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YOU CAN MELT ROCK WITH NOTHING BUT SUNLIGHT

A large Fresnel lens focuses 1.5 square metres of sunlight into a 2 cm spot hot enough to turn granite into glossy black obsidian in 20 seconds.

by Dan and Denise Rojas (GreenPowerScience)

FULL CAD BOM FIRMWARE DOCS

ScienceWorkshop

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time
an afternoon
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WHAT YOU’LL NEED

Videos
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COMPAREE VERDICT

GreenPowerScience (Dan and Denise Rojas) concentrate 1.5 square metres of sunlight through a large Level 2 spot Fresnel lens to a focal point of about 2 cm; the maximum temperature they report with this lens is 2093 °C (3800 °F). Granite melts at 1260 °C (2300 °F), so after 20 seconds of exposure the rock develops a lava-like pool that turns into glossy black obsidian. Dipping another piece of granite into the molten pool pulls long hair-like strands whose tips average one-eighth the diameter of a human hair. This is a demonstration of extreme solar concentration, not a step-by-step build with a parts list. The lens in the video is their large spot lens, not the TV lens conversion documented in their Instructables guide; they say their Level 3 spot lenses (49 inches corner to corner or larger) can do the same, and they sell spot lenses on their website. The biggest risk is underestimating the danger: wet rocks can explode from flash steam, and the rock in the video took almost 15 minutes to cool below 150 °F (65 °C). GreenPowerScience recommend an IR thermometer to check the rock is safe to handle, proper eye protection, and treating a solar collector like a blowtorch. Never leave a Fresnel lens unattended in sunlight.

GOOD TO KNOW

  • —GreenPowerScience publish a free Instructables guide on converting a linear Fresnel lens from a rear-projection TV into a spot lens. The guide is listed as CC BY-NC-SA.
  • —The lens in the lava video is their large spot Fresnel lens, not stated to be a TV conversion.
  • —No repository, BOM, or CAD files; the Instructables guide covers the TV lens conversion only.
  • —GreenPowerScience sell spot Fresnel lenses on their website; the blog post gives no prices.
  • —The blog post and videos document the experiment; they are not a step-by-step build guide.

Parts to buy

2 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • Large spot Fresnel lensFind
  • Rear-projection TV lens converted with their Instructables guideFind

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Can I build this?

Printnothing required
BuyA large spot Fresnel lens (GreenPowerScience say their Level 3 spot lenses, 49 inches corner to corner or larger, can do the same), or a rear-projection TV lens converted with their Instructables guide. An IR thermometer is recommended.
ToolsDry granite, proper eye protection, a safe outdoor workspace away from combustible materials, an IR thermometer to check cooling.
SkillsUnderstanding of concentrated solar hazards and thermal safety. No fabrication skills needed unless you do the TV lens conversion; extreme caution and adult supervision required.
TimeAn afternoon for the experiment itself; the TV lens conversion is extra work.
Cost$$; the blog post gives no prices
SafetyExtreme hazard: GreenPowerScience report up to 2093 °C at the focal point. Touching hot rock causes instant burns. Never use wet rocks; flash steam can make molten fragments explode. Always wear proper eye protection. Supervise children. Never leave the lens unattended or leaning against a vehicle or structure in direct sunlight, and keep it away from dry brush. Treat it like a blowtorch.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

Videos

Fresnel Lens melting GRANITE 2300˚ Fahrenheit make Obsidian FRESNEL LENS1:29

GreenPowerScience, April 2016. Granite melting and obsidian formation.

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Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the GreenPowerScience blog post and watch both videos to understand the risks and temperatures involved. (Do not proceed without understanding the hazards.)
  2. 2.If converting a TV lens, follow the Instructables guide step-by-step. (Listed as CC BY-NC-SA.)
  3. 3.Source a large spot Fresnel lens or complete the TV conversion.(The lens in the video is not stated to be a TV conversion. GreenPowerScience say their Level 3 spot lenses, 49 inches corner to corner or larger, can do the same.)
  4. 4.Set up in a safe outdoor area, far from combustible materials, with proper eye protection and an IR thermometer.(Keep away from dry brush and combustible materials, and never leave the lens unattended in sunlight.)

Resources

Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.

KNOWN ISSUES

  • The large spot lens used in the video is not a converted TV lens. The Instructables guide covers the TV lens conversion only.
  • Wet rocks can explode from flash steam when heated. Only use completely dry rock.
  • The rock took almost 15 minutes to cool below 150 °F (65 °C). Use an IR thermometer to check it is safe to handle.
  • Always wear proper eye protection for intense light; ordinary sunglasses are not designed for this.
  • Never leave a Fresnel lens unattended, or leaning against a vehicle or structure, in direct sunlight, and keep it away from dry brush and combustible materials.
  • GreenPowerScience stress that children must be supervised around solar collectors.

Can I use a Fresnel lens from a rear-projection TV?

GreenPowerScience publish a free Instructables guide on converting the linear Fresnel lens from a rear-projection TV into a spot lens. The granite in the video was melted with their large spot Fresnel lens, not a TV lens, and they do not say whether a converted TV lens can melt granite.

What is the actual temperature at the focal point?

GreenPowerScience report a maximum of 3800 °F (2093 °C) with the large spot lens. Granite melts at 2300 °F (1260 °C).

How long does it take to melt granite?

About 20 seconds of focused sunlight to develop a lava-like pool. Cooling takes almost 15 minutes to get below 150 °F (65 °C).

How thin are the obsidian strands?

Dipping another piece of granite into the molten pool pulls long hair-like strands whose tips average one-eighth the diameter of a human hair. GreenPowerScience call it possibly the thinnest obsidian on Earth.

Where should I set this up?

Outdoors, away from dry brush and combustible materials. GreenPowerScience warn never to leave a lens unattended or leaning against a vehicle or structure in direct sunlight.

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Discussion1

FROM THE COMPAREE TEAM

GreenPowerScience report that the rock in the video took almost 15 minutes to cool below 65 °C, and the obsidian strands pulled from the pool have tips one-eighth the diameter of a human hair. If you were setting up a safe outdoor spot for this, what would you change about your workspace first?

CompareeTEAM1d ago

Practical notes from our verification: the lens in the lava video is a large spot Fresnel lens, not stated to be a TV conversion, and the Instructables guide covers the TV lens conversion only; those two things are separate. The blog post and videos demonstrate what concentrated sunlight can do; they are not a step-by-step build guide with a BOM. The Instructables guide is listed as CC BY-NC-SA. GreenPowerScience sell spot lenses on their website but the blog post gives no prices. The single biggest safety note is theirs: never use wet rocks, because flash steam can make molten fragments explode.

Dan and Denise Rojas (GreenPowerScience)

Dan and Denise Rojas run GreenPowerScience, which publishes solar experiments on its blog and YouTube channel and sells spot Fresnel lenses. They also have a free Instructables guide on converting a rear-projection TV lens into a spot lens.

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