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What causes logs to turn white while burning, and why do moving black patches appear?

I was camping this weekend, and I noticed that the burning logs that were already white had black patches moving up and down the log. What causes this phenomenon?
gpt
The ash that is produced is composed of several products.  Some of of it is incompletely combusted carbon which tends to be black.  Other components are due to other elements in the wood such as magnesium which forms white magnesium oxide when burned.  The colour fluctuations you see while it is burning is due to blackbody radiation. When a surface doesn't emit light of its own (luminescence) it is considered black.  When heated, the body starts to emit radiation at low frequency first (infrared, which we feel as heat).  As it gets hotter,  the frequency of the radiation increases to red, orange, and then white.  The hotter the object the higher the frequency of radiation emitted.  The temperature in the fire fluctuates due to convection currents causing areas of the wood to change temperature resulting in the colour of the blackbody radiation to change as well.  When it's very hot it appears white, if it cools off it appears black again.