In alchemical texts we sometimes see the number 40. It is always associated with the first stage of the Great Work, that is, Nigredo or Blackness, when the Earth or some other term for the Matter is putrefying in order to cleanse it from its impurities. Nigredo is explained in many terms and allegories. Basically, it is a period of time when the alchemist retreats in oneself, in meditation, contemplation, fasting or other means of self-isolation, sometimes symbolized by going into a cave, or the underworld. In Jungian terms, one could say that he makes his own subconscious contents conscious. He comes into contact with his own repressed emotions and other psychological issues, that he needs to work out (Purification).

Whenever an alchemist uses the 40 days, he is not talking about a laboratory process, even when he uses laboratory terms. Why 40 days? As Western alchemy originally religious and Christian symbols and references, and we find the number 40 many times in the Bible, where it is used to denote a period of solitude, prayer, fasting, preparation, purification, regeneration, and sanctification. We can ask again, why do we find these 40 days in the ancient scriptures? I am just guessing, but maybe it refers to 40 weeks or 280 days of the pregnancy period.

The 40 day period was well-known among alchemists, and Antoine-Joseph Pernety (1716–1796) in his Dictionnaire mytho-hermétique (1758), called it the Philosophical Month:

"Philosophical Month. The Hermetic Chymists make their months of forty days, which is the time of the putrefaction of matter. But they say the month is a time that imitates the movement of the Moon; that is why some give it it thirty, others forty days. It is called philosophical, because the Hermetic Philosophers count it so for the time of their operation. However, it is not necessary to imagine that they mean forty natural days, it takes a lot less; but they express themselves thus enigmatically for time, as for material and for the vase."

Below you will find quotes from alchemical manuscripts which mention the 40 day period, and quotes from the Bible.

Viatorium spagyricum, by Jamsthaler, 1625

Alchemy

Aurifontina Chymica, by John Frederick Houpreght, 1680:

You must expect to have it exceeding Black, within 40 days after you have put your Composition into the Glass over the Fire; if it be not black, proceed no further, for it is unrecoverable: it must be as black as the Ravens Head, and must continue a long time, and not utterly to lose it during five months.

The Crowning of Nature (Coronatio Naturae), by anonymous, 16^th century:

I shall add this, instead of the Crown concerning the ruling of Saturn, let a mineral be taken of the same, existing in his first being. Let it putrefy in the belly of a horse, according to the Philosophical month, to wit, 40 days.

Medulla Alchymiae (The Marrow of Alchemy), by George Ripley, 1654:

XXXVI. 9. De Sublimatione: Sublime not the matter to the top of the Vessel, for without Violence, you cannot bring it down to the bottom again; by a temperate heat below, in the space of 40 days, it will become black and obscure. When the Bodies are purified, let them be sublimed by degrees more and more, till they shall be all elevated or converted into Water.

Allegory of Bernard of Treviso, 15^th century:

Therefore does the fountain attracts them all. It humects the King to death, after which it shows itself through the same kingdom [it shows its power throughout the metallic kingdom] and everyone takes his own.
Saturn takes the gold coat and keeps it forty days. Thereafter, through the desire and nature of the King, Jupiter keeps the black jacket also 40 days. Then Jupiter delivers it over to Luna who is the third person, who keeps it 80 days or more. Then comes the regimen of Mars the fourth person, and under him the whiteness becomes very brilliant: he guards it 80 days or more according to the abilities of the Artist or Philosopher, and thus the King is stript to his shirt, which is white as snow, dry without redness. Thereafter the King takes off his shirt and gives it to Venus to keep, who is the fifth person and watches it 40 days: then gives she it to Sol, so that the King becomes naked, sees his body as it were leprous and all over red as blood"

An alchemist's diary, from a manuscript in the British Library, MS Sloane 2222, middle of the 17th century:

The matter did magis in superficie nigrescere vel membrane circuente [become more black at the surface around the membrane] the membrane on the one side did pucker like a purse mouth: but still with a shining blackness, swelling, simpering and bubbling more and more, the Subcitrine colour at the circumference, growing now to Saturnine and seemed to make inwardly to his centre or bottom of the matter.

At 40 days and my matter swelling bubbling and blustering, there appeared veins of marvellous diaphaneity that might easily be seen in the dark.

Allegory of the Fountain, by Bernard of Trevisan, 15^th century:

Then the King is in a shift, pure white as snow, or like fine pearls or a white lily, which also he puts off and delivers to Mars, who keeps it 40 days and sometimes two more. Afterwards Mars by the will of God, delivers it to yellow Sol, not clear Sol, who keeps it 40 days.

Secrets reveal'd: or, an open entrance to the shut-palace of the king, by Philalethes, 1669:

When you have prepared our gold and Mercury in the manner described, put it into our vessel, and subject it to the action of our fire; within 40 days you will see the whole substance converted into atoms, without any visible motion, or perceptible heat (except that it is just warm)

...It is in regard to these first 40 days that the student requires additional light. This period represents the regimen of Mercury (of the Sages), which is alone active during the whole time, the other substance being temporarily dead. 

Hermaphrodite Child of the Sun and Moon, by Anonymous, 1752:

At the end of this time period the raven is at the door. The Raven is deprived of its body or its darkness after lasting 40 days.

De Natura Rerum (Of the Nature of Things), attributed to Paracelsus, in a manuscript from 1571

Let the Sperm of a man by itself be putrified in a gourd glasse, sealed up, with the highest degree of putrefaction in Horse dung, for the space of forty days, or so long untill it begin to bee alive, move, and stir, which may easily be seen. After this time it will be something like a Man, yet transparent, and without a body. Now after this, if it bee every day warily, and prudently nourished and fed with the Arcanum of Mans blood, and bee for the space of forty weeks kept in a constant, equall heat of Horsedung, it will become a true, and living infant, having all the members of an infant, which is born of a woman, but it will be far lesse. This wee call Homunculus, or Artificiall [Man?].

Medicina practica, or, Practical physick shewing the method of curing the most usual diseases happening to humane bodies, by William Salmon, 1692:

XXIX. Likewise melt borax and wax, ana, one ounce, to which put of the former medicine 1 ounce: Put all these upon mercury, or any other metal 3 pound, and it will be most fine Sol or Luna, to all judgments and assays. Thus have I ended this process, in which, if you have any practice or judgment, and know how to follow the work, you may finish it, or complete in in 40 days.

Pretiosissimum Donum Dei (The Most Precious Gift of God), by Georgius Aurach de Argentina, 1475:

Here is required by how long time the stone is to be turned into blackness and what is the figure of true solution of the stone when blackness appeareth the first time it is figure of putrefaction and solution of the stone, but when it is vanished away and clearly gone it is a sign of the whole putrefaction of the stone and of the dissolution thereof, or else it is demanded if the black clouds endure in the foresaid stone by the space of 40 days,...

The Key of Alchemy, by Samuel Norton, 1421:

Namelie, that at 40 days it be black; The reason ascribed is that heate working in moist bodies, ingender blackness, which the Philosophers calle Caput corvi, which is a sure token of putrefaction.

Brief Guide to the Celestial Ruby, by Philalethes, 1694:

The four principal colours (white, black, white red), always follow in the same order; but the order of the intermediate colours cannot be so certainly determined, and you ought to be content if within the first 40 days you get the black colour.

A Golden and Blessed Casket of Nature’s Marvels (Pandora magnalium naturalium aurea et benedicta), by Benedictus Figulus, 17^th century:

Seal hermetically, and, in God’s Name, place in a vapour bath, giving it such warmth as an egg, or a child in the womb, requires. You will then behold within 40 days how the soul, lying in his spirit, again assumes its body, and is the only medium for the reuniting of body and spirit.

Secreta Alchymiae, by Kalid ben Jazichi, 1541, English translation 1597:

Bury it in as very hot Horse-dung as you can get, the vessel being well luted with Lutum Sapientiae: And in this manner let it digest. But when you perceive the Dung to grow cold, get other fresh Dung which is very hot, and put your vessel therein to digest as before. IV. Thus shall you do for the space of 40 days, renewing your Dung soften as the occasion or reason of the Work shall require, and the Medicine shall dissolve of itself and become a thick White water.

...put it again into hot Horse-dung (which is hot and moist) to digest, not omitting to renew the dung when it begins to cool, till the course of 40 days be expired.

...and put it into a Vessel and bury it in Horse-dung for 40 days till it be dissolved.

Alchemy Unveiled (Die entschleierte Alchemy), by Johannes Helmond, 1963

The adamic Earth is now a middle substance between the body and the Spirit, an intermediate thing between the fixed and the volatile. It is not as fixed, that it would be like gold, but also not as fleeting as mercury. Nevertheless, this new body is fixed enough, that it can endure the fire, which is necessary for this work and all the torture and pain of this purgatory of the Alchemists, wherein it remains for 40 days and nights without smoke or steam, as Ripley explains this in detail.

...This is the condition of the alchemistical Blackness, whose Central-Darkness, as Ripley taught, extends over three days and nights, even though the total blackness last for 40 days

When an alchemist tells a story of how he discovered the philosopher's stone, and he mentions the forty days, it is a sure sign that he is making it up, as is the case with Thomas Norton. Thomas Norton (c.1436–c.1513), born in Bristol, England was already familiar with alchemy, when at age 28 he sent a letter to the alchemist George Ripley, who answered by inviting him to meet face-to-face to discuss the secrets of the art. Ripley promised to make him "my heir and brother in this art." Norton said that he stayed with Ripley for 40 days. However, Ripley declined to give him the secret of the process of making the white elixir into red elixir because of his youth, and the danger of using it improperly. Only after Norton convinced him of his integrity did Ripley at last yield up the secret. Norton returned to Bristol with the secret for making the Great Red Elixir. But his first elixir was stolen by a servant, sending Norton into such a deep depression that he nearly gave up alchemy altogether. He rallied and made more elixir, but once again he was robbed, this time by a woman who was said to be the wife of William Canning, the mayor of Bristol. In 1477 he would wrote the famous The Ordinall of Alchimy, a large alchemical poem.

The Bible

In the Old testament, Noah's Ark survives the 40 days and nights of rain and flood waters. Noah isolates himself inside a vessel and then a transformation of the earth takes place. Alchemists also use a vessel, not a boat but a glass flask, in which the transformation takes place.

In Gen. 7.4. The lord says: "Seven days from now I will send rain on the earth. It will rain for 40 days and 40 nights. I will destroy from the face of the earth every living creature I have made."

Gen. 7.12. : Rain fell on the earth for 40 days and 40 nights.

Gen. 7.17. : For 40 days the flood kept coming on the earth. As the waters rose higher, they lifted the ark high above the earth.

Gen. 8.6. : After 40 days Noah opened a window he had made in the ark.

Is it a coincidence that the Hebrew letter Mem has a numerical value of 40? The word mem stands for mayim, which means water.

Moses spends 40 nights fasting on Mount Ararat while receiving the 10 Commandments.

Exodus 34.28 : Moses remained there on the mountain with the Lord 40 days and forty nights. In all that time he ate no bread and drank no water. And the Lord wrote the terms of the covenant—the Ten Commandments—on the stone tablets.

The Hebrews wander the desert for 40 years before entering the Promised Land.

Exodus 16.34 : The Israelites ate manna forty years, until they came to a land that was settled

Moses sent spies to explore the land of Canaan (promised to the children of Israel) for forty days.

Numbers 32:25 : And they returned from searching of the land after forty days.

Several early Hebrew leaders and kings are said to have ruled for "forty years", that is, a generation. Examples include Eli (1 Samuel 4:18), Saul (Acts 13:21), David (2 Samuel 5:4), and Solomon (1 Kings 11:42).

Goliath challenged the Israelites twice a day for forty days before David defeated him.

1 Samuel 17:16 : And the Philistine drew near and presented himself forty days, morning and evening.

The prophet Elijah had to walk 40 days and 40 nights before arriving at mount Horeb.

1 Kings 19:8 : Then Elijah walked for 40 days and nights to Mount Horeb, the mountain of God. There Elijah went into a cave and spent the night.

In the New testament, Jesus spends 40 days with his disciples before finally ascending to heaven. The 40 days here seem to denote a period of transition, a transformation period necessary before he was able to go to the spiritual realms.

Acts 1:3 : After his death, he showed himself to them and proved in many ways that he was alive. The apostles saw Jesus during the forty days after he was raised from the dead, and he spoke to them about the kingdom of God.

Jesus spends 40 days fasting in the wilderness before being tempted by the Devil. The fasting and tempting corresponds with the Nigredo phase in the alchemical work, when introspection is necessary to clear out negative emotions.

Matthew 4:4 : Then Jesus was led by the Spirit into the wilderness to be tempted by the devil. After fasting forty days and forty nights, he was hungry.

Luke 4:2 : Being forty days tempted of the devil. And in those days he did eat nothing: and when they were ended, he afterward hungered.

Jonah is commanded by God to warn the city of Nineveh that they will be destroyed in 40 days because of their sins, though they are spared by God after having repented and changed their ways. Again the 40 days are a period in which the inhabitants can cleanse themselves of their sins.

Jonah 3:4 : Jonah began by going a day’s journey into the city, proclaiming, Forty more days and Nineveh will be overthrown.