Cone guides

[Gonepotty]

Hi everyone, wondering if you  could help please. Im still learning how to interpret witness cones and not sure if these have overfired past cone 8?

(cones are from right to left :7,8,9 )

thanks so much,

kathryn 

[Jeff Longtin]

Good Morning Kathryn,

fyi: typically the cones are set in the opposite of what you have done here. That way they can melt and not influence the neighboring cone. (So when 7 melts, there's no chance it will fall on 8, and affect it.) The sequence results in a shape that looks like the Sydney Opera House. (The first cone bent alot, the middle cone bent a bit, and the third cone just starting to bend.)

Digitalfire has a great page about these things. Pyrometric Cone (digitalfire.com)

While Orton suggests a specific angle at which the cones indicate specific temperatures I've found cone plaques, and what they tell you, to be more subjective. If your glazes look beautiful when fired to these temps than thats the bend you want to maintain. If your glazes seem overfired than you would want to lower the temperature a bit.

The other thing that cone packs help you understand is consistency. These two cone packs look fairly similar. Thats good, If they're two different firings or two different areas of the same firing it looks like you have a measure of consistency. So thats good. 

I always look at the "guide" cone to decide if a kiln "overfired". If the guide cone has bent alot then its a definite overfire. If the guide cone has bent a little then its not a bad thing. It really depends upon what the glazes look like.  Are you happy with the glazes?

 

 

 

 

 

[Gonepotty]

1 hour ago, Jeff Longtin said:

Good Morning Kathryn,

fyi: typically the cones are set in the opposite of what you have done here. That way they can melt and not influence the neighboring cone. (So when 7 melts, there's no chance it will fall on 8, and affect it.) The sequence results in a shape that looks like the Sydney Opera House. (The first cone bent alot, the middle cone bent a bit, and the third cone just starting to bend.)

Digitalfire has a great page about these things. Pyrometric Cone (digitalfire.com)

While Orton suggests a specific angle at which the cones indicate specific temperatures I've found cone plaques, and what they tell you, to be more subjective. If your glazes look beautiful when fired to these temps than thats the bend you want to maintain. If your glazes seem overfired than you would want to lower the temperature a bit.

The other thing that cone packs help you understand is consistency. These two cone packs look fairly similar. Thats good, If they're two different firings or two different areas of the same firing it looks like you have a measure of consistency. So thats good. 

I always look at the "guide" cone to decide if a kiln "overfired". If the guide cone has bent alot then its a definite overfire. If the guide cone has bent a little then its not a bad thing. It really depends upon what the glazes look like.  Are you happy with the glazes?

Hi Jeff, 

thanks for your reply and details. All noted:)

I’m not that happy with the firing, have experienced quite a few pinhole issues so wasn’t sure if the kiln got too hot and started creeping above cone 8 temperature. 

1 hour ago, Jeff Longtin said:

 

 

 

 

 

 

[Min]

I'ld call that cone 8. Do you do any sort of drop and hold/soak on the cool down? What is your rate of climb towards the end of the firing? Are the pinholes a new issue? Glaze recipe(s)?

[Gonepotty]

On 12/8/2023 at 6:23 PM, Min said:

I'ld call that cone 8. Do you do any sort of drop and hold/soak on the cool down? What is your rate of climb towards the end of the firing? Are the pinholes a new issue? Glaze recipe(s)?

Hi Min, 

firing schedule (Celsius) is 1) 90o per hr until 300oC. 2) 115o per hr until 1080c 3) 60o per hr until 1220oc with a 20min hold. Pin holes could have been caused by a glaze too thickly applied

[Bill Kielb]

3 hours ago, Gonepotty said:

1220oc with a 20min hold

just an observation
The 20 minute hold likely gets you to cone 7, not sure if that is what you are trying for. Do you use cones?

[Min]

4 hours ago, Gonepotty said:

firing schedule (Celsius) is 1) 90o per hr until 300oC. 2) 115o per hr until 1080c 3) 60o per hr until 1220oc with a 20min hold. Pin holes could have been caused by a glaze too thickly applied

Pinholes can have a number of causes, if it's an issue that you haven't been able to fix I would be looking at trying a drop and hold (soak) schedule as a first possible fix. To do a drop and hold let the kiln free fall in temp to approx 40C below top temp then hold there for 20 minutes then kiln off (if you don't do a slow cool). If the glaze is fairly stiff then yes for sure having it too thick could be the cause of the pinholes. 

I also think you could try increasing  your second ramp to about 175-200C per hour up to 1120C. It shouldn't have any effect on the pinholes and it will decrease your length of firing. It's the last 100C of the firing that is typically slowed down to 60C/hr.

 

[Gonepotty]

6 hours ago, Bill Kielb said:

just an observation
The 20 minute hold likely gets you to cone 7, not sure if that is what you are trying for. Do you use cones?

Thanks Bill. I previously tried top temp of 1230oC and it overfired quite a bit fully bending cone 9. Yes I use witness cones and this schedule gives me a perfect cone 8

[Gonepotty]

5 hours ago, Min said:

Pinholes can have a number of causes, if it's an issue that you haven't been able to fix I would be looking at trying a drop and hold (soak) schedule as a first possible fix. To do a drop and hold let the kiln free fall in temp to approx 40C below top temp then hold there for 20 minutes then kiln off (if you don't do a slow cool). If the glaze is fairly stiff then yes for sure having it too thick could be the cause of the pinholes. 

I also think you could try increasing  your second ramp to about 175-200C per hour up to 1120C. It shouldn't have any effect on the pinholes and it will decrease your length of firing. It's the last 100C of the firing that is typically slowed down to 60C/hr.

 

Thanks Min, I’ll try fix the pin holes (potentially too thick and glaze recipe issue) first but the drop and hold sounds like next step if I can’t get ontop of the pineholes.

with regards to increasing the second ramp, is this likely to alter the appearance of any glazes or cause any blistering etc? I’m quite new to using my own programmes rather than using the standard presets of the skutt kiln.

[Bill Kielb]

6 hours ago, Gonepotty said:

Thanks Bill. I previously tried top temp of 1230oC and it overfired quite a bit fully bending cone 9. Yes I use witness cones and this schedule gives me a perfect cone 8

Sorry did not read the whole thread above…. Of course you use cones! Just a thought, some glazes will pinhole and not heal well when fired hot. Counterintuitive, they do liquify more but don’t necessarily spread out and heal as one might think. Drop and hold is often an effective way to get these types of compositions to heal.
I also assume you are firing cone 8 because it matches your clay and glaze maturity requirements. If not, firing too hot can have its draw backs.

[Hulk]

My (limited) experience indicates that pinholes can (also) be caused by gasses expelled by the clay.
In that case, I don't know if dropping and holding a) slows the gassing/fizzing down, b) allows time for the particle to complete its decomposition, or c) combination of both.
Any road, the glaze would have to still be fluid enough at the hold temp to flow back together...

Pinholing (digitalfire.com)

Other factors that may be in play:

hotter and/or longer bisque fire, to burn out offending/gassy bits more thoroughly;
supply adequate oxygen during bisque and glaze fires, to burn out offending/gassy particles more thoroughly;
ramping up the bisque fire time for thicker wares, as it takes longer.

[Min]

8 hours ago, Gonepotty said:

with regards to increasing the second ramp, is this likely to alter the appearance of any glazes or cause any blistering etc? I’m quite new to using my own programmes rather than using the standard presets of the skutt kiln.

As long as the final segment is slowed down (as you are doing) then no, speeding up the second segment should't cause any issues. Have a look at the "slow glaze firing profiles" from L&L kilns, regardless of final cone reached the middle ramp goes at 400F / hr.  (204.4C / hr). 

A heads up though, when using a set temperature as your final cutoff point rather than a cone you will need to keep an eye on your results and or cones as your elements wear. Given that the kiln will need to fire longer to reach a set temperature the extra heatwork needs to be taken into account. If you look at the Orton cone chart you can see that for cone 8 (using self supporting cones) for the last 100C of rise using 15C/hr gives a top temperature of 1211C, at 60C/hr it raises to 1249C and at 150C/hr it needs to go to 1271C. So if you program in 60C/hr but the kiln can't actually keep up with that then the final heatwork will be greater. This is not a big deal if you are keeping watch on what your cones are telling you. 

One other point re your pinholes, and the possibility it is being applied too thick, are you measuring the glaze specific gravity? Is your claybody a grogged one? 

[Gonepotty]

On 12/10/2023 at 3:38 PM, Bill Kielb said:

Sorry did not read the whole thread above…. Of course you use cones! Just a thought, some glazes will pinhole and not heal well when fired hot. Counterintuitive, they do liquify more but don’t necessarily spread out and heal as one might think. Drop and hold is often an effective way to get these types of compositions to heal.
I also assume you are firing cone 8 because it matches your clay and glaze maturity requirements. If not, firing too hot can have its draw backs.

Thanks Bill. Yes I am firing to cone 8

[Gonepotty]

On 12/10/2023 at 4:19 PM, Hulk said:

My (limited) experience indicates that pinholes can (also) be caused by gasses expelled by the clay.
In that case, I don't know if dropping and holding a) slows the gassing/fizzing down, b) allows time for the particle to complete its decomposition, or c) combination of both.
Any road, the glaze would have to still be fluid enough at the hold temp to flow back together...

Pinholing (digitalfire.com)

Other factors that may be in play:

hotter and/or longer bisque fire, to burn out offending/gassy bits more thoroughly;
supply adequate oxygen during bisque and glaze fires, to burn out offending/gassy particles more thoroughly;
ramping up the bisque fire time for thicker wares, as it takes longer.

Thanks very much

[Gonepotty]

On 12/10/2023 at 5:50 PM, Min said:

As long as the final segment is slowed down (as you are doing) then no, speeding up the second segment should't cause any issues. Have a look at the "slow glaze firing profiles" from L&L kilns, regardless of final cone reached the middle ramp goes at 400F / hr.  (204.4C / hr). 

A heads up though, when using a set temperature as your final cutoff point rather than a cone you will need to keep an eye on your results and or cones as your elements wear. Given that the kiln will need to fire longer to reach a set temperature the extra heatwork needs to be taken into account. If you look at the Orton cone chart you can see that for cone 8 (using self supporting cones) for the last 100C of rise using 15C/hr gives a top temperature of 1211C, at 60C/hr it raises to 1249C and at 150C/hr it needs to go to 1271C. So if you program in 60C/hr but the kiln can't actually keep up with that then the final heatwork will be greater. This is not a big deal if you are keeping watch on what your cones are telling you. 

One other point re your pinholes, and the possibility it is being applied too thick, are you measuring the glaze specific gravity? Is your claybody a grogged one? 

Thanks Min, all great tips and info. Yes I am measuring specific gravity - approx 1.45. Clay is slightly grogged. I think I will try make sure the glaze is not too thickly applied and see how I get on. 

[Mark C.]

weigh your glaze with a 100ml plastic syringe for quick gravity test

[Min]

22 hours ago, Gonepotty said:

Clay is slightly grogged

Some people find it helps to dip pots in water very briefly before glazing if the body is grogged. Theory is the water expels any tiny air pockets cause by the grog so when the pot is dipped in glaze there isn’t trapped  air which in turn can cause pinholes in the glaze firing. If the body is burnished with a rib after trimming this is reduced.