Perhaps the earliest reference to a furnace in Blake's poetry come in his poem in Songs of Experience named The Tyger. The fourth verse states:
"What the hammer? what the chain, In what furnace was thy brain? What the anvil? what dread grasp, Dare its deadly terrors clasp!" Songs of Innocence & Experience, Song 42, (E 24)
Hammer, chain, furnace, anvil: these words all direct our attention to the shaping of metal, a hard and resistant material, into some desired or useful form. That the process would induce terror if their use were suggested to be applied to a living thing is axiomatic. Yet Blake finds the furnace an appropriate metaphor for the measures which are required for altering the human brain which has through the ages produced societies which result in so much suffering and pain in their members. He returns to the symbol of the furnace often, especially in the hands of Los, as the tool to which he must resort in opposition to mental constructs which produce oppressive conditions.
The title page of The Book of Los shows Los situated in a confining enclosure surrounded on all sides by unyielding rock. This is the condition which requires the use of a refining, reshaping furnace.
Commentary introducing The Book of Los in Blake's Poetry and Design, Edited by Mary Lou Johnson and John E. Grant indicates that:
"It is evident in both [The Book of] Urizen and [The Book of] Los that the blacksmith Los is a skilled craftsman; he has built his own tools and knows how to use them. But he is not yet the artist he is in Jerusalem or a true prophet as he is in Milton; the product of his imaginings in Los is only the 'Human Illusion,' a conception of mankind as mind contained and confined in flesh.' (Page 169)
Book of Los, Plate 5, (E 94) "2: Upfolding his Fibres together To a Form of impregnable strength Los astonish'd and terrified, built Furnaces; he formed an Anvil A Hammer of adamant then began The binding of Urizen day and night 3: Circling round the dark Demon, with howlings Dismay & sharp blightings; the Prophet Of Eternity beat on his iron links 4: And first from those infinite fires The light that flow'd down on the winds lie siez'd; beating incessant, condensing The subtil particles in an Orb. 5: Roaring indignant the bright sparks Endur'd the vast Hammer; but unwearied Los beat on the Anvil; till glorious An immense Orb of fire be fram'd 6: Oft he quench'd it beneath in the Deeps Then surveyd the all bright mass. Again Siezing fires from the terrific Orbs He heated the round Globe, then beat[,] While roaring his Furnaces endur'd The chaind Orb in their infinite wombs"
The work of the furnaces has begun and will continue until the return to Eden is accomplished.