The sun is out, so it’s tough to be inside working on the computer. Poor Erin is stuck inside working on her book, but I can still find excuses to go outside. A couple days ago I decided to break out the lens.

Timelapse fog and sunset from Bretwood Higman on Vimeo. We’ve had incredible fog rolling out Kachemak Bay and Cook Inlet for the last several days, and it made for quite a sunset this evening. In the distance you can see Iliamna Volcano, the least active of the four volcanoes you can see from our yurt.

Origins

In this case I’m not talking about a camera lens (plenty of that too though.) When I was 13 years old or so, I came across an advertisement in Edmund Scientific magazine for a giant plastic Fresnel lens. The ad said that it could focus sunlight to produce temperatures of 3000 F. I had just watched a documentary on the eruptions in Hawaii, and remembered the narrator talking about how hot the lava was… 2000 F. So could the sun melt rock?

Strips of gravel fused into black glass using our fresnel lens solar furnace.

I bought one. $150 was quite a bit, but I found a key ring that had a $100 reward on it… my mom thought I was going to get ripped off. My dad built a frame and stand for the lens when I got it, and I set it up on Schooner Beach where I lived then. And would you believe it, it really did melt rock! The gray gravel of the beach fused into a line of shiny black glass as the sun moved through the day. Clearly it was time for some experimenting. ! Into the focus went all manner of interesting things. I burned wood and plastic, a Bic lighter, melted wire and coins, finding that modern zinc pennies catch fire. I melted shale, olivine, and schist, stood back as carbonate cemented sandstone popped apart in a machine-gun of superheated rock flakes. And I melted bits of beach glass.

Giant fresnel lens as a solar furnace from Bretwood Higman on Vimeo. In this video I show how you can use the lens to cut paper. It can take a minute to download, so be patient!

Melting gravel using the sun from Bretwood Higman on Vimeo. This video is a time-lapse showing the sun melt a stripe of gravel so that it fuses into glass. When the focus moves (most dramatically in the second clip) that’s when I adjust the focus.

Melting Glass with the Sun

Glass is particularly interesting to melt using the sun. The 31 x 41 inch lens is large enough that it provides heat about equivalent to a table-top torch that one might use to do lampwork sculpting of glass. But the way it delivers that heat is very different.

In the case of a torch flame, the heat comes from a chemical reaction (combustion) resulting in a hot gas. The temperature is dependent almost entirely on the reactants, so for example an acetylene flame in pure oxygen, the hottest flame known, is about 5700 F. When this gas hits the surface of glass, the heat of the flame is conducted into the glass from the surface. So the first things to melt are sharp corners and protrusions that have a lot of surface area relative to their volume.

In contrast, light itself does not have a temperature. Heating only happens when the light is absorbed. If I focus my lens into empty air, the air at the focus is not appreciably hotter than any other air, since it is nearly perfectly transparent and absorbs almost no light. If instead I put a lump of black iron oxide in the focus, it absorbs almost all the light, and rapidly heats up. We know that with a black target like this we can get temperatures of over 3000 F since a steel bebe or a small piece of quartz will melt easily. Both of these have a melting temperature of about 3000 F.

When you put glass into the focus, the way it melts is very dependent on its color. Pure crystal doesn’t really heat up at all, window glass or clear bottle glass gets too hot to touch but doesn’t melt, colored bottle glass melts quickly, and dark opaque art glasses will “boil,” or overheat so they exolve gasses, within seconds.

So transparent colored glasses, both from bottles and art glass, are quite easy to melt using the lens.

It turns out that there are advantages to heating glass with light over using a flame. First of all, it doesn’t require fossil fuels with their associated climate impact and cost. But there are technical advantages as well. Because the light is absorbed not just on the surface, but through the whole volume of the glass, it can heat the glass faster. There are two reasons for this. First, heat is delivered to more of the glass at one time. In a flame, the heat only hits the outside, and that heat must conduct inward to heat the rest of the glass. Second, glass is sensitive to uneven heating. If the surface is heated too quickly, it expands and shatters the glass. So when you heat with a flame you have to heat slowly at first to let the inside of the glass heat up, and then you can apply full heat. But with the lens the heating is much more uniform, so you can more quickly heat the glass to melting.

Also, because the air around the glass is cool instead of hot, sharp corners and other surface features are the last to melt under the lens. This, along with odd possibilities like differential heating of glass by color opens up some intriguing potential to working glass with focused sunlight. I certainly can’t claim to have explored all the possibilities.

Fresnel lens solar furnace from Bretwood Higman on Vimeo. Here I’m melting glass making little droplet bedes to be made into earrings.

Sundrop Jewelry

That said, I did build a business using sunlight focused through the lens to make glass jewelry called Sundrops. Now along with Erin and our friends Shaun and Tawny in Minnesota (where they get much more sun!) we make jewelry that is sold at retail shops throughout the US and on a couple online sites (The Hunger Site and our own Sundrops store). Tawny has been the power-house behind this business for the last couple years, and brought some actual sense of order to our business.

For more on Sundrop Jewelry, visit our blog.

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