The "Copper" (from various sources) explanation is probably the right one for a boiler fire with just an armload or two of wood, but I frequently get blue and blue-green flames from my largest, hotest bonfires towards the end of the evening. It is my theory that some of the un-burned wood on the bottom of the stack gets buried under ash from the wood above it before it is fully burned. It then is "Destructively Distilled", reduced by heat in the absence of oxygen, liberating comparatively pure unburned hydrogen, which ignites with a blue "Gas" flame as it ascends into the oxygen rich inspiratory zone of the fire. Hydrogen gas with the right mix of oxygen burns with a blue flame. That blue flame comingles with the yellow flame of the rest of the fire. Mix blue and yellow and you get green. I have duplicated this effect by burying a piece of live oak under hot coals, then building a good hot open fire on top of it. Did you by any chance or accident bury some un-burned wood under hot coals before you added your next load of fresh wood?
Regardless, don't have to understand it to enjoy it! Aint it purty? Just tried to understand it so I could make it happen more often. Visiting my sister in California last week I built some fires for her family. She had some pine cones she had purchased on-line for the occasion that were coated with copper sulfate. They produced an effect very similar to what I see in the late stages of my biggest bonfires.
