Don Kopyscinski on tue 17 sep 96
Hi everyone,
Richard Boyd wrote:
>Am I expecting too much?
The bag in which the elements are packaged says to expect "100's of firings".
<
There should be in very fine print somewhere (*"as had by a little old greenware
lady that never takes the kiln over cone 06, YOUR MILEAGE MAY VARY!" ).
(I'm not slamming lowfire, don't flame me, it's a joke!)
I've been firing electric kilns regularly to cone 10 for about 15 years. The
problem of element life and deterioration of the top end of the firing curve has
plagued me as well. I have made a few observations, and would like to share them
here. Without modifications 50 firings to cone 10 is about what one might expect
from many electric kilns on the market. Kanthal did say that this figure may be
doubled by proper design of the kiln and correct design of elements (I've yet to
see this kind of life, but in an optimum situation this might be obtainable).
I'm getting somewhere in the neighborhood of 70+ with some extreme degradation
of those last few, with firing time *greatly extended*.
Let me be so bold as to say that one of the main causes for this problem is that
most electric kilns built in the U.S. and sold in the mass marketplace are
simply underpowered (as well as insufficiently insulated) to be called real
cone 10 kilns. I researched the reason for this and came up with an unusual
answer (although no-one will come right out and say it). The main reason kilns
are underpowered in this country are because of insurance practices. In order
for a kiln manufacturer to get a considerable break on their insurance cost they
seek Underwriters Laboratory certification. U.L. considers a kiln to be a
"heating appliance". The maximum amperage for a heating appliance to be given
the certification is 48 amps. and as I understand it, that's the cut-off.
So......most kiln makers max out at 48 amps (which is borderline acceptable for
a 7 cubic foot kiln IMHO). The market then demanded larger kilns and they
compensated by adding another 1/2" of brick or some fiber or in some cases just
making the kiln larger and claiming it will make it. Skutt at least has the
decency to tell you from the start that their 10 cubic ft. electric can only be
expected to make cone 8 (you will note that their cone 8 specs are the same as
those for other's claimed cone 10).
The reasoning behind this limit is that a kiln of sufficient power to regularly
make it to cone ten has a greater potential for disaster in the event of
failure. As it was explained to me, at 48 amps the kiln if not shut off won't
get so hot as to cause the lid to fail and the resulting disaster that would
create. So for the sake of safety in a worst case scenario, performance is
compromised. Most of the lower end three ring stackable configurations stick to
this formula.
I consider there to be three classes of kilns:
[I use specific brands for illustrative purposes, I'm sure I left out some
makers]
1-The three ring stackable, lowest price, and underpowered despite claims (for
regular cone 10 firings).
2-Those who break with tradition and put some thought into performance
requirements and element calculations (the two most notable are ConeArt/Bailey
(only the nameplate has been changed, built in Canada thus C.S.A. approved), and
L & L most notably their DaVincci series.
3-The real industrial heavy duty (and extremely expensive compared to the
others). The two that come to mind are the Fredrickson and Unique. Real
workhorses, built and priced like tanks.
Compare the specifications (one quick way is amps used for similar sized kilns
and you will see quite a range) of these kilns as published in various catalogs
or the makers' own literature and you will soon see the problem.
I've been struggling with 5 kilns from "column A" for a long time. I have made
some modifications which I prefer not to pass on, as they would void any
warrantee, they have room for improvement, and if not done properly could cause
problems. In doing so, I have at least made these kilns "tolerable".
I have tried a myriad of solutions and one of the most annoying failures was the
purchase of a "special alloy" element claiming a "ten-fold extension in element
life at cone 10" for $500+ a set from Axner (they no longer carry these) . At
1.7 times the life of a regular set of Kanthal A-1, they failed just as the
others had.
One of the improvements has been had with the use if the ITC coating also
available through Axner. The kiln uses about 10% less power to follow the same
firing curve as an identical untreated kiln. and I got about 15-20% more firings
from a set of elements.
L & L does have a sheet "Factors That Effect Element Life" that is free for the
asking.
One of the factors that most greatly effect the life is calculation of the
correct "watt density" of a given element. I wish I had the time to really learn
this, but to date haven't, other than a superficial understanding. Apparently
the lower the calculated watt density of a given element, the less stress is
placed on it in firing and it's life can be expected to be longer. L & L claims
to have taken great care in proper calculation for greatest element life
(however I was disappointed with their proctice of not balancing the elements,
one is expected to control time proportionate switches with a time proportionate
controller, without knowing how one will affect the other, I much prefer having
elements balanced so that temperature is even throughout with compensation made
for top and bottom elements and their lack of neighbors on both side).
I would expect that Cone Art is comparable in this area (thoughtful element
calculation) given the quality of the wiring and connectors I recently noted
when inspecting one of their kilns (virtual pat on the back to Frank Tucker,
nice job!, glad to hear that you upgraded the relays as well).
Some of the marketing is a bit deceptive. One major maker told me that their
kilns "were designed so that the firing would naturally slow down at the upper
end of the firing which gives you the most beneficial firing curve, allowing the
glazes to smooth out" (read underpowered). I would prefer they gave me the
power, and let me decide how I want to use it.
Some more thoughts:
1-Any attempt to "reduce" in an electric kiln will greatly reduce the life of
the elements.
2-Unvented kilns will most likely deteriorate more quickly due to damage to the
protective oxidation coating on the elements which must be replaced by
subsequent oxidation of more wire thus decreasing the diameter ever so slightly.
3-Expect elements to be a wear item consumed a little by each firing and replace
at signs of significant deterioration.
4-It seems that the replacement element market has quite a mark-up. Significant
cost savings may be had by sourcing these through a place like Dura-lite but be
sure that you know the exact specifications of your elements (they and others do
have the specs for most major brands, be sure these are the specs you want. You
should know such things as wire gague, O.D. of wind, Ohms of resistance, and if
you want they will pre stretch to your specifications. Custom elements may be
had, but be sure you've done your homework.
5-Use nothing less than A-1 Kanthal or it's equivalent for high fire.
6-A lower firing temperature will ***significantly extend*** the useful life of
your elements. Kanthal quotes a reduction of 70*F will double the useful life of
the elements, this is nearly exponential so that a 140* reduction will provide a
fourfold increase in life. Conversely the higher you fire, the longer the kiln
stays there, the amount of overshoot, and ultimately a higher peak wire
temperature will dramatically reduce your element life. (Perhaps holding back
those first few firings , slowing down the end of the cycle and acheiving cone
bending (glaze melting) by a slightly longer time at a slightly lower
temperature would stop the greatest damage from wire exceeding it's useful
maximum temperature and eliminate the associated overshoot). Kanthal recounted a
story of a manufacturer who increased their peak process temperature by 200*F
and cut their coil life to about 1/8th of it's previous life. My dedicated
busque kilns haven't been re-elemented in years, while the glaze kilns are
changed about twice a year each.
7-Kanthal A-1 actually loses heating capacity at elevated temperatures due to
changes in resistance (4.2% loss at 2370* as shown in Fornier's "Electric Kiln
Construction For The Potter" - long out of print), so the problem of high end
deterioration of borderline elements is compounded.
8-When choosing a kiln, in general, you get what you pay for.
9-Don't take everything some manufacturers tell you as the complete story.
10-Anyone who labels a bag of elements with only the words "Good for hundreds of
firings" is misleading and underinforming you.
11- A cycle of regular replacement of all the elements in the kiln when they are
tired will be more costly but in the long run much more advantageous than
replacing single elements.
12-As a kiln gets a little older and tired they have a greater heat loss through
those cracks and gaps, some loss in the insulative capacity if the bricks
themselves, as well the loss of reflective capacity of the polished stainless
steel jacket if it is allowed to deteriorate.
13- The use of "on/off" type of contacts in kiln controllers may contribute to
early element deterioration (Kanthal suspects this, but the difinative jury is
still out).
14- A kiln on High will often cause line drop, subsequent lower voltage and
heating capacity, use of multiple kilns can compound this most dramatically
15-In general, use of underpowered kilns for cone 10 firings is a drag, one
which I resent regularly. It looks like after having said all this, I better
take by own advice and replace my kilns, as needed with those with the real
power to do the job.
16-If attack of volatiles to the elements is indeed cause for reduced life, a
reasonable protective coating of oxidation my be had by firing the kiln empty
the first time to 1900*F for 5 hrs to build up the protective coating before the
volatiles have a chance to attack [Source - Kanthal Corp.]
17-Paraffin, sulfer, and carbonates "do not significantly deteriorate A-1
Kanthal, they will damage AF alloys though" [from Kanthal Corp.] (zinc and other
volitile's can be a factor).
18-Kanthal has recommended (to me...today) that an increase of wire gague with
an equal total resistance figure will in effect reduce the end temperature of
the wire, but that the heating characterictics of the kiln should remain nearly
the same as the increased mass of the slightly reduced wire temperature will
compensate to a degree. This might well greatly extend element life. This is
accomplised by reduced peak temperature of the wire and having a greater amount
of material to deteriorate. Sounds reasonable. Simply increasing power to an
existing kiln without adjusting the gague of the wire, may help in the first few
firings but the greater potential overshoot and maximum wire temperature will
ultimately most often shorten the life of the elements.
19-True proportionate power control (such as zero crossover SCR - getting in
over my head a little) capable of cutting off individual cycles may extend
element life because the power switching is so fast that element temperature is
reduced, thus it's useful life can be expected to increase.
20-I have insufficient knowledge of the subject to be recommending that anyone
change the specifications of their elements. The definative sources on the
subject are:
The Kanthal Handbook -The Kanthal Corp.
Electric Kiln Construction For The Potter - Robert Fornier
The Electric Kiln - Harry Frasier
There is a new one I recently saw on the "Potter's Shop" booklist but I haven't
seen it, nor do I have the title at hand.
21- Be aware that any change in resistance can cause problems and potentially
dangerous failures in other components of a kiln or wiring if their limitations
are exceeded.
22- Be safe, do your homework, know your limitations, and use good judgment.
23-A tired wall socket or lose electrical connection will also greatly affect
the firing time and efficiency of your kiln (a hot socket is an indication of a
loose connection). Get in the habit of regularly checking (plug out, power off,
breaker off, never take chances) the integrity of the system as a whole. Direct
wiring, eliminating the wall socket, seems to have helped some potters I talked
to improve their firing cycle.
So.......are there any kiln makers, element calculators, electrical engineers,
or the like out there who would like to add their expertise to the discussion?
Home watching the Muppets with a sick kid,
Don Kopyscinski
Bear Hills Pottery
Newtown, CT
74134.2672@compuserve.com
or
DonKopy@aol.com
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