A hard heart is compared to a lightning bolt, to gold and to shellac. The lightning bolt is very strong and never becomes soft. Similarly, the hearts of those who are engaged in severe austerities and penances do not become very easily softened. The golden heart becomes melted at high temperature, as in ecstatic love. And the shellac heart is very easily melted in slight temperature.
A soft heart is compared to honey, to butter and to nectar. And the condition of the mind is compared to sunshine. As honey and butter become melted even in slight sunshine, softhearted persons become easily melted. Nectar, however, is by its nature always liquid. And the hearts of those who are in pure ecstatic love with Kṛṣṇa are by nature always liquified, just like nectar.