What is the meaning of this phrase in "The Last Leaf" by "Behrman took his seat as the hermit miner on an upturned kettle for a rock." "The Last...

In O.l Henry's "The Last Leaf," Old Behrman lives
downstairs from the two aspiring artists, Johnsy and Sue, in the little Greenwich
Village apartments.  He is over sixty years old, and has always thought of painting a
masterpiece, but he has yet to begin it.  A failure at painting, Behrman earns a meagre
living by posing as a model for the young artists who cannot afford professional
models.

Sue has Behrman come to her apartment to pose, but
she points out to him the ivy vine upon which leaves has fallen.  They both look at each
other; then, Behrman takes his seat to pose.  In Sue's painting, he is supposed to be a
miner who stays to himself and is seated on a kettle set upside down; this kettle will
be painted as a rock.