Kiln Firing Hot

[cracked pot]

My L&L e23T seems to be firing hotter than the cone 6 I set.  In the last three glaze firings the cone packs were as shown in the attached photo.  Packs are cones 5,6 and 7.  I'm a little intimidated to try and use the thermocouple offset in the controller.  I thought maybe I could just fire to cone 5 and have a hold at the end to get to cone 6.  Would this be bad for the elements??

BTW, thanks to all the experienced potters who contribute to this forum.  I learn so much every day from you.

 

[Norm Stuart]

When our Cress E23 kiln was new I fired it fairly empty with cones and discovered that the kiln was always one cone too hot at all cone fires.

Cone 017 actually fired as ^016, Cone 05 provided a ^04 bisque and Cone 5 provides a ^6.

 

I suggested to Bartlett Instrument that they might possibly have an indexing error coded into their EPROM, but they say they looked into it and the program is choosing the correct profile.

 

So rather than use Thermocouple Offsets, or reprogram the final temperature of each cone, I realized it was far easier to accommodate this kiln's eccentricity by always choosing the next lower Cone-Fire.  For a short time we reprogrammed the Cone 5 to a 5.5, but later dropped this.

 

But there are a number of ways higher cones can occur later, even if the kiln fired correctly when new.

 

1.) Type-K thermocouples can measure the temperature less correctly as it ages.  This is the most likely cause of your problem if your kiln initially fired correctly.  After replacing out Type-K annually we have now replaced it with a Type-S platinum, which is initially more than three-fold more accurate, ages better, and supposedly lasts far longer.  We aren't far enough out one our Type-S usage to give you an anecdotal story about longevity.  The Type-S cost us about $220 instead of $65, including the shielded Type-S thermocouple lead wire. We also had to change the position of the only jumper on the Bartlett board and access the Hidden Menu to change Thermocouple from Type-K to Type S.  Incidentally, with the new Type-K thermocouple our kiln continues to provide exactly one cone too much firing for each cone-fire program.

 

2.) If you add a hold at the end of the cone 6 firing, this increases the cone - I think about one cone per hour.  Likewise adding a slow-cool (down-firing) to the glaze fir will increase the cone-work registered by the cones.

 

3.) Cone placement can affect the results, but this not likely your problem.  One way to determine if you are firing too fast is to place a cone pack under a bowl and another outside the bowl.  If the cone pack inside the protection of the bowl doesn't match the cone pack outside of the bowl, the kiln is firing too fast to let the heat penetrate through the thickness of ware you're firing.  So a firing which is too fast can result in a different cone in different placements.

[cracked pot]

Thanks Norm. 

I do use the slow cool down program provided in the controller so maybe that is why it is firing higher.  The kiln is only three years old and glaze fired about once a month.

I think I will try your suggestion of putting a cone pack under a bowl to check any difference. I would like to find a solution without having to buy new thermocouples.  I tried firing to cone 5 with a 10 minute hold and cone 6 did not fall, so guess I will try the longer hold.  So much to learn but I'm having a ball!

[Bob Coyle]

It's too bad you can't enter a "fudge factor" into some of these controllers. My thermocouple at this time is almost 8% too low. I use a cheap hand held temperature probe with a section of small gauge  K thermocouple wire that I stick into one of the peep holes. It gives me a reality check as to how well the dedicated K thermocouple is working that is connected to my controller. I cut off a new section each time I fire.

 

As Norm said, they do drift over time and the only way you will know by how much, is to have a second thermocouple to measure them. If you have read some of my other posts, you know how much I trust controller readings. When the gazes turn out right, follow what the cones tell you, use the same ramp, and don't pay any attention to the reading.

[Norm Stuart]

Bartlett V6-CF controllers give you the choice of adding a fudge-factor in two ways, if you think that is the best way to handle the problem.

 

TCOS  --  (Thermocouple Offset  Page 22)   Add or subtract a maximum of 50 F (28C) to the peak temperature for all thermocouple readings.  If your thermocouple is off by, or has drifted off to a set percentage, rather than a fixed number of degrees, for all readings - then this is not your solution.  Enter 0035 to add 35 degrees F, or 9935 to subtract 35 degrees F.  But this typically fixes the problem when the Type-K always reads a fixed number of degrees wrong.

 

 

Since there are almost a random number of degrees difference between each cone, see below, this would not fix our problem where the Witness Cones indicate the controller has always fired to the next hotter con, regardless of the Speed chosen or the Cone-Fire target.

 

 

CNOS  --  (Cone Offset  Page 21)   Choose a specific cone, say Cone 6, then add or subtract a maximum of 50 F (28C) to the peak temperature for that particular Cone-Fire.  Enter 0035 to add 35 degrees F, or 9935 to subtract 35 degrees F.  We could have chosen this method to reprogram the peak temperature for each Cone.  For us, it was easier just choosing the next lower Cone for each firing.

 

http://www.bartinst.com/KILN/V6CF/v6cfman.pdf

 

Type-K thermocouples when new should provide an error no larger than the greater of 3.6 degrees F or 0.75% of the temperature (8 degrees at Cone 6).  Drift over a hold time can add or subtract an additional 9 degrees F at Cone 6, for a potential total error of +/-17 F at Cone 6.  As this type of thermocouple ages, it will tend to display a different temperature depending on how far it is placed inside the kiln, as the thermocouple wire becomes non-uniform as the metal crystallizes.

 

Type-S thermocouples when new should provide an error no larger than the greater of 2.7 degrees F or 0.75% of the temperature (for a total error of +/- 5.6 degrees F at Cone 6.

 

 

It's too bad you can't enter a "fudge factor" into some of these controllers. My thermocouple at this time is almost 8% too low. I use a cheap hand held temperature probe with a section of small gauge  K thermocouple wire that I stick into one of the peep holes. It gives me a reality check as to how well the dedicated K thermocouple is working that is connected to my controller. I cut off a new section each time I fire.

 

As Norm said, they do drift over time and the only way you will know by how much, is to have a second thermocouple to measure them. If you have read some of my other posts, you know how much I trust controller readings. When the gazes turn out right, follow what the cones tell you, use the same ramp, and don't pay any attention to the reading.

 

F           F diff   CONE

2,361      16        11

2,345      45        10

2,300      20          9

2,280      18          8

2,262      30          7

2,232      65          6

2,167      43          5

2,124      18          4

2,106      18          3

2,088        9          2

2,079      33          1

2,046      30        01

2,016      29        02

1,987      42        03

1,945      57        04

1,888      60        05

1,828      39        06

1,789      61        07

1,728      40        08

1,688      31        09

1,657      50      010

1,607      25      011

1,582      43      012

1,539      54      013

1,485      29      014

1,456      34      015

1,422      62      016

1,360      41      017

1,319      67      018

1,252      93      019

1,159      47      020

1,112      25      021

[neilestrick]

Unless you have confirmed that it is firing hot at all cones, you should use the cone offset feature, rather than the thermocouple offset. This will allow you to adjust cone 6 while leaving all others untouched.

 

A slow cool could also be the culprit, as Norm said.

 

If your glazes look good, don't worry about any of it......

[cracked pot]

Neil, my glazes actually look great, just a little different than early firings.  What I don't like is that my red clay (Red Rock)now looks more brown after firing. 

I think your suggestion to use the cone offset instead of the thermocouple offset is probably right since when I fired at cone 5 with a 10 minute hold the cones melted just right. 

Thanks for the information.  I didn't realize that there were two different offset options.  Should have read further in the manual.

[Natania]

My kiln fires hot too, so I just use a cone 5 cone in the kiln sitter instead of a 6. The clays I use are vitreous at both temps, and the glazes aren't overfired when I go down one cone. Besides, I think I am probably firing closer to cone 6 temp., but I don't see any particular reason to be overly precise about it. Of course, my kiln has no computer....so that makes it easier.