Firing cycle for single firing

[Bill Curillo]

Hi  I want to give raw firing a shot, cone 6 electric ( of course!) I have a digital programmer> I know the cycle is longer than a biscut firing> Can I have some help with some ideas on what type of sequence would be a good starting point. I know that one must jump into the water to learn to swim, but a little guidance to get me started would go along way. I will be glazing my pots in the bone dry stage.
Thanks

Bill

[George Lewter]

Suggested schedule for a cone 8 firing.

1 200 deg/hr to 220 hold 1 to 3 hrs depending on wetness and thickness
2 100 deg/hr to 500
3 300 deg/hr to 1700
4 100 deg/hr to 2250 hold for 1 hr till cone 8 falls (for cone 6 maybe hold around 2120 until cone 6 falls)
5 free fall to 1900
6 100 deg/hr to 1600 hold 30-60 min then cool naturally.

Above is single fire schedule from a Steven Hill 2009 workshop handout. It is not cast in stone. The most critical period is at the start of the firing when you are driving off water from both the clay and the glaze.

[Victoria Cochran]

Carbon burn out is occurring through 1400 degrees. More carbon is burning out of stonewares, especially heavily red, than out of porcelains. So, the more "impure" the clay, the more time you should allow for burnout. Burnout also takes longer for thicker pieces. Adequate venting is important; makes sure peep holes are open through burnout. I leave bottom and top open so that there is airflow, then close bottom at the last ramp.

If you go too fast, two things can happen. The clay can begin to vitrify before all gases are burned out and you will get bloating. If gases are still exiting as glaze is forming, you are likely to get pinholes. (I share both of these because they will be indicators, if you get them, that you've gone too fast.)

I'm still firing manually, infinite switch! For porcelain at 1/4" thickness to Cone 6:

1hr Low
1/2hr Low/Med
1hr Med
1hr Med/High
1hr High (cherry red at this point: burn out zone)
2.8 hrs High Fire

This is basically a bisque firing, followed by a climb to vitrification temps.

[Bill Curillo]

Thanks George and Victoria. Really useful information. I note from the Steven Hill article I found from your web site link, that he uses B mix which is also a white clay body. I would imagine that the low impurity factor is one of the reasons he uses it.
My clay supplier doesn't even recommend once firing because of all the pinholing. When I spoke to them , I came away very discouraged, feeling there would be lots of glaze quality compromises. But I remember what Paul Soldner would tell his students when they asked if this or that would work? Why don't you try it!
Another question please. Would an open clay body be better suited for single firing to assist with driving out the water and outgassing during firing? B mix is not really a gritty clay body, so coarseness may not be a requirement to successful results. Also, I note Steven Hill now pours glazes on the interiors only and sprays on the outside. Does the inside pour create problems because of the heavier application( as opposed to spraying)? It's next to impossible to sprays interiors.

Thanks again for the help. I hope I can help someone here at some point.

Bill

[Victoria Cochran]

Found a concise, informative summary on bloating and carbon core, pertinent to single firing at Laguna's site: http://www.lagunaclay.com/support/bloating-and-blackcoring.php

We were still having core, despite dramatically dropping bisque firing ramp through burnout temps. Key is plenty of available oxygen: the organics can't be burnt out until they combine with sufficient oxygen (you can imagine the packing for a series of college classes, not to mention an insistence on tradition that all peepholes but one remained plugged!).

[George Lewter]

Thanks, Victoria for linking to that article. I was not sure what bloating is until I read it. It is most interesting that this is a firing related problem, yet the defect produced ends up being pretty much the same as I have described when raw glazing bone dry greenware. That post is copied below.

"I fired another load that contained some raw glazed pots, and though the glaze effects were nice, I had some serious body issues. Richard Bush Nutmeg glaze and an accent glaze of Varigated Slate Blue from Hesselberth. I poured the interiors and dipped the exteriors and poured the accent color around the outside of the rims. It didn't take long until I saw blisters appearing mainly where the pots had been wetted three times. I was able to press the blisters flat but the clay was delaminated and it got worse again in the firing.

As you can see, the blistering was pretty severe, and was present in all the pieces with this glaze combination. There was one blister in one of the mugs where only the Nutmeg glaze was located."

From now on I will refer to bloating when talking about expanded voids in the clay body, and reserve blisters for expanded voids in the glaze. I do want to emphasize that bloating is now known to have this wetting cause when raw glazing, and it is separate and distinct from the firing cycle related cause.

[George Lewter]

The following were moved from General Comments on the Single Firing Group main page to this discussion topic where they belong.  Please take no offense. This is just me acting in an editorial role to try to maintain a degree of organization on the network.

Comment by George Lewter just now

Re: Kathy Ransom's question, "I know that I need to go slow until at least cone 06 and I wonder if I need to go slow again on the cool cycle?"

After carbon burnout on heating up (1400 deg F to be on the safe side), you can complete your firing on your regular firing schedule, for at this point there is no longer any reason for a difference between single firing and glaze firing on bisque ware.  Put another way, cooling depends on what glaze effects you are going for, rather than something different about single firing.

Comment by John Lowes 3 hours ago

re: Kathy Ransom question

I have fired a full kiln of single fired ware recently (my first) and I used my bisque schedule as if firing to Cone 04.  My kiln is an Olympic 25 Oval and is kiln sitter equipped and has an Orton vent.  It has three L-M-H switches, and I turn one up per hour going bottom to top. It takes 8 hours to go from cold to all switches on high, then 5 more hours for this load to bend Cone 7 in the sitter. When the sitter dropped, I fired down using the bottom two switches on medium and the top on low to achieve a 150 deg F fall to 1,650 deg F (about 4 hours going down) and then shut off everything.  I had some iron saturate red in the load I wanted to slow cool.  When I opened up 24 hours later the load was just fine.

Comment by Kathy Ransom 4 hours ago

I have done a small amount of single firing with pretty good success and will soon be doing an entire kiln load because Christmas is coming and I'm not ready!  I don't have space for a spray booth so I'm painting my glazes on and letting the pieces dry thoroughly before applying a 2nd coat and firing.  I know that I need to go slow until at least cone 06 and I wonder if I need to go slow again on the cool cycle?

[John Lowes]

re: Kathy Ransom question

I have fired a full kiln of single fired ware recently (my first) and I used my bisque schedule as if firing to Cone 04.  My kiln is an Olympic 25 Oval and is kiln sitter equipped and has an Orton vent.  It has three L-M-H switches, and I turn one up per hour going bottom to top. It takes 8 hours to go from cold to all switches on high, then 5 more hours for this load to bend Cone 7 in the sitter. When the sitter dropped, I fired down using the bottom two switches on medium and the top on low to achieve a 150 deg F fall to 1,650 deg F (about 4 hours going down) and then shut off everything.  I had some iron saturate red in the load I wanted to slow cool.  When I opened up 24 hours later the load was just fine.

[Kathy Ransom]

I have done a small amount of single firing with pretty good success and will soon be doing an entire kiln load because Christmas is coming and I'm not ready!  I don't have space for a spray booth so I'm painting my glazes on and letting the pieces dry thoroughly before applying a 2nd coat and firing.  I know that I need to go slow until at least cone 06 and I wonder if I need to go slow again on the cool cycle?

[George Lewter]

I'm doing a single fire today with my new Orton controller - No constant messing with knobs! This is a mixed load with Steven Hill and other glazes.  My program for today is: (Temperatures given in Fahrenheit)
150 deg./hr to 220 deg. and hold for 1 hr (preheat)
200 deg./hr to 1400 deg.
100% power to 2145 and hold for 1 hr. (My kiln is underpowered and only gains temp very slowly at the top so I reduced max temp 15 deg.)
0% power down to 1700 deg.
100 deg./hr down to 1400 deg. and hold for 1 hour.
Kiln off.

Currently I'm at the maximum temperature 1 hour hold and I'm thrilled to see the controller holding to within 1 degree plus or minus of my set temperature of 2145.  I never have achieved that tight of control when trying to hold manually.

Pictured here is the controller mounted temporarily on a shelf.

[George Lewter]

This is my current firing schedule for newly glazed and still damp ware. I'm currently using Laguna B-mix 5 clay. I have had no body defects in my recent firings. (Units are in degrees farenheit)

Ramp up 100/hr from room temp to 220. (With kiln top propped open, peep hole plugs out, vent on)

Hold at 220 for 2 1/2 hrs.

Ramp up 200/hr to 1400 - No hold here.  Close kiln (lid and peeps) around 800 & make sure vent is running negative pressure inside kiln (should pull smoke into kiln at a small hole in the lid) 

Ramp up at full power to 2140 (Use cones to adjust this temperature setting for your kiln to achieve cone 6 after your one hour hold. Your kiln/pyrometer/controller combination may necesitate a considerably different maximum/hold temperature) This schedule gave me cone 6 touching down and cone 7 starting to bend.

Hold at 2140 for 1 hour. 

Ramp down at free fall to 1750.  You may want to slow this ramp for matte glazes to have time to crystalize out.

Hold at 1750 for one hour. I use this hold for iron red color development.

Kiln off and free fall to room temperature or cool enough to unload.

[George Lewter]

This is the Steven Hill firing schedule copied from the Oregon Potters Association newsletter from Feb-March 2011. It is probably outdated as he tweaks that kind of thing very often, but it gives you a place to start.  It is NOT CAST IN STONE!

STEVEN HILL'S FIRING SCHEDULE: Steven Hill, as you may be aware from his ads for Skutt Kilns, has recently begun firing his work in electric kilns. He is know for his raw glazing (not bisquing his work prior to glazing). The following schedule is used in his Skutt 1027 kiln. Temperatures are degrees F.

Segment:

Ramp: End Point: Hold: 

1   200/hr 220  1-3 hrs, depending on wetness/  thickness of work 

2  100/hr 500  0 

3  400/hr 2100  0 

4  100/hr 2180  1 hr, according to Skutt manual this end point is cone 5, but with 1 hr soak cone 6 falls 

5  9999/hr 1700  0 

6  50/hr 1600  30-60 minutes, then cool naturally  

“As of 10/10, this is my schedule for a ^6 single firing of greenware. Nothing is static with my firing schedules, however, so it will most likely change in the future. One thing is certain… there is no sense in extending the firing beyond what is necessary for desired results, as this is wasteful of electricity. As I do further experiments, I will work on shortening the firing cycle.” Steven also mentioned that he was constantly changing his cooling cycle, and would probably never settle on one particular cooling schedule. He is working on an article, probably for Ceramics Monthly, about his new electric firing techniques.

His probably schedule for a firing of bisqued glaze ware is: 

Segment: Ramp: End point: Hold: 

1  200/hr 220  30 minutes, this is only necessary if you have raw cone packs 

2  400/hr 2100  0 

3  100/hr 2180  1 hr, according to Skutt manual this end point is cone 5, but with 1 hr soak cone 6 falls 

4  9999/hr 1700  0 

5  50/hr 1600  30-60 minutes, then cool naturally

[matthew mejia]

once you have hit cone six and the cone sitter is triggered... are you simply leaving the kiln to cool on its own or are you down firing in some manner?

[Karen Mandeville]

I'm in the same boat as you, my kiln is loaded all I need to do is fire it.... sooner than later!

[Simon]

It's an old thread, but thought I'd add my 5 cents anyway ;)

On the initial stages of a once fire, the 'drying out' phase, I personally try to stay clear of having a precise time in mind. Yes, good to know for planning (will it finish before I'm too tired to stay awake !?) .. but inaccurate in reality.

Different kilns, different loads, different pots, different weather ... all lead to variations.

Think of the 'reason' you are holding the temperature, it is to let all the water out of the clay/glaze before moving onto the high temps where it would otherwise cause explosions.

Hence, try to only move on from the initial drying temperature when the water has indeed finished escaping, for fact.

A cold piece of mirror/steel held over the vent of your kiln for a second or two, will mist up and be wet to the touch when the water is being pushed out.

Doing this at regular intervals during the drying stage, you will also get a good feel and connection with what is happening inside, you'll see it being lite, become heavy and saturated, and yes eventually decline.

It's a nice feeling to be 'in touch' with the physical events rather than simply watching the hand on a clock.

Yet most importantly, you will know 'for sure' when it is good and safe to ramp up to the next temp.

I could go on and on, but try to connect with your kiln in a similar fashion at other stages of the cycle.
Yes you will need a time schedule in the early days just starting out, but with time you will be able to judge for yourself how to tweak for individual circumstances.

Look through the spyhole and note when the atmosphere becomes hazy, when clear, the textures and reflectance of the glazes.

Anyway, I'm rambling now !

.. happy firing :)