Thursday, March 7, 2013

Word of the Day: Auto- ignition - Pyrolysis - Spontaneous Combustion


I just read Ceramic Review for this month and they had a letter in there by a potter who had been in business for 34 years and his kiln was fired 432 times. He went to bed and the shed caught on fire.  He attributed it to Pyrolysis. He had never heard of this!! That I found astounding! This is actually a common occurrence with old time potters.

So there are three words of the day : 


Pyrolysis - Pyrolysis is the thermal decomposition of organic material at elevated temperatures (800ºF– 1200ºF) in the absence of oxygen. Exothermic reaction.

This is what happens when wood is heated. The wood is not what is initially burning but the gases that are given off.  Click here for more information.

So what happened to this and many other poor potters is more like Auto-ignition.

Auto-ignition - is the minimum temperature required to ignite a gas or vapor in the air without a spark or flame being present. 

What happens is over years of heating the wood in the shed it is so dried out and charred it can ignite at a VERY low temperature (under 300 F). This is detailed in this paper (click here or read below). What they do is to heat wood over years at various temperature and then test where it ignites.

Only potters conduct the experiments over 30 plus years ! BEWARE!!!!!

That is the reason you make your shed of METAL. This potter had fiber around the chimney, etc. Another option is to separate your kiln shed from your house!

There is also spontaneous combustion.

Spontaneous combustion-The bursting into flame of a mass of material as a result of chemical reactions within the substance, without the addition of heat from an external source. Oily rags and damp hay, for example, are subject to spontaneous combustion.
Excerpt: With prolonged exposure at all of the temperatures used there was a gradual
darkening of the wood, accompanied by  loss of weight and shrinkage in the transverse dimensions of the specimen. Chemical destruction of the specimens, as indicated by their loss of weight, was not associated with any one critical temperature. Instead, at each temperature of exposure, the specimens lost weight at a rather regular rate, and the rate became faster as the temperature was raised.

Samples which had been exposed to 107° C. for 1,050 days assumed a light chocolate shade. Those exposed to 120° C. for 1,235 days became appreciably embrittled, were of a dark chocolate color, and when moistened were strongly acid to litmus paper. Those exposed to 140° and 150° C. had the appearance and friability of charcoal even before they had lost 65 percent of weight or their original air-dried weight at 6 to 8 percent moisture content, but none was ignited during its exposure.

...In a confined space, however, the opportunity for escape of the gases and the heat accompanying oxidation would be lessened, and the danger of developing spontaneous ignition would be increased. This may account for the fires that have been reported to have started in wood in direct contact with low-pressure steam pipes or in wood heated at temperatures below that where the exothermic reaction normally becomes a factor ( 9 ). There are also indications from experience with wood in dry kilns, steam tunnels, and other places that long continued intermittent heating and exposure to damp conditions accelerate the decomposition of wood.

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