The typical blue color as seen on your home gas stove is the color
associated with the temperature. Blue is richest in high temperatures.
When there are other colors, like red, yellow, green, are due to cation
impurities in the gas stream. Stimulated irradiation colors for the
elements are extensively listed in many physical chemistry handbooks. I
think sodium might be the yellow source, but can't find my pchem book.
Likely to be from the oxygen steelmaking, the gas from this process is
harder to process as fuel than the coke oven or blast furnace gas. It would
also fit with the change in colour of the gas. The composition of the gas
from steel making varies a lot during the blow while the coke oven and blast
furnace gasses are more stable in composition.
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