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yarddog

Firing Schedule, Cone 6 Glazes

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yarddog    1

Anyone advise me on the advisability/dangers to fast firing of cone 6 oxidation glazes? I don't mean soak or hold times, just how quickly I bring the kiln up to temp.

What little training I ever received in firing only differentiated bisque and glaze firings by the desired temp or cone, not the length of the firing. So I've always ramped up slowly, bisque or glaze.

My consideration here is not just time, but the expense of lengthy firings. If I can shave a few hours off a glaze firing without compromising my results, then it just makes sense.

Any advise much much appreciated.

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Chris Campbell    1,081

Apologies if you have heard this before, but "Mastering Cone 6 Glazes" is a fabulous resource text, well researched so you are reading facts rather than opinions. I don't have my copy beside me now, but as I recall, they address firing schedules with images to show the differences.

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Bill T.    4

Here is the MC6 suggested firing schedule (with a slow cooling if you want to use it).

 

                    Deg F/hour          Temp & hold

Ramp 1           100                       220/0

R         2          350                      2000/0

R         3          150                     2190/15       or where cone 6 is bent correctly, depends on your kiln

                                                                         mine bends at 2185

To continue with slow cooling cycle

R         4            -500                   1900/0

R         5            -125                    1400/0

 

 

"For bisque firings we recommend slow firing (100C, 180F per hour from 100C to 900C, 212F to 1650F).  This is when chemically combined water and organics are burn off.  This is especially important with iron bearing bodies to stop any reduction of iron which can over flux clay."  " For bisque firing a 5-10 minute soak at the end of the firing cycle can be advantageous......."   MC6 John Hesselberth & Ron Roy.    

 

My glaze firing with at Skutt 1027 pretty full is about 8 1/2 hours and bisque about 12 1/2. 

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yarddog    1

Perhaps I should ask it this way: assuming the bisqued ware has been glazed a few days ahead of time and is thoroughly dry, why not just power through with a very fast firing up to 2000, then slow way down at R3, soak at 6, and follow with a slow cooling?

My electric bill, and my not-so-green footprint, would both benefit...

I guess a small test kiln could answer it for me, but I don't happen to have one.

And thanks, Chris, for reminding me about the Hesselberth/Roy book. Reading it right now.

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neilestrick    1,379

Depending on the size of your kiln, firing really fast can cause problems with unevenness if you don't have zone control. But slowing down for the last 240 degrees should even it out.

 

As for the firing costing less, maybe maybe not. In a slower firing, the elements are cycling on and off. In a fast firing they are staying on longer, or even all the time. So which uses more- less usage for a longer time, or more usage for a shorter time? Hard to say without putting a meter on it. Over all, the cost of firing an electric kiln is pretty minimal compared to the value of the ware you're firing. So while you don't want to waste electricity with unnecessary really slow firings, the difference between an 8 hour firing and a 5 hour firing will be quite small.

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Stephen    139

Well one problem would be the possibility of blisters although I think this is more likely with over firing. I have not seen this myself but have it down as one of my concerns from research on firing schedules. I don't think the answer is universal so I would not just load up a kiln load of ware and fast fire it until each of your glazes have been tested with your new firing schedule. Looked at another way, I think a ramp firing schedule will not cause problems with any glazes whereas an all out to 2000 schedule may cause problems with some glazes so I would not do it. It cost so little to fire an electric kiln in comparison to the time to make the ware I just don't see the point.  

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neilestrick    1,379

We have a little manual baby test kiln in my studio that fires very fast, like 4-5 hours to cone 6. You can slow down the beginning and middle of the firing, but once it's on High it just goes like crazy. Most of my studio glazes come out awful in it. It does okay for small pieces like jewelry, which is what we use it for mostly, but for pots it's no good. I've recently hooked up an external digital controller to it, and with a fast ramp program and slow cool I can totally match the results from my giant kiln.

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neilestrick    1,379

In the baby kiln? I've done a very simple program of 400F degrees per hour up to 2000F, then 120 degrees per hour to 2232F (cone 6). So about 7 hours total. It's really the rate of the last couple hundred degrees that are most important. I also add a cool down of 175 per hour to 1400. In my larger kilns I go to 1500, but they cool much slower at the low end. Many cooling programs say to crash cool to 1800 or so and then do the controlled cooling, but my kilns are such different sizes that the crash cool rate is radically different, so I do the controlled cool from the peak temp. It's about a 4 hour cooling cycle, but at the low end the big kiln (and to some degree the medium kiln) is hardly cycling at all. I've found that my baby kiln (1 cu/ft), medium kiln (4 cu/ft), and big kiln (21 cu/ft) have nearly indistinguishable results this way.

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MichaelP    21

Thank you. I more or less follow the "Mastering Cone 6 Glazes" schedule with my 4.6 cu.ft kiln, and the whole cycle takes about 12 hours  (IIRC, 8 hours of firing plus 4 hours of gradual cooling to 1400F). Yours is 11 hours or so.

 

The highest increase rate the authors suggest is 350F/h, but I strongly suspect they had a larger kiln that was unable to heat faster. Mine can. But based on what you said, you wouldn't exceed 400F/h, right? I have a gut feeling that bad results in your small kilns were not because of the high initial rate, but rather high rate up to cone 6. Your final temperature had to be quite high to compensate for loss of exposure time, Maybe the glaze didn't like this (although, stricktly speaking, it was Cone 6. Just one with a high temperature and short exposure, vs. the "standard" 108F/h... or even 270F/h if your kiln was able to achieve this rate above 2000F). Questions, questions...

 

Gradual cooling is hard on relays... (I didn't check what type of relay my Skutt uses, but it doesn't sound like a mercury relay). And it's hard on my patience, :)

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bciskepottery    925

The highest increase rate the authors suggest is 350F/h, but I strongly suspect they had a larger kiln that was unable to heat faster. Mine can. /quote]

 

A couple years ago John Hesselberth sold the kiln used for testing their glazes; it was a 7 cu.ft. or so kiln common among potters.

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yarddog    1

It seems that on some issues potters tend to be fairly certain of their positions, while rarely agreeing with one another...

My old Gare defaults to 400F/hr until it gets within a couple hundred degrees or so of the target temp, when it slows down and tiptoes to the end. So it appears that I am already firing toward the high end of the recommended rates.

Experimentation would be best, bu at 7-plus cubic feet, my kiln is by no means large but certainly big enough to preclude a lot of test firings at different rates.

thanks for all the input. I guess for now I'll stick with the current ramp, and continue to holler at my granddaughter to turn off her lights and close the damn refrigerator door.

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neilestrick    1,379

Thank you. I more or less follow the "Mastering Cone 6 Glazes" schedule with my 4.6 cu.ft kiln, and the whole cycle takes about 12 hours  (IIRC, 8 hours of firing plus 4 hours of gradual cooling to 1400F). Yours is 11 hours or so.

 

The highest increase rate the authors suggest is 350F/h, but I strongly suspect they had a larger kiln that was unable to heat faster. Mine can. But based on what you said, you wouldn't exceed 400F/h, right? I have a gut feeling that bad results in your small kilns were not because of the high initial rate, but rather high rate up to cone 6. Your final temperature had to be quite high to compensate for loss of exposure time, Maybe the glaze didn't like this (although, stricktly speaking, it was Cone 6. Just one with a high temperature and short exposure, vs. the "standard" 108F/h... or even 270F/h if your kiln was able to achieve this rate above 2000F). Questions, questions...

 

Gradual cooling is hard on relays... (I didn't check what type of relay my Skutt uses, but it doesn't sound like a mercury relay). And it's hard on my patience, :)

 

Exactly. My big kiln certainly can't go 400F/hr. My middle kiln can, and the baby kiln does it easily. And yes, it's that final ramp that made the glazes look bad. There was no easy way to slow that down since it was a manual kiln. Goes to show that cone 6 isn't always the same. How you get there has a lot to do with the results.

 

There hasn't been a noticeable decrease in the life of my relays since I started using a controlled cooling schedule.

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Marcia Selsor    1,301

There is a great article on "Firing Up and Down" in the recent Jan/Feb Pottery Making Illustrated by Deanna Ranlett. It includes some very distinct color differences from a medium firing schedule to down firing. Photos and ramp information included.Good article.

 

Marcia

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bciskepottery    925

Looking at various models of kilns (morbid curiosity) shows the several factors may influence rate of climb. My L&L e23T3 is rated for 11520 watts per hour and could fire to cone 10. A smaller L&L e18t3 is rated for 8400 watts per hour, also cone 10. A small L&L test/doll kiln dlh11 is rated for 2160 watts per hour using 120 v supply and 2800 watts per hour using 208 v service (and could go cone 10). So, rate of increase possible appears influenced by several factors, including top cone firing, watts per hour for elements, and poqwr supply (120 vs. 208 vs 240 and single vs three phase).

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neilestrick    1,379

Watts are what bring the heat. When run at the same voltage and phase, smaller kilns tend to have more watts per cubic foot than larger kilns. Watts = amps x volts. If you look at the L&L e23T, it uses 11520 watts when wired at 240 volts (240 x 48amps) = 11520. The E28T has the same wattage because they have to stay at 48 amps or less in order to put a plug on it. But the E28T is rated for cone 8, because 11520 isn't enough watts to get to cone 10 in a kiln that size. So less wattage per cubic foot in the larger kiln, which is also why they can't heat as fast. Skutt 1027 vs 1227, or any brand of 10 vs 12 sided, 27 in tall kiln, with a 50 amp plug, are the same way. Most brands make a higher amperage, and therefore higher wattage, version of their 12 sided kiln, to give it more power. But these kilns must be hard wired due to the higher amperage.

 

When you run a kiln at 208 volts, single phase, it comes out even worse. 208 x 48 = 9984, which is not enough wattage to get an E28T or KM1227 (12 sided, 10 cubic foot kiln) to cone 10, or even cone 8. These kilns are rated at cone 5.

 

If you go up to 3 phase power, staying under the 48 amps is simple because the power is divided out to 3 hots instead of 2, so the amperage is lower at each leg to create the same wattage. They can increase the wattage and still stay under the 48 amps. An L&L E28T at 240V, 3 phase uses 16620 watts (240 volts x 40 amps x 1.73 (3 phase factor)). Plenty of power to get to cone 10.

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I fire almost all of my work to cone 6-8 and the way that has always worked for me is to get to temperature as fast as my kiln possibly can (immediately turning the switch to hi fire on a manual kiln or pressing the fast fire button on an automated kiln), holding the temp for 15 minutes and cooling naturally. This is a lot less expensive and simpler than firing slowly or with some fancy schedule, and i've had success firing all kinds of glazes like this with no problems whatsoever.

 

Best,

Darrel

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MichaelP    21

Darrel,

 

Could you please tell us how long it takes your kilns to get to a certain cone (let's say, Cone 6), and how quickly the temperature drops to 1400F when you cool them naturally?

 

Do you use matte and crystalline glazes?

 

Thank you.

 

Mike

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neilestrick    1,379

 

The Fast setting on our Cress E23 is steady 570 F.  

 

Nothing wrong with using the Fast setting on your controller.

 

 

570F/hr for Fast in Cone Fire mode? It surprises me that they would put in a program like that for their Fast setting. Most controllers, like the Bartlett, Orton and Sentry do not put in anything that fast, and still do a series of ramps to deal with quartz inversion, and slowing near peak temperature. Did someone alter it? 570F per hour would be about as fast as turning it on full speed.

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neilestrick    1,379

So I just looked through the firing profiles for the Cress kilns, and all of their profiles except Slow run at 500 degrees per hour or more for the first 1000 degrees or more, even on firings they recommend for thick greenware. You would have to do a good preheat to run that fast with thick greenware! This is why I like the Bartlett controllers- they have settings for bisque or glaze, that take into account the specific needs of each type of firing.

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MichaelP    21

 

 

570F/hr for Fast in Cone Fire mode? It surprises me that they would put in a program like that for their Fast setting. Most controllers, like the Bartlett, Orton and Sentry do not put in anything that fast, and still do a series of ramps to deal with quartz inversion, and slowing near peak temperature. Did someone alter it? 570F per hour would be about as fast as turning it on full speed.

 

 

Neil,

 

My Skutt kiln in the Fast Cone Fire mode goes from the ambient up at the constant rate of 600F/h. It only slows down to 108 F/h about 2 hours (~200 degrees) short of reaching the dialed Cone number.

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Mart    23

This is one of those fun but endless topics :)

At the moment (lots of thick pieces) I use the following schedule (everything is in C):
 
 

New schedule:1h 20m ->  1204h 45m -> 11062h 05m -> 124510 min @  1245End

 
 
 
Maybe this is a total overkill because
pieces are bisque fired previously.
Maybe it's better to go 120C/h to 200C and then just go at 150C/h to 1160 C etc.
 
Why I scheduled the last 90 min to 60C/h, because this is what this kiln actually does.
For details (about cooling etc) see How Fast Is Your Kiln Heating Up And Cooling Down? (lots of pretty charts and even a small video)
PS! My last post in that thread has a question still waiting for an answer.

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neilestrick    1,379

 

 

 

570F/hr for Fast in Cone Fire mode? It surprises me that they would put in a program like that for their Fast setting. Most controllers, like the Bartlett, Orton and Sentry do not put in anything that fast, and still do a series of ramps to deal with quartz inversion, and slowing near peak temperature. Did someone alter it? 570F per hour would be about as fast as turning it on full speed.

 

 

Neil,

 

My Skutt kiln in the Fast Cone Fire mode goes from the ambient up at the constant rate of 600F/h. It only slows down to 108 F/h about 2 hours (~200 degrees) short of reaching the dialed Cone number.

 

 

So it does. And that would work for glaze firing just fine since they have a slow down at the end. When I first responded to Norm he had not specified that there was a slowing at the end of the firing, which there actually is. And Skutt does have a medium speed that is quite reasonable for bisque firing. But I think Cress saying that 500F/hr is okay for greenware is careless.

 

I never understood why Skutt didn't go with the separate bisque and glaze profiles that Bartlett puts in their controllers.

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