Pliny on Nitre Production
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Pliny on the Production and Uses of Nitre
Pliny the Elder, Caius Plinius Secundus lived from 23 to 79 CE. He wrote his encyclopaedic Natural History towards the end of his life.
And here we must no longer defer giving an account of nitrum (seemingly potash or soda) ......
In Egypt, again, it is made artificially, and in much greater abundance, but of inferior quality, being tawny and full of stones. It is prepared in pretty nearly the same manner as salt, except that in the salt-pans it is sea-water that is introduced, whereas in the nitre-beds it is the water of the river Nilus; a water which, upon the subsidence of the river, is impregnated with nitrum for forty days together ...... On occasions when there has been a fall of rain, a smaller proportion of river water is employed.
As soon, too, as any quantity of nitrum has formed, it is immediately removed, in order that it may not melt in the beds. This substance, also, contains a certain proportion of oil, which is very useful for the cure of scab in animals. Piled up in large heaps, it keeps for a very considerable time...... The lightest part of nitrum is always considered the best, and hence it is that the froth of it is so much preferred. Still, however, when in an impure state, it is very useful for some purposes, colouring purple cloth, for instance, and, indeed, all kinds of dyeing. It is employed, also, very extensively in the manufacture of glass, as we shall more fully mention on the appropriate occasion.
The only nitre-works in Egypt were formerly those in the vicinity of Naucratis and Memphis; those near Memphis being inferior to the others, the piles of nitrum there prepared being as hard as stone, and many of the heaps having become changed into rocks. When in this state, vessels are made of it, and very frequently they melt it with sulphur on a charcoal fire. When substances are wanted to keep, they employ this last kind of nitrum.
In Egypt there are also nitre-beds, the produce of which is red, owing to the colour of the earth in the same locality.
Froth of nitrum, a substance held in very high esteem, could only be made, according to the ancients, when dews had fallen; the pits being at the moment saturated with nitrum, but not having arrived at the point of yielding it. On the other hand, again, when the pits were in fall activity, no froth would form, it was said, even though dews should fall. Others, again, have attributed the formation of this last substance to the fermentation of the heaps of nitrum. In a succeeding age, the medical men, speaking of it under the name of "aphronitrum," have stated that it was collected in Asia, where it was to be found oozing from the soft sides of certain mines--the name given to which was "colyces" --and that it was then dried in the sun. The very best is thought to be that which comes from Lydia; the test of its genuineness being its extreme lightness, its friability, and its colour, which should be almost a full purple. This last is imported in tablets, while that of Egypt comes enclosed in vessels pitched within, to prevent its melting, the vessels being previously prepared by being thoroughly dried in the sun.
To be good, nitrum should be very fine, and extremely spongy and porous. In Egypt, it is sophisticated with lime, an adulteration easily detected by tasting it; for when pure, it liquefies immediately, while that which has been adulterated, remains undissolved sufficiently long to leave a pungent taste in the mouth. It is burnt in a close earthen vessel, as otherwise it would decrepitate: except in this last case, however, the action of fire does not cause it to decrepitate. This substance neither produces nor nourishes anything; while, in the salt-pans, on the other hand, we see plants growing, and the sea, we know, produces immense numbers of animated beings, though, as to plants, sea-weed only.
It is evident, too, that the acridity of nitrum must be much greater than that of salt, not only from the fact last mentioned, but from the circumstance also, that at the nitre-beds the shoes wear out with the greatest rapidity; localities which are otherwise very healthy, and remarkably beneficial for the eye-sight. At the nitre-works ophthalmia is a thing unknown: persons, too, that come there with ulcers upon them experience a rapid cure; though ulcerations formed upon the spot are but slow in healing. Used as a friction with oil, nitrum is a sudorific, and acts emolliently upon the body.
That of Chalastra is used as a substitute for salt, in making bread, and the Egyptian nitrum is eaten with radishes, it having the effect of making them more tender; though as to other edibles it turns them white and spoils them. To vegetables it imparts an additional greenness.
Pliny, Natural History, Book XXXI, chapter 46 - (eds. John Bostock, H.T. Riley)