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.
Kind Of Flash Material. Pure magnesium ribbon is a very convenient and perfectly safe form of illuminant to employ for small views of still-life subjects, and after one becomes accustomed to its use, it will be preferred to any other method of lighting. It burns slowly as compared to the other illuminants and it is possible to move it while it is burning so as to soften or accentuate various lights.
From the fact that it does not give an instantaneous flash it is not suitable for portraiture or where figures are in the view. Short or long pieces of the ribbon can be burned in different places so as to secure a perfectly correct illumination throughout the entire room. Great care must be exercised, however, that no strong light be placed in front of the lens and that the light from the burning ribbon be not included in the angle of view. It may, however, be ignited in the rear portion of the room, provided there is something between the light and the camera. For instance, if there is a piano at one side of the room, you could stand at the end of the piano and hold the ribbon so that the rays from it would not reach the lens. In this way you will illuminate some shadows at the rear of the room which could not be reached by the light when it was burning in a position near the camera.
531. For the average interior with medium colored walls and dark furniture, about 20 inches of ribbon will be required. Twelve to 15 inches should be burned on one side of the camera and five inches on the other. This will produce a sufficient amount of contrast; the secondary light illuminating the shadows enough to do away with any harshness or lack of detail.
532. When the walls of the room are very dark and everything in the room is exceptionally dark, it might require more ribbon burned as a secondary illumination, for the shadows in this case will be exceedingly deep and will require a considerable amount of light in order to obtain the desired amount of detail. Under no circumstances should the secondary light, however, be more than one-half as strong as the main light.
533. When it is deemed advisable to burn either magnesium ribbon or magnesium powder near the rear of the room in order to illuminate shadows, it will not require as much ribbon or powder in the main and secondary positions. However, if the room is exceptionally long, the same amount of powder will be required near the camera, as the additional flash for the shadows in the rear of the room will only take the place of the additional amount of ribbon or powder which would otherwise be required in the main or secondary light to fully illuminate the far distance. The results will be superior if the extra light be employed, for by this means there will not be as much contrast as when all of the source of illumination is very near the camera, or, in other words, from the front of the room. The shadow flash should be only a light one, from 1/4 to 1/8 the strength of the main light.