Ibn Qayyim al-Jawziyya (Raheemahullah) said in his book, Al-Wabil as-Sa’ib, p. 52,
There is no doubt that the heart oxidizes, just as copper and silver oxidize. Its polishing is the dhikr (Remembrance of Allah ta’ala), which will make it like a white mirror. The oxidation of the heart is due to heedlessness and sin. Its polishing is by means of two actions: repentance and dhikr. If someone’s heart is cloudy, the reflections of images will be unclear, he will see falsehood as truth and truth in the image of falsehood (batil). When there is too much oxidization on the heart, the heart will be darkened, and in the darkness the images of the Truth and Reality never appear. The best way to polish it is through Dhikrullah.