Which Purāṇa is completely authentic, free from interpolation?
According to Swami Harshananda, not a single Purāṇa available today is authentic (written/composed by Vyāsa).
The purāṇas are extremely popular in the Hindu society even today. Many of the religious rites and practices as also temple rituals are based on, or, connected with, the purāṇas.
The basic material of these purāṇas lies in the gāthās (metrical songs or proverbial sayings), ākhyāyikās (ancient tales), upākhyānas (anecdotes) and kalpoktis (old sayings), often referred to even in the Vedic literature.
The sage Vyāsa is said to have compiled them all into one treatise called the Purāṇasaṁhitā, which became the basic work for the later purāṇas and upapurāṇas.
Though it is conceded that the purāṇas are of hoary antiquity, extreme paucity of information leaves us in complete darkness with regard to their character or contents, none of which seems to have come down to us in the original form. No doubt Hindu tradition attributes the authorship of these purāṇas—the eighteen Mahāpurāṇas (the main purāṇas) and the eighteen Upapurāṇas (the subsidiary purāṇas)—to the sage Kṛṣṇa Dvaipāyana, better known as Vedavyāsa or Vyāsa. However, this cannot be substantiated by the evidence available. The original purāṇa referred to in the Vedic and allied literature was, perhaps, a conglomeration of ākhyānas (tales), upākhyānas (anecdotes), gāthās (metrical songs or proverbial sayings current in the ancient society) and Kalpakoṭis (sayings that had come down through the ages). The sage Vedavyāsa might have compiled these into one Purāṇasaṁhitā. His disciples and their disciples as also others in that tradition might have composed more detailed works which gradually took the present form, the eighteen purāṇas as we know them today. This surmise is confirmed by the accounts given in some of the more ancient purāṇas like the Vāyupurāṇa, the Brahmāṇḍapurāṇa and the Viṣṇupurāṇa. According to them, after compiling the original Purāṇasaṁhitā, Vyāsa imparted it to his disciple Sūta Romaharṣaṇa (also spelt as Lomaharṣaṇa), who in his turn made it into six versions and taught them to his six disciples. Of these, three disciples viz., Kāśyapa, Sāvarṇi and Śāṁsapāyana made three separate saṁhitās which were named after them. These three, along with that of Romaharṣaṇa, are known as ‘mūlasaṁhitās’. The later purāṇas were evolved out of these.
He also gives a timeline for when the present Purāṇas were composed:
Growth of the Present Mahāpurāṇas
When exactly the original purānic material began to give rise to different purāṇasaṁhitās, it is difficult to say. Since the Taittirīya Āraṇyaka as also the law-books of Manu and Yājñavalkya have used the word ‘purāṇāni’ (the ‘purāṇas,’ in plural number) it cannot be denied that three or more purāṇas had come into existence long before the beginning of the Christian era. By the time of Āpastamba (450-350 B. C.) the term ‘purāṇa’ had already become restricted to designate a particular class of books. It is not known how many purāṇas existed during Āpastamba’s time and how they went on growing in number. But we do find a tradition recorded in almost all the extant purāṇas and other allied works, that the purāṇas, or rather the Mahāpurāṇas, are eighteen in number. The names of these eighteen purāṇas as given in different purāṇic works, are more or less the same as those of the works now extant under the general title ‘Mahāpurāṇa’. Based on the evidence of the Matsya and the Kūrma purāṇas as also some other Sanskrit works, we can safely assume that by A. D. 700, the evolution into eighteen Mahā-purāṇas had become complete and the number got rigidly fixed there.
The following table gives an idea of these eighteen Mahāpurāṇas as known to us now:
||Name of the Purāṇa
||Number of Ślokas
||Period of Composition