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The shiny metal teapot reflects heat back in on itself and has a lower thermal mass than a brown china teapot so it stays hot longer. I have tried this by wrapping a copper can with aluminum foil and spraying an identical copper can with matte black paint. Filling both with the same amount of boiling water and adding thermometers I took readings every minute for about 20 minutes It became clear that the shiny can's water stayed about 4 degrees C hotter than the heat radiating black can. Case solved!
dull
Shiny
dull
dull
dull
dull
It is shiny.
Shiny
Shiny
Shiny.
It is shiny.