Kiln is not reaching cone 6 temperature

[neilestrick]

2 minutes ago, elenab said:

I checked videos on Youtube, potters show that the glaze is drying within seconds, mine doesn't even with severe dilution!

How hot did you fire your bisque?

[elenab]

Cone 6 temperature. I don't measure the temperature, I only use a kiln sitter. My glaze is Amaco Snow, maybe it's a glaze?

[Bill Kielb]

1 hour ago, elenab said:

Cone 6 temperature. I don't measure the temperature, I only use a kiln sitter. My glaze is Amaco Snow, maybe it's a glaze?

Bisque firing (your first firing) is generally done at a low temperature like cone 04.  The leading zero is very important, it is much cooler than cone 4. The purpose of the bisque firing is to sinter the material together and burnout organic impurities which could off gas and affect your finished glaze results. The low temperature or bisque firing leaves the clay in a state where it is very absorbent and ready for decorating.

Do you bisque fire and if so what cone do you bisque to? 

[neilestrick]

1 hour ago, elenab said:

Cone 6 temperature. I don't measure the temperature, I only use a kiln sitter. My glaze is Amaco Snow, maybe it's a glaze?

If you bisque fired to cone 6, then your clay is no longer porous and will not accept glaze well. You need to bisque fire to a much lower temperature like cone 04. That will harden the clay but it will still be porous for glazing.

[elenab]

Hello everyone! I had my pottery on hold for some time, but I started firing a few months ago and new problems emerged. Ironically, my kiln is overfiring now.

I fire porcelain and stoneware cone 6 with Amaco glazes and glazes that I made, my bowls are warped and sagged, and the glaze is all bubbles. I understood that firing was too fast, my kiln gets to cone 6 temperature in 6 hours. I will fire slower next time.

But for now, I would like to figure out how to find the true temperature of the kiln. I have a cheap pyrometer and thermocouple. I stopped using them because the pyrometer was showing a much higher temperature, which seemed unreal to me. However, we checked the wires on resistance (or something) and it was good. I thought of buying witness cones and using them with a pyrometer, recording the data, and using those temperatures for firing. Basically, it is ignoring the kiln sitter. But why would the kiln sitter allow overfiring? Should the holes be open all the time - would it stabilize even temperature in the kiln? I don't fire a lot, two or three bowls at a time, so there is a space for air circulation. 

Please, let me know if you have any suggestions. Thank you!

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[Hulk]

Good questions.
Good to be back at it too!
I had some big gaps over the last fifteen months, ramping back up lately.

Given your pyrometer gives consistent readings, you may be able to get very close to the desired heat work by correlating the temperature and time to the cones' bends.
Your goal, very likely, is "heat work," not so much temperature.
A kiln that heats up quickly (over the last hundred degrees or so) will either run up to a higher temperature, or hold at peak temperature, to bend the cone, where a kiln that heats up slowly at the end will bend the cone at a lower temp.
Pyrometric Cone (digitalfire.com)

Some atmosphere exchange is likely good, allowing fumes to escape and oxygen in.
Likely one peep open - typically the top one - not all.
The heat, especially once past low red (cone 6 is light orange to yellowish), is getting to your wares almost all by radiation, very little convection.
Expect that there will be hotter and cooler areas in the kiln. This can be mitigated somewhat by:
  loading the hotter areas with more ware, shelving, and posts (iow, mass);
  loading the cooler areas with less ware - tall is good, for shelves are very heavy;
  manipulating the controls.

My first kiln, fitted with a sitter, was controlled by three switches, each with off, low, medium, and high settings.
First firing, way overfired.
Subsequent firings, I watched cone packs set by the peep holes*, took notes (times, temps - via brand new pyrometer, switch changes, final results) and tweaked the setter until it was fairly close.
After several firings, I was confident that reading the pyrometer was sufficient, however, I continued to place cone packs in each firing and note results.
I bought the portable pyrometer Skutt sells (it's a TPI 343** )

I never did trust the setter to shut down when just the amount of heat work I wanted was achieved.
The setter mechanism, to me, is a failsafe, which will shut the kiln down (either by the cone melting, or the timer expiring) in the case I fall asleep or somewhat***.
That said, there are many potters that trust their sitter and get good, consistent results.

See online tutorials for repairing, refurbishing, and adjusting your sitter.

*kiln glasses are super important for protection against harmful rays and any superheated flying bits! !!
** 343 Dual Input K-Type Thermocouple Thermometer with Field Calibration | TPI USA (testproductsintl.com)
***I'm also watching the new kiln, which is fitted with three zone automated firing control, and taking notes.

 

 

 

Edited January 19 by Hulk
punctuation

[neilestrick]

7 hours ago, elenab said:

But why would the kiln sitter allow overfiring?

If the sitter isn't calibrated properly or the sitter rod is bent then it could over fire. It really shouldn't be more than 1 cone at the most, though. If you're loading the cone wrong that could also cause problems.

I would try the pyrometer with witness cones and see how it goes. Leave the kiln on medium as long as possible, like until the rate of climb drops below 50C/hr, before turning it on high.

[elenab]

Thank you, guys. Great links, Hulk. I checked my sitter, looks ok, and feels ok... For now, I will assume that the sitter is ok, I will slow down firing, accenting on medium heat,  I will have one hole(top hole) open all the time while using the middle hole for the thermocouple. Would it be ok for the thermocouple to be near some draft air? We could make a new hole anywhere else in the kiln...

I ordered witness cones, but I just couldn't digest the notion that one cone in the kiln sitter and another one on the shelf in a closed kiln with more than 1200C might behave differently, even taking into account that witness cones are melting in 9 d lower temperatures. I can't believe that 9 degrees makes my bowls melt and glaze bubbling...

In both cases when I fire the kiln fast (overfiring) and slow, the difference between the witness cone and the sitter cone would be 9 degrees plus a few more because of the location of the witness cone. I believe in both cases witness cone is insignificant. The total difference will be too small compared to the final >1200 degrees. In my next firing, I will put sitter cone as a witness and I will measure the difference.

Before the kiln sitters, how did potters control their firings? Did they have witness cones?

Also, I do not understand the mechanics of using 3 witness cones. I do understand that cones can help to understand a heat 'map' of the kiln. I don't understand why to use 5,6,7 witness cones when firing to cone 6. Cone 7 will be always standing high, and cone 5 melted,  am I right?

Yesterday I found out about temperature measuring rings. They literally can measure the temperature of the items in the kiln. Now I realize that the temperature might not be the most important factor.  In the firing process, what is the one main factor that is far more important than others? Is it a certain amount of heat 'spreaded' in a certain time?  If the answer is yes, then all I need to do is to slow down the firing and wait for the witness/sitter cone to bend.

If I fire fast with a witness cone 6, would my wares warp and glaze bubble? 

 

[Hulk]

Temperature can be measured fairly accurately*.
For our ceramics, it's time and temperature, which is typically called "heat work."
I'm about to head out for the day, will check in again later.

*precision typically costs.
For our kilns, consistency is likely more important than accuracy.

[Bill Kielb]

3 hours ago, elenab said:

Yesterday I found out about temperature measuring rings. They literally can measure the temperature of the items in the kiln. Now I realize that the temperature might not be the most important factor.  In the firing process, what is the one main factor that is far more important than others? Is it a certain amount of heat 'spreaded' in a certain time?  If the answer is yes, then all I need to do is to slow down the firing and wait for the witness/sitter cone to bend.

Hopefully this is more helpful than mysterious. Our clays without fluxes would not melt at a reasonable kiln temperature. So proper full melting is achieved using fluxes and raising the energy of the fluxed mixture enough to get everything to melt. The melting process is not instant so it takes a combination of time and temperature to get this to happen. This predictable melting takes place in the last 200  f of the firing. Cones are made of glaze (including flux) and made to bend with the proper amount of temperature and time at their stated cone value.

So a very cool way to measure this total amount of heatwork quite accurately, with one requirement. The final segment speed matches what is written on the Orton cone chart. The final segment is an educated guess as to when we have enough energy to begin these fluxed reactions. Current Orton cone charts I believe set this in the final 200 degrees of the firing, then establish based on the speed being fired what the finishing peak temperature will be.

So the most important part is the final 200f and the speed you go in the final segment. 108 degrees per hour gets you the center column of the chart. Overall though, the faster you fire the more uneven a kiln can fire. Slower rates generally lead to potentially more even firings than very fast rates. For cones though and finishing, the final 200f is most important.

Oh before kiln sitters and cones etc… potters learned by color of the firing. @Pres can still do this I think (appropriate IR eye protection) sadly in the old days this was a really good skill but without eye protection a likely short lived one.

Edited January 19 by Bill Kielb

[Pres]

Peep holes of newer kilns are smaller, However I still check color from time to time using a mirror on a stick with a flex arm. This allows me to get the right angle to see inside especially if looking for a cone. In truth though this is not needed on my new kiln as I find it fires SPOT ON!

 

best,

Pres

[neilestrick]

5 hours ago, elenab said:

If I fire fast with a witness cone 6, would my wares warp and glaze bubble? 

Cones react to heatwork, which is temperature over time. But just because the cone melted doesn't meat your glaze will be happy, because firing too fast can cause problems. When glazes melt, they bubble up and then settle down and smooth over. If you're firing too fast they may not have time to smooth over, and they end up cooling with bubbles. You can also get bubbles from over- firing, however in my experience glazes are more likely to simply run when over-fired, unless it's an unstable glaze to start with, or two layered glazes that don't agree with each other. Warping can be caused by over-firing, but it could be related to the construction of the piece, thickness issues, etc.

Witness cones are the most accurate method of measuring heatwork, so trust what the witness cones say. The reason we use 3 is so we can see just how under- or over-fired it is. Was it a half cone or a full cone too hot? Having the next cone will help us answer that.

[neilestrick]

Think of cones and heatwork like cooking a roast. You can cook it at 350F for an hour, or you can cook it at 275F for 4 hours. Both get the roast cooked through, but the slower cooking tastes better.

[elenab]

this! is exactly what I was thinking

when Pres said "SPOT ON', honestly, i got  nervous. I don't know what it means.  I am only learning how to 'cook' clay here.

[elenab]

Good news, learning is infinite. I fired a stoneware bowl with black Amaco glaze on it. Lots of bubbles, but no distortion of the shape. I was able to fire slowly to 1000C, but after that, I just couldn't control it - the speed of firing was much more than 50C per hour. I do not have a digital control, remind you. I tried to open the top peephole, open the lid a little, put it back on medium, and combine some of the above, but still, I wasn't able to get slow firing up at a temperature above 1000C.  And that was my lesson.

I am shopping for a controller now. 

I also wanted to mention, that it looks, like Bill said, that the last 200 degrees are what matters most. It took more than 14 hours for me to slowly bake my clay, but it didn't help because, in the last hours, my kiln wasn't cooperative.