BLAKE'S FURNACES

To appreciate Blake's use of the furnace it is helpful to have observed some process of shaping metal through the use of heat and force. Seeing a blacksmith shaping horseshoes or hardware impresses one with a process which requires considerable energy and violence. Add to this the force required in extracting ores and the heat required to change a solid metal into liquid. With this in mind, the furnaces of Blake gain a added dimension which emphasizes the extreme measures to which he alludes.   
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.   
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