"....the duration of an ordinary magnesium flash is about one-seventh of a second, and although this is brief enough for ordinary portraiture, it is too long when rapidly moving objects have to be photographed. Much greater rapidity of combustion is obtained by mixing the metal with certain substances rich in oxygen, such as potassium chlorate. A larger quantity of magnesium can be burnt in this way than is practicable in flash lamp or with gun-cotton.

It is very important to bear in mind that these magnesium flash powders are really explosives, and must be treated with all the respect due to such compounds. Several fatal accidents have been caused in the past by carelessness in this respect. With proper precautions, however, they can be used with safety.

The points that must be specially observed are as follows: - The ingredients must be kept separate, and must only be mixed immediately before they are required for use; any rubbing or powdering must be done whilst the ingredients are separate; mixing should be done on a sheet of paper by means of a knife, or by merely shaking the powders together, great care being taken to avoid pressure or friction; no more should be mixed than is required for use, and the mixed powder should never be kept, much less carried about from place to place.

The powder is placed on a shallow iron or lead tray, and is best ignited by touching it with burning touch-paper-i.e. paper that has been soaked in a strong solution of potassium nitrate, and dried. One end of a small strip of touch-paper may be placed in the flash powder, and when the other end is ignited it slowly burns down to the powder. For portraits, however, the burning paper should be applied directly. A quantity of powder containing 15grs of magnesium is amply sufficient for an ordinary portrait. The powder should not be piled in a heap, but should be arranged in a long narrow strip, as in the case of the gun-cotton. If necessary, reflectors and screens must be used as already described.

A very good mixture is: -
Potassium perchlorate 3 parts
Potassium chlorate 3 parts
Magnesium powder 4 parts

The chlorate and the perchlorate may be very finely powdered and mixed together, but the chlorate mixture and the magnesium must be kept separate until wanted for use. 20 grains (2 parts) of magnesium is mixed on a sheet of paper as already described, with 60 grains (3 parts) of the mixture of perchlorate and chlorate (20 grains = 1.3grammes). The duration of the flash is from one-eightieth to one-twentieth of a second.

Whilst this powder may be safely burnt in small quantities, the combustion is so violent that it cannot be used in large quantities without danger. When a large quantity of light is required, a mixture that burns more slowly must be used. A mixture of magnesium powder with an equal weight, or one and a half times its weight, of potassium nitrate, prepared with all the precautions previously insisted on, may be burnt in large quantity without detonation or explosion. When a great space has to be illuminated, and large quantities of powder are burnt, it becomes of the greatest importance to spread it out in a moderately long strip, and not to pile it in a heap. In such case it should be fired at the middle with a fuse of touch-paper, the photographer standing at a safe distance.

When a broad flash is wanted, paper which has been converted into what is practically gun-cotton and thickly impregnated with magnesium is very convenient. It is sold under the name of flash-bags.