Hot Bark

[jokar]

Had a little visit to Lidl today.  They are selling bags of bark so picking one up I noted it was hot to the touch.  I therefore had a little dig deeper into the palleted stack and it was very hot.  Good response from the manager though as I explained it and he had it moved straight away.  Would have been a nice fire.

[CivvyFSO]

Very strange. Is there any sort of special chemicals/treatment used on the bark?

[Phoenix]

I don't think you need any chemicals - just organic material, the right moisture level, good insulation and a little time.

[John Webb]

Well spotted! Was the store sprinklered at all?

I came across an interesting bark fire some years ago, outside one of the laboratories at the Fire Research Station when I received a call to a 'Flowerbed on fire'! Turned out that the flowerbed was covered in bark chips and we'd had a couple of weeks without rain. One of my colleagues went outside for a smoke (as his lab contained flammable gases) and threw away his stub without extinguishing it...... A quick dousing with a hose-reel put out the fire and perked up the plants as well.

[Phoenix]

Just in case anyone's not clear what's going on here;

What you are referring to, John, is the simple ignition of dry combustible material by a heat source.

What jokar spotted was spontaneous combustion.  No heat source is required for this, chemical processes provide the heat.  For this, there must be moisture in the organic material.  Some materials are more prone to this than others, pistachios and linseed oil soaked rags are commonly quoted ones.  There are many others.  Bark.

Many, many years ago my job was agricultural fire insurance and we used to have 'hot rick inspectors' whose job was to go round visiting hay ricks with a thermometer on a long probe to check that they weren't getting too hot (spontaneously).  Sometimes they used to climb up on top and there was a story that one fell in once and was consumed by the hot hay rick.  Possibly a myth but I believed it at the time.

I think quite a high moisture content is required for the process to start.  Many fires have started in this way.

Stu




 

[Mr. P]

Also with a plastic based wrap, even with breather holes in, moisture or probably condensation can build up and, with the weight/pressure of the stacked packs ...

[Phoenix]

Also with a plastic based wrap, even with breather holes in, moisture or probably condensation can build up and, with the weight/pressure of the stacked packs ...

Indeed, relatively cool plastic wrapping probably offers a mechanism for the formation of condensation that would not otherwise exist.

Stu

[Mike Buckley]

If I remember correctly the whole thing is biology based. What happens is that bugs start to digest the material and grow and multiply. Whilst they are doing this they are producing heat, as the heat builds up the bugs in the centre die but the bugs on the outside still grow, the heat continues to build up until spontaneous combustion temperature is reached in the centre. When the build up of heat reaches fresh air up it goes!

This also leads to the formation of a area of digested matter in the middle of the stack which is less dense than the rest of the stack hence Pheonix's tale.

As we are dealing with bugs the whole thing does depend on organic material being present, hence the need for damp conditions and the emphasis on hay, straw, linseed oil soaked rags and bark.

This has nothing to do with the farmer who was panicing when the local brigade was damping down a stack fire as he was hiding a Roller in the middle of the stack. Something to do with tax I believe!

[Mr. P]

I've heard them [tax people] called alot worse than 'bugs'...

[John Webb]

A further example of the right mixture of organic material and damp being needed is in archery. When I took up archery in the late 1970s targets were still of straw. The club I belonged to had to build special racks so that the targets could be spaced away from each other, particularly if wet. These racks allowed the targets to dry out quickly and disperse any heat generated. By the time I stopped shooting about ten years later more modern materials which absorbed less water and did not contain micro-organisms had come along and the problem was resolved.