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.
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#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)
