Saltpeter: A Concise History and the Discovery of Dr. Ed Polenski

Saltpeter:  A Concise History and the Discovery of Dr. Ed Polenski
By Eben van Tonder
October 2016
Cape Town

the_noord-nieuwland_in_table_bay_1762
The Noord Nieuwland in Table Bay 1762

Overview

Many years ago people used saltpeter to cure meat.  I wondered what it was and who was the first to link saltpeter to nitrite formation in curing brine and meat and ultimately the creation of nitric oxide which cures meat.  This is the story.

Background

Edward Smith says in his book that “the art of preserving meat for future use, with a view to increase the supply and lessen the cost of this necessary food (meat), is of very great importance to [England] and all the available resources of science are now engaged in it.” (Smith, 1876: 22)

He lists the main ways that this is bring done as “by drying, by cold, by immersion in antiseptic gases and liquids, by coating with fat or gelatin, by heat, salted meat and by pressure.” (Smith, 1876: 22 – 38)  All have their benefits and disadvantages and interestingly enough, they are still all being used to this day and unique products have developed around each of these.

Edward Smith says that pork is particularly prized over beef and mutton because of the  “taste, but chiefly perhaps [due] to the universal habit among the peasantry of feeding pigs, which has descended from Saxon times.  Moreover, there is a convenience in the use of it, which does not exist with regard to beef and mutton, for in such localities the pork is always pickled and kept ready for use without the trouble of going to the butcher, or when money could not be spared for the purchase of meat.”  Pigs proved to be an equally prized meat in the new world due to the “ease with which pigs are bred and reared, and the meat preserved, whilst there is great difficulty in obtaining a sufficient number of persons, in a thinly populated country or a small village, to eat a sheep or ox whilst meat is fresh.  (Smith, 1876: 59)

“Bacon is made when cuts from the pig is preserved by salt and saltpeter.”  (Smith, 1876: 64).  This gives bacon its characteristic pinkish/ redish colour, a nice flavour, and it lasts a long time before it tastes “off”.

A typical curing mix that was used during the late 1800 to the middle of the 1900’s for dry cured bacon was a mix of 10 pounds salt, 3 pounds of brown sugar, 6 ounces of black pepper and 3 ounces of saltpeter.  They use 10 pounds of this mixture per 100 pounds of meat.  (1, 2)

Dr. Ed Polenski (3)

The power of saltpeter is the fact that it contains nitrogen.  The two substances that contains nitrogen most familiar to us are saltpeter and ammonia.  The nitrogen in saltpeter and ammonia makes it very reactive, giving it an explosive power.  In saltpeter it has a particular effect on blood, explaining the fact that it gives cured meat its pinkish/ reddish colour.  Nitrogen comes into our world through plants that take it from the air and use it as food, thus making it part of the plant’s structure.  This is why saltpeter is a great fertilizer.

Dr. Ed (Eduard) Polenski (1849-1911) was a chemists at the Imperial Health Office in Berlin, Germany.  He was born in Ratzebuhr, Neustettin, Pommern, Germany on 27 Aug 1849 to Samuel G Polenski and Rosina Schultz. Eduard Reinhold Polenski married Möller. He passed away in 1911 in Berlin, Germany. (Ancestry.  Polenske)

The Imperial Health Office was established on 16 July 1876 in Berlin,focusing on the medical and veterinary industry.  At first it was a division of the Reich Chancellery and from 1879, fell under the Ministry of the Interior. In 1879, the “Law concerning the marketing of food, luxury foods and commodities” was adopted, and the Imperial Health Office was tasked with the responsible for monitoring compliance with it. Established in 1900, the Reichsgesundheitsrat supported the Imperial Health Office in its tasks. (Wikipedia. Kaiserliches Gesundheitsamt)

In 1891 Dr. Polenski made a remarkable discovery when he discovered nitrite in the brine and the meat (4).  The next few years saw this knowledge being harnessed by industry and advanced upon by science as they worked out that it is indeed the nitrite doing the curing work (through NO) and developed methods of accessing nitrite directly in curing brines.  This sped up curing from 21 days to curing in a few hours.  On farms, long curing is less of a problem, but for a commercial curing operation, it means that you keep large stocks of bacon that are in the process of curing.

In an experiment Dr Ed mixed saltpeter and salt as his curing brine.  He tested for nitrite. (5)  Nitrite and saltpeter are closely related, but it is has different properties.  Saltpeter is sodium or potassium or calcium, combined with nitrogen and three oxygen atoms with an extra electron, called nitrate.  Nitrite is the same as nitrate, but it lacks one oxygen atom.

He had a hunch about nitrate after work that was recently done in the 1880’s on conversion of nitrates to nitrites in soil and water through bacteria.  He tested for nitrite in the brine and meat and there was nothing present.

He then left the brine and the meat in brine for a week.  The meat started curing as expected.  He again tested for nitrite and as he suspected, nitrite was present.

What is Saltpeter?

For many years we did not know what saltpeter is composed off.  We knew what one could do with it.  It is a salt that is used for explosives, meat curing, to fertilize crops, cool beverages and lately even as a heart medicine.

At a few places, some peoples of the ancient world cured their meat with saltpeter and enjoyed its reddening effect, it’s preserving power and the amazing taste that it gave.  It was however, not widely used until the 1700 when it became more widely used and by 1750, its use was probably universal in curing mixes.

These ancients could not tell if saltpeter occurred naturally or was it something that had to be nourished or cultivated by humans.   When they managed to get hold of it, they wondered how to take the impurities out of the salt which gave inconsistent curing results.

People were baffled by its power.  Almost every great civilization used it in one way or the other.  Romans used it to cure meat as early as 160 BCE.  The Chinese and Italians used it to make gunpowder.  There is a record of gunpowder being used in India as early as 1300 BCE, probably introduced by the Monguls.  It was used since ancient times as medicine and as fertilizer. (Cressy, David, 2013:  12)   Saltpeter was used in ancient Asia and in Europe from the 1500’s to cool beverages and to ice foods.  (Reasbeck, M:  4)  The first reported references to the characteristic flavor of cured meat produced by the addition of saltpeter during meat preservation and curing were made as early as 1835.  (Drs. Keeton, et al;  2009)

One magical substance achieved all these amazing results, saltpetre!   Some speculated that it contained the Spiritus Mundi, the ‘nitrous universal spirit’ that could unlock the nature of the universe!

Peter Whitehorney, the Elizabethan theorist who wrote in 1500’s said about saltpeter, “I cannot tell how to be resolved, to say what thing properly it is except it seemeth it hath the sovereignty and quality of every element”.  Paracelsus, the founder of toxicology who lived in the late 1400’s and early 1500’s said that “saltpeter is a mythical as well as chemical substance with occult as well as material connections.” The people of his day saw  “a vital generative principle in saltpetre, ‘a notable mystery the which, albeit it be taken from the earth, yet it may lift up our eyes to heaven’”   (Cressy, David, 2013:  12)

From the 1400’s to the late 1800’s scientific writers probed the properties of this magical compound.  “Saltpeter encompassed the “miraculum mundi”, the “material universalis” through which ‘our very lives and spirits were preserved.  Its threefold nature evoked ‘that incomprehensible mystery of … the divine trinity,’ quoting Thomas Timme who wrote in 1605, in his translation of the Paracelsian Joseph Duchesne.  “Francis Bacon, Lord Chancellor and Privy Councillor under James I, described saltpeter as the energizing “spirit of the earth.””   (Cressy, David, 2013:  14)

“Robert Boyle who did experiments trying to understand saltpeter found it, ‘the most catholic of salts, a most puzzling concrete, vegetable, animal, and even mineral, both acid and alkaline, and partly fixed and partly volatile.  The knowledge of it may be very conductive to the discovery of several other bodies, and to the improvement of diverse parts of natural philosophy” (Cressy, David, 2013:  14)

Saltpere is found around the world, but nowhere in bigger and better volumes and quality as in India.  Here they mined saltpeter from the earth and thousands of small villages are engaged in its production.  (Crookes, W.  1868/ 69:  153)  In countries such as these, with a warm climate and high rainfall, “ammonia resulting from the putrefaction and decay of nitrogenous materials is washed into the soil by rainfall, to be oxidized by bacteria, yielding nitrate. . .  In India, saltpeter is leached from the ground as sheets of water left by monsoon flooding evaporate. A crust of saltpeter, including mineral salts, spreads across the ground, and can be dug up and refined into pure potassium nitrate.”  (Frey, James W.   2009)

It was this vast quantities of Indian saltpeter and the European and English desire to get their hands on it that resulted in the establishment of the East Indian Companies in England and Holland.  The Dutch East Indian Company, in turn, established Cape Town and in a way, we can say that Cape Town exist because of the saltpeter trade.   It was in the first place not used to cure meat, but in the production of gunpowder and as a fertiliser to increase crops to feed the ever growing population of the world.  It was the arms-race of the 16th 17th and early 18th  century and the desire of all great nations.

Its not only mined from the earth, but it also grows like fungus on the walls of cellars and around toilets.  (Drs. Keeton, et al, 2009:  6)  (6) It seems as if it occurs wherever urine and dung occurs such as in bat caves.  Peter Whitehorn, the Elizabethan theorist said that saltpeter ‘is a mixture of many substances, gotten out of fire and water of dry and dirty ground.’    “It could sometimes be found as an efflorescent or ‘flower that growth out of new walls, in cellars, or of that ground that is found loose within tombs or desolate caves where rain can not come in.’  But saltpeter could also be nourished or encouraged to grow by adding ‘the dung of beasts’ to the earth.  A distinction was made between ‘natural saltpeter’ which only needed to be scraped from walls, and ‘artificial saltpeter’, which required digging and refinement.  The two kinds ‘partook of the very same virtue’ (according to Whitehorn, relaying Biringuccio) except that some /beasts, converted into earth, in stables or in dunghills of long time not used”. (Cressy, David, 2013: 16)

In Germany they developed technology whereby they created their own saltpeter.   The average German farmer was so skilled in its production that Germans authors, writing about it did not even bother to give the detail of its production.  The German method centered around the use of niter beds (managed heaps of vegetable matter mixed with excrete and dung) created by farmers from which niter rich material was removed and saltpeter extracted.

The best description of its production in Europe from the mid to late 1500’s came to us from Lazarus Ercker (1530-1594), chief master of the mines of Emperor Rudolph II in Bohemia.  He wrote arguably the most detailed account on the production of saltpeter in ‘The right and most perfect way of the whole work of saltpetre’.   A German translation appeared in Prague in 1574 and in Frankfurt in 1580.  (Cressy, D.  Saltpetre, State Security, and Vexation in Early Modern England:  5)

Still, despite the impact of the renaissance and tremendous advances in fields of biology and the natural sciences, people could only appreciate saltpeter by what it did without any understanding of how it worked.  (Cressy, D.  Saltpetre, State Security, and Vexation in Early Modern England:  5)

In the late 1770’s a chemical instructor said about saltpeter, “‘we are much in the dark as to the origin and generation’ of saltpeter, though we knew it to be ‘found among earth and stone that have been impregnated by animal and vegetable juices susceptible of purification, and have long been exposed to air. . . . It is the product of the elements deposited in the bosom of the earth, and may not be improperly called the universal and unspecific mercury”.  (Cressy, David, 2013:  14)

“Written knowledge of saltpeter filtered into England in 1540 with Vannoccio Biringuccio’s De la pirotechnia.  This work was eventually translated into English and included accounts of the making of explosives. (Cressy, David, 2013:  14)  Saltpeter being one of the main ingredients in gunpowder.

In England they harvested saltpeter earth and extracted it from the niter enriched soil.  Saltpetermen scoured the English countryside and dug up any place where the ground could have been impregnated with animal and human urine and dung.  Saltpetermen received authority from the king to dig up any soil from which saltpeter could be extracted to the great frustration of the people of England.  Their work and the irritation it brought to the English people became a political issue during the time when England was a monarchy and after the civil war.  Dove coves or pigeon houses were favourite sites because of their concentration of the sheltered droppings.  “A skilled prospector would know how to soil rich saltpeter for ‘by the taste of the tongue it may be felt if it be biting, and how much”.  (Cressy, David, 2013:  16)

In 1588 a man by the name Lucar wrote in Collequies Concerning the Art of Shooting in Great and Small Peeses of Artillerie, ““digging the earth out of floors in cellars, vaults, stables, ox-stalls, goat or sheep cotes, pigeon house, or out of the lowermost rooms in other houses.”  This reflect the practice in Tudor England’s roving saltpetermen.”  (Cressy, David, 2013: 17)

Barrels full of dung rich earth would be set on a framework so that water introduced at the top would trickle down to a catchment tube at the bottom.  Further refinement took place through filtration.  The whole process would take a week or more.  (Cressy, David, 2013: 17)

In general, Europe and England struggled to produce enough saltpeter.  After 1850’s the East Indian Company solved the supply problem of Saltpeter by importing it from India where it occurred naturally and was also efficiently produced. (Cressy, David, 2013:   25)  Some of the saltpeter were imported in a refined state and some to be refined in England.

Trade in saltpeter from India to London, Amsterdam, Lisbon and Stockholm and Copenhagen dominated.  Saltpeter was one of the largest commodities by volume for the Dutch East Indian Company.  (Frey, James W.   2009.) (7) Its ships that sailed past our Cape Town brought vast quantities of unrefined saltpeter to be sold to England and European countries.

The state of ignorance as to the workings of saltpeter continued until in France, Antoine Lavoisier wrote in 1777 his groundbreaking works Instruction sur l’établissement des nitrières et sur la fabrication du salpêtreand  Publiée par ordre du roi, par les Régisseurs généraux des Poudres & Salpêtres where the link with nitrogen was made.  The breakthrough came in 1770 when he analysed nitric acid and its connection with saltpeter. (Mauskopf, MSH.  1995:  96)  

Lavoisier is the father of industrial chemistry.  By the time of his death a good understanding of saltpeter’s chemical origin had been achieved.  (Mauskopf, MSH.  1995:  116)  The mystery was unraveled (8).  The mysterious compound – potassium nitrate.  It is the nitrate in Saltpeter that Dr. Ed Polenski’s speculated, is turned into nitrite through bacterial action that is ultimately responsible for Nitric Oxide (NO) production and the curing of meat.

Conclusion

Saltpeter’s mysterious composition was unlocked by Lavoisier.  This, together with discoveries in the 1880’s about the conversion of nitrite to nitrate and nitrate to nitrite by ground and water bacteria lead to Dr. Polenski’s speculation that nitrite may be created in a mix of saltpeter which is potassium nitrate.  He tested for it.  His hunch was right.  This simple discovery later precipitated an avalanche of academic discoveries about nitrate, nitrite, nitric oxide and the mechanisms behind curing.

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Notes:

(1)  A survey was done in the US in the 1950’s to determine the most common brine mix used for curing bacon at the time.  Even though it is 60 years after this letter was presumably written, I include it since methods and formulations in those days seemed to have a longevity that easily would have remained all those years later.  The survey was also done among farmers, in an environment where innovation are notoriously slow.  (Dunker and Hankins, 1951:  6)

(2)  How salty was this bacon in reality?  The recipe is used by most US farmers by the 1950’s was 10 lb (4.54kg) salt, 3 lb (1.36kg) of brown suger, 6 ounces (170g) of black pepper and 3 ounces (85g) of saltpeter.  10 pounds (4.54kg) of this mixture per 100 pounds (45.36kg) of meat.

The total weight of dry spices is therefore 6.07kg of which salt is 74% or  3.4kg.  This was applied at a ratio of 3.4kg salt per 45kg of meat or 1 kg salt per 13 kg of meat.  Not all salt was absorbed into the meat, but the meat was regularly re-salted during then curing time which means that this ratio would be applied many times over before curing was complete.  Compare this with the salt ratio targeted by us in 2016 of 25g per 1kg final product, this means that the bacon made with this recipe would be extremely salty, irrespective of the use of sugar to reduce the salty taste.  The bacon would have to be soaked in water first to draw out some of the excess salt, before consumed.

(3)  Spelling of his surname varies between Polenski and Polenske.

(4)  Dr Ed Polenski’s findings has been published in “Arbeiten aus dem Kaiserlichen Gesundheitsamte , 7. Band, Springer, Berlin 1891, S. 471–474” (http://books.google.co.za/books?id=R_YAAAAAYAAJ&pg=PA471&redir_esc=y)

(5)  Qualitative and quantitative techniques for measuring nitrite and nitrates in food has been developed in the late 1800’s.  (Deacon, M;  Rice, T;  Summerhayes, C,  2001: 235, 236).  The earliest test for nitrites is probably the Griess test.  This is a chemical analysis test which detects the presence of organic nitrite compounds. The Griess reagent relies on a diazotization reaction which was first described in 1858 by Peter Griess.

Schaus and others puts the year of the discovery by Griess as 1879.  According to him,  Griess, a German Chemist used sulfanilic acid as a reagent together with α-naphthylamine in dilute sulfuric acid.  In his first publication Griess reported the occurrence of a positive nitrite reaction with human saliva, whereas negative reactions  were consistently obtained with freshly voided urine specimen from normal individuals.   (Schaus, R; M.D. 1956:  528)

(6)  Wall saltpeter (calcium nitrate), formed by nitrifying bacteria and found as an efflorescence on the walls of caves and stables, was gathered in China and India long before the Christian era. (Drs. Keeton, et al, 2009:  6)

(7)  “BETWEEN 1601 AND 1801, ships made thousands of voyages carrying goods from Asian ports to the primary European markets for East Indian commodities: Amsterdam, London, l’Orient, Copenhagen, Lisbon, and Stockholm. On average, these Indiamen measured 1,000 tons burden, with approximately 2,830 [m.sup.3] of cargo space. Sixteen percent of this cargo space, according to the normal practice of East India captains, consisted of saltpeter–some 452.8 [m.sup.3] of nitrates, weighing 1.6 metric ton”  (Frey, James W.   2009)

The British took over the Subah of Bengal from the VOC in 1757.   “The subsequent defeat of a VOC expeditionary force at Bedara in 1759, and the British defeat of the Mughals at Buxar in 1764 secured Company control over Bihar and permitted the monopolization of the saltpeter trade. The significance of these events cannot be underestimated. By seizing Bengal, the British exerted mastery over 70 percent of the world’s saltpeter production during the latter part of the eighteenth century.”  (Frey, James W.   2009)

(8)  One of Lavoisier’s major achievements of the 1770’s (and the one of particular interest to us) : the analysis of nitric acid.”  (Mauskopf, MSH.  1995:  96)

“Lavoisier’s chemical discoveries and reformulation, enabled him to delineate the chemical reaction that produced (explosive) gases : between the nitric acid component of saltpeter and the carbon in the charcoal.” (Mauskopf, MSH.  1995:  105)

He wrote the landmark paper in the spring of 1776 “in which Lavoisier demonstrated that nitric acid was a compound of nitrous gas with the portion of the pure air (la portion la plus pure de l’air) by decomposing the acid into these gases and then reconstituting it from them, and asserted for the first time that this “was a constituent of all acids and a principle of acidity” (la portion la plus pure de l’air).  (Mauskopf, MSH.  1995:  107)

“Saltpeter is a “borderline” substance between the organic and the inorganic world. Although clearly a neutral salt like other mineral salts, it only seemed to come into existence in the presence of organic substances in a state of decay. There was therefore
a debate in the century before Lavoisier took up the subject over how this substance originated and whether or not vital processes were essential for its formation.”  (Mauskopf, MSH.  1995:  108)

“According to Macquer, three theories were current. The oldest one posited a non-vital origin of saltpeter (or at least of nitric acid) in the air. The other two, formulated in the eighteenth century, related nitric acid’s origin to organic processes. Lemery the Younger
postulated a completely organic origin : nitric acid developed in vegetable and animal substances. The theory of Georg Ernst Stahl, was, in a sense, the mean between the other two vis-a-vis organic and inorganic origins; moreover, it offered a chemical analysis
of nitric acid. To Stahl, this acid was a compound of the universal acid (vitriolic) and phlogiston. Although the source of vitriolic acid lay in the ambient air, this acid could only be converted into nitric acid through the agency of organic putrefaction, which liberated
the necessary phlogiston.”

“The origin and chemical nature of the fixed alkali component were only somewhat less obscure than that of nitric acid. “Fixed vegetable alkali” was one of a class of three alkaline substances (the other two were mineral and volatile alkali) that possessed many
common features. Two of these alkalis (vegetable and volatile) were thought to have organic origins while the mineral alkali, or, at least, its principal salt, common salt, was felt to be, in Macquer’s words, “a production of nature, and […] it does not belong to the plant kingdom or the animal kingdom, it is stored in the class of minerals. It is for this reason that we gave his name alkalile iTAlkali mineral” (une production de la nature, & […] il n’appartient ni au règne végétal, ni au règne animal, on le range dans la classe des minéraux. C’est par cette raison, qu’on a donné à son alkalile nom iTAlkali minéral).

. While alkalis, like acids, were distinct chemical substances operationally, they were not yet ontologically separate (nor would they be fully for Lavoisier).” (Mauskopf, MSH.  1995:  108)

“Lavoisier succeeded both in decomposing nitric acid with mercury into its components gases in succession “the rudy,  nitrous gas” and then “the portion of the pure air” (la portion la plus pure de l’air)  from the calx of mercury which had formed in the meantime and in reconstituting the acid from just these components gases over water. With this analysis, Lavoisier was moving towards a clear differentiation of acids into chemically distinct species (albeit with a common “acidifying principle”, a point also enunciated in this paper for the first time.” (Mauskopf, MSH.  1995:  108)

“By 1777, Lavoisier had reached the conclusion that the traditional (i. e. Macquerian) view of the role of the wood-ash treatment was indeed correct : it was necessary to employ wood-ash preferably both intermixt with saltpeterish earth and as a lessive in that earth’s first purification. His conclusion was based on systematic study of the tamarisk ash used by Parisian saltpetermen ; the crucial tests were of : 1° washed ash mixed with a) mother liquid of raw saltpeter and b) an artificial liquid of “chalk cham loin cloth” (craie de champagne)  dissolved in very pure nitric acid; and 2° the same tests using unwashed ash. In the first set of tests, no saltpeter was obtained; the second yielded 7 ounces. Prior to the reports on the tests, Lavoisier had given the results of his analyses of the Parisian ash itself : he had found various salts including «sélénite», «tartre vitriolé» (selenite, tartar vitriolated) , Glauber salt and common salt.

At first surprised at the production of saltpeter in the second set of tests with unwashed ash, Lavoisier soon produced more tests and a chemical explanation : «… il ne m’était pas possible de douter qu’il ne se fût opéré unedouble décomposition en vertu d’une double affinité; que, d’une part, l’acide vitriolique qui entre dans la composition de ces sels ne se fût combiné avec la terre calcaire de l’eau-mère pour former de la sélénite, et que, de l’autre, l’acide nitreux ne se fût emparé de la base alcaline du sel de Glauber et du tartre vitriolé pour former de véritable sal pêtre.»  Translate:  “it was not possible for me to doubt that he would have made unedouble decomposition under a dual affinity; that, on the one hand, the corrosiveness acid in the composition of these salts was not combined with the ground limestone mother liquor to form selenite, and, on the other, acid nitrous was not captured the alkali and Glauber’s salt to form tartar Vitriolated real sal Petre”

As confirmation, Lavoisier combined the mother liquids of « very pure » niter with solutions of vitriolated tartar and other vitriolates. In all cases, selenite and a niter of whatever base of the vitriol were formed. He concluded :« // est évident, d’après tout ce qui vient d’être dit, que les cendres qu’on emploie dans la fabrication du salpêtre ne servent pas seulement en raison de la partie alcaline qu’elles contiennent à nu; qu’elles agissent encore en raison de la partie alcaline qu’elles contiennent dans un état de neutralité, et combinée avec l’acide vitriolique. Ainsi, peu importe qu’on emploie, pour décomposer l’eau-mère et pour la convertir en vrai salpêtre, un alcali fixe à nu, ou un sel vitriolique à base d’alcali; l’effet est le même, et l’acide nitreux, dans les deux cas, va chercher de préfé rence l’alcali contenu dans le sel, et en déloge l’acide vitriolique. »  Translated:  Is clear from all that has been said, that the ashes that employed in the production of not only serve saltpetre because they contain the alkaline part exposed; they act yet because of the alkaline part in a state that they contain neutrality, and combined with the vitriolic acid. Thus, regardless of we used to split water mater and to convert it into real saltpeter, alkali fixed bare or vitriolic based salt alkali; effect is the same, and nitrous acid, in each case, pref fetches ence alkali content in salt, and removes the vitriolic acid.”  (Mauskopf, MSH.  1995:  112, 113)

References

Cressy, D.  2013.  Saltpeter.  Oxford University Press.

Cressy, D.  Saltpetre, State Security, and Vexation in Early Modern England.  The Ohio State University

Crookes, W.  1868/ 69.The Chemical News and Journal of Physical Science, Volume 3.  W A Townsend & Adams.

Deacon, M;  Rice, T;  Summerhayes, C.  2001. Understanding the Oceans: A Century of Ocean Exploration,   UCL Press.

Dunker, CF and Hankins OG.  October 1951.  A survey of farm curing methods.  Circular 894. US Department of agriculture

Frey, James W.   2009.  The Historian.  The Indian Saltpeter Trade, the Military Revolution and the Rise of Britain as a Global Superpower.   Blackwell Publishing.

Jones, Osman, 1933, Paper, Nitrite in cured meats, F.I.C., Analyst.

Drs. Keeton, J. T.;   Osburn, W. N.;  Hardin, M. D.;  2009.  Nathan S. Bryan3 .  A National Survey of Nitrite/ Nitrate concentration in cured meat products and non-meat foods available in retail.  Nutrition and Food Science Department, Department of Animal Science, Texas A&M, University, College Station, TX 77843; Institute of Molecular Medicine, University of Texas, Houston Health Science Center, Houston, TX 77030.

Kocher, AnnMarie and Loscalzo,  Joseph. 2011.  Nitrite and Nitrate in Human Health and Disease. Springer Science and Business Media LLC.

Lady Avelyn Wexcombe of Great Bedwyn, Barony of Skraeling Althing
(Melanie Reasbeck), Reviving the Use of Saltpetre for Refrigeration: a Period Technique.

Mauskopf, MSH.  1995.  Lavoisier and the improvement of gunpowder production/Lavoisier et l’amélioration de la production de poudre.  Revue d’histoire des sciences

Newman, L. F.. 1954.  Folklore. Folklore Enterprises Ltd.

Smith, Edward.  1876. Foods. D. Appleton and Company, New York.

http://hansard.millbanksystems.com/commons/1938/mar/01/meat-prices

Pictures:

A cargo ship at the Cape:  https://en.wikipedia.org/wiki/Economy_of_the_Western_Cape

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