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Pottery Kiln Automation

03/01/2017 4:43 PM

I've recently purchased two old pottery kilns; a Paragon A66B 14" x 20" small kiln, and an L&L K230 23" x 30" medium sized unit. After replacing a switch and an element each, I have both working in their old manual ways. Only one kiln or the other can be used at one time due to high current need.

I'm reading as much as possible to educate myself on what a clay pottery kiln does. The first step is low setting for a set time to get the water out of already air dried clay creations, without boiling water and breaking them. I thought a kiln vent system with a humidity sensor could actually automate this step. Instead of just cooking away at 200-250F for (x) hours, I could wait for a set point humidity within some standard deviation stability time and automatically ramp temperature up to medium and do the same at medium heat. The final clay quartz inversion step will probably remain a timed event, as I don't know what else I could measure to determine when the final step is done.

The hardware needed would be an N type thermocouple for each kiln, a blower motor with metal piping to pull air through a 1/4 hole in the bottom of both kilns, a sheet metal manual mix valve to allow enough air into the vent system to keep it cool enough not to cook the 356F capable humidity sensor in the exhaust flow pipe. Two SPST, and one DPDT 240V relays per kiln to control the elements. I would use an NI-USB 6008 to interface the sensors and relays to a Labview program I could write in a day.

There are commercial kiln automation controllers available, but none actually monitor the process, they just adjust temperature per time. These also cost about 3 times what the parts I've identified do.

Has anyone here tried this yet?

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#1

Re: Pottery Kiln Automation

03/01/2017 6:14 PM

I would use an Arduino. There are a plethora of 'shield' boards out there for moisture, temperature...

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#8
In reply to #1

Re: Pottery Kiln Automation

03/02/2017 6:29 AM

Or even a PICAXE if he is not very computer savvy, as its even cheaper to buy and all the software and manuals are free to download and use, plus there is a great forum when you meet some really clever people!!

Such ideas are perfect for the fast BASIC PICAXE uses....

What-Is-PICAXE

Picaxeforum.co.uk

Best of luck to the OP and do tell us how you get on no matter what you use!!

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#2

Re: Pottery Kiln Automation

03/01/2017 6:42 PM

I'm not sure checking the humidity will be reliable enough to be worth the effort, but worth some experimentation I guess....To get consistent readings the humidity and cfm flow of the entering air would have to be consistent, I'm not sure that's doable on a hobbyist's budget...you realize the amount of moisture is very small....

http://www.bigceramicstore.com/info/ceramics/tips/tip49_load_fire_kilns_part2.html

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#3

Re: Pottery Kiln Automation

03/01/2017 8:30 PM

A couple of suggestions:

You probably need to arrange for some heat exchange to preheat the incoming air using the exhaust. This will help to minimize cold drafts, thermal shock, saddened pot throwers. A length of coaxial intake and exhaust could be sufficient.

Rather than the entire exhaust, it will be far easier to route a small amount of the exhaust flow to be cooled and measured for water content. No need to admit outside air to cool, just run long enoufh pipe to allow enough heat exchange with ambient.

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#4

Re: Pottery Kiln Automation

03/01/2017 8:53 PM

I have a ceramic potter friend. I don't recall her ever performing a drying cycle as the 'work' was installed in the kiln after being fully air dry.

There may be a initial low temp setting to final dry, but you don't ever put wet clay in the kiln. Your bigger problem is learning the techniques to prevent cracking while drying, as different thickness of the piece will shrink differently pulling the work apart. That is what she fought more then any other problem. She has a Paragon Kiln sitter. This uses cone firing temperature references. They make these to melt at different temperatures. Paragon makes a temperature switch that has 3 fingers that are mechanically connected to a power switch to the heat elements. When the cone melts, the spring loaded fingers move to open the power switch.

Cone 4 through 14 are the typical firing temperatures of clay (Vitrification). Different clay bodies requires different temperatures to melt the glass silicon. As well glazing has it's own cone temps based on the ceramic base piece and proprietary glaze ingredients.

If you fire more then one layer of work, you will need kiln furniture to make use of the volume of the kiln. A solution for my paragon heat treat furnace (a glass makers kiln front load) came with a programmable controller that allows time segments, with temperature and ramp, and hold times.

http://www.paragonweb.com/Sentry_Digital_12-key_Non-Ceramic.cfm

Note this controller has an output to control a kiln vent. The link references a Orton KilnVent.

https://www.dogwoodceramics.com/product/orton/orton-kiln-vents.htm

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#5
In reply to #4

Re: Pottery Kiln Automation

03/01/2017 11:07 PM

'Fully air dry' is not typically dry enough. Greenware (airdry) pieces are fired once at somewhat lower temp regiment to become bisque. Glazes are typically then applied and the piece can be fired again to complete the glaze.

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#6
In reply to #5

Re: Pottery Kiln Automation

03/02/2017 1:45 AM

Proper firing is a little more complicated than that.....

..."Mechanical or “free” water is driven off during the initial stages of firing, from 100 to 200 degrees C, and the chemically combined water begins to leave the clay structure in the 450 to 600 degrees C range. As the temperature increases, organic matter is oxidized or driven off, and the clay body begins to sinter or form itself into the first stages of melting.

Vitrification starts when the primary flux in the clay body (frit or feldspar) begins to react with silica, which is generally in the form of flint or is tied up with other materials in the clay body. (For example, many frits and/or feldspars found in clay bodies contain silica.) As the temperature increases, more silica is drawn into the flux, causing increased amounts of glass formation.1

The optimum vitrification temperature depends on the type of clay being fired. For example, most stoneware clay bodies have a maturity range of two or three cones (cone 6 [2232 degrees F] to cone 9 [2300 degrees F]). This means they will be non-absorbent if they are fired within these cones. Porcelain clay bodies, on the other hand, must be fired to cone 9 (2300 degrees F) to achieve full vitrification.

If a clay body does not reach its maturity range during firing, the finished product will be porous and absorbent. Alternatively, if the clay body is fired past its maturity range (or if it contains too much flux), it can warp, stick to the kiln shelf, exhibit a glassy appearance, and eventually slump and deform into a glass puddle.

The glass phase in a clay body is also influenced by the amount of time required to fire the clay to maturity. A clay body that is placed in a kiln and fired to its endpoint temperature in three hours will not be anywhere near as dense, hard and vitreous as the same clay body fired over 12 hours to the same temperature."...

http://www.ceramicindustry.com/articles/84673-ppp-clay-body-shrinkage-absorption

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#7
In reply to #6

Re: Pottery Kiln Automation

03/02/2017 2:48 AM

In reality, it is even more complicated than that....

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#9

Re: Pottery Kiln Automation

03/02/2017 8:40 AM

Both kilns have "Sitters" which cuts power via mechanical switch when the cone melts at its set temperature. We had our first successful bisque firing last night. The Orton kiln vent is what inspired this idea.

My thought is to monitor and automate the process with measurement of the atmospheric conditions inside the kiln. The free water stage is specifically what I was after on this first step. Looking down the road to the vitrification step, I'm imagining an x-ray for density change could work, but not in my basement! The kilns I have max out at 2300F/1260C, so cone 9 is the top of the heat settings.

TE Measurement specialties has a humidity sensor at DigiKey that can handle 140C temperature for $6.46. I've found N-type thermocouples in the $35 range that handle 1287C. I didn't consider flow measurement, (Thanks SolarEagle). A tee fitting with two humidity sensors, one pre kiln input to measure ambient humidity, one post to monitor kiln conditions, The MAF from my van $28, and a temperature probe in the kiln and one in the pipe, post kiln should do it. I will also explore the long pipe/ no mix route, but I'm leary of condensation and time lag.

A raspberry Pi may be the final product, but in the mean time a $150 NI-USB 6008 will allow me to log sensor readings to determine if this is a feasible endeavor.

Andy... A Pic controller is what I used on my senior EE project in college. I interfaced a stationary bicycle with a video game, you had to pedal for the guy to move (PWM the up arrow) and steering was done via the handlebars.

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#10

Re: Pottery Kiln Automation

03/02/2017 10:19 AM

Oh, kiln process is just like drying malt and some crops like rice and wheat.

You can do some hygro calculations and come up with the temperature, the length of kiln, rpm and hot air speed.

Some cement plant use IR monitoring like flir.

Go ahead its doable.

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#11

Re: Pottery Kiln Automation

03/02/2017 10:51 AM

You might want to actually add a small pipe to the kiln to draw air off from the kiln, cool it and measure the humidity rather than monitoring the exhaust. That way you can cool the sample air in a controlled manner and measure the humidity and then just shut off the draw off air with a solenoid valve during high fire.

We used this method on gas-fired heat treating furnaces to monitor and control humidity levels.

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#12
In reply to #11

Re: Pottery Kiln Automation

03/02/2017 12:33 PM

That is an excellent idea! Quite surprising no one made that suggestion until now.

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