The furnaces of Los seem to represent the difficulties of gaining experience as psychological processes are undergone to transform divided minds into unified minds. Entering the furnaces is not done willingly because it is expected to be painful.
One of the processes of creating Blake's engravings is metaphoric of the smelting process in the furnace. On the copper plates Blake drew his text and images with a varnish which was resistant to acid. The prepared plates were dipped in acid to remove the uncoated surfaces which would not be holding ink. The furnaces are the acid bath which remove from the psyche the accumulated prejudices, misapprehensions, unfounded fears, unresolved projections, and other unfinished business that can be dissolved to reveal the outlines of identity. Los undertakes the operation of the furnaces as Blake undertakes the production of his engraving, to provide a process of construction through destruction.
In reading from Plate 9 of Jerusalem, (E 152)I was struck by these words:
"I lifted them into my Furnaces; to form the spiritual sword.
That lays open the hidden heart:"
A passage from the book of Hebrews seems to echo the import of these phrases:
Phillips NT, Hebrews 4:12
"For the Word that God speaks is alive and active; it cuts more keenly than any two-edged sword: it strikes through to the place where soul and spirit meet, to the innermost intimacies of a man's being: it exposes the very thoughts and motives of a man's heart. No creature has any cover from the sight of God; everything lies naked and exposed before the eyes of him with whom we have to do."
Blake of course is not explicit in referring to Hebrews or in identifying the purpose of the furnaces with the activities of the Holy Spirit. But reading in Hebrews of the sword which strikes to the inmost heart, I associate the passage with Blake's words. It becomes clear that the process in the furnaces may be for the purpose of stripping one naked as one is in the sight of God who sees everything as it is. "Then we will see as we are seen." (I Corinthians 13:12)