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Candling temperature / Drying greenware in kiln


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In a recent thread the subject of drying less than bone dry pots in the kiln came up in which the suggestions were to keep the temp at or below 180F to avoid blowing things up. This jogged my memory of an article by Tony Hansen where he states it's okay to candle / dry out greenware up to 240F without issues.  I know it seems logical to keep the temp below the boiling point of water but is it necessary? If one can get the same results with drying the pots out at 240F rather than below 212F it seems logical to do so. In a perfect world we would have temperature, humidity and air flow control to have the perfect drying conditions but that's not likely for most studio potters.

I haven't gone up to 240F in temperature for candling greenware, wondering if anybody has and were there any issues?

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I've never done it, but some things to consider - The preheat segment on a Bartlett controller is 180-200F, depending on the age of the controller (some of the older models I have preheat at 200F, the newer ones at 180F). If 240F is right, why does Bartlett keep it lower than boiling? With an electric kiln w/ controller, there may be some thermocouple offset programmed in, so the apparent temperature showing on the screen may not be the actual temperature of the atmosphere in the kiln? It takes time for the temperature to penetrate the clay body, so the surface may be already dry and hotter than the interior which is still cool enough that it doesn't turn to steam? A principle of phase change is that evaporation leaves the surface from which it evaporated cooler, so 240F may be the perfect atmospheric temperature to balance the evaporating moisture and surface temperature of the ware?

dw

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1 hour ago, Min said:

I haven't gone up to 240F in temperature for candling greenware, wondering if anybody has and were there any issues?

Since boiling point is relative to the atmospheric pressure and the accuracy of most pyrometers is plus or minus and their ability to detect and read is not instantaneous and the thermal mass and lag of a kiln could be a thing and there is some differential built into most controllers ……… my guess is Bartlett could not find a practical reason to get close to boiling and so lowering the temp from 200 to 180 was still effective at drying things out, a bit safer and just not worth the risk of elevating it to theoretically save some time. I have seen folks come out of a 180 degree 4 hour hold on their massive sculpture and have it explode on them. Next go around was 180 at eight hours for the remade piece - no explosion.
 

Its interesting theoretically, but with artists work at stake, we offer up infrared so their full size bust of their son graduating ranger school can dry slowly over a few weeks time, then encourage a long hold at 180 degrees and even help to tend the gas kiln and maintain a slow rate to their finished tested cone for color.  Call me chicken I guess, not going there with others work or my own.

 

65618D19-9C6E-4232-890F-2554A8FEF170.jpeg

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If nobody here has tried it I'll measure the accuracy of one of my kilns with an infrared heat gun (it's as accurate as I can get) to get to 240F and run some tests on 1/4" slabs and see what happens. (Genesis controller on a 3 zone kiln) 

In industry the use of humidity combined with increasing temp plus air motion has drying down to an art, from some of the reading I've done commercial dryers can go up to 130C / 266F. I realize this might be with specialized bodies and not the broad range of bodies that studio potters use but at some point somebody tried it and had success. As a change in practice can happen with new information and trial and error testing I'm of the feeling that just because we typically haven't done this in the past it's not a reason not to do our own due diligence and see what happens, with test pieces of course.

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40 minutes ago, Min said:

As a change in practice can happen with new information and trial and error testing I'm of the feeling that just because we typically haven't done this in the past it's not a reason not to do our own due diligence and see what happens, with test pieces of course.

No doubt many things can be done and are done. Reducing the atmosphere is a common thing. Drying evenly is usually key so adding less heat is usually a thing because it costs money in industry. As far as advising potters based on that, I worry that they would not understand the qualifiers nor have the equipment to do it effectively.  My guess, it’s done daily in many industries. Typical potters, I would try and err on the conservative side of things. I am not sure what we are saving actually 40 minutes? Maybe it’s significant, it will be interesting to learn what you find.

I can add that I made some speed dryers once. Large commercial cake transport with a computer fan inside and an adjustable inlet and outlet. Evenly dried things fairly quickly as most of the air was recirculated. Never made one for anyone else though for fear they would just go for max fresh air to speed things up.

Edited by Bill Kielb
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I know for certain that different clay bodies shed their water at different rates. The most remarkable I’ve experienced was a body made of two parts OM4 and one part pumice (it was pumice I dug up from a deposit in Utah). I regularly fired it from greenware in a raku kiln in less than three hours. It was pretty awful as a clay body, even with all that ball clay it still wasn’t plastic and never got very strong. It melted around cone 1. Never blew up though. 

The large scale industrial producers definitely have their process fine tuned. I got to tour a tile factory in Pennsylvania once, it was enlightening. Their clay body had a very high percentage of non plastic material, just enough clay to hold it together. It also had an exact percentage of water and was used in powder form. A measured amount of powder was dropped into a steel mold, pressed hydraulically, then popped out as a tile. From there it was on a conveyer under a glaze sprayer then into a tunnel kiln. Nothing ever stopped moving. 

180° is a good temperature to candle at, you’ve just got to do it long enough to get the water out. Digital kiln controllers make this easier than it’s ever been in the history of ceramics. We live in good times. 

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12 hours ago, Dick White said:

It takes time for the temperature to penetrate the clay body, so the surface may be already dry and hotter than the interior which is still cool enough that it doesn't turn to steam?

I think this is the answer. Every time I've actually heard pieces blow up, the kiln was around 500F. It takes a lot longer for the heat to penetrate and cause problems than we realize.

So why does Bartlett program their preheat to hold at 180F?  Probably because it makes their lives a lot easier. If it was set at 240F, or anything too close to 212F, they would have to educate their users on the subject, and they'd get blamed every time something blew up. They'd be dealing with phone calls and email on the subject every day. 180F works just fine, and it's far enough from the boiling point that if something blows up it's definitely not their fault.

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If this helps I have loaded an electric solid with serval hundred slip cast lamps (yes they have thin walls and are stronger egg shapes) all wet and bisgue fired then with zero issues. This was in the 90s and the kiln steamed for a long time. Kiln is manual no  fine control .Thicker wall pots will take longer and slower.Steam is a powerful thing. I'm a firm beliver of the 212 boiling point of water-that said puching the limits is what I am known for. Then you find the limit and back down

Edited by Mark C.
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No scientist, and just rwmember reading If folk who put pots i kiln straight off the wheel, sealed kiln and fired.

Wonder what the firing schedule was, prob pre controllers.

Will try to find that source.

Off topic sorry Min.

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2 hours ago, Babs said:

No scientist, and just rwmember reading If folk who put pots i kiln straight off the wheel, sealed kiln and fired.

Wonder what the firing schedule was, prob pre controllers.

Will try to find that source.

Off topic sorry Min.

I vaguely remember Ian Gregory (of fibre kiln fame) making a similar comment during a demonstration -- about being able to fire a kiln-full of freshly thrown pots without trouble. He then immediately said that with somewhat  dryer pots this  didn't work. 

The impression I got that there was a brief sweet-spot where this was possible (and then only for a kiln-full of such pots), not that the state of dampness wasn't important.

I've no coherent theory to explain/understand this claim. Although its possible that some sort of wicking process draws the water to the surface as the pot dries, and the large number of wet pots in the kiln limits the temperature ramp while there is still surface water to evaporate. Surface drying before firing could  prevent such wicking of water to the surface during firing.

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The boiling point of water changes with the atmospheric pressure above it so in general at about 500 foot elevation (Chicago)I it boils at 211 degrees but at higher elevation say 1000 feet more like 210 degrees. There is also truth  as they say that water boils easier on a cloudy day than on a clear one. Low and high pressure respectively. So I guess the point would be folks should dry their stuff with whatever risk is acceptable to them or has proven to be risk free. If you regularly fire your own things from a wet state to dry with no issue and watch steam coming out of the kiln, you are probably comfortable with doing it each time with whatever your schedule is. For many - 180 hold is conservatively safe for them. As to the boiling temperature of water, it varies. 
 

Teaching AC techs we always had the demonstration of evacuating the pressure in a flask of water and making it boil with the heat from our hand when we picked it up.

Edited by Bill Kielb
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Boiling point of pure water should go up as concentration of solutes increase.
...I've no idea how salty the moisture in clay is...

Another variable may be the amount, size and type of grog bits. I believe that bits of grog allow clay to (a) get wetter, faster, when exposed to water, and also (b) dry out faster once exposure to water stops.

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Since for years I had now way of knowing the temp of the kiln with not pyrometer or thermocouple, I would just start with the bottom switch at 10% overnight with lid 2". Next morning crank all to 10% for 1-2 hours, lid down,  until I no longer felt dampness in the kiln by putting my hand up to the peeps. Firing from there was pretty much as most others. Worked well with student handbuilt pots and student wheel thrown pieces.

 

best,

Pres

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I rolled out a  7mm / just over 1/4" thick slab 2 days ago. Used a very soft white clay that contains some ball clay, silica, lots of kaolin and fluxed with nepheline syenite plus has some bentonite, no grog or sand, and ribbed both sides of it then stood it on end and bent it into a gentle curve. This particular clay dries fairly slowly, it will still be damp when I candle it. I've weighed and measured it, will preheat at the Bartlett rate then (after checking the accuracy of my kiln temp) will heat it to 240F and hold for 3 hours. 3 hours is just a guess, if it's going to blow up I think it will have by then. If there is still steam coming out of the top peep I'll leave it another hour and check it again. I'm not running a vent as many people don't use them. Trying for the worst case scenario, if anyone has some more ideas on testing this theory I'm happy to hear them.

I'm wondering if Mark's experience with firing wet pots plus what Babs and Peter remember is more about creating a very humid environment that allows the clay to dry out more evenly? Sort of how the commercial dryers work but without the air turbulence?

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Maybe I also heard what @Babsis remembering. Back in the 70s I heard of an enterprising itinerant Raku potter with just a wheel and a little propane kiln. He would make tea bowls and fire them immediately to sell immediately to thrilled customers. At the time, it was thought to be a similar method to older Japanese Raku traditions. I think the goal of the old method would be to produce many useful items in a short time. 

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Haven't been able to find anything more about this:

http://www.potters.org/subject98825.htm

Frank Colson on sun 4 feb 07

Some many years ago, I published an article called: Total Wet Firing. I
would take a freshly thrown pot off the wheel head and fire it immediately
in a preheated raku type kiln Did a workshop for a group of local art
teachers, so we were able to "experiment" this type of firning with more
than 100 pots so we were able to get all te wrinkels worked out. You might
call
the approach a one-upmanship raku technique. Allthough I still have one pot
on my shelves, it was great fun but not very peranent. The entire firning
proces took about 5 minuets!

If anywone would like to see how to do "Total Wet Firing", I'll put it on my
website as a free download.

Frank Colson
www.R2D2u.com

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I have candled pots at 235F for many years with out any ill effects.

Here are some thoughts, I'll let you decide.

1) In a closed system such as a tea kettle or a car's radiator water boils at a higher temp. In a car it can reach  to 265F before boiling. Is greenware drying in a closed kiln a closed system?

2) When drying timber in a wood kiln  and the wood is dried at to quickly and at to high a temp, the outside of the lumber will harden preventing the inside moisture to escape. This is called case hardening . When a board of wood that is case harden and is cut on a tablesaw it will warp which can be dangerous even fatal is safety precautions aren't observed.

I posit that the greenware in a kiln hasn't case harden while candling @ 235F and may not have reached it's boiling point 

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I frequently have to fire student pieces that are freshly made and way too thick, and to keep classes on schedule I have to fire them before they have a chance to dry.  I soak at 180F for 10-16 hours before ramping up, depending on the thickness/wetness of the pieces.  The cost of electricity to keep the kiln at 180F is relatively small, and I have never had any student pieces explode.  It is probably overkill to soak that long, but it works, so I'm sticking with it.

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I used to candle overnight wet to leather hard pots for the kids. Would start the kiln right after school 2inch lid gap, work until around 530, and head home. Next morning would check kiln for peep moisture around 645, then depending start a slow ramp to bisque 06. Worked most times, especially when there were heavy spirit houses or coil  pieces in the kiln.

Years ago, mid 70's I went to a workshop at Indiana State College of PA. They were doing fast firing one day with pearlite addition. Threw like crap, but the pots survived and were glaze fired in two days of the workshop! Took students to the workshop also, and they were impressed!

 

best,

Pres

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Posted (edited)

This is just one test with one claybody, please don't try this without running your own tests with your kiln and your clay!

Some details:

- Slab from this clay, smooth white high kaolin, grog and sand free

-  Clay run 3 times through a slabroller to get it to 7mm  X 26.7cm  X 15.25cm (approx 1/4" X 10 1/2" X 6 1/2") weight of 853 grams

- Slab ribbed on both sides, stood upright and bent slightly to stand upright

- Slab left for 2 1/2 days loosely covered with plastic

- Weight of slab going into kiln 801 grams. Clay was still very damp feeling, lost 52grams weight

- Candling schedule 80F hr to 240F then 3 hour hold, no vent

- Weight of slab after candling 676 grams, lost 125grams from start of candling weight

- I'm at 260' above sea level, humidity today at 51% (don't know if this matters but it was brought up so I thought I'ld include it)

So doing rough math the clay lost approx 22% of it's weight which sounds about right as the info sheet for it states it contains 21.7 - 22.7% water. I'm calling this bone dry.

No explosions or damage to the slab whatsoever.

edit: forgot to add, lid propped open 1" for 2 hours then closed, 3 spyholes left open for the entire time

 

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Edited by Min
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I asked Tony Hansen about this given it was his article that got me thinking about it. His reply: "These are assuming that water smoking should be done below the boiling point of water. That sounds good in theory, but in practice, it does not remove all the mechanical water. In industry, they candle at 250F or higher. Actually, their driers probably take the water to that temp"

Whatever the explanation it would be interesting to get the science behind why it appears to be okay candling higher than the boiling point of water as Hansen states and my simple test seemed to validate.

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30 minutes ago, Min said:

Whatever the explanation it would be interesting to get the science behind why it appears to be okay candling higher than the boiling point of water as Hansen states and my simple test seemed to validate.

Seems to work. Not sure I will depart from significantly below boiling point though as I am rarely looking to save a specific amount of time and like to be cautious with the wares.

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