Spontaneous Combustion? HELP needed to identify cause

OP - Great to hear you got to the bottom of it (runs around house covering mirrors)

Brendan - brilliant link
 
Mysterious burn marks on uPVC window frame - just happened to me too!

Hi. I have just looked this up on Google as it has just happened to me and that is how I found this post. I have been living in the house for 14 years and the first occurrence of a slug-shaped burn mark on the inside uPVC frame of one of my bedroom windows was about 6 months ago. I painted over it and did not pay it much attention but it happened again y/day - this time, the size and shape of an adult slug - about 3 inches long. There were what appeared to be scorch marks above the shape but not below - hence it could not be chemical as presumably it would have dripped down whereas heat rises.

I live alone so there is no culprit as such who may have caused this damage but who does not want to admit it to it.

Now, I read the mirror theory and, indeed, there was a two-sided shaving mirror there - which I have just removed. The problem is that the 'burn' mark appeared over-night between the hours of 9.30 pm when I closed the curtains and 8.00 am when I opened them this morning. Today's date is September 17th and I live on the Isle of Man. The window is south-facing thus no direct sunlight would have been shining into the room at that time and, in any case, it was not an especially bright or warm morning. Furthermore, the curtains would have prevented any direct light from outside shining on to the mirror.

Thus, the mirror theory does not appear to cover this mysterious occurrence. Thus, I'm left with a real mystery. How did this spontaneous burn mark appear over night on the uPVC window frame while I was asleep in that very room?

I have ruled myself out as a suspect because, if I had done it whilst sleep-walking - and I have no history of sleep-walking and nor do I smoke or have ready access to a lighter or blow torch - then I am sure I would have set off the night alarm which was still set when I got up at 8.00 am this morning.

I would really appreciate any theories as this is a very strange and perturbing event.

P.S. I have managed to repair the appearance of the frame through use of polyfilla and three applications of white gloss paint to the affected area.
 
Shaving mirror culprit?

Just looked again at the link to 'blackmuseum' posted by irishmoss in this thread.

The photo of the shaving mirror adjacent to the scorched uPVC frame looks very convincing. I had left the house at 4.50 pm to go to work last night. The sunlight was strong and would have been shining directly into my south-facing bedroom window where the shaving mirror was sitting on the sill with the magnifying side facing outwards.

The damage in one of the photographs at above link is very similar in ways. As unlikely as it at first seemed, I must have simply not noticed the damage last night when I closed the curtains. As it was dark when I arrived home at 9.30 pm, it is possible that I closed the curtains before putting the bedroom light on. I have a habit of doing this and this would explain why I only noticed the damage this morning.

I have since found a number of articles on the internet which describe house fires beginning in this way - so thank God the curtains did not ignite!

Anyway, the mirror is now inside a closet - and many thanks to irishmoss for that brilliant insight and for possibly saving my house from burning down at some point in the future. The internet is truly a treasure-trove of knowledge!

And I believe that this mystery is now solved!
 
My sister had a fire in a rental property when, after cleaning the house between tenancies, she left cleaning cloths with Pledge / Mr Sheen / bleach / kitchen cleaner / etc piled on a window board. The cloths ignited due to reactions between the cleaning chemicals and she had to replace the uPVC window and have the kitchen area professionally cleaned and redecorated. Her loss adjuster said it not an uncommon event.

I also know from personal experience that teak oil and linseed oil should be washed out of cloths and brushes after treating doors and windows as they can also self ignite.
 
Another no-no is mixing cleaning products. Many a person has been found slumped over the toilet bowl after mixing products that react to give off chlorine.

Regarding the mirror burning problem, another one to watch out for is binoculars or spotting scopes on stands. I have several lying around due to an astronomy interest. I always check that lens caps are on.

One thing I notice from the photos posted by the OP is that the brown marks radiating upward from the primary scorches are extremely even, with vertical edges. That is symptomatic of constant heating producing a laminar updraft. A flame from a blowtorch or some such would produce a turbulent updraft and you would not get those vertical edges. The slug shape, of course, is because of the off-axis image formed by the sun.
 
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