So how did the fire start again?

[mr x]

I take home old coffee grinds from work to keep cats out of the garden.

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[GuingesRock]

I don't get the question? What's going on in the pictures Rob?

[mr x]

That is the remnants of a round coffee filter. I left it outside in the bowl to dry out.

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[Jimmy]

I'm going to guess the shape/reflective properties of the bowl caused the fire?

[mr x]

That's the best I can guess. Sunshine lit it up.

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[GuingesRock]

Some other thoughts .....

Campers use dried coffee grounds as fire lighters, which suggests to me that they have a lot of oil in them. If that is the case, then spontaneous combustion might be another possibility. The paper filter might have soaked up the oil and then spontaneously caught fire.

The bowl doesn't look the right shape and the paper would have had to be at the optical focus point of the bowl, and it doesn't look like it would have been. A lot of the reflective curved area of the bowl would have been covered by the filter and grounds.

Also, the coffee grounds might have acted like a mini compost heap, and compost heaps heat up and catch fire. So do stacks of damp hay bales.

http://en.wikipedia.org/wiki/Spontaneous_combustion" onclick="window.open(this.href);return false;

[mr x]

These are possible as well. I have also considered that a bee flew into the focal point of the bowl, was shot down in flames, and lit the filter on fire.

[GuingesRock]

A "May Bee?"

Might be a spontaneous combustion process assisted by the sun heating up the metal bowl and creating a kind of oven.

But, the curved part of the bowl has a tight radius and the focal point would therefor be close. So that theory is still a strong contender.

[GuingesRock]

But, the curved part of the bowl has a tight radius and the focal point would therefor be close. So that theory is still a strong contender.

Can you put a plain paper filter in the bowl out in the sun, without the coffee grounds?

[mr x]

yeah, next time the sun comes out.

[akr71]

But, the curved part of the bowl has a tight radius and the focal point would therefor be close. So that theory is still a strong contender.

Can you put a plain paper filter in the bowl out in the sun, without the coffee grounds?

I suspect that the coffee grinds, or rather their colour, were a large contributing factor. A plain filter might not do much.