"remembering again that he can make his love last forever in his writing," Seems so doesn't it Pat - he really had an expectation didn't he that writing lasts or at least his writing would last forever or maybe at least last long enough to "cheat time's ravages"
Became more and more curious about Brass - included in both days Sonnets - could find nothing online so turned to my trust Encyclopaedia of Traditional Symbols by JC Cooper and there it is: Brass; Venus, the sensuous world of unregenerate man, embryonic in the womb of the earth.
Unregenerate: Not spiritually or morally reformed; sinful or unrepentant. Persistently unwilling to accept change; obstinate
Wow this reference to Brass is serious - a persistent, obstinate, basic to the essence of the earth representing; Venus, a morning and evening star representing the union of opposites, she follows the moon and precedes the sun, as the drawer of bow and thrower of javelin she launches the new moon on the sea of night and defends the moon against all monsters of darkness, the passions, desires, creative mother if imagination.
That is a lot all in one word Brass - and the next, Stone is equally saying more than something on the surface that we can mean as timeless.
Stone; durability etc. indestructibility of the Supreme Reality, the cosmos in its entirety, the Philosopher's stone, the supreme quest, the reconciliation of all opposites, unity. Ovid "the bones of mother earth". Associated with the cult of Apollo, associated also with Saturn.
Earth; the corruptible body, with salt as the immortal spirit. The Great Mother, universal generatrix, the Nourisher, the nurse, inexhaustible creativity and sustenance.
Sea; primordial waters, chaos, endless motion, the source of all life, containing all potentials, the sum of all possibilities in magnifications. the unfathomable, the anima mundi, the Great Mother, the sea of life that must be crossed, exoteric knowledge.
And he is saying "sad mortality o'er-sways their power" Sounds like he is suggesting Love is the source of all and the power of Brass, Stone, Earth and Sea is wiped away with our mortality - lots of words, an entire Sonnet of words to say, the essence of life is love and within his black ink, love is timeless. His love could even represent a person but it is still love and only a miracle could change the swift boot of time - the ravages of time appear as the mitigator of power, recognized among the living as the powers represented by Brass, Stone, Earth and Sea.
This sonnet can sure be read on several levels can't it...