water bag fire

[pritch]

Hey I just tried making a focul point with one of these but i couldnt seem to get one. Has anyone suceeded in using this method and have you any tips?

 

[Moonraker]

A few more details would help I am guessing it is using a clear plastic bag full of water used as a magnifying device to light tinder etc, something like a magnifying glass?

 

[pritch]

Yup thats exactly what I mean. I have read it mentioned on here but nobody said exactly how to do it. I tried making a kinda lense shape but i couldnt seem to get a hot enough focal point to burn stuff.

 

[alick]

Never done it but...

The more water the light has to go through, the more of it's energy will be absorbed in the water. So try and keep the water "lens" thin.

Water has a refractive index (ability to bend light) around 1.3 compared to 1.5 for ordinary glass and 1.7 for very special glass. This means you'll need a thicker, more rounded water lens than a magnifying glass but i'd GUESS you don't need more than an inch of water at the thickest part. A wide lens will gather more light ...

Makes me interested to try it out now

Cheers

 

[Lost in civilisation]

This website shows what you are trying to do

http://trackertrail.com/survival/fire/water/index.html

 

[Tantalus]

saran wrap is cling film pretty much



Tant

 

[pritch]

I tried again today with no sucess its very hard to get a perfectly round with no lines bag I found. So the light difracts in all different directions and you cant get a good focal point. I tried cling film and a condom but no success.

 

[Hogan]

I tried again today with no sucess its very hard to get a perfectly round with no lines bag I found. So the light difracts in all different directions and you cant get a good focal point. I tried cling film and a condom but no success.

Sorry, I misread the post three times as:

I tried cling film as a condom but no success.

before I got it !!

 

[pritch]

lol
i read what you wrote bottom up and thought thats what id wrote! was slighly shocked for a second.

 

[AJB]

Hi,

This is brilliant, but I doubt I could get it to work in Lancashire ;o(

Just to be anal, you should be talking about refraction not diffraction, two different things. (Had to put that in or my dad would shout at me!)

 

[rich59]

I have been working on ways to make a fire with water for a while. Yes I did get a plastic bag of water to light some char cloth. Interestingly the configuration looked little like a conventional lens. I found the best focus was down into a corner of the bag, possibly something to do with reflecting off the inside down into a cone shape I suspect.

More times than not though I just got my tinder wet when the bag sprung a leak.

 

[rich59]

Just to say that I had another go today as it was a gloriously sunny day. I used an extra strong freezer bag 12 long and 8 inches wide when open but only about 5-6 inches wide at the bottom. I filled it about 1/3 full with water, excluded air and tied it off by knotting the bag itself. Then I put pressure on the bag so that all the wrinkles on the two flat surfaces disappeared (except at the very edge). Then I had a passable lens between the two smooth surfaces. I then spent about 15 minutes squidging this about over some charred cotton string (char cloth would have done as well) until I finally got a hot spot that caught the string that then started to glow.

 

[Beakytzw]

You may have started something now the price of clear plastic bag will skyrocket now they can be used in bushcraft!!!