Let's Melt Granite using a Fresnel Lens
In this video a large Level 2 Spot Fresnel Lens is used on a clear sunny day to concentrate 1.5 square meters of sunlight to an optimal focal point of 2 cm. The maximum temperature build achieved with this lens is 3800 fahrenheit or 2093 celsius. This granite rock has a melting temperature of 2300 fahrenheit or 1260 celsius. When sunlight is focused on the dark surface of the granite rock, it begins to surface melt almost instantly. After 20 seconds of exposure, the rock develops a lava like pool that turns into a glossy black structure becoming obsidian. Using another piece of granite to dip into the hot pool of molten rock, long strands of hair like obsidian are pulled away rapidly cooling. This is possibly the thinnest obsidian on Earth. Strands at the tip average a diameter 1/8th that of a human hair.
Molten granite exceeding 2300 degrees fahrenheit.
While we only have one lens this large, the level 3 Spot Fresnel Lenses on our website can do the same thing (49" corner to corner or larger). When working with extreme temperatures, it is very important to keep children, pets or unexperienced family members away from the work area. Touching a 2000 degree surface is an instant finger disaster. Granite does not require annealing like soda glass does so simply removing the concentrated sunlight allows for the rock to gradually cool to ambient temperatures. More caution is required as this rock in the video took almost 15 minutes to cool below 150 degrees fahrenheit. If you attempt backyard obsidian with the power of the sun, use an IR thermometer to ensure the rock is safe to handle. Never attempt this experiment on wet rocks as flash steam can cause molten fragments to explode outward causing serious injury. Always wear proper eye protection.
Glowing rock and cement block after 1 minute exposure.
When a Fresnel Lens or Parabolic Mirror is not in use, store in a covered box away from windows that receive direct sunlight. Never leave unattained or leaning against a vehicle or structure in direct sunlight. Never install any solar collector on a rooftop or in an area near dry brush or combustable materials. Solar collectors are not toys and while they are fantastic teaching tools of science for kids, they require 100% experienced adult supervision. DO NOT LET KIDS PLAY WITH THEM. Treat solar collectors like a blow torch, barbecue grill or power tool.
Obsidian strands from pulled rock.
High End Infrared Thermometer
Solar energy from Concentrated solar for heating projects. Solar Furnace.