Green Flame Compositions
Pyrotechnic compositions containing a barium compound and a good chlorine source can generate barium monochloride, BaCl, in the flame and the emission of green light will be observed. BaCl - an unstable species at room temperature - is an excellent emitter in the 505-535 nanometer region of the visible spectrum - the "deep green" portion.
Barium nitrate - Ba(NO 3) 2 - and barium chlorate - Ba(C103)2 - are used most often to produce green flames, serving both as the oxidizer and color source. Barium chlorate can produce a deep green, but it is somewhat unstable and can form explosive mixtures with good fuels. Barium nitrate produces an acceptable green color, and it is considerably safer to work with due to its high decomposition temperature and endothermic heat of decomposition. Barium carbonate (BaCO3 ) is another possibility, but it must be used in low percentage due to its inert anion, CO 3.
An oxygen-deficient flame is required for a good-quality green flame. Otherwise, barium oxide (BaO) will form and emit a series of bands in the 480-600 nanometer range, yielding a dull, yellow- green color. The reaction
will shift to the left-hand side when chlorine is present in abun- dance and oxygen is scarce, and a good green color will be achieved. A flame temperature that is too high will decompose BaCl, however, so metal fuels must be held to a minimum, if they are used at all. A "cool" flame is best.
This temperature dependence and need for chlorine source are important to remember. A binary mixture of barium nitrate and magnesium metal will produce a brilliant white light upon ignition, from a combination of MgO and BaO emission at the high temperature achieved by the mixture. Addition of a chlorine-containing organic fuel to lower the temperature and provide chlorine atoms to form BaCI can produce a green flame. Several green flame compositions are given in the table.
Green Flame Compositions