Bengals are flare candles mainly used to achieve indirect pyrotechnic lighting of surfaces or buildings e.g. castles. The unnatural illumination is made use of to evoke feelings of warmth etc. in the observer and to stage the lighted areas.Edit
The items are expected to emit a brightly coloured light for a long period of time and for this purpose the composition must meet special requirements:
a) The compositions should produce maximum colour,
b) should burn efficiently but as slowly as possible (40-60 sec. per 25mm) and
c) should not emit too much smoke. (a completely smokeless performance is not desirable, as the smoke reflects light and thereby partially contributes to the effect.)
Against the background of this criteria it is plain that the compositions generally are of the more expensive grade.
The candles come in various sizes, the larger units being about 50mm in diameter, 30cm long and showing a wall thickness of about 1,5mm (kraft paper). The latter means that the tube will burn away with the composition. The lower end of the candle is equipped with a wooden plug and screw eye for attachment to a holder. Bengals are fixed horizontally to allow the dross to drip off and prevent it from running down the wall and accelerate burning. They are furthermore always placed behind a shield, which blocks the spectatorÂ´s views onto the burning unit (they will just see the illumination).
These are typical compositions. Parrafin oil is frequently added to act as a retardant and to protect the unit against moisture, which can be a problem employing strontium nitrate. The compositons are not heavily compressed, often loaded using a drift and hand pressure only. In practice there are many snags such as shrinkage with some compositions (which increases burning time due to a gap between the comp and the tube) and unreliable burning. Mixture red A is particularly liable to shrinkage and must not be stored for a long time. Maybe this issue could be partially solved by replacing shellac with red gum. The compositions may need adaquate priming.
Compositions for bengal illumination:
|Red A||Red B||Green A||Green B|
|Shellac 30-200 mesh||15|
|Approx. burning time per 25mm (sec.)||45||30||20||40|
Sources: Lancaster, Ronald: Fireworks, Principles and practice, 3rd ed., p.222ff