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Calculating and comparing Heatwork


Julie P
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I have been trying to be scientific about arranging my firing schedule in a small electric kiln with a controller. I understand the concept of heatwork and use cones on all shelves in my kiln to monitor results. I soak at top temperature and fire down so want to compensate for the additional heatwork these procedures involve when I set the top  temperature on my controller.  I understood that this could be done by looking at the area under a graph of total heatwork, but having compared theoretical models using the Orton cone firing tables, if this is indeed the case, I must be missing something.

The following example uses °C, as I am in the UK, and I am referring to the Orton theoretical temperature values. 

1. In the Orton table Cone 7 is reached at 1237°C when firing at 60°C /hr, if this is a straight ramp, this would take 20.3hrs, assuming a starting room temp of 17°C ((1237-17)/60).  If the x-axis is time and the y-axis temperature, the area under the graph is a triangle and can be calculated as (1237 x 20.3)/2 = 12,555

2. When firing at 150°C/hr cone 7 is reached at 1255°C. As a straight ramp this would take 8.25hrs ((1255-17)/150). Then the area under the graph is also a triangle calculated as (1255x8.25)/2 = 5179

SO, these areas which are supposed  to represent total heatwork to achieve cone 7 are not the same.

I am clearly missing something here. Can anyone help please. Thank you

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@Julie P The heat work calculations only apply to the last 100C or 180F of the firing. Prior to that it doesn't really matter how fast or slow you go. Also be aware that controlled cooling will increase heat work if you're cooling from the peak temp. If you want to avoid that, crash cool for 100 degrees before starting your controlled cooling.

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Thank you very much, that is most helpful. It doesn't make that at all clear in many of the Orton cone tables. I particularly wanted to compensate for the cooling cycle and I take it down 260 C/hr for the first 200 degrees C. Perhaps I'll try a faster crash for the first 100 degrees. Thanks again

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2 hours ago, Julie P said:

Thanks Neil. That's really helpful. I'm just surprised that the heatwork before that stage doesn't impact the final result but I will keep on experimenting. Thanks again for your help.

It takes a certain amount of actual heat to get thing started before heat work kicks in. I ran a test of my cone 6 glazes once, firing to cone 4 and then holding until cone 6 dropped. About half of my glazes worked that way, but the other half were still quite dry. They just hadn't gotten hot enough for the melt to kick in enough that a soak would keep it going. The exception would be very, very long firings. I once fired a large wood kiln, and after holding temp at 1850F for 5 days, cone 5 went down, about 300 degrees earlier than it normally would.

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There are a lot of things that go on in a firing that it would be good to be aware of: where biologicals/organics burn off, quartz inversion, and others. Very good areas of reading in several ceramics books, Rhodes, Nelson and others, but one of the ones I always find myself returning to is Hamer. Heat work is extremely important, in rate of firing, but understanding these other areas can also help with planning a firing cycle.

 

best,

Pres

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2 hours ago, neilestrick said:

It takes a certain amount of actual heat to get thing started before heat work kicks in. I ran a test of my cone 6 glazes once, firing to cone 4 and then holding until cone 6 dropped. About half of my glazes worked that way, but the other half were still quite dry. They just hadn't gotten hot enough for the melt to kick in enough that a soak work keep it going. The exception would be very, very long firings. I once fired a large wood kiln, and after holding temp at 1850F for 5 days, cone 5 went down, about 300 degrees earlier than it normally would.

It is so good to hear about these experiences, which obviously take many years of ceramic practice. I really appreciate you giving me so much helpful information. Thanks again

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33 minutes ago, Julie P said:

It is so good to hear about these experiences, which obviously take many years of ceramic practice. I really appreciate you giving me so much helpful information. Thanks again

Great comments! I will give you another reason that it’s hard to be so precise. Clay and glazes are made of silica and alumina which do not melt on their own below 1648c so we use fluxes to help them melt sooner.

 So they (Glazes and clay) melt by composition, not necessarily by temperature. Heatwork is a great idea in that you put a cone in (basically made of glaze with a flux ratio of 0.3:0.7) and it melts at a level of heatwork every time. This provides a great consistent baseline for each firing but does not necessarily act the same as your glazes and  their composition other than you learn whether you like how your glazes  look at a specific cone  ......... which is repeatable because of cones.

The last 100c of the firing is where most of the heatwork will have its effect as these compositions begin melting in that range, because of ........... the fluxes. Like boiling water,  in a covered pot it doesn’t matter how long it simmers at 50c  you still have water. The heat energy put in early on is not as relevant as when things begin to melt. Basically it’s reversible,  and can be stopped. If stopped though, basically no change has taken place so the prior heatwork is not cumulative and ya likely got to put it all in again when restarting.

Heatwork and fire down - If I drop 100c and hold  for a bit I’ve dropped from cone 6 to cone 1 or less. Probably just not significant additional heatwork at this point actually and everything that did melt by composition has already melted. Most electric kilns and many gas for that matter cool extremely fast in the first few seconds when the power is turned off so it’s hard to add a bunch of heatwork after shut down with simple drop and holds.

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Slow cooling the kiln can have a dramatic effect on how some glazes turn out. Couple images below taken from here showing the difference in a natural fast cool from the kiln shutting off after top temperature/cone reached versus a slow cool program. Fast cooled on the left, slow on the right of each pair. Same glazes, both taken to the same temperature/cone. Slow cooled between 1900F - 1450F (1040C - 788C). Hesselberth and Roy really got the ball rolling with how slow cooling electric kilns can benefit many glazes.

image.png.d9780d1e669b203bf0a38257496491b2.png  image.png.b604874152ae96e34fd72ff318364002.png

I use a drop and hold then slow cool for all my firings. The drop and hold definitely has a positive effect on reducing pinholes, especially in my glazes that contain rutile. After the cone 6 has dropped I have the kiln cool at 9999 (maximum freefall) by 100F (38C) and hold there for 20 minutes. (with my kiln this would be 2090F) It does a bit of heat work. 

1 hour ago, Bill Kielb said:

Clay and glazes are made of silica and alumina which do not melt on their own below 1648c

BTW at the eutectic point, when 10% alumina and 90% silica are mixed, the melting point is 2813°F (1545°C).

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56 minutes ago, Min said:

BTW at the eutectic point, when 10% alumina and 90% silica are mixed, the melting point is 2813°F (1545°C).

 

2 hours ago, Bill Kielb said:

Great comments! I will give you another reason that it’s hard to be so precise. Clay and glazes are made of silica and alumina which do not melt on their own below 1648c so we use fluxes to help them melt sooner.

Yes, thanks!
There is a a very sharp inflection at 10%  alumina and 90%. Silica. I didn’t mean to imply combined they did not have a melting point below 1648. I was hoping to convey that these things melt by composition  because of flux though so adding the area under the curve is often impractical in calculating a specific amount of heatwork. Especially in the lower temperature ranges.

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11 hours ago, Min said:

Slow cooling the kiln can have a dramatic effect on how some glazes turn out. Couple images below taken from here showing the difference in a natural fast cool from the kiln shutting off after top temperature/cone reached versus a slow cool program. Fast cooled on the left, slow on the right of each pair. Same glazes, both taken to the same temperature/cone. Slow cooled between 1900F - 1450F (1040C - 788C). Hesselberth and Roy really got the ball rolling with how slow cooling electric kilns can benefit many glazes.

image.png.d9780d1e669b203bf0a38257496491b2.png  image.png.b604874152ae96e34fd72ff318364002.png

I use a drop and hold then slow cool for all my firings. The drop and hold definitely has a positive effect on reducing pinholes, especially in my glazes that contain rutile. After the cone 6 has dropped I have the kiln cool at 9999 (maximum freefall) by 100F (38C) and hold there for 20 minutes. (with my kiln this would be 2090F) It does a bit of heat work. 

BTW at the eutectic point, when 10% alumina and 90% silica are mixed, the melting point is 2813°F (1545°C).

Thank you Min, I saw this and Dave Finkelnberg's piece on Heatwork in the CAN pamphlet on electric kilns. It was that which started me on the idea of using graphs to adjust my kiln programming. I've noow changed approach and only compare the top 100 degrees in deciding on the top temp to set my controller with a given period of soaking and am going to try our the results. Really appreciate your help

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15 hours ago, Min said:

It also has that info in the third line down at the top of the chart.

845442551_ScreenShot2020-04-27at8_36_07AM.jpg.f065ce11ce3ee244819f0c96bcd8774e.jpg

Thanks Min, found the table on the Orton website. The reason I got confused is that the books I am using with the Orton tables in them don't mention this important fact. Linda Bloomfield's appendix  in her glaze handbook implies that it is the pace of the total temperature rise, not just the last 100°C which would confuse a lot of beginners I think,

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13 hours ago, Bill Kielb said:

Great comments! I will give you another reason that it’s hard to be so precise. Clay and glazes are made of silica and alumina which do not melt on their own below 1648c so we use fluxes to help them melt sooner.

 So they (Glazes and clay) melt by composition, not necessarily by temperature. Heatwork is a great idea in that you put a cone in (basically made of glaze with a flux ratio of 0.3:0.7) and it melts at a level of heatwork every time. This provides a great consistent baseline for each firing but does not necessarily act the same as your glazes and  their composition other than you learn whether you like how your glazes  look at a specific cone  ......... which is repeatable because of cones.

The last 100c of the firing is where most of the heatwork will have its effect as these compositions begin melting in that range, because of ........... the fluxes. Like boiling water,  in a covered pot it doesn’t matter how long it simmers at 50c  you still have water. The heat energy put in early on is not as relevant as when things begin to melt. Basically it’s reversible,  and can be stopped. If stopped though, basically no change has taken place so the prior heatwork is not cumulative and ya likely got to put it all in again when restarting.

Heatwork and fire down - If I drop 100c and hold  for a bit I’ve dropped from cone 6 to cone 1 or less. Probably just not significant additional heatwork at this point actually and everything that did melt by composition has already melted. Most electric kilns and many gas for that matter cool extremely fast in the first few seconds when the power is turned off so it’s hard to add a bunch of heatwork after shut down with simple drop and holds.

Thank you very much Bill, this is all great stuff. I do want to try to be deliberate in decisions about kiln programming, rather than guessing and hoping for the best. However I do appreciate that there is no such thing as precision in ceramics. Thanks again for the helpful response

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2 hours ago, Julie P said:

Thank you very much Bill, this is all great stuff. I do want to try to be deliberate in decisions about kiln programming, rather than guessing and hoping for the best. However I do appreciate that there is no such thing as precision in ceramics. Thanks again for the helpful response

If it helps you at all,  most automatic control manufactures will try and  nearly match the published Orton speed with the published temperature in the final prox. 100c (220f)) of the firing. They get pretty darn close using that segment and speed rather than trying to calculate cumulative heatwork.
Most decent literature I have read  on programming your own schedule says subtract 100c (220f)) from your final temp and program your last segment accordingly in rate and final cone temp. it generally works  pretty darn good.

This would be for bisque and glaze, however Bisque overall is generally slower and longer to effectively and slowly remove combined water and organics.  Bisque schedules probably average 10-12 hours, glaze schedules can be really fast and conclude in four hours or so. Each generally concludes with a final segment of prox. 100c (220f) below final cone temp at or near the published rate to get the Fire cone to bend.

In the last ten years I have spent a crazy amount of time explaining the Orton chart to many, so it seems maybe it’s not obvious or explained well on the Orton chart.

Final bit of trivia, cones are made of glaze approximately 4:cones more than their rating. Grind up a cone six cone  .... and fire it to cone ten .... it comes out a nice glaze. Seger (August Herman, I believe) actually is more the father of cone theory as he tried to devise a way to identify glaze maturity points. Orton, I believe brought this work to the states.

Edited by Bill Kielb
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1 hour ago, Bill Kielb said:

If it helps you at all,  most automatic control manufactures will try and  nearly match the published Orton speed with the published temperature in the final prox. 100c (220f)) of the firing. They get pretty darn close using that segment and speed rather than trying to calculate cumulative heatwork.
Most decent literature I have read  on programming your own schedule says subtract 100c (220f)) from your final temp and program your last segment accordingly in rate and final cone temp. it generally works  pretty darn good.

This would be for bisque and glaze, however Bisque overall is generally slower and longer to effectively and slowly remove combined water and organics.  Bisque schedules probably average 10-12 hours, glaze schedules can be really fast and conclude in four hours or so. Each generally concludes with a final segment of prox. 100c (220f) below final cone temp at or near the published rate to get the Fire cone to bend.

In the last ten years I have spent a crazy amount of time explaining the Orton chart to many, so it seems maybe it’s not obvious or explained well on the Orton chart.

Final bit of trivia, cones are made of glaze approximately 4:cones more than their rating. Grind up a cone six cone  .... and fire it to cone ten .... it comes out a nice glaze. Seger (August Herman, I believe) actually is more the father of cone theory as he tried to devise a way to identify glaze maturity points. Orton, I believe brought this work to the states.

Love this Bill thank you so much. Julie 

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On somewhat of a tangent, mother nature has excellent examples of heatwork.  Granite and rhyolite have the same composition.  The former is allowed to slowly cool (a controlled fire down and long soak over a few million years). It is able to form a beautiful lattice structure and large crystals.  The latter is extruded in an eruption and has a rapid cooling.  Its matrix is composed of finer grained crystals.

Next tangent:  quickly fill a large box with  ping pong balls.  After the box is full, apply  a constant gentle vibration to the box.  The balls will work their way into a matrix or lattice structure.  This vibrational energy is analogous to heatwork.

Thus you see my ranting.  I really am just procrastinatng moving a few tons of gravel from my driveway to the backyard.

Great thread,

Jed

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3 hours ago, jrgpots said:

On somewhat of a tangent, mother nature has excellent examples of heatwork.  Granite and rhyolite have the same composition.  The former is allowed to slowly cool (a controlled fire down and long soak over a few million years). It is able to form a beautiful lattice structure and large crystals.  The latter is extruded in an eruption and has a rapid cooling.  Its matrix is composed of finer grained crystals.

Next tangent:  quickly fill a large box with  ping pong balls.  After the box is full, apply  a constant gentle vibration to the box.  The balls will work their way into a matrix or lattice structure.  This vibrational energy is analogous to heatwork.

Thus you see my ranting.  I really am just procrastinatng moving a few tons of gravel from my driveway to the backyard.

Great thread,

Jed

Great explanations. Shame the gravel can't vibrate itself into the backyard. I had to do the same last year. Exhausting

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