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Firing Schedule Variables

There are several key issues that effect the final firing schedule selected. 
1. Functional or Non-functional use.
2. Wall thickness: thrown or hand built up to 3/8". Structural starting at 1/2" up. Sculptural with varying thickness/ parts. 
3. High iron/ carbon bodies vs. white body.
4. Single fire vs. bisq.

For the bulk of most firings; functional or non-functional and single fire vs. bisq fire comprise most firings. The additional variable is how thick are these pieces? Several universities across the world have done studies using X-ray defraction to measure heat work in gradient kilns. The general consensus being that it can take up to 30 minutes for the atmospheric temperature to reach the core of the clay body in the 3/8 to 1/2" thick range. 

In order for clay to fully mature, this variable has to be included in the firing schedule for functional wares. Absorption rates increase, COE values can change and firing defects such as pin holing, blistering, and shivering can be attributed to firing schedules. Sodium (Nep Sy) is the flux of choice in the USA and Canada; and is commonly used in other parts of the world. It is a cheap body flux but it does create issues. Sodium begins to melt at 2044F, and potassium at 2012F, as the clay is converting from spinel to mullite at 2050F. In application; at the same time sodium begins to off gas vigorously, the porosity of the clay is beginning to close up. Extending the time climbing to peak temperature allows the feldspars to completely off gas; thereby resolving pin hole issues while maturing the clay.

Selecting a preset ramp speed or programming your own depends upon the clay body, piece size, weight, and foot ring contact. In addition, starting at single fire or from bisq also decides ramp speed. Pieces with wall thickness above 3/8", heavy pieces above 7lbs, or pieces with large shelf contact such as platters need slower ramp speeds to allow for even heat distribution. Slower speeds during the quartz inversion range is also advisable for large format pieces

Quartz inversion occurs at 573C (1064F) when quartz changes from alpha to beta phase. Silica (quartz) actually expands at this temperature: part of an exothermic reaction. Just prior to this phase change and to just above this temperature: molecular moisture is being driven out of the body resulting in overall shrinkage. These two processes are occurring relatively at the same time: overall shrinkage from the loss of molecular moisture, while silica is expanding during inversion. If pieces are heavy enough, have weight, or have large shelf contact such as platters: cracking can occur. 

The remedy for this issue is programming a 100F per hour climb from 1000 to 1100F before resuming higher ramp speeds. You can actually increase firing speed to 180 to 270F an hour if firing porcelain or white stoneware. The overall size and weight of the piece may still justify a slow ramp cycle once you pass the inversion temperature range. Wadding, sand, or alumina may be placed under large/heavy pieces to facilitate movement during the firing cycle.

Dark and red bodied stoneware produce buff, terra cotta, and brown bodies that potters love. While they produce warm toasty colors, those colors come from iron disulfide. (Pyrite) in addition, lignite coal particles are common contaminants. Both sources of sulfides require special firing cycles to prevent blistering, bloating, and carbon coring. Inorganic carbons burn off from 1250 to 1750F, and require heavy oxidation during this temperature range. Rather single firing or bisq firing: programming a slow cycle of 108F an hour (slow speed) from 1250 to 1750f an hour while oxidizing the kiln is required.

If single firing; you are simply programming a bisq fire, while incorporating the final ramps to peak temperature. If firing large, heavy, or large foot ring pieces: then adding a quartz inversion cycle is required. If firing dark or red bodied stoneware; then programming a slow ramp (108F) from 1250 to 1750F while oxidizing the kiln is required to avoid blistering, bloating, and coring. Once you reach 1800F in a single fire, then you can increase ramp speed to 180 to 270F until you hit 2050F. At this temp, speed is then reduced to 108 to 125F an hour to allow escaping spars to escape before the clay body vitrifies.

University studies from around the world all report an endothermic reaction at 2050F as observed by X-ray defraction. It is a key reaction temperature in the firing cycle; when the porosity of the body begins to close rapidly. Most clay bodies in the USA and Canada use Nep Sy (sodium) as a body flux. At 2044F, sodium becomes reactive and off gasses vigorously; which appears as pin holes in the glaze. Rather single firing or starting from bisq; slow ramping from 2050F to peak hold allows the extra time for off gassing spars to dissipate. Recommended ramp cycle from 2050 to 2232F is 108-125F an hour. A commonly used peak temperature is 2190F with an extended hold (cone 6 ), use the recommended ramp cycle for this program firing. This slow ramp cycle towards peak range also has the added benefit of extending element life. 

Tom

 

 

 

Edited by glazenerd
Cone value added.

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Looks like pretty decent correlation to 108. - 120 degrees per hour in the last 200-250 degrees of firing practice which generally gets your cones to bend on rated temperature for the given rate column for general wares. Anything unique with later fire conduction influence as convection becomes a small fraction with  respect to the X-ray diffraction analysis?

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Bill:

Clay goes through three endothermic (absorbs heat) phases, and one exothermic (releases heat) phase.  

200-250F  atmospheric moisture is driven off. Potters already know what happens when they blow through this ramp too fast. The reason most controller programs have a hold at this temperature.

573C/ 1064F quartz inversion.  Pottery books explain this as alpha quartz converting to beta quartz. The technical aspect: at this temperature kaolin becomes metakaolin. Metakaolin is a fancy word for: all molecular water has been driven off. What is missing from the explanation is: silica (quartz) expands at this temperature: the reason bisq is so absorptive. However, molecular water begins to be driven off at 1030F.. So you have two processes going on at inversion: silica is expanding, and kaolin is shrinking because molecular moisture is being driven off. Porcelain is typically 50% kaolin and kaolin can have up to 15% molecular moisture.  The high silica content of porcelain is expanding, at the same time the high molecular moisture content of kaolin is shrinking: which explains why porcelain bodies are much more susceptible to dunting at inversion.

At 950C/1745F metakaolin begins to convert to spinel. Spinel  is composed of alumina and silica: with excess silica being ejected as crystallyttes. The spinel conversion is what gives bisq its mass: sodium and potassium begin to melt above 2000F. The safety police get bent out of joint over many things pottery related: but in my opinion bisq is the most dangerous respiratory stage of pottery. As mentioned before: at 1730 metakaolin converts to spinel. When spinel forms, it ejects excess silica in the form crystallyttes. That white powder on bisq is almost pure crystalline silica in the 30,000 mesh range. Which is why I wash bisq straight out of the kiln; actually I converted to single fire a few years back so I would not have to deal with it. Let the glaze absorb it. 

At 2050F is when spinel begins to convert to mullite: the only exothermic change clay goes through. Like quartz inversion, several things are going on at one time. Potassium begins to melt at 2012F, sodium begins to melt at 2044F, and spinel begins to convert to mullite at 2050F.  Sodium and potassium convert from solids to gas as they melt, which are the source of pinholes and blisters. As mentioned earlier: it can take up to 30 minute for the atmospheric temperature to penetrate a 1/2" wall. When you fast ramp in this upper temp range: your glaze might melt, but the core of the clay body is still immature. This equates to higher pinhole problems, and higher absorption rates. The slower you ramp above 2050F to cone 6; the more mullite develop occurs, and spars off gas better.

This all reverts back to clay chemistry: porcelain bodies are held to a 4:1 SiAL ratio to: 1. Achieve maximum mullite development. 2. Minimize the amount of crystallytes produced. We only see the crystallyttes on the surface after bisq, the core is filled with them as well. Again back to clay chemistry: spar addition are there for glass content development, but also there to absorb crystallytes into that melt. If a clay body is not mixed according to certain parameters: you can fire it till he'll freezes over: it will still weep or absorb water.

the final note is above 2230F. If the flux levels are off, and silica is too high, and alumina too low: crystallytes will not be absorbed into the melt. Free crystallytes then convert to cristabolite instead of mullite. You will know if that happens around 450F (cooling) and strange pinging noises are coming from your kiln during cristabolite inversion. As porcelain is more prone to quartz inversion due to its kaolin and silica content: stoneware is more prone to cristabolite inversion due to its much lower flux content. Porcelain typically has 25-30% flux content! and stoneware only 10-14%.

Tom

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“Several universities across the world have done studies using X-ray defraction to measure heat work in gradient kilns. The general consensus being that it can take up to 30 minutes for the atmospheric temperature to reach the core of the clay body in the 3/8 to 1/2" thick range. “

I am good with the process my question goes to the X-ray diffraction analysis and rates. Most kilns I come across are gradient to some extent but you mentioned rates and thickness and confirmation by analysis. Two part question:

  • Did the analysis differentiate between convection, conduction, and radiation, and  if yes to what extent did these thermodynamic processes differ in the efficiency of warming the bodies so to speak. ( Just curious for my relay / shelves  project and mass reduction)
  • The rate of 108-125 degrees in the final 250  degrees is needed for an Orton cone to bend at its prescribed temperature. I was just curious if this was their recommended rate based on the diffraction analysis?

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Sorry Bill, I broke my two cup rule. Never answer questions until I finish the second cup.

As I recall: one study was done in Brazil , one in India for their Government, and one in Germany?  All three used gradient kilns with 6-10 chambers and 10-15C variation between chambers. The one in India was testing laterite, and reported an exothermic reaction at 2050F. The study in San Paulo? Actually used bars in various thickness 1/4 to 1/2 in a multi chamber gradient kiln: that studied produced the time of heat absorption and release at 2050F: conduction being the focus as I recall. The one out of Germany was studying local materials, and reported the reaction at 2050F. So 2050F spinel to mullite temp has been confirmed numerous times.

U of I (Champaign/Urbana) has numerous studies up on their Ceramic Technology site. I posted a link in one of my ramblings somewhere : stoneware study thread I think. They used X-ray diffraction to analyze heat work and the phase changes in potassium and sodium. At 2190F, sodium and potassium are spent- no longer visible. I cannot confirm this: but I suspect this is where the commonly used 2190F peak with hold firing cycle came from.

Orton Sr. Did extensive studies back in 1909-1919 range(?) noted in my Nerds Firing Schedule thread. He proposed the 108F ramp speed for several reasons: primarily to burn off inorganic carbons, secondly for heat work purposes. As you well know, cones are based on Segers work, but Orton did the initial testing on calibrated heat work.

did I answer them that time, or do I need a third cup?  Check my Stoneware study thread, Nerds Firing Schedule thread, and possibly my Porcelain thread. I have links to studies floating all around. I do know some studies are no longer accessible: Wiley Library has been buying them up and archiving them. 

Tom

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No enough said, it sounds like an interesting study. We surmise that convection is minor, conduction somewhat contributory and radiation the predominate thermal engine after red heat. Just curious if someone quantified this throughout the firing and related it to core temps is all. We have no effective way to test this in a practical manner.

 *Seger my unsung hero BTW much like Nikola Tesla*

As to gradients, the difference in ware temperature top to bottom is usually significant as well as outside to in, yet quartz inversion happens across a fairly  specific temperature so our  supposition has always been clay can take a good bit of stress and movement since parts of a  body are inverting while other parts are not. I was hoping the study would maybe provide some quantifiable evidence in this respect to expand on the clay can take it theory. 

As to final leg of the firing, consistent with what we teach so no new revelation from the X-ray analysis is my take.

Can you get your hands on the equipment? I have a handful of tests I need  to do.

thanks!

Hey, last random thought R.T. Stull taught at Uof I!

Edited by Bill Kielb

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If anyone ran measured effects of heat work, it would be Orton Sr.  He wrote several abstracts for American Ceramic Society, I will nose around and see what I can find. The other source would be Ougland and Brindley from the British Ceramic Society: "Effects of a High Temperature on Kaolinite"  I read that abstract, and quoted some of it in my threads. It has since been pulled down, sadly. 

I have been pricing gradient kilns: and potters gasp at the price of an electric. Keep waiting for a good used one to come along.  From my observation pending kiln size: there can be up to 40F difference in a large chamber. Years ago I started mixing my crystalline glazes via PH meter. I raise the PH in cold spots, and lower it in cold spots: works fairly well. 

I would put conduction up to 2000F, and radiation there after. 

Edit add: If Edison did not have Telsa: his inventions would have been few.  IMOIMO

Tom

Edited by glazenerd

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15 hours ago, glazenerd said:

Recommended ramp cycle from 2050 to 2232F is 108-125F an hour. A commonly used peak temperature is 2190F with an extended hold: use the recommended ramp cycle for this program firing.

Tom, for the sake of clarity (for the newer potters on the forum) it would be a good idea to say this would be for a cone 6 firing. Thanks.

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Good point - just to add,

For newbies the 108 - 125 final rate segment in the last 200 - 250 degrees will get your cone to bend at  or near the published temperature. The cone chart is based on it.

Cone 04 - cone 6 - cone 10.

The heatwork in the final segment is most significant to maturity and I find the simplified thermodynamic approach  is simpler for most to grasp.  

We often use a simple analogy when students ask about firing to lower cones and adding a hold. At some point it becomes non functional,  insignificant and doesn’t work. Our simple analogy is cooking a pizza. Cook it forever at 200 degrees, probably never gonna work. Cook it at 425 for 10 minutes and it’s a masterpiece! (Hopefully)

 I just don’t want folks to feel this is a cone 6 thing, it’s a maturity and heatwork thing that applies reasonably well throughout.

Just my thought though ..... I think I will warm up the oven, pizza for lunch!

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Bill: results from Ougland & Brindley study on heat work.

2192F (1200C) minimum further development of the clay body after this point. Minor decreases in absorption, along with minor increases in glass content. See chart below. Typical cone six ramp hold temperature for maturity.

-----------------           ---------------------

2192F (1200C.). Glass 62.   Silica 21.    Mullite 19
2372F (1300C).  Glass 66.   Silica 16.    Mullite 21

(Ougland & Brindley)

 

Off topic,  Ron Roy emailed me: he is doing a work shop nearby in June. Looking forward to seeing my friend. 

Tom

 

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Staying off topic

We have Kenyan Hansen here for today and tomorrow for our residents. Should see some nice soda work Monday morning by all.

Edited by Bill Kielb

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”If anyone ran measured effects of heat work, it would be Orton Sr.  He wrote several abstracts for American Ceramic Society, I will nose around and see what I can find. The other source would be Ougland and Brindley from the British Ceramic Society: "Effects of a High Temperature on Kaolinite".   

Tom,  please post the complete citation information (author name, article title, journal official name, volume, issue, page, and publication date).  

LT

 

 

 

 

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LT: there are 100 plus references to journals, thesis, books, and other resources on the effects of temp on clay bodies. 

https://books.google.com/books?id=pQpCDCPqlS4C&pg=PA58&lpg=PA58&dq=W.H.+Sutton;+factors+influencing+the+strength+of+clay+bodies&source=bl&ots=AfkxuypAxo&sig=ACfU3U2W_Zh8NTluxpwvcnIN4zVOWBV3vw&hl=en&sa=X&ved=2ahUKEwjB-_rwj7jiAhUPnq0KHaUpC_YQ6AEwAXoECAQQAQ#v=onepage&q=W.H. Sutton%3B factors influencing the strength of clay bodies&f=false

you need to hire a research assistant: I am a little busy.

as I have told you in PM before- W.G. Lawrence  " Ceramic Science for the Potter." 

F.H. Norton " Fine Ceramics, Technology and Application".  And the book referenced above are the best encapsulated information resources. 

For the record: I have journal references lying all over the place in my various threads. Feel free to go find them. Long past tired of having post proof every time I make technical posts.

Tom

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