This is an extract and symbol key from the celebrated alchemical text by Kenelm Digby, A Choice Collection of Rare Secrets (1682). I bung it up, here, because I'm thinking about 'The Ancient Mariner', and I discover that the alchemical symbol for copper is the same as for 'Venus', there. So after passing through the iron-grey land of ice and snow the mariner's craft enters the Pacific [107-14]:
Down dropt the breeze, the sails dropt down,I've always taken the copper sky as merely descriptive of colour, and perhaps of smoothness and apparent hardness; but maybe there's an alchemical gloss here: the ship has moved from the martial, cold, vigorously windy, iron-coloured masculine Atlantic into the feminine, hot, lassitudinously windless, venusian female space of the Pacific; from Death to Nightmare-Life-in-Death.
’Twas sad as sad could be;
And we did speak only to break
The silence of the sea!
All in a hot and copper sky,
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the Moon.