SUNS LIGHT / SOLAR RAYS CAUSING FIRES

[Guest]

IHi folks

Was intrested by a serious broadcast last year on the BBC - it was a documentary about LFBs FITs

They mentioned some fires can be caused by sunlight focussed through windows igniting materials inside buildings.

How many cases have you come across where the fire was started by this?

And what kind of glass is more prone to allow this to happen - ie; stained glass, bowed glass etc etc :idea:

[colin todd]

There are certainly cases where the rays were focussed by glass jugs of water or similar.

[Guest]

Also shaving mirros are good for focusing energy. The Isle of White Fire Service have a picture of the effects of sunlight being concentrated, this picture is in Principles of Fire Investigation by Ide and Cooke published by the IFE. Consideration should also be given to the orientation of the room and the position of the sun in the day in question. Bare in mind, if the room of origin is facing North then it can't be the sun and if the day was cloudy then it is also unlikely. Stained glass windows or windows with patterns in the glass which have a concave arrangement will provide focusing of light much like a magnifying glass. The sun would have to be of a good intensity and the material close too i would suggest. Any questions, please make out a self addressed envelope to the usual address.

[p.b.morgan]

Bull's eye glass was a favourite at one time.  The Fire Research Station did a lot of work on this a very long time ago. Several sitting-room curtains and hall carpets were ignited this way. There was a BRE Report on the subject but can't find it on the website.

Glass items such as paperweights have been implicated from time to time as well; dressing tables with mirrors can be helpful here.

[Jim Munday]

It can be a pretty tempting cause when you've run out of other ideas but be careful!  The essential thing is that the sunlight has to be focused on the fuel package (transmitted light by a convex surface and reflected light by a concave one - the other way round won't work!).  Also you need to confirm by experimental measurement or calculation (check out your GCSE physics books) that the focal length of the supposed focusing medium is appropriate when compared to the distance between it and the fuel.  I've seen some classic non-possibilities, including a bulls-eye window in a front door which had supposedly caused ignition of a hallway carpet 3 metres away and a plastic sucker on a car window which allegedly ignited a seat back over a metre away.  When the focal length was measured, it was about 10 cm.
Our old MPFIU boss Adair Lewis had one incident similar to the example in Cooke and Ide, involving a make-up mirror, and I think the Acton FIT of the FB also had a similar case some time back.  Now that I work here in Oz it is generally accepted that broken bottle bases can start bush fires this way but only if they come to rest in the right orientation.

[Stan Ames]

The minimum radiative heat transfer required for ignition is in the region of 15-20 kW/m2. The sun provides a maximum of about 0.8 kW/m2 so a concentration factor of at least 20 must be provided.

I'm certain I'm not the only one who ignited paper with a magnifying glass as a child and suitable lenses can easily provide the concentration required on a hot summer day. The most unusual object I have come across that was thought to be responsible for causing a fire in this ay was was a goldfish bowl filled with water and goldfish. Concave make-up or shaving mirrors are common culprits.

In his excellent new book, Ignition Handbook, Vyto Babrauskas provides a detailed discussion and list of products that can produce ignition by concentrating the sun's rays.

Hope this is helpful

Stan

[Brian Downes]

I investigated an incident a couple of years ago where rays from the sun shone through a window and ignited a sofa after  being focussed by a free  standing magnifying glass used for needle work.

[Dave Townsend]

These fires are one of our constants. All the items mentioned above (apart from Jim's sucker on the car window and bulls eye in the door) have been documented. Plus - a goldfish bowl!
Flat or bowed (non-mirror) glass, stained or not, will not do it. If the goldfish bowl had been empty there would have been no fire. Filled with clear water it acted like a solid glass ball.

The most vulnerable periods appear to be the last rays before winter and the first rays after. The sun is low at those times compared to high summer and penetrates deeper into rooms. The rays may also be stronger when there is less heat haze to pass through.
There is probably a human factor here also: those are perhaps the periods when people are most likely to throw the curtains, and the nets, open to make the most of it.

[messy]

I attended a call which was eventually recorded as sun through a magnifying glass which was stored upright in a desk tidy thingy.

When asked whether we should take some of the broken magnifying glass outside to test it's focal length (ie to determine whether the magnifying glass was close enough from the perceived 'item ignited first') a rather nervous LFB FIU officer (perhaps worried about losing face) was adamant that such a test wasn't necessary.

As previously stated, sun thru glass maybe a useful 'get out of jail card' by some investigating officers. However, as a result of this incident, although I accept that this is a possible ignition source, I am rather sceptical about the number of  fires so recorded, and wonder why more of these decisions aren't challenged.

[Brian Downes]

Very interested by Dave Townsends comments about the spring/Autumn rays.
as the case I investigated was on an early spring day with the Sun low in the sky.