Blake used 'furnace' for many things:
From AMERICA a PROPHECY Plate 4; Erdman 52-53) :
(Here 'furnace' suggests the outbreak of the Revolutionary
War initiated by the American leaders, such as 'Boston's Angel.)
"Here on their magic seats the thirteen Angels sat perturb'd
For clouds from the Atlantic hover o'er the solemn roof.
Fiery the Angels rose, & as they rose deep thunder roll'd
Around their shores: indignant burning with the fires of Orc
And Bostons Angel cried aloud as they flew thro' the dark night.
He cried: Why trembles honesty and like a murderer,
Why seeks he refuge from the frowns of his immortal station!
Must the generous tremble & leave his joy, to the idle: to the
That mock him? who commanded this? what God? what Angel!
To keep the gen'rous from experience till the ungenerous
Are unrestraind performers of the energies of nature;
Till pity is become a trade, and generosity a science,
That men get rich by, & the sandy desart is giv'n to the strong
What God is he, writes laws of peace, & clothes him in a tempest
What pitying Angel lusts for tears, and fans himself with sighs
What crawling villain preaches abstinence & wraps himself
In fat of lambs? no more I follow, no more obedience pay.
So cried he, rending off his robe & throwing down his scepter.
In sight of Albions Guardian, and all the thirteen Angels
Rent off their robes to the hungry wind, & threw their golden
Down on the land of America. indignant they descended
Headlong from out their heav'nly heights, descending swift as
Over the land; naked & flaming are their lineaments seen
In the deep gloom, by Washington & Paine & Warren they stood
And the flame folded roaring fierce within the pitchy night
Before the Demon red, who burnt towards America,
In black smoke thunders and loud winds rejoicing in its terror
Breaking in smoky wreaths from the wild deep, & gath'ring thick
In flames as of a furnace on the land from North to South"
From The Four Zoas (Vala; Night the Fifth;
"And first he found the Limit of Opacity & namd it Satan
In Albions bosom for in every human bosom these limits stand
And next he found the Limit of Contraction & namd it Adam
While yet those beings were not born nor knew of good or Evil
Then wondrously the Starry Wheels felt the divine hand. Limit
Was put to Eternal Death Los felt the Limit & saw
The Finger of God touch the Seventh furnace in terror
And Los beheld the hand of God over his furnaces
Beneath the Deeps in dismal Darkness beneath immensity
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In terrors Los shrunk from his task. his great hammer
Fell from his hand his fires hid their strong limbs in smoke
For with noises ruinous hurtlings & clashings & groans
The immortal endur'd. tho bound in a deadly sleep
Pale terror siezd the Eyes of Los as he beat round
The hurtling Demon. terrifid at the shapes
Enslavd humanity put on he became what he beheld
He became what he was doing he was himself transformd"