# Fresnel lens



## Anonymous (Apr 19, 2009)

While out on my milk run the other day came across a large television set with a huge Fresnel lens.

Has anyone melted metal with one of these lenses.


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## Oz (Apr 19, 2009)

It would depend on how tightly it is focused. You have a bit under 1kw per square meter available to you in perfect sunlight. If you really want to do this I would use a parabolic dish lined with Mylar with your crucible held at the focal point with a ceramic support.


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## bmgold (Apr 19, 2009)

I say give it a shot. You have the lense anyway. 

I played around with a smaller lense in high school and it would easily ignite paper and even wood. I never tried melting metal with it but have read that it can be done with the larger ones. 

Wear dark glasses when you play with it or you will be seeing spots for a while. Also, be careful of where you store it since when the sun moves across the sky it could start an unintended fire with no one around to put it out. 

I'm interested to know how this works out for you. Other than your time it could eliminate your fuel costs for melting small amounts of gold. Solar powered gold refining!


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## ck (Apr 19, 2009)

deleted


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## peter i (Apr 21, 2009)

The wattage of even a very small blowtorch is normally more than 1 kW, and that makes using even a 1 square meter Fresnel a lost cause for anything but a "proof of concept".

"Solar desoldering" might work, but the shiny solder will reflect a lot of the heat.


(But try searching the web for "solar cooker" now thats something!)


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## ElectricTreasure (Aug 28, 2022)

Anonymous said:


> While out on my milk run the other day came across a large television set with a huge Fresnel lens.
> 
> Has anyone melted metal with one of these lenses.


yes ive melted all kinds of stuff. had to weld up a little stand so i wasnt holding the lens all the time and didnt want to use wood because the second any wood comes under the light it burst into flames. very fun to mess around with but can be dangerous. wood doesnt smoke it instantly burst on fire, pennies instantly melt, rocks explode or turn into glass and magma, you can refine small amounts of metal for free, only issue that sucks is having to adjust as the sun moves. Hazards would be lighting things on fire if left unattended. burning yourself if you dont turn the lens away from the sun, and exploding rocks are very sharp, if you burn your arm you see a gloom of smoke and wonder what the hell that was from and a second or two later you realize that was you.....clothes burst into flames. i dont have kids so no danger for me there but i left the lens unattended one day and it burned the assphalt on my driveway, then warmped my garage door as well as burning the paint. so when your done put the lens in a safe place so it doesnt get broken or have the ability to burn anything. But i was able to fix the dented metal on my jeep on day. it got it hot enough for me to easily bend the steel back into place on the roll cage. id say i had alot of fun with my lens even though your neighbors are confused and everyone drives by slowly. but not all lenses are equal some suck and some are amazing so try a few different ones. by best lens can get to about 3000 degrees in seconds while the other didnt focus the light well and only warmed things up to maybe 800 degrees. instant fire is really amazing  not a little fire either like grab any log and its on fire more then a torch can do in almost 30 minutes of constant flame. so dont burn your arm get some welding gloves and welding glasses or dont look at the light no matter how beautiful it is. its like looking at a welding arc.


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## olawlor (Aug 29, 2022)

I use #10 welding goggles for my 0.6 x 0.4 meter fresnel lens. It has no trouble melting cement silver powder to nice centimeter-sized blobs, but it's only about 400 watts total heating, so larger pieces of metal are more difficult to melt on a graphite crucible. Because a melted silver blob mostly reflects the light, it can be difficult to remelt a previously melted blob. It's possible some high temperature insulation wrapped around a small graphite crucible could retain heat and increase the melt quantity above a few grams. 

Safety issues do abound--any trace of salts will volatilize to extremely unpleasant halogen gas.


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