Sulaiman - 21-3-2016 at 10:41
I put one of my wife's kitchen cloths on a wooden chair near the door of my lab/shed
to cushion a 2 litre Florence flask of distilled water from the lab
and I poured (hence outside) the water into 500 ml hdpe bottles
so that I could use the flask later
then I put the flask safely in the lab..
Later whilst gardening my wife asked; how did I burn the cloth?
it did not look like a (prime suspect) cigarette burn,
the cloth was eaten through and charred around the edges,
the paint on the wood was charred and bubbled ....
the only possibility that came to mind was used iodine that I washed out earlier, looked like iodine corrosion too,
whilst getting some ammonia and sodium thiosulfate my wife worked it out.
To test her theory I filled the flask with water and stood it on the cloth
within seconds there was a new smouldering spot !
Considering that the Florence flask must be one of the oldest designs
and it can almost perfectly focus the sun to a spot
I'm amazed that
. I did not think of it
. I have not heard of it.
Now you have all heard of it,
a Florence flask full of clear liquid may focus the sun to a very hot spot.
ave369 - 21-3-2016 at 12:04
An RBF can do it, too.
elementcollector1 - 21-3-2016 at 12:32
I wonder how high the temperature can get? Can't be that high, if the wood didn't burn.
Sulaiman - 22-3-2016 at 00:11
The full flask was only standing for a short while as I poured it into four bottles,
So the full flask was stationary only briefly.
If it is sunny and I remember, I will video something being ignited
Herr Haber - 22-3-2016 at 08:37
In march and already complaining about the sun. Ahh, those brits 
Though at least, you had the excuse it was nature, not you doing holes in the clothes / furniture !
unionised - 22-3-2016 at 13:25
My dad used to use a RBf full of water as a cigarette lighter (in Kenya in about 1960) where sunlight was more dependable than most things.
The same broad idea has been used for this
https://en.wikipedia.org/wiki/Campbell%E2%80%93Stokes_record...
and vases full of water on windowsills have occasionally caused house fires.
yobbo I - 22-3-2016 at 16:54
Bottles conaining water or liquid have been blamed for starting forest fires