Digestion is the alchemical process of applying steady heat to a substance in a flask or container. Digestion can last hours or days. The favorite heat source for this process is fresh horse manure, due to the action of unseen bacteria.
Digestion might be part of several processes or a process in itself. It's commonly quoted on lists of necessary processes to discover / make the philosopher's stone, written by people who haven't achieved it but somehow know how.
Processes for attaining the Philosophers' Stone, appearing as lapis philosophorum in medieval writings, can range from three to a dozen and several in between. They are ultimately based on teachings of Maria the Jewess c. 100 AD in Alexandria, Greco-Roman Egypt.
Digestion is a process used by alchemists since the early days. The practitioner applies gentle steady heat to a substance over a certain time period. Digestion can be used for several purposes:
separation of substances
liquification or blending of substances
heat decomposition
acid decomposition
gradual addition of other materials (where flask neck or opening protrudes - opening is otherwise corked unless for special purpose)
color or texture changes in substances
It's historically done by sealing a substance in a flask and keeping the flask in horse dung. Direct sunlight might also do the job. Maria the Jewess mentions full sunlight in descriptions of iosis or ios, sometimes called the reddening or rust. Later Latin refers to this stage as rubedo.
Digestion is a core alchemical process, along with simple distillation and fermentation. In some later practices it's associated with the zodiac sign Leo due to a series of lists published in the late Renaissance. Astrology is considered a science at the time.
For example it's part of the fundamental knowledge of physicians such as Paracelsus. The doctor does a horoscope reading to discover the nature of the person's disease. This is in combination with other methods such as uroscopy to examination the patient's urine.
The process of digestion involves decomposition of manure, usually that of horses. During this event the flask can heat up between 130°F to 155°F. At this temperature impurities such as ungerminated seeds and disease pathogens die.
Fresh horse manure heats up quickly due to busy microbes who break down components and generate heat. It's a readily available material. A horse of 1,000 lb (454 k) produces 40-50 lb (18-23 k) of manure every day. This comes to about a ton of manure a month.
Slow warming is less shocking to the substance in the flask and even the integrity of the flask itself. The vessel can be made of iron, copper or lead, depending on the level of heat and effects desired.
Glass is also an option if warmed before being heated, but it can have hidden weaknesses and break due to heat-driven expansion of its molecules. If heated quickly, glass can have both hot and cool spots, encouraging breakage. Today we have Pyrex.
The manure maintains temperature for several days to a week or longer. Elements in horse manure include nitrogen (N), carbon (C) phosphorus (P) and potassium (K). To heat through bacterial action the manure needs oxygen, moisture, and a proper carbon:nitrogen ratio.
Nitrogen and carbon are plentiful in horse manure. The practitioner need only place the bottle or flask in the manure and let the heat-producing bacteria do their work. The flask is set with the neck above the manure , or completely buried. It often has a very long neck. One style is oval, known as the "philosophers' egg".
In the initial stages of decomposition (< 40°C), mesophilic bacteria are the main types present, many from topsoil. Once the temperature of the dung rises over 40°C, thermophilic bacteria become dominant. At this stage, the microbes are largely Bacillus genus.
Acid digestion is significant in the recovery of different analyte contents in complex matrices, like herb and plant materials. In dissolving herbal product samples acid digestion methods are favorable. Nitric acid is a popular solvent.
Wet/acid digestion offers the advantage of effectively acting on both organic and inorganic substances by disintegrating the sample matrix to reduce interference. Acid digestion can be a critical phase in the overall process.
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