Obscure, cryptic and steeped in layers of rich, pictorial allegory the symbolism of alchemy seems to us today to be very strange indeed. Known as the "Royal Art" the root of the word is the Arabic al-kimiya a symbolic reference to the dark first matter of the earth. 
The practice of alchemy was at one level the precursor of modern chemistry as well as being the repository of the metallurgical wisdom of the ancient world. It encompasses a physical practice, an artisans craft out of which exquisite alloys, dyes and composite materials were developed. While at the same time being a spiritual discipline holding the ultimate symbolic description of the transformation from the dark leaden physicality of the earthbound consciousness to the refined gold of the spiritual illuminated being and the purification of the body, mind and soul.  It is this strange fusion of the physical an psychic where the modern mind seems to stumble most.
Evolving in days when wisdom was not compartmentalised the leaden and earthbound referred to Saturn and the purified gold, the Sun.
The goal of alchemy or "The Great Work" was the transformation of one into the other, and as many of the texts make clear, this as much a spiritual as a physical process. The search for the philosophers stone and the quest for the grail are similar renaissance metaphors whose truths abound in such sister disciplines as the Tarot, Numerology and Astrology.  Following the enlightenment and more recently depth psychology we now view all of these as metaphors that describe the self.
These are all traditions, by the way, containing many rich commonalities and overlays although any attempt at rigid one to one correspondence will be fraught with difficulty.  For instance many of the texts abound with astrological references and one of the most famous alchemical sequences the Splendor Solis consists of 22 magnificently inscribed plates, the same number of cards as the major arcana. More specifically to be an alchemist was also to be an astrologer, for astrology was an inseparable part of the language – the timing of the various stages of the Opus was was crucial.  Similarly there are optimum moments for conscious effort at development and there are times when it is better to leave well enough alone.  In Your Personal  Astrology Report I liken such time to the 1oo year sleep. 
In common with astrological symbolism this illustration depicts the Sun as male and the moon as female. consider too the lion representing the Sun (Leo) and the serpent which this Lion is devouring as the raw psychic energy of the soul. The serpent further being one of the emblems of Scorpio.
 Alchemy, quotes Jung, as a nature philosophy of great consideration in the Middle Ages, throws a bridge to the past, the gnosis, and also to the future, the modern psychology of the unconscious. Such is its importance that he goes on … Only by discovering alchemy have I clearly understood that the Unconscious is a process and that ego's rapport with the unconscious and his contents initiate an evolution, more precisely a real metamorphoses of the psyche.
In terms of astrology the stages of the opus, the nigredo,  coagultio, solutio and calciatio also provide rich metaphors for understanding the evolving condition of the psychic and describe with excruciating accuracy different type of descent that we can undergo and the different planetary combinations that can precipitate them.
Similarly alchemical terms can equally vividly describe in word pictures various emotonal stages, for instance  the frustration described  by the image of a 'wolf in a sealed container having had its paws cut off and being slowly heated.'

The alchemical union

Here we have enterd a dream world and it is the processes of incubation and active imagination that hep us navigate a pathway through their labyrinthine meanings. Like tarot pictures everything in the picture has meaning, for example in the leading woodcut note the rooster in the foreground of the illustration (top) to the boats in the distance.
This illustration (left) depicts the goal of alchemical work, the innier conjuctio or union of male and female, anima and animus. In this example we see broher an sister, two halves of the one, equally the final acheievement is symbolised by the union of the king and queen, a theme similar to that of so many fairy tales.