Cones And Temperatures

[Joe_L]

I've only had my kiln a short time and mostly fired test tiles so far. I'm using an electronic controller for temperature control.  For the first time I decided to fire some actual pots as I wanted to see how some of the more interesting glaze results from the tiles would look on a real pot. I thought I'd also pop some cones in to see what they look like for the schedule I've been using (I'm doing this only for information and comparative purposes and not relying on the cones - the controller will handle repeatability for me).

 

This glaze firing was to 1240C at 130C/hr for the last few hours and then a 20 minute soak. I'm using standard large cones in a holder and read the Orton cone leaflets and decided that was probably about Cone 7ish:

 

The table says Cone7 is 1237C @60C/hr and 1255 @150C/hr.  Cone 8 is 1247 @60C/hr and 1269 @150C/hr.

If you interpolate this cone 8 would be something around 1260C @ 130/hr and cone 7 ~1245C. So I put in Cones 6,7 & 8.

 

After the firing all 3 cones were fully down, all lying horizontally on the holder like sleeping dogs. Obviously I can do it all again and use higher cones but I'm a bit surprised and want to understand a bit better. Am I mis-reading the table, or placing the cones badly, or would you expect this schedule to be >cone8?

 

The kiln is a new Rohde 43 litre. It wasn't heavily loaded - only 3 jugs on the bottom shelf and a few test tiles plus the cones on the top shelf in case that's significant.

 

By the way the pots came out really nicely so no problem there!

[bciskepottery]

Your 20 minute hold likely increased the heat work to the equivalent of Cone 8.

[Mark C.]

So cones measure the heat work like your 20 minute soak which adds a bit of extra heat work.

Firing is a time temp relationship not a static temp reading on an electronic gauge

Only cones do this exact - that is measure the time temp heat work on the wares in that fire

The cones never lie so you are firing higher than you thought

Put in some higher number cones next fire to determine what temp you are really firing to.

I,m a big believer in cones

The next closest acurate measurement is a S thermocouple but it will not measure heat work.

Most kilns have type k thermocouples and they are even less acurate.

[Min]

Does your kiln have a spy/ peephole? If it does then I would put a cone pack in front of it and monitor those towards the end of the firing. Kinda flying blind if you are just checking the cones after the kiln has fired. Sounds like you got to at least ^9 if the 8 was flat. What cone were you aiming for, ^7?

 

I’m glad your pots came out really nice : )

[Joseph Fireborn]

If your cone 8 was flat then your probably fired in the 9+ range. The soak can add a lot of heatwork to your pots depending on many variables(load mass, size of kiln, etc). To get an accurate cone reading for a preset temp you want to fire without a hold and no slow cool as that will effect cone readings as well. Another way is to do as Min says above and check the cones through the spy holes near the peak temp. This will give you an idea of the heatwork that the kiln is achieving at the preset without the soak. But remember that is just a snap shot of the heat work at that moment, as the kiln soaks the heatwork is still added to the cones. This is why cones are so useful because they tell you the true amount of heat that a pot takes. Remember that not all glazes work the same and some can go to high heat works and not have a problem, also some clays even if marked to a certain cone can handle higher temps without any real problems, some however can't and it will be obvious when they can't as they will bloat, warp and melt. 

 

If your kiln has a controller you can adjust the firing easily to get the temp that you want. It will just require testing. Your glazes will probably look different as that much heat work will change a lot of glazes compared to a lower cone. My glazes change a fair bit between 6 and 7. Some look better at 7 and some look better at 6. My usual firing goes to about 6.5 as my 7 is bent slightly.

 

I went through a similar process when my kiln started firing hot and my clay started bloating. I had to adjust my Cone offset for cone 6 by 30 degrees. Now it fires a perfect cone 6 without a soak or slow cool. However I have changed my firing process to cone 5 with a 50 minute soak, and I reach about cone 6.5+. I am much happier with the quality of my glazes at this temp and my clay body tolerates it fine so that is what I fire to.

 

There is no rule as to what temp you should fire to, if you find your work looks great at those temps then nothing is keeping you from firing that schedule, just be aware the hotter you go on your kiln the more wear on the elements. The same applies for soaks and slow cools as well.

 

So if you wanted to solve this issue, the first order would be to fire the kiln to the program you set with no soak. You could do this with just kiln furniture and cones. Put the cones in front of the peephole to check temp when it hits the highest temp you set. Then see how much is added when the kiln is cooling. Some kilns cool faster than others depending on size and mass in the kiln. Obviously if you have a full load that kiln will cool slower and add more heatwork to your pots, not very much though, but it is something to be aware of when you unload your cones and think your overfired slightly. 

[MatthewV]

A nearly unloaded kiln will fire differently -- usually hotter if I recall correctly. If there is only one probe in it, having a shelf somewhere above it will give more consistent results.

 

At the last studio I worked in a perfect ∆6 firing had to be set to a different temperature for each kiln. This included 1185°C with a 15 minute hold and another at 1205°C +15 minutes. These were Type-K thermocouples.

 

Overfiring: It doesn't surprise me that you liked the results. As long as problems with the glazes running or bodies bloating didn't happen, the glazes will be glossier and generally very pretty.

[Joe_L]

Thank you all for your replies. It sounds like I'm misjudging how much the heat work affects the cones. The Orton datasheet says "as a general rule, a 1 to 2 hour soak is sufficient to deform the next higher cone number." so I was expecting 20 minutes to only make a small difference, perhaps only 1/2 a cone. In reality it must be more than this, and probably exacerbated by a small load. Next time I run this program I'll put higher cones in.

 

I don't have a spy hole so have to rely on final results, but that's not a problem. I am planning to do extensive tests at different temperatures and schedules and use the electronic controller's magic to reproduce the results I like. As I said the test pots came out really well and don't look overfired so I don't need to modify the schedule.

 

With an electronic controller you don't really need cones to repeat a favourite firing of course but I decided to use them occasionally anyway because it gives me a way of comparing results with other people when discussing glazes later on and gives me a point of calibration in case anything changes with the kiln - aging elements and thermocouple etc.

 

On on on with the tests!

[Chilly]

 

With an electronic controller you don't really need cones to repeat a favourite firing 

 

 

Nooo, but......  

 

If you always use cones you will know when your elements or other parts of your kiln start to deteriorate.

[neilestrick]

 

 

With an electronic controller you don't really need cones to repeat a favourite firing 

 

 

Nooo, but......  

 

If you always use cones you will know when your elements or other parts of your kiln start to deteriorate.

 

 

I agree that you don't really need cones in a digital kiln. I've done 1100 firings in my smaller kiln over the last 7 years and I've never used cones. Cones are a good indicator of thermocouple drift and such, but I think that using them in every firing is unnecessary. A good glaze can handle a little variation in heat work, and if you know your glazes well, they can be just as good an indicator. A cone won't tell if you the problem is in the thermocouple or the elements, nor prevent an error code. But if you keep up on inspecting your thermocouples and keep track of how many firings you've done and check your element ohms you'll know if problems are coming. The only times I think cones are necessary are when you're using a custom program to a specific temperature and need to figure out what cone you're actually hitting, or if your kiln doesn't have zone control and you suspect it's firing unevenly.

[Min]

I'm assuming you are testing glazes for functional pots right? It's important to fire the clay to it's maturing temp if so. Are you using a ^10 clay? If so then going as hot as you did is probably fine but if it's a ^6 clay then sooner or later you will run into problems firing that hot.

[Callie Beller Diesel]

. The Orton datasheet says "as a general rule, a 1 to 2 hour soak is sufficient to deform the next higher cone number." so I was expecting 20 minutes to only make a small difference, perhaps only 1/2 a cone. In reality it must be more than this, and probably exacerbated by a small load. !

Yep.

This information is most certainly accurate when you're firing a tightly packed 20 cu ft gas kiln at high altitude, going to cone 10 temps. Sometimes it can take a while to shift those suckers. If you're dealing with a 3 cu ft electric, ten minutes is an eternity of a soak. You're not heating the same mass.

[Joe_L]

I'm assuming you are testing glazes for functional pots right? It's important to fire the clay to it's maturing temp if so. Are you using a ^10 clay? If so then going as hot as you did is probably fine but if it's a ^6 clay then sooner or later you will run into problems firing that hot.

That is a very good question!  I'm using Valentine's Toasted Stoneware which the manufacturer says is 1120°c - 1280°c and I'm firing to 1240. So far so good, but your question has prompted me to notice the datasheet mentions temperature and not heat work or cone equivalent. Hmmm I can see this is a rich minefield for my enquiring mind!

 

What do I need to look out for if I'm overfiring the clay body (as well as glazes running etc)? I've fired some test bars on stilts and only getting minor deformation on this schedule.

[MatthewV]

Overfired: the piece will start to slump, blister, or bloat. An earthenware clay fired to stoneware will be a messy puddle.

Underfired: not as vitrified. Water in a cup or vase will slowly leak through.

[neilestrick]

The Orton datasheet says "as a general rule, a 1 to 2 hour soak is sufficient to deform the next higher cone number." so I was expecting 20 minutes to only make a small difference, perhaps only 1/2 a cone.

 

I've done a number of soaking tests in my electric kilns, and combined with that experience and everything I hear about here on the forum, the degree to which soaking affects cones seems to be very inconsistent. I used to fire my big electric to cone 6, then soak for 40 minutes to achieve cone 8. Much shorter than the 1-2 hours you mention from the Orton sheet. When I started firing cone 6, I tried a similar soak to go from cone 4 to 6. It took 80 minutes, and the glazes did not respond well at all. Many were under-fired. And every couple of weeks here on the forum we hear about someone's kiln that accidentally soaked for several hours, with surprisingly few poor results. A couple of pots stick to the shelves, but by and large things seem to survive. Soaking is a strange thing, and while short soaks (under 20 minutes) seem to have noticeable affects on glaze melt, as the soaks get longer the effects seem to diminish. It seems that at some point you need higher heat, not just heat work.

 

Just my thoughts, which may or may not be at all accurate.

[Min]

That clay has a huge firing range of 160C. There is is no way it will be suitable for functional pots at the low end of that range and will probably be over fired at the top end, I have no clue what the optimal final cone / temp should be. 

 

The manufacturer is saying it will go from 1120C all the way up to 1280C, that would be from ^01 all the way up to ^10.

 

Clay for functional ware needs a tighter range,  midrange would be approx ^5 - 7, and high fire approx ^9 - 11. So, in Celsius, (using a ramp of 60C for the last 100C of temp rise), the span for midrange would be 1184C - 1237C (53C difference), for high fire it would be from 1257C - 1293C (36C difference). The Valentine Toasted is supposed to work with a 160C span, don’t think that’s possible.

 

It might be fine firing it to 1240C, only way to find out is to test some samples. I would run shrinkage, warping and absorption tests. The absorption test is really important for functional ware. If the absorption is too high either the pots will leak or the glazes will craze due to moisture in the body. 

[Joe_L]

Min, that's interesting, particularly your comments about functional ware. I'm also using Earthstone ES5 but at the moment playing with the Toasted for the colour.

 

I've measured shrinkage at 14%, warp about 2.5mm droop on a 120mm test bar suspended on stilts at 100mm spacing, absorption about 2g for 44g bar after 24hrs immersion (but my scales resolution is 1g so this could be anything from 1.5g to 2.5g)

 

For comparison ES5 measures at 12%, <1.5mm, 51g/48g (both clays bisqued at 999 then 2nd firing at 1240 soak 20mins). Testing method following Paul Wandless's article on this site, sorry can't find a link to it at the moment.

[Min]

So, using your absorption figures the Toasted clay is somewhere between 3.4 and 5.68% for how you fired your test. If it's 3.4 then absorption is probably okay but not so much for higher than that. I would make 3 or 4 test cylinders, (they can just be tiny ones, like a couple inches across and tall) and fire them in different parts of the kiln in the same way you would fire a glaze firing but don't put any glaze on them. After firing fill them with water and leave them on a sheet of newsprint for a week. If after a week the paper under the pots doesn't have any wrinkles then you are probably fine.

[Joe_L]

ok I'll give that a try.

 

And thanks again to everyone for your comments, I'm learning so much. When my wife suggested to me to get my own wheel and kiln I don't think she quite realised what was going to be involved!