FLASHLIGHT POWDERS The chief two kinds of flashlight powders are (1) pure magnesium powder and (2) mixtures of magnesium and other substances. Magnesium powder used alone is blown through a flame, the brightness and duration of the flash depending upon the quantity of powder burnt and upon the length of time taken in passing it through the flame. Some arrangements for firing permit of a slow passage of the powder through the flame, in which case the light is continuous, and not an instantaneous flash ; others permit of a large quantity of the powder being passed through a flame very quickly, in which case there is fre quently .1. risk of much of the powder being wasted.
Flashlight mixtures are explosive, and in their action behave like gunpowder ; they must not be blown through a flame, but must be placed in a heap or a ridge and the light applied, the result being a momentary flash of high actinic power. Such explosive mixtures must always be looked upon as being more or less dangerous. The addition of chemicals to the magnesium is for the purpose of increasing the rapidity of the combustion and the actinic power of the light. When such mixtures are made by the worker, the ingredients should be purchased in the powder form and then mixed carefully together on paper with a dry feather. Large quantities should never be mixed for fear of explosions, and for the same reason there should be no lumps of any kind in the mixtures. The ordinary photo grapher will be well advised in buying his flash light mixtures ready prepared. Some of the best known formulw are : (1) Magnesium . . . 6 parts Potassium chlorate . . 9 (2) Magnesium . . . 6 Potassium chlorate . 41Potassium perchlorate 4/ (3) Magnesium . . . 6 Potassium chlorate . . 12 Antimony sulphide . 2 (4) Magnesium . . . 16 Potassium perchlorate . 12 Potassium nitrate . . 12 (5) Magnesium . 48 Ammonium nitrate . . 3 Strontium oxalate . 5 II Sodium oxalate 5 II (6) Magnesium . . 40 Potassium perchlorate . 6o Sodium chloride (salt) . 5 Barium tartrate 7 II Nos. x and 2 are good average mixtures for home work or professional portraiture. No. 3
gives a very good light, but its fumes are poison ous, and it should therefore be used in the open air or in a well-ventilated room where the fumes can escape quickly. No. 4 burns rapidly, and is less liable to explode. No. 5 is for isochro matic plates, and a yellow screen should be used in the lens. No. 6 is for isochromatic plates, but a yellow screen need not be used.
Aluminium is said to give less smoke than magnesium, but it yields only about two-thirds of the actinic light. The following mixture of aluminium and magnesium not only gives less smoke than a mixture containing chlorate, but the smoke quickly passes away and the powder is non-explosive : Copper sulphate (anhydrous) 6 parts Magnesium powder . . 3 Aluminium powder . . 1 This gives much less smoke than mixtures con taining chlorate, and the smoke passes away quickly, thus allowing of a series of successive exposures in a room. (See also " Aluminium Flashlight.") There are many other formulw for flashlight mixtures, but they are similar to the above.
Great care is necessary when firing flashlight mixtures ; and when no proper lamp is used the powder is best placed on a small iron slab or tray and fired by means of touch-paper (which see), or by means of a long taper or of a match fixed to a stick, the operator turning his head away when the actual flash takes place.
Slow-burning mixtures may be made, the following being a typical formula : Magnesium powder . . ioo parts Ceric nitrate . . . 7o Strontium carbonate . . 3o Eighty grains of this powder burn in about six seconds.
Flash-sheets are made by soaking thin blotting paper in a strong solution of saltpetre, drying, and then spreading over the paper pure dried, unoxidised magnesium powder, leaving the edges free. Such 6 sheet constitutes a com bined slow-match and flashlight, it burning slowly until the smoulder reaches the powder, which then bursts into a bright flame. Flash sheets are quite safe in use ; a commercial form is a mixture of fine magnesium powder and cellu loid spread on glass and allowed to dry.