In the forests of the night:
What immortal hand or eye
Could frame thy fearful symmetry?
In what distant deeps or skies
Burnt the fire of thine eyes?
On what wings dare he aspire?
What the hand dare seize the fire?
And what shoulder, and what art
Could twist the sinews of thy heart?
And when thy heart began to beat
What dread hand? & what dread feet?
What the hammer? what the chain?
In what furnace was thy brain?
What the anvil? What dread grasp
Dare its deadly terrors clasp?
When the stars threw down their spears
This section of the poem is clearly referring to God as a blacksmith. In forges, the furnace is generally used to heat up metal before shaping it into weapons or tools. It would have taken an extraordinary sort of forge to have fired into existence something as fearsome as the tyger. 
 Blake makes a connection between Satan and God. The free-willed brain was made in the furnace; which has an obvious resemblance to hell.
The Tyger