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Sondahl's
Guide to Home Electric Kiln Repair
Electric kilns consist of:
An outer shell of stainless steel
Channeled insulating brick with
peephole plugs
Lid and bottom of kiln and support.
A power supply cable, hooked to an electric
circuit breaker
A means to turn up power incrementally
and shut off power when the kiln is finished
Then there are the elements themselves...
Cheap Electric
Kiln exhaust
Each of these is discussed on this page. To quickly
access a specific part, click on the links.
Remember to disconnect the power to
the kiln before accessing any of its electrical parts, including elements.
The opinions I express on this page represent my own practices, presented
for discussion purposes only. If you follow any of the advice given
here, you assume responsibility for your own actions.
For advice on specific kilns, contact
the manufacturer.
An outer shell
of stainless steel
This shell holds the firebrick in place, and should last
longer than the rest of the kiln. However it is attached at the ends
usually with screw type hose clamps, and these tend to rust out, or at
least the sheetmetal screws holding them. If
you see this happening, buy some new hose clamps, cut them in half with
heavy tin snips, drill holes in the two halves near the ends for sheet
meal screws, drill new pilot holes for the screws, and attach the replacements.
Tighten when finished.
Channeled insulating
brick with peephole plugs
This firebrick holds the elements in and provides insulation
for the kiln. The main problem with it is that chunks of the brick
fall off with time, particularly when elements swell after long use, and
take out chunks of brick as they are removed for replacement. Generally,
once a piece has come loose it does little good to try to reattach it.
The parts that come loose are invariably outer pieces of the channel which
support the elements. The elements, once freed, start to stretch
when red hot, and if they contact their neighbors, can arc and ruin both
elements. To avoid this, I use the two items shown below:
They are both rustically made from pure clay. I use
fireclay, EPK, or ball clay, or any combination of these, mixing up a small
batch in a bowl. The larger piece I call a bridge, and the smaller
a pin. Pins are used to support the element over a short distance
where the channel is degraded. Drill a hole slightly larger than
the pin into the brick where desired, keeping the angle of the hole such
that gravity will keep the pin in place. See instructions under elements
for getting elements back in place...
The bridges are made by rolling out the clay to about
1 cm, cutting into strips, and adding on with slip two small coils which
are used to attach the bridge to the kiln. Take care that the small
coils are straight and parallel as attached, as bent ones will not work.
Fire them to your hottest kiln temperature before use.
To use a bridge:
Bridges are to provide support for elements where a large
section of firebrick is missing. I make them about 4 inches
long, and use several to span larger distances. Using a drill with
a larger bit than the coils (usually 1/4 inch), drill into the kiln at
an angle where one end of the bridge will go, about 3/4 inch deep (the
angle should be such that gravity will hold the bridge in place.
Set the bridge in place, and mark where the other end should be drilled.
Remove the bridge and drill the second end. Check for fit-- if necessary
enlarge the holes by wiggling drill side to side while the drill is going.
After installing the bridge, follow instructions
under elements for getting elements back in place...
Peephole plugs:
These crack and break from thermal stress. You
can make your own by mixing a small batch of pure clay (kaolin, ball clay,
EPK, and or fireclay) and water, with sawdust in it to give it air space.
After mixing, shape the plug by hand into a cone shape approximating the
size of the hole on the kiln. Allow for shrinkage. Dry thoroughly
and fire in your highest normal firing before use. If you use sawdust,
expect smoke as the peephole heats up.
Lid and bottom
of kiln and support.
The lid of the kiln tends to develop cracks with age.
To some extent this can be avoided by maintaining sufficient tension on
the lid, with the screw type hoseclamps used to secure the steel flange
around the outside. This flange is critical in supplying support
for the lid--if it were to become detached, catastrophic failure may result.
These hose clamps and the sheetmetal screws used to attach them can become
rusted with age, and can be replaced as necessary (instructions).
If chunks of pottery are falling out of the lid, or any sag is detected
in it, or wide cracks are appearing, replace the lid. Even though the bottom
supports the whole kiln, it is not stressed as much as the lid, which is
opened and shut frequently, so the bottom will likely last longer than
the top. But another concern is the base which holds the whole kiln.
Sometimes this is not made of stainless steel, and if not, it can rust
out. Check for rust on your base. If necessary the kiln can be set
on regular masonry bricks or probably cement blocks, preferably with some
space to allow heat to dissipate into the air.
The power
supply cable and circuit breaker.
These are both possible causes of frustration for the
kiln user. As the cable ages, it can build up corrosion where it
plugs in, and if the corrosion is thick enough, it can cause the plug to
arc. Since kilns are designed to draw a lot of amperage, this situation
will not necessarily trip the breaker. The closest I've come to a
fire from my business was when my plug in connector arced and started burning
the plastic housing for the receptacle. My kids noticed it and I
shut off the power and extinguished the fire before the wood wall caught.
Since then I always hard wire the kiln with screw type connectors inside
a junction box. Kilns don't need to be that portable... You may want
an electrician if you want to do this.
The circuit breaker can become a problem, if it starts
shutting off during a firing. If you have this problem from the beginning,
you probably need a bigger breaker. But breakers are matched sometimes
to wire size, so make sure your wiring can handle a bigger breaker. Also
make sure you check your elements for arcing before the expensive step
of a larger breaker. And as breakers age they can start shutting
down with lower amperage, so that may be grounds for replacing also.
Remember that you should probably not exceed the recommendations of your
kiln provider, as too large a breaker may promote dangerous arcing and
potential fires.
Switches
and shut off
I don't use a computer controller, so you'll have to
chat with your supplier on that. I did use a thermocouple shut off
system from Seattle Pottery (they no longer offer it), but it cost about
$30 per year for thermocouples, and finally the electronics burned out.
I think computer controllers are overpriced for what they do. I recommend
the Dawson Kilnsitter with timer as the simplest, cheapest, and most dependable
way to shut off your kiln. This means turning up your power manually--which
most of us are capable of. The kiln sitter can need new metal posts
which support the cones, a new porcelain tube assembly, or new contact
block in the switch. If you need advice on repairing these, email
me or talk to your supplier.
The manual switches are usually either 3 position(low,
medium, or high) or infinite (no click setting from low to high).
I have kilns with both styles. Infinite switches can sometimes be
recycled from old electric stoves. 3 position switches can handle
more amperage, so you get by with fewer of them, and are generally more
reliable than infinite switches. With either of them I turn kilns
up just three times, every two hours of the firing until they are on high.
If a switch burns out, usually there will be a post that
is wiggly, or something will rattle when it is shaken. Whatever wire
is connected to that burned post needs a new connector crimped on, to prevent
damage to the new switch. The wire itself may need replacement--if
so, contact your pottery supplier, as this is special high temperature
wire. You replace the switch by removing the knob, loosening the
large nut under the knob, and moving the wires to the new switch.
It's generally easier to reattach the wires before sticking the knob on
again.
Elements
Elements will last until they burn out, but their efficiency
goes down with the number of firings (particularly high fires) they have
done. To check elements, turn on the kiln to high for a couple minutes.
Shut off the power, and drip a bit of water on each element. The
hissing tells you which ones are working. If they are not heating
inspect the element for an obvious gap or break. If this does not work,
run a small screwdriver or your fingernail along the whole route of the
element, checking for gaps or burnout holes. If you still haven't found
a problem, or if two elements on the same switch are not working, suspect
the switch (it's sometimes easiest to replace the switch rather than test
it.) A continuity tester can be used also to see if the element has
continuity throughout its length.
If you do find a burned out section of element, remember
to put kilnwash over it before putting in the new element. This will
prevent a hastened repeat burnout, caused by the metal residue which still
conducts some electricity.
If you want to roll
your
own elements or buy premade ones: Duralite still sells the
bare Kanthal A-1 wire. Duralite in Connecticut custom rolls elements (860-379-3113).
Current prices for 16 gauge wire are 19.50/lb. There's a minimum 25.00
order, so I ordered two pounds. That will make, as I recall, about 6 elements
for my old Crusader kiln . If you send them all the pieces of your old
element, they will size and price it for what they would make, or tell
you what how long to make yours. Then it's a question of using a rod the
right length, and winding the wire on compactly. Duralite has for the asking
a photocopy of info on resistance per foot per gauge of wire, etc. I've
used a welded crank and steel rod (with a slot cut in the end to
hook the wire in to start. Lately I've used a variable speed drill with
rod stuck through a hole in mounted wood to stabilize it. It would help
to have one person run the drill and another to guide the wire. The ends
are fairly easy if you use loop ends-- tougher if they're supposed to be
just tight twists.
When you're ready to put in the new
elements, clean the channel thoroughly of bits of old element and dust.
A lot of kilns use little pins in each corner to hold the element in place.
I don't like them much--most elements come sagging out of the middle flat
areas, not the corners. I just use them in problem areas (see
insulating brick repair)
Cheap Electric Kiln exhaust
I recently added a cheap kiln vent to my electric kiln.
I bought some flexible aluminum clothes dryer vent and attached it to the
bottom peephole of my kiln with three sheetmetal screws with added washers
to keep from ripping through the thin metal. At the other end of
the flexible section I used standard rigid aluminum dryer vent tubing to
get it out of a window. Outside I added a small fan mounted in an
A-frame of plywood (so as to protect it from rain/snow). From inside
I can push the vent tube closer to the fan to create more suction, or farther
from it for less. When I fire a bisque, I want more suction, to vent
the sulphur fumes. You can tell how effective the venting is by how
hot the pipe is getting. It never seems to get too hot to touch.
No longer the taste of sulphur when I fire... Do bear in mind
that in venting out fumes, you are also removing some air from the room,
which must be replaced somehow. If you are in a confined area with
fuel fired appliances (gas hot water, furnace, or woodstove), the draft
created by drawing out kiln gases can also draw into the room some of the
exhaust gases from these other appliances, including carbon monoxide.
So you may need to run a vent hose into the kiln space to equalize pressure.
The following are related pages
on my website:
For a large
list of original Sondahl Pottery glazes (Cone 8-10 Oxidation), click here
Sondahl's
short guide to crystalline glazes
Collection
of my Ceramics Monthly Articles and other pottery thoughts
Multiply your glaze tests with
my Glaze Hybridization system page
Offsite related links:
http://www.euclids.com/
Another supplier of elements and kiln info
Sondahl.com
