Single Fire Question

[Karlan Emery]

I am currently involved in sharing Steven Hill's cone 6 single fire method with my fellow students at a community college. Last semester, I followed all of Hill's glaze recipes and firing schedule and was thrilled when the culmination of my efforts resulted in some beautiful finished work. This semester right off the bat we encountered a firing issue....after 23 hours of firing (again, using Hill's single fire schedule) we had ramped to only 2016 degrees f. One of the resident instructors modified the schedule at segment 3, which is an increase from 500 degrees to 2100 degrees at 400 degrees per hour. He changed the temp increase to 270 degrees per hour and started the firing again. His comment was that the 400 degree increase was too hard on the elements. By the by, his attempt also resulted in a failed firing. My question is has anyone who is or has used Hill's single fire schedule encountered any similar problems? I think the kiln is the problem and that it is not necessary to modify the firing schedule as I have successfully fired in this same kiln with Hill's Surface Techniques schedule. Any feedback is appreciated.

[George Lewter]

According to the firing schedule published in Ceramic Arts Daily - Feb 2012, the heating up and hold at the high end should be over in 13 hours or less. (With Ramp 3 at 400/hour)

The free and controlled cooling with a hold at 1600 to the Kiln Off at 1500 degrees F should take less than 6 hours. 

The total time from Kiln On to Kiln Off should be less than 19 hours.

KILN ON

Ramp 1 - 200 deg/hr - Room Temp to 220 <1 hr

 Hold 1 @ 220 -  <= 3 hrs

Ramp 2 - 100 deg/hr from 220 to 500 <3 hrs

Ramp 3 - 400 deg/hr from 500 to 2100 = 4 hrs

Ramp 4 100 deg/hr from 2100 to 2175 <1 hr

 Hold 4 @ 2175 = 1 hr

Ramp 5 Fast cool to 1700 (assume 500 deg/hr) <1 hr

Ramp 6 cool at 100 deg/hr to 1600 = 1 hr

 Hold 6 @ 1600 <= 1 hr

Ramp 7 cool at 100 deg/hr to 1500 degrees = 1 hr

KILN OFF

My kiln is too under powered to rise 400 degrees per hour at the top of the heating curve. It will only go up a couple of hundred per hour above about 1800.  

[Victoria Cochran]

I am surprised (more like a bit shocked) to see 400/per hour from 500 to 2100.

1. Cristobalite transition at 439 degrees F (226 degrees C) Quartz inversion at 1063 degrees F (573 degrees C), require a slow ramp. Reforming of the quartz structure takes place at these points and if it is forced, the vitrified clay will have faults--commonly dunting (sharp cracks). I've always passed through the first at 100/hour and the second at no more than 150/hour.

2. Carbon coring: if you're passing that quickly through burnout stage (1200° F - 1400° F), it's likely (especially with red/brown/black clays) you've not outgassed all of your clay's carbon. It makes for a brittle black glass-like core that has very low strength/is easily broken, pinholing in the glaze. You may never see this if you don't break open a test piece, but the wear will not be durable and the fault will eventually reveal itself). Alternative, you get bloating. I wouldn't go faster than 250/hour.

This first stage of single firing is a *bisque firing* -- after you've passed it, glaze firing temps are ok-- usually 300/per hour.

[George Lewter]

Victoria raises Interesting points.

1. Steven's firing schedule handles the cristabolite transition with the same rise she uses.
2. It does appear that he ignores the convention to rise (and cool) slowly through the quartz inversion range. Perhaps his porcelain is more forgiving in this regard than many other clays? I've noticed that pieces made from B-mix5 have a pronounced tendency to crack across the bottom when I've tried to reglaze and glaze fire a second time. It causes me to think that quartz inversion is a cumulative issue that gets more severe through multiple heating/cooling cycles. It this is so, then single firing eliminates one of the cycles and possibly the the issue is much less pronounced than for those who bisque. - This is purely speculation.
3. I know he uses a manufactured porcelain exclusively as his white canvas to get better glaze color saturation, since Iron bearing stonewares tend to muddy glaze colors. If he isn't using a lot of recycled clay, I think he has few worries about carbon burnout.

Steven is often in workshop mode where he throws pots one day, quick dries, and glazes and fires them the next day. Perhaps his methods might not produce the highest possible durability, but the beauty of his work is undeniable.

We have other discussions of firing schedules going such as:

http://cone6pots.ning.com/group/sahillproject/forum/topics/cone-6-f...

http://cone6pots.ning.com/group/fireitonce/forum/topics/firing-cycl... 

[Victoria Cochran]

Yes, quartz inversion is cumulative; good point. And, using a porcelain does reduce the time needed for burnout. (If his work is thin-walled, this further reduces burnout time.)

Thanks for more info on his process-- I do think his methods are risky--for good/predictable outcome--in the school environment, where so many variables are introduced that are otherwise highly controlled in an individual setting (clay purity, thickness, load size, oxygen atmosphere, kiln element wear/damage). Also, there should be more info going out to students regarding the difference between decorative and functional ware, especially if they plan on selling their work. There is an awful lot of potential liability in the faults that may show themselves at a later date (consider families with children), as well as "false advertising" issues arising from claims around cone (an accurate cone is a vouch for low porosity and impermeability/safety of the glaze)-- I see too many kiln owners confusing pyrometer reading with cone/not using witness cones...which means their claim to "Cone X" is unsupportable.

[Maggie Jones]

Yes, students need to learn about quartz inversion and realize that single firing is a technique with its own properties.

My kiln too needs a slower ramping after the 1750f mark. 

But,,,I have learned from the digital fire folks that after hitting the target temp and a 15 min hold, dropping 100 degrees and holding 20 min will vitrify a clay to a much more mature state and smooth out glazes, pinholes and the like, remarkably. This practice has reduced worries about getting that last cone to bend properly.