27" before lining.

#2 Line the base first, then the sides, then put the base piece of shelf in and cut the burner hole just above the base of the shelf, drill the sides at intervals and secure the fibre to the walls with the buttons. Don’t pull them so tight it looks like a leather armchair as the blanket will tear on its initial shrinkage.

#2a Cut the hole for the burner then cut the blanket like a star and fold the ends out. You can either glue the ends down or wire them down as in the drawing. Tricky but easier to renew when worn. N.B put the wire loops in place before you cut the blanket. See the picture on the left.

#2b If you want something longer lasting for the burner and/or flue ports see "Flue and Burner block alternatives" later on in this leaflet.

#3 The lid is obviously assembled like the base. The flue hole is cut first and the loops put in place before any blanket is put in.

FINAL JOB: spray the inside with RIGIDISER (W) using a garden hand spray. This puts a hard skin about 2mm thick on the surface of the fibre, which although brittle, prevents the flame from the burner tearing up the kiln wall and reducing it to dust. So it's good for the kiln, your pots, and your lungs! Rigidiser (w) is coloured blue (which burns out) so that you can gauge how much you have put on.

CONSTRUCTION (B) INSIDE-OUT KILN.

#1 Make a former around which layers of blanket are wrapped,each later held in place with sellotape while the next layer is applied. Joints must be staggered, to prevent heat loss. Use the strongest metal mesh you can handle for the outside wall, and allow plenty of overlap as it makes pulling it up tight to the blanket a lot easier. Don't trim off "ends" you'll need them to hold it all together!

#2a You can put a double layer of kitchen foil between the blanket and the mesh as it protects the former and it`s reflective qualities help.#2 The base and top are made the same way except the hole for the flue which is cut and blanket folded out before final assembly.

#3 Complete the cylinder then cut to separate top from bottom. Buttons are then applied to the inside, handles, rim stiffeners etc. are fixed to the outside and the hole is cut for the burner. Finally rigidiser is sprayed over the inside and allowed to dry. The tall handles incorparate feet (diagram) to help protect fibre when kiln lid is dumped on the ground when unloading.

#4 For a top hat version the same process may be followed but obviously leave out the base. The firebox is best made separately out of brick.

#5 When all assembly is complete, spray the insides a deep blue with rigidiser, but leave 1"(25mm) either side of the lid/base join so that the fibre can compress and form a good seal.

When firing any of these kilns, always angle the burner as shown. The flame spirals up around the pots, and then reluctantly out of the flue vent. If you point the torch straight in, the fire more or less avoids everything in its hurry to get out, resulting in a very uneven firing.

FLUE AND BURNER BLOCK ALTERNATIVES

Standard brick size soft firebrick can be shaped like this #1 using very simple tools i.e. old wood drills, hacksaw blades, and a woodworker's rasp, (cut the hole as near to a slot as you can,this seems to help to keep the heat in by acting as a sort of baffle.)

The brick fits into the kiln lid like so #2 and is held in place with nichrome wire through the holes shown.

Drilling soft firebrick does not require an electric drill, quite often the bit can be used just held in the fingers! I usually use an old bicycle spoke for small holes. Remember that soft firebrick will break easily so carry out all operations on it with great care, however once in place it resists abrasion better than fibre, and means you can alter the size of the vent easily and accurately without damage.

Burner

As can be seen from the drawing, the burner block is a more complex shape, but it can be done with care, and its benefits outweigh the fiddle involved in making it. As with the flue brick, it is fitted with the "flange" on the inside, and is held in place by stitching it with nichrome wire. Keep any offcuts of brick, as they will be useful for controlling the size of the flue opening during early stages of the firing (economy) and during reduction (if any).

Note 1: When pushing either of these bricks into place, use another brick or a piece of wood as a pusher to prevent breakage as they will be relatively frail once they have been cut to shape

Note 2: If the kiln is going to be used for high temperatures on a regular basis, it is worth cutting a shallow groove where the nichrome wire crosses the hot face, and burying the wire under a thin layer of fire cement to protect it.

TOOLS,

Standard tools:

Small Angle Grinder-to cut the drum.

Electric Drill and bits -to drill holes in same.

small nosed Pliers.

Hacksaw blade.

Old Tenon Saw.

Wood Rasp.

Old but sharp Carving Knife-to cut blanket.

Small Garden Hand Spray.

One metre straight edge.

Non standard tools

Bicycle spoke drill.  

Tube borer: made from a shortish length of steel tube with teeth cut into one end. Makes the job of boring holes in brick for the flue or burner a bit simpler.

Ceramic button threader: For this you need a piece of metal rod about 8" long sharpened at one end, and a short length of plastic tube, the sort used to protect the ends of fine paint brushes. The rod is pushed from the outside of the kiln through the fibre to the inside, the piece of tube is then pushed over then end, the ceramic button with its piece of wire attached is offered up on the inside, the wire ends tucked into the tube, and the whole lot pulled back through to the outside. It saves a lot of time (and temper). The size of the piece of plastic tube will dictate the size of the holes you drill in the drum of the "outside in" kiln!

Tips & Wrinkles

For cutting the fibre to fit, measure the inside circumference, cut the fibre to length, then lay it out flat and cut it horizontally into the lengths for the top and bottom. It is a good idea if you are using a combination of blanket thickness' to sandwich the thinner material between the thicker for support.

When laying fibre inside a drum, I find it helps to use a rolling pin in a stroking rather than rolling fashion to compress the blanket against the wall

With the "inside out" Kiln, a good way of getting the mesh really tight round the outside is to use a length of strap pulley fashion (see drawing) and pull hard!

If you are building a kiln using a cut down drum, keep the bit you cut out of the middle of the drum; with the edges cleaned up, it makes an excellent cutter for the discs of fibre for the top and bottom.

If the inside of the kiln becomes damaged during use, repairs can be made by spraying the damaged bit with rigidiser, and while it's still wet pushing a scrap of fibre into the hole and spraying over it again.

When reducing, keep the length of flame at the flue vent pretty short (2"-3"), too much reduction can actually force the temperature down.

Keep a log of each firing , noting as much detail as you can,it will help in subsequent firings.

SETTING UP ETC.

With your kiln built and the inside rigidised, you'll need a base shelf to go on the kiln floor, three props between 3" and 4" long, and the first shelf of your setting. As far as the latter is concerned I have found that contrary to what you might expect, round batts are the last thing you want in a kiln of this type, especially if you are proposing to use it for firings other than raku. Square batts, the corners of which miss the kiln wall by about 1/2" seem to be the most effective. The fire circulating round the kiln hits the corners and is deflected into the setting, rather than skating round the outside as would be the case with round batts.

Should you decide you need a spy hole, use a tubular prop, sticking as much out of the kiln as into it and plugged with a knob of fibre. By having it sticking out it means you can either secure it to the outside with wire and a bracket,or hold it with a gloved hand while you peer in! A spy hole may be unnecessary as you can usually see your cones through the flue vent.

FIRING

Site the kiln in a well-ventilated area with no overhanging material within 5 feet of the flue vent.

Stand the kiln on three bricks to allow air to circulate beneath it.

Ensure the burner is down wind, as cool air blowing into the burner port can adversely affect the quality and controlability of the firing.

Important: Site the gas supply to one side and downwind of the kiln. Protect the hose with some sort of cover to prevent it being tripped over or trodden on.

ALWAYS INSPECT THE HOSE FOR DAMAGE OR AGEING BEFORE FIRING

Always increase gas pressure in very small amounts, the same applies to changing the size of the flue opening with your brick offcuts. These kilns are scaled down versions of large kilns, so a damper movement in the latter of 1" would scale down to approximately 1/16".

When reducing, keep the length of flame at the flue vent pretty short (2"-3"), too much reduction can actually force the temperature down.

MATERIALS

Below are the materials used to line a cut-down 45 gallon drum to 2.5" thickness. This kiln was built primarily to fire to cone 8/9, but has also been used a lot for raku. If the latter only is intended, 1.5" to 2" thick lining would suffice.

Fiberfrax blanket, 8lb density, 1260o grade, 25mmX610mm 5.5m

Ditto--12.5mmX610mm 2.5m

Bisc. buttons (home made) 18, 10 top, 8 bottom

2 X 10' coils Nichrome wire,

2 X K23 or TC 25 Brick

Rigidiser (W) 500cc

Scrap 45 UK gallon drum

Plus assorted self tapping screws and 7' X 1/2"X 1/8" mild

steel strap which I had in the workshop.

FURTHER READING

The Self Reliant Potter by Andrew Holden (out of print)

The Kiln Book by Fredrick L Olsen (a bit of a "Bible")

The Energy Efficient Potter by Regis C  Brodie

Kilns, Design Construction and Operation by Daniel Rhodes (out of print)