Blake used that image in Plates 6 and 7 of Jerusalem like this:
"For as his Emanation divided, his Spectre also divided In terror of those starry wheels: and the Spectre stood over Los Howling in pain: a blackning Shadow, blackning dark & opake Cursing the terrible Los: bitterly cursing him for his friendship To Albion, suggesting murderous thoughts against Albion. Los rag'd and stamp'd the earth in his might & terrible wrath! He stood and stampd the earth! then he threw down his hammer in rage & In fury: then he sat down and wept, terrified! Then arose And chaunted his song, labouring with the tongs and hammer: But still the Spectre divided, and still his pain increas'd! In pain the Spectre divided: in pain of hunger and thirst: To devour Los's Human Perfection, but when he saw that Los was living: panting like a frighted wolf, and howling He stood over the Immortal, in the solitude and darkness: Upon the darkning Thames, across the whole Island westward. A horrible Shadow of Death, among the Furnaces: beneath The pillar of folding smoke; and he sought by other means, To lure Los: by tears, by arguments of science & by terrors: Terrors in every Nerve, by spasms & extended pains:"
What might he mean? Los=Blake created with his furnace, like the alchemist did with his crucible; he tried to keep his mind on the heavenly visions, but the spectre (read Satan) constantly intervened, distracting him from 'Heaven' to the purely material: to jealousy, to enmity, to hate, to worldly ambition (it happens to me, too; we pass through the valley of the shadow of death pretty much every day). (It's an image of Blake's everyday life, and of ours.)
"Multitudes, multitudes in the valley of decision: for the day of the LORD is near in the valley of decision." (Joel 3)
Jeremiah 7 verses 31-32 "And they have built the high places of Tophet, which is in the valley of the son of Hinnom, to burn their sons and their daughters in the fire;
which I commanded them not, neither came it into my heart. Therefore, behold, the days come, saith the LORD, that it shall no more be called Tophet, nor the valley of the son of Hinnom, but the valley of slaughter: for they shall bury in Tophet, till there be no place."
Now look at Jerusalem Plate 15 (Erdman 159):
"I see Albion sitting upon his Rock in the first Winter And thence I see the Chaos of Satan & the World of Adam When the Divine Hand went forth on Albion in the mid Winter
And at the place of Death when Albion sat in Eternal Death
Among the Furnaces of Los in the Valley of the Son of Hinnom"
And in Plate 22 we read:
"Albion thy fear has made me tremble; thy terrors have surrounded me
Thy Sons have naild me on the Gates piercing my hands & feet:
Till Skofields Nimrod the mighty Huntsman Jehovah came,
With Cush his Son & took me down.
He in a golden Ark,Bears me before his Armies tho my shadow hovers here
The flesh of multitudes fed & nouris[h]d me in my childhood
My morn & evening food were prepard in Battles of Men
Great is the cry of the Hounds of Nimrod along the Valley
Of Vision, they scent the odor of War in the Valley of Vision.
All Love is lost! terror succeeds & Hatred instead of Love
And stern demands of Right & Duty instead of Liberty"
IMO Blake is saying that the present generation is giving their sons a fiery future. The furnace is the cauldron of misery in this vale of tears; with our indolence, ignorance and political stupidity we're preparing that for our children. (Do you agree?)