for fire lighting
Lenses of sufficient optical quality for fire lighting, basic photography and magnifying glasses and the like may be fashioned from various pieces of glass and rigid clear plastic by anyone with the determination and the time to do so.
For example, a small square cut from a clear plastic ruler can be turned in to a lens by first using coarse abrasive paper to grind it in to a circular disc, then by using the same abrasive paper to give one of the flat surfaces a spherical curvature, grinding in an arc whilst slowly turning the lens in the making. Once the rough shape is obtained, successively finer abrasive papers should be used to sequentially remove the abrasions caused by each previous abrasive paper until the surface is as smooth and free from scratches as possible. Then the whole lens should be polished with cloth, such as a shirt sleeve or a skirt pulled tight over one knee, until the surfaces shine. Test the lens in a shaded room by holding the lens towards a window and focussing the image of the scene coming through the window on to a sheet of white paper or card. Once you see the image resulting from your own handicraft using nothing more than hand and eye coordination you will be thrilled. You will also be inspired to make bigger and better lenses and to learn more about the fascinating subject of optics.
If instead of focussing the scene from a window on to a white surface you use your home made lens to focus the Sun's rays on to some dark coloured tinder, such as hand rolling Tobacco, in the centre of a pad of dry leaves or grass the tinder can be made to smoulder. If the smouldering tinder is blown or fanned gently it can be made to ignite. Thus you have made fire, and from that thrilling point onwards you may prefer never again to use matches and lighters for fire lighting. Such a home-made lens is also ideal for lighting cigarettes.
The process is similar but far more demanding and time consuming when making a home-made lens out of glass, yet, conversely, the resulting satisfaction is greater. When making a lens starting with a flat piece of glass use a hand powered grinder to make it circular and then give one face a spherical curvature. Make sure that you wear goggles to protect your eyes from flying chips of glass and also wear a face mask over your mouth and nose so that you do not inhale glass dust. Proceed slowly with the initial grinding, otherwise localised heat may cause the glass to crack or chip. Once the basic shape has been obtained, use successively finer grades of wet and dry abrasive paper with plenty of water until you have smoothed the surfaces of the lens to your satisfaction.
An interesting variation of the home-made magnifying lens is the liquid filled lens, which can be improvised from any manageable spherical or largely spherical clear glass vessel, such as a Coffee jug, a Tea pot, a Goldfish bowl, or the now increasingly rare incandescent light bulb. If using an old light bulb, carefully remove the connector base and break the sealing nipple. Then try to remove the filament without breaking the bowl of the bulb. Fill all such containers with water. Such lenses are ideal for fire lighting. You might also like to experiment with different liquids. Almost any liquid will do, even if it is coloured, just so long as it is transparent. Sherry works fine, as does Lager, vegetable oil, and household bleach, to name but a few. In a survival situation, clear urine may also be used.
Gelatine used for culinary purposes can be used to make a lens sufficient at least for fire lighting. Dissolve the Gelatine in hot water, making the mixture as strong as possible so that the resulting lens is as firm as possible. Pressurised containers such as gas canisters for torches and portable stoves, also cans of hair spray, shaving foam, and compressed air, and various paint and drinks cans, have a concave depression in the base, as indeed do the old style of regenerative blow lamps, and such depressions in exhausted cans and empty blow lamps make ideal casting moulds for Gelatine lenses. Since Gelatine lenses are generally poor in quality and end up with a colour cast they are really only suitable for fire lighting, and they therefore need to be made as large as possible. With one of the concave depressions uppermost, pour in the Gelatine mixture and let it set. Then carefully tease it out, briefly wet it, and then set it flat face down on to a sheet of glass. The purpose of wetting the lens after the mixture has set is to hopefully make it stick to the glass so that as it dries and hardens warping is kept to a minimum. Once the lens is hard enough to handle, carefully prise it from the sheet of glass. If the surface of the Gelatine lens has become cloudy whilst stuck to the sheet of glass it can be made to shine by the careful application of a little water whilst holding the lens by the edges. Then use the lens in the usual manner to focus the Sun's rays to make fire. Gelatine lenses deteriorate with time and are therefore generally only serviceable for fire lighting in the early days after they are cast.
Here are a few of my glass and plastic home-made lenses.