This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Preparing The Flash-Powder. There are different ways of operating or igniting the flash-powder. The most convenient for amateur use, however, are either the flash cartridges or flash-sheets, the latter of which are used by simply pinning them up against a cardboard on the wall and igniting the lower corner. The same general principles will apply whether the lamp, the cartridge, or the flash-sheet be used. Another simple method, where Luxo or other flash-powders are used, and where you have no machine for igniting the powder, is to use an ordinary flat tin pan.
683. To prepare Luxo or other bulk powders for flashlight, when a flat pan is used, place a little cotton batting or absorbent cotton on a pan (one with a handle preferred). Pour sufficient Luxo, or whatever powder you use, on the cotton, well scattered. Allow a small piece of the cotton to hang over the edge of the pan, say five to seven inches. Use this as a fuse for igniting and setting off the flashlight. When you are ready, place the pan on a table or step-ladder, and light the end of the cotton with a match or lighted candle. Always light the fuse from below, never from above. Do not look into the flash, but as soon as you light the fuse turn your face away from the powder, as it is very dazzling and extremely hot.
Placing The Flashlight. The light should always be placed two feet behind and two or three feet to one side of the camera. It is very essential to avoid having the flash come into the field of the lens, which would blur the picture. This would be the case if it were placed in front of or even on a line with the front of the camera. It should be placed to one side, as well as behind, so as to throw a slight shadow and give a little relief in lighting. It is best to have the flash a little higher than the camera. A sheet of white cardboard may be set up a couple feet behind the flash, and as this will act as a reflector the strength of the light will be increased.
Flash-Sheet. Pin a flash-sheet by one corner to a piece of cardboard, which has previously been fixed in a perpendicular position. If the cardboard is white it will act as a reflector. Now, having the camera and the powder in position, open the shutter of the camera as for time exposure, using the stop ordinarily employed for snapshots. Stand at arm's length and touch a match to the lower corner of the flash-sheet. As soon as the flash is over, close the shutter.
686. If it is necessary to use two or more sheets, they should be pinned to the cardboard, one above the other, the corners slightly over-lapping. As a matter of precaution, place a piece of cardboard beneath, as well as one behind, the flash-sheet. If a piece of burning powder should fall, it will then do no injury.
Flash Cartridges. Remove the cover and rubber band from the cartridge and place it upon a cardboard. Now, open the shutter, ignite the fuse while at arm's length, protecting the eyes, in the meantime, from the brilliant flash. After the flash close the shutter. If two cartridges are required, the contents of one of them can be poured into the other, care being taken, however, that if any of the powder spills off it does not lie in such a position as to practically shorten the fuse. The fuses work very quickly; therefore you should be very careful to get away before the flash goes off. Never stand with your face over the powder, nor even have your hand above the powder. When the flash takes place the flame shoots upward ; therefore, if you keep below the flash there will be much less danger. Do not become excited or nervous, as there is no need of it. You should work carefully, and by observing the above cautions there will be absolutely no danger.