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Firing Interruption And Continuation (Especially Terracotta)


gowrip

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Disclaimers : 1) This is more of a theoretical question. 2) I am not a potter/ceramic person of any sort. 3) all my knowledge of pottery, tile making come from youtube videos and internet. 4) actually my question has more to do with making TerraCotta  (or any other low fire, easily available clay - not the high quality, high temp porcelain etc.) articles like tiles, pipes etc.

 

Short Version: If for some reason you have to shut off the kiln halfway during firing (when the pieces are at around 1080F/600C), let them all cool down and restart firing again and go all the way till bisque/glaze temperatures. Will this work or is it just not acceptable and ruin the pieces completely because you broke the process halfway?

 

The real reason for asking however is this: I am wondering whether I can design a solar kiln to do pottery & tile making (at least partially replace fuel/electricity needs) but it can only function the following way... So, will this work?

 

All heating cycles can only be 7-8 hours long, i.e., when the sun shines; 

 

- Day 1 (or more than one day): drying using warm air at <212F/100C, assume this is waste heat coming from somewhere else - think of warm dry air from Air Conditioners' condenser coils. Slow self cool down once sun goes away every day. Can be more than one day if need be.

- Day 2: heating from room temperature to 1080F/600C, once again during 7 hours long period. But can't go above this temperature. Heat source is in the form of hot air at a temperature a bit higher than this. Slow self cooling once the sun goes away - may not go all the way down to room temp but definitely losing a lot of heat.

- Day 3: heating from room temperature (again, worst case scenario) to 1800F/1000C or whatever bisque temperatures are, in just 7 hours (yes!). Source of heat: electric heating elements. Slow self cooling once the sun goes away.

 

Will this work? Or the interruption in the heating midway is unacceptable? Also, can the pieces be moved around a little bit between Day 2 & Day 3?

 

I am differentiating these two days in case you're wondering, apart form what energy pattern is possible, by chemistry: End of Day 2 to me means calcining the clay - losing all chemically bound water and converting to metakaolin; hoping Day 3 energy input will be lower but only demanding high temperatures. I can give you the expected energy breakdown if anyone is interested in correcting me if I am wrong.

 

I have TONs more questions; I just don't know whether I am and my questions belong here. Please let me know if I am. I really will/do appreciate your inputs. I am sitting and dreaming about moving mirrors covering at least 100 sqft if not more.

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If your pots are well constructed, thin, even, and dry, they can handle a 7 hour firing to cone 04 no problem. I do it with my porcelain all the time. The second firing should not be necessary. If you don't plan to glaze anything, then only 1 firing should be necessary.

 

From what has been discussed here, making a kiln that runs directly from solar panels is a mighty tricky thing, if possible at all. You'd be better off using the panels to feed the grid all the time and offset the cost of firing the kiln. Even small kilns need 5000 watts- that's a lot of panels, and a lot of faith in the weather. A 250 watt panel only produces that amount for a portion of the day. Fluctuations in power are not something you want with your kiln.

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There are numerous commercial tile manufacturers that fire tiles cold to cold in less than one hour.  I have fired small bone dry terra cotta stamps with a propane torch in about 10 minutes.  It is all about controlling the temperature gradients inside the material as a function of time.  The gradient depends on the thermal conductivity and the heat capacity of the materials; these are not constants but are related to temperature as well.  Stated differently the heating rate is non-linear.  There is also a constraint on thermally induced internal stresses due to dimensional changes from thermal expansion and grain rearrangement due to evaporation and chemical reactions.

 

Do an internet search on 'solar kilns' and 'solar furnaces'.  I recall some experiments being done about 10 - 15 years ago.  It will be a challenge, not likely immediately efficient, but also not impossible, just maybe non-practical.

 

You will need some heat transfer knowhow.

 

lt

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Thanks for the replies, I really appreciate it. 

 

I want to dig a little deeper; my original question is still unanswered, it will be nice to look deeper. My first question is whether you can heat the piece to to around 1000F in one day using hot air, let it cool down over the night and do a second round of heating to Cone 04 on another day (which will be electricity that comes from Solar). Is this break in the heating allowed in the firing process?

 

To give you a better idea (better to see something that relates to this process): please look at this; https://urvishdave.files.wordpress.com/2012/08/solar-crematorium1.jpg. This in an experimental project obviously. What the creators are trying (and running into problems) is to focus a lot of the sunlight (50 m2) into a small cremation chamber and use that heat to cremate the body (they found that the heat isn't enough though). No electricity involved. 

 

neilestrick:

 

Thanks for confirming the 7 hrs schedule, puts me at ease.

 

Solar Electricity. Yes. But I am not talking about panels. But now as you can see why I am trying to break the process into two steps. the mirror structure focuses energy into a small area (can be a hole to let heat into the kiln) and things can heat up until some point (I keep coming back to my magic number 1080F/600C) aided by some air circulation to evenly distribute heat (cheaper, high output). Above this, the hole will be blocked by a electricity generating system (expensive, low output).

 

I even have the estimated cost of the solar cremation furnace numbers - it can work out to be cheaper if all you want is hot air and a lot of solar heat coming in thru' a small hole. Ideally, my biggest problem is fact is this: In the solar furnace design, you can see that the mirrors or somewhat standing up at say 45 degrees but enabling the target chamber on the ground (not the best use of the mirrors). 

 

I could elevate the kiln (yes, the entire kiln, say 20 feet, with everything goes inside it - pieces, insulating walls) and all the mirrors (they'll actually be more like thin reflective plastic  mirror films fit to a structure, covering a 40 ft diameter area) will be on the ground rotating a bit to follow the sun. You would have, say, a  2 feet dia hole (could be smaller but I am comfortable with this precision) in the bottom of the kiln with *all* the reflected solar heat coming in could be 60 kW avg over 7-8 hours (will be more like 45 kW at 8 AM & 4 PM, lowest and 90 kW at noon).

 

But you have to calculate how much heat will escape back thru' the same hole - if the temperature is lower, say 1080F, the net input might still be around 30 kW avg. But if I actually try to do entire firing (almost impossible) reaching cone 04, I just don't think it will work; the hole at this point is going to radiate more heat than what is getting as input (I can even give you numbers if you are curious).

 

So, after a certain point, it will have to be electric (an stirling engine + generator, if you care for the details can block this hole instead and make electricity. and yes, electricity can also provide some form of buffering/stabilizing.

 

But the engine+generator is expensive and it won't make sense to say the kiln is in solar thermal mode until 2 PM and electricity kicks in for the next two hours. Very low return on the investment.

 

So, for me it has to be a two step process, cost wise; one place (or one day) where the pieces can reach 1080F and do its thing and the other day it will be electric, ideally. The whole thing will be cost effective ONLY IF a good deal of heat can be direct heat (or thru' hot air) - not the solar electric route.

 

 

Magnolia Mud Research: 

 

One hour total fire for tiles. That's great news (even if it comes with complex heating requirements); I assumed it would be impossible; requiring a minimum of 7-8 hours in general for a reliable & repeatable process.

 

It is hard to control heat supply at high precisions though; Solar cremation example for instance is focusing 50m2 energy but all they care about is burning the body (if all we want is heat @ say 1080F/600C their target is 800C, comes to say Cone 015 in the pottery world). In theory at least a 20 kW kiln. 

 

Is that an acceptable heating rate? I am talking structures that will most be tiles, pipes & some clay tanks: highly uniform structures though with thicknesses from 1/2 an inch to as high as but not more than 1 inch (But definitely not complex like pots with neck, base, lip, handles etc).

 

Marcia Selsor:

 

Thanks. In fact this exactly what I want (I will search for the article and see whether I can find it). For instance, mirrors from say 10m x 10m area can focus at least 50 kW the energy onto 2 ft dia hole. so, essentially you'll be looking at a kiln where you can keep the pieces and a whole lot of heat coming thru' a small hole - which can be a long alumina pipe to buffer and distribute the heat somewhat.  Putting pieces directly here is just too complex, whatever is here will heat up quite fast and has to be moved around (I originally started thinking if each piece can be done in say 15 mins... well, you know the deal). 

 

I know temperatures can reach 800C but I am guessing the heating will be very uneven and definitely would not reach 1000C-1010C (Cone 04 range). At this stage, what happens is that the hole thru which I am expecting the heat to come thru' will start radiating heat back too much.

 

 

just FYI, In the solar crematarium project, they are hoping to save tons of wood BTW, impressive numbers (100 kg wood for every two hour session) if it works (I don't want to claim that  I am expert but I don't think it is possible as they have designed - starting with the mirrors orientation). But I know devil (absolutely no pun intended) is in the details though - in my case for instance that 50 sqft mirror just doesn't magically transform to a kiln that runs thru 100 kg of wood (let's say around 20 gals of propane or 400 kWh electricity equivalent) as the process is different.

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Well, I have to write a lot but let me keep this simple (I know I am taking a lot of time already). No matter how many times I tried to make a short reply all I end up with are posts as long as these. and for that (lack of my ability to be crisp) I apologize.

 

You can take a quick look at this url: http://www.solartronenergy.com/solar-concentrator/specifications/; you can get a better idea about the what I am talking about. These things are becoming more common but as you can see the entire structure rotates. They are parabolic mirror structure focusing sun light onto a tiny spot

 

Original designs for solar furnaces, and other big things (people thought will change the world in furnaces etc): https://en.wikipedia.org/wiki/Solar_power_tower. These concentrate sunlight using a lot of mirrors onto one stationary receiving tower.

 

Story starts with what that dish does for instance; that 30ft dia dish can concentrate most of the sunlight onto a 10"x10" square area; It could be a 45 kW heat source if all you want is to boil some water. But the heat you can extract will go down as you want higher and higher temps. You could also put a high temp solar panel thingy for instance (yes, just the 10"x10", no typo) and that generates as much electricity as if the entire 30ft dia was a solar panel. Or it could be some old fashioned system that use a small engine and a generator. 

 

For all you know a crazy solution is to put the entire kiln above in the focal point (15 feet above the ground in the center of dish - only the dish will rotate a bit) with a 10" dia hole in the bottom thru' which all that heat comes in. obviously whatever sits in this path, close to hole will probably just melt, a different story. This heat needs to be diffused and distributed, different topic. Too many things I have to explain if I try to explain the picture.

 

As you already know my magic number is 1080F. I can still get a good deal of heat (at least 20 kW) from this arrangement (without needing conversion to electricity) if I just need hot air (hot enough that it can travel back and forth between this focus point and the kiln - say minimum 15 feet distance) and there are also losses on the way. Lot of number crunching. But let's say this is still a cheap enough heat source.

 

Now the whole cool down and next day discussion is a more complex topic and I wrote his response thrice and delete it all because it was too long. But let's say it is because of many factors and I was just curious what happens; If you understand everything on my mind though, you'll be able to see why such crazy questions come up (which otherwise seems like an obviously stupid question)

 

See this for example (I have tons of calculations like these). I am not trying to explain what these numbers are but based on inputs from these forums I can figure which way to go. But I guess you can see how I have broken down diff things that I think happen during firing. (600C is 1080F). The numbers have to do with what happen to water, and clay (the ceramic part of it) itself. And the calculations continue even after that.

 

Drying 100C 758 30% SH#C1 600C 500 20% SH#W 600C 336 13% Dehyd 600C 361 14% SH#C2 1100C 550 22% Total Input 2505 100% Non Drying 1942   Stored Energy 810 1980 Refire Energy 900  

 

It is just that, I have lot of questions in my head and I really need to get the burning ones answered without which my brain just can't be at ease. It is 3:30 AM here and I am still awake. The notion of breaking the cycle in that range (even if it means a total cool down) was one such burning question. It doesn't mean that's what I want though. It is basically trying to do a bunch and cost and material analysis - the cost of energy is different in the first case and something else in the second case.

 

So, once again, thanks for your time.

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