Fresnel Lens Furnace
Overview
Large lenses are exceedingly heavy and expensive to make because they are ground from thick slabs of glass. Imagine slicing a large lens into thousands of concentric rings each with a prism shape corresponding to sections of a lens. The focusing of the lens is due to the angles between the top and bottom surface of the lens. If each concentric ring were to be pressed into a flat plastic sheet, you would have a Fresnel lens that is light, powerful and inexpensive.
The Fresnel Lens Furnace described here is especially powerful and dangerous. Just looking at the focal point with unprotected eyes can damage your vision. Always use welding goggles and look at it only for short periods of time. This furnace has enough power to melt glass and small stones and can burn your
hand in an instant if you accidentally touch the focal point.
The Fresnel Lens Furnace described here is especially powerful and dangerous. Just looking at the focal point with unprotected eyes can damage your vision. Always use welding goggles and look at it only for short periods of time. This furnace has enough power to melt glass and small stones and can burn your
hand in an instant if you accidentally touch the focal point.
Materials
1) large plastic fresnel lens
6) 3/4" plastic pipe elbows 2) 3/4" plastic pipe "T's
14'of 3/4" plastic pipe
8' of plastic storm window molding and gasket
Plastic pipe cement
3/4" sheet metal screws
Tuna fish can
High temperature insulation
Instructions
1) Cut the plastic window frame so it fits around the lens. Miter the corners. Set the lens into the rubber gasket and press into frame. Secure the corners of the frame with screws.
2) Cut the 3/4" pipe into specified lengths. Assemble the base as shown in the exploded view. Glue pieces in place with pipe cement, making sure that the "T"s point straight up. Do not glue the 30" uprights in place; this way the frame can be easily disassembled and stored.
3) Cut a notch in a 1-1 /2" section of 3/4" pipe to fit the lens frame. Mount this bracket halfway up the long side of the lens with glue and screws. Cut a slot in a 3/4" elbow and slide the bracket into this elbow using some petroleum jelly for lubrication. Lock the bracket in place with a screw through the slot.
4) A crucible is a heat-proof container that can be held in the focal point of the furnace. Make a crucible from a tuna fish can packed with high temperature insulation from a ceramics supplier. Attach a section of broomstick to the side of the can for a handle. The broomstick can be supported in a ring stand or held in the hand. Unexposed, developed photographic film (it should be jet black) mounted in 35mm slide frames can make temporary goggles, but it is better to use dark arc welding goggles.
5) Beautiful jewelry can be made by melting colored glass on copper discs with the furnace. Obtain 1" copper discs, flux and enamels from an art supply store. Wet the copper with water and sprinkle it with powdered glass flux. Place the coated copper in the crucible carefully and position it in the ring stand so that the focal point is as big as the blank. When the flux melts, remove it from the focus, add colored enamels and reheat. Don't forget your eye protection.
Sources