Making a lens

[rich59]

I've had a plan (never got round to it yet) of taking a silvered survival blanket, stretching it over a large pot and removing some of the air from the pot, ?with a tube tucked down the side, to make a concave mirror to focus the sun.
As I say, it's a plan. One of these days......

John - like the idea. Have you tried taking a torch bulb out, replacing with tinder/ char cloth and then pointing at the sun? Works well.

But, going back to your parabolic reflector and sucking the air out. How about if you stretched the survival blanket as described and then pour water on it - filled to the brim. The weight should make it sag and therefore make the parabolic shape. Would work best at mid day in the tropics though.

 

[KIMBOKO]

Making a fire hot enough to melt glass - just had a memory of going round a glass factory. For small work like bending a glass rod they would simply put it into a hot blue flame like a bunsen burner at school.

The difference between the flame that comes from burning wood (yellow) and a hot blue flame (e.g. bunsen burner, blow lamp, kitchen gas ring) is the air / gas mix. Therefore is may be possible to get a hot blue flame from ordinary wood.

First one must be able to control/ focus the gas coming off the burning wood. An example of this is when one makes char cloth. Put your material - be it cloth or wood or whatever in a closed tin with a hole and put it on the fire. Once it is hot you get a jet of gases that ignite and burn quite excitingly in a jet of flame.

So, now all I have to do if figure on the best configuration to get a good gas/ air mix before it comes out of the hole. It should probably not be under pressure, so a larger outlet hole would be good. Then I probably need a hole in the bottom or sides to draw in air. But then air around a fire is probably a bit starved of oxygen so better to get air input from an axtra source such as bellows. Also air coming into the tin would simply allow the wood in it to burn normally. So, I need the gas to leave the wood before adding the air. I see a tin with a bunsen burner sitting on top in my mind's eye.

I'll work on this. Has anyone else any thoughts or experience on burning hotly the gases coming off burning wood?

In another thread the reverse downdraft gasifyer is mentioned .... it burns the gasses given off by the wood without smoke by allowing copious amounts of air to be mixed with the "wood gas". I have made one with great success.

Jacqui Wood is an experimental archaeologist has suggested a use for finds of pot type objects with many holes in them. She suggests they are in fact a very early type of "bunsen burner". The pot acts as a chimney with a central hole and the side holes add the extra air to provide a very hot flame. Possibly used for jewelery making.
The pot is put over a naked flame as is very effective.
You may have to Google to find out more.

 

[rich59]

I guess several people have tried making a water drop lens. But has anyone succeeded in making fire? The comment on the web is that it "takes a long time". Is this a euphamism for "it doesn't work when I try it"?

There are a few ways I have tried to this:-

- drop water in the end of a straw,
- water drop in a hole in a leaf,
- water drop lying on the surface of a leaf - this works best on a leaf that refuses to wet - Water iris is my best find yet. If the sun is not too high in the sky one can get the focal point on the surface of the leaf just off the drop.
- droplets hanging in a tangle of threads.

Each one will at times give me a drop of a few mm and a tiny focused pinpoint of sunlight that one can train on some char cloth. No smoke and no fire yet.

The tiny pinpoint - what is my best use for this? Clearly any spot of heat gets as hot has allowed by energy in less energy lost by convection, conduction and radiation. With my tiny speck - just a mm or two from the water drop - how could I reduce the heat losses?

In experiments with a camera lens and adjusting the aperture I found it difficult to get enough heat to ignite char cloth from apertures of less than about 10mm.