Blue Flame Compositions
The generation of an intense, deep-blue flame represents the ultimate challenge to the pyrotechnic chemist. A delicate balance of temperature and molecular behavior is required to obtain a good blue, but it can be done if the conditions are right.
The best flame emission in the blue region of the visible spectrum (435-480 nanometers) is obtained from copper monochloride, CuCl. Flame emission from this molecular species yields a series of bands in the region from 428-452 nanometers, with additional peaks between 476-488 nanometers.
In an oxygen-rich flame, and at temperatures above 1200°C, CuCl is unstable and will react to form CuO and CuOH. CuOH emits in the 525-555 nanometer region (green!) and substantial emission may overpower any blue effect that is also present. Copper oxide, CuO, emits a series of bands in the red region, and this reddish emission is often seen at the top of blue flames, where sufficient oxygen from the atmosphere is present to convert CuCI to Cu0.
Paris green - copper acetoarsenite, (CuO)3 As2 O3 Cu(C2H302) was widely used in blue flame mixtures until a few years ago. It produces a good blue flame, but it has all but vanished from com- mercial formulas because of the health hazards associated with its use. (It contains arsenic!)
Copper oxide (CuO), basic copper carbonate - CuCO 3 • Cu(OH)2 , and copper sulfate - available commercially as CuS0,, •5H20 - are among the materials used in blue flame mixtures. Potassium perchlorate and ammonium perchlorate are the oxidizers found in most blue compositions. Potassium chlorate would be an ideal choice because of its ability to sustain reaction at low temperatures (remember, CuCl is unstable above 1200°C), but copper chlorate is an extremely reactive material. The chance of it forming should a blue mixture get wet precludes the commercial use of KC1O 3 . Several formulas for blue flame compositions are given in the table.
Blue Flame Compositions
Several formulas for blue flame compositions are given in the table. An extensive
review of blue and purple flames, concentra- ting on potassium perchlorate mixtures,
has been published by Shimizu.