SB 11.27.14

asthirāyāṁ vikalpaḥ syāt
 sthaṇḍile tu bhaved dvayam
snapanaṁ tv avilepyāyām
 anyatra parimārjanam
Word for word: 
asthirāyām — in the case of the temporarily installed Deity; vikalpaḥ — an option (as to whether the Deity is to be called and sent away); syāt — there is; sthaṇḍile — in the case of the Deity traced upon the ground; tu — but; bhavet — do occur; dvayam — these two rituals; snapanam — the bathing; tu — but; avilepyāyām — in the case when the Deity is not made out of clay (or paint or wood); anyatra — in the other cases; parimārjanam — thorough cleansing, but without water.
Translation: 
The Deity that is temporarily established can optionally be called forth and sent away, but these two rituals should always be performed when the Deity is traced upon the ground. Bathing should be done with water except if the Deity is made of clay, paint or wood, in which cases a thorough cleansing without water is enjoined.
Purport: 

Various classes of devotees worship the Deity of the Personality of Godhead according to their various stages of faith in the Lord. An advanced devotee of Lord Kṛṣṇa understands his eternal loving relationship with the Lord and, seeing the Deity as the Lord Himself, establishes an eternal relationship with the Deity based on loving servitude to Him. Understanding Lord Kṛṣṇa to be the eternal form of bliss and knowledge, a faithful devotee makes a permanent arrangement for Deity worship, installing the Lord’s form made of, for example, stone, wood or marble.

The śālagrāma-śilā is considered to be automatically installed even without the formal ceremony, and so it is forbidden to call the Deity with mantras or ask the Deity to leave. On the other hand, if one prepares a Deity form by drawing on sanctified ground or by constructing a sand image, one must call the Deity with mantras and then ask the Deity to leave the external form, which will soon be demolished by the natural elements.

The general principle is that a pure devotee of the Lord understands his relationship with the Deity to be eternal. The more one surrenders in loving devotion to the Deity, the more one can understand the Supreme Personality of Godhead. Lord Kṛṣṇa is a person, but He is the Supreme Person, possessing His own unique feelings. One can easily please the Lord by devotional service offered to His Deity form. By pleasing the Lord one can gradually progress in the mission of human life and eventually go back home, back to Godhead, where the Deity personally appears before the devotee and welcomes the devotee to His personal abode, known throughout the world as the kingdom of God.