The natural tendency of clay to harden

But the Lord hardened Pharaoh’s heart and he would not listen to Moses and Aaron, just as the Lord had said to Moses. Exodus 9:12

There is much debate regarding the phrase "the Lord hardened Pharaoh's heart" because it seems to imply God caused Pharaoh to sin.  In other words, did God work evil into Pharaoh's heart to prevent him from obeying?  Martin Luther did not think so.  In his words, God does not "work evil in us (for hardening is working evil) by creating fresh evil in us." Luther continued:

When men hear us say that God works both good and evil in us, and that we are subject to God's working by mere passive necessity, they seem to imagine a man who is in himself good, and not evil, having an evil work wrought in him by God; for they do not sufficiently bear in mind how incessantly active God is in all His creatures, allowing none of them to keep holiday. He who would understand these matters, however, should think thus: God works evil in us (that is, by means of us) not through God's own fault, but by reason of our own defect. We being evil by nature, and God being good, when He impels us to act by His own acting upon us according to the nature of His omnipotence, good though He is in Himself, He cannot but do evil by our evil instrumentality; although, according to His wisdom, He makes good use of this evil for His own glory and for our salvation. (Martin Luther, The Bondage of the Will (Westwood: Fleming H. Revell, 1957), p. 206.)

The last time I took an art class was in middle school.  My favorite assignments were when we got to work with clay to make pottery.  The trick about working with clay as a medium is if you leave a block of clay sitting on a counter it will naturally harden and dry out.  Clay requires constant kneading with water to keep it malleable.  So every few minutes I would stick my fingers in a cup of water and rub the water into the clay to keep it soft.  The human heart works the same way spiritually.  Left to ourselves we dry up, crack, and harden.  God's grace is the only reason we can avoid sinning all the time.  So the phrase "the Lord hardened Pharaoh's heart" means the Lord withdrew his grace, allowing Pharaoh's heart to harden, all the while knowing Pharaoh would naturally respond out of the hardening with disobedience.

In Romans 1, Paul says that because man suppressed the truth, exchanged the glory of God for a lie, and did not acknowledge Him, "God gave them up" to "the lusts of their hearts", "dishonorable passions", and "a debased mind to do what ought not to be done" (Romans 1:22-28).  As a result,

29 They were filled with all manner of unrighteousness, evil, covetousness, malice. They are full of envy, murder, strife, deceit, maliciousness. They are gossips, 30 slanderers, haters of God, insolent, haughty, boastful, inventors of evil, disobedient to parents, 31 foolish, faithless, heartless, ruthless. 32 Though they know God's righteous decree that those who practice such things deserve to die, they not only do them but give approval to those who practice them. Romans 1:29-32

Thankfully that's not the end of the story.  Yes, left to ourselves we naturally harden, but God in his love for us graciously has not left us to ourselves.  In my church, at the end of every worship service my pastor leaves us by quoting directly from the book of Jude.  Jude ends with this wonderful doxology:

24 Now to him who is able to keep you from stumbling and to present you blameless before the presence of his glory with great joy, 25 to the only God, our Savior, through Jesus Christ our Lord, be glory, majesty, dominion, and authority, before all time and now and forever. Amen. Jude 1:24-25

Christ through the Holy Spirit is able to keep us from stumbling.  Literally, from sinning.  Christ is able to present us blameless before himself.  And this He will do if we are in union with Him.