It is not merely metaphorical or symbolic in Alchemy schools - Western, or their more ancient Asian predecessors from Tantra and Daoist schools. Mercury was literally used in alchemical potions, and like many other things, got sublimated into a philosophical/symbolic concept over a period of time.
There are two schools of Alchemy in Indian tradition - Physical and Spiritual - which overlap in many aspects.
The Physical Alchemical schools focused on physical metallurgy, using Mercury, Sulfur, and a host of minerals and herbs to stabilize and transform these raw objects into alchemical potions. Their goal was one or more of the below:
- Kāyākalpa - Body rejuvenation, extending life, preserving youth, etc.
- Svarṇakalpa - converting metals to gold
- Siddhikalpa - attaining supernatural powers (siddhis) such as telepathy, invisibility, etc.
Then there were spiritual alchemical schools that did use physical methods, but they did not rely on the physical techniques of metallurgy alone. They used Mantras, Sexual Alchemy, Life-force beaming, etc. to purify and transmute the potions. The main of these schools was the Rasēśvara school which traces itself back to Shiva himself. These alchemists used potions that included mercury, sulfur, and other rare herbs to awaken Kundalini, stabilize the energy, achieve supernatural states of consciousness, etc., while also preserving their physical body and lifeforce till the point they were ready to voluntarily discard the body. They invoked the mantras of Rasēśvara (Shiva) and Rasabhairavī (Goddess) to transmute alchemical potions. They also internalized some of the concepts from physical alchemical schools to interpret them as spiritual concepts.
Alchemy found its glory in Southern India and all accounts trace it back to a sage named Agastya (also known as Kumba-muni - the sage who is forever in the state of Kumbhaka, aka a state of effortless breathlessness), an immortal and a direct student of Shiva himself. He is known to have taken a long and laborious journey from the Himalayas to Southern India and brought with him secrets of Alchemy, Mantras, Kundalini Yoga, certain forms of martial arts, and grammar, etc. He is present on astral planes to this day and can be easily accessed for guidance by adherents of these methods.
His student, a Siddha (immortal) named Bhoganātha is considered the father of alchemy. He used a combination of nine poisons (including mercury and sulfur), certain mantras, a breathing technique, and his immense lifeforce to transmute and stabilize a potion named Navapāśāṇa (Navapashana, Navapashanam are the colloquial terms) which is meant to cure diseases, extend life, etc. Several alchemists today claim to know this technique, but I am not convinced. Bhoganātha, before disappearing from public life, created an idol of Subrahmanya (the son of Shiva and the deity of sacral chakra) in a shrine at a place called Palani in Southern India, which exists to this day. Priests at this shrine bathe the idol with Panchamrita (milk appears bluish) and distribute it to people, who report miraculous cures after consuming this milk. This idol is extremely powerful and I could see the aura around the idol spread for miles and miles endlessly (in fact, right now, I was trying to focus on the idol and pay my respects to the deity and a surge of energy flooded me as the deity in this shrine is very awake unlike other shrines).
Several folks claim to have recreated this formula, but I have not personally seen any evidence that proves these claims. In our school of Tantra, we no longer use physical alchemy, but instead use a mantra of a deity named Sparshamani Kali (Kali in her aspect as the philosopher’s stone) and Chintamani (a form of Shiva as Ardhanarishvara - Shiva and Shakti in one) to activate a similar alchemy.