By projectingveryfinely powdered magnesium (to pass sieve No. 120) into a spirit flame, a flash lasting from one-fifth to one-tenth of a second can be obtained. A continuous flame can be obtained by using an ordinary powder bottle sprayer or various types of lamps, the manu facture of which has been discontinued since the advent of the flash powders. It is necessary otherwise he will be completely dazzled for some minutes and will be unable to look after the combustion of the metal.
The table given below shows the length of magnesium ribbon required, according to the distance of the light from the subject, there to emphasize the fact that these lamps, par ticularly those fitted with powder reservoirs, when used either for single or continuous illumination, must only be charged with pure magnesium powder, not with any other mix tures (flash powders), which will almost cer being no other effective lighting. The photo graph is assumed to be taken on a plate of the rapid ordinary type, with lens working at F/8. It should be pointed out that in order to obtain even illumination the light should be produced at a distance much greater than the width of the subject.
The ribbon may be ignited with a match or with a spirit flame, but the ends of the mag nesium should first be cleaned with glass paper tainly cause a serious explosion in the lamp or reservoir.
298. Combustion of Magnesium or Aluminium Wire or Foil in a Bulb of Oxygen. A recent type of lamp (J. Ostermeier, 1929) gives a single flash of very great actinicity (colour temperature 3,500° K. approx.) and of very short duration by burning crumpled aluminium foil or magne sium (alloyed with aluminium) wire in pure rarefied oxygen' within a sealed bulb, i.e. with out external smoke. For the purpose of elec trical ignition by all currents from 2 to 220 the bulb is fitted with a cap like that of an incandescent lamp. Ignition is effected by explosion of a wire acting as a lighter. This lighting wire is sometimes coated with metallic salts (such as sodium nitride) to increase the proportion of yellow and red rays and thus improve colour rendering.
The table shown gives the average charac teristics of some of these lamps according to the measurements of W. E. Forsythe and M. A. Easley (1936) on one hand, and of M. van
Liempt and J. A. de Vriend (1935) on the other.
In Press photography the need for compact and very portable apparatus has led to the mounting of the lamp or lamps' on the camera, which is obviously the worst position aesthetic-. ally, as all shadow modelling is thereby sup pressed.
Some attempts have been made to use in the same way flashlamps containing only a gaseous mixture (carbon bisulphide and nitric oxide).
299. Flash Powders. These powders invari ably consist of a mixture of magnesium (or one of its with one or a combination of oxidizing substances, which supply the oxygen necessary for the combustion immediately the ignition is started by the application of heat to any part of the mixture.
The photographer who has no knowledge of chemistry or pyrotechnics should never attempt to prepare flash powders. The majority of the formulae for these preparations, which have been copied in many cases from one book to another, are really formulae for dangerous explosives, and it is not unlikely that any such experiments may lead to a serious accident, sometimes fatal. Even mixtures which are said to be fairly safe may prove to be dangerous if, instead of using substances of known purity, as the manufacturers do, commercial products of unknown composition are used. For instance, perchlorates often contain as impurity a small amount of chlorate, which for other purposes is quite harmless. While flash powders containing pure perchlorate are perfectly safe, any which contain chlorate are very dangerous to handle since they are liable to explode through shock or by friction against a hard substance. It must be thoroughly understood, therefore, that the various formulae given later are included for use by competent people only, knowing the precautions necessary for the handling of such mixtures.' If there is any doubt about the safety of a flash powder, 2 all the precautions necessary with an explosive should be taken while handling it.
A certain number of flash powders are sold with their constituents separated, to facilitate transport. In mixing powders of this type, care should be taken to avoid shock or friction against hard substances. Sufficient homogeneity may be obtained by gently shaking the powder, preferably in a cardboard box.