The above image is a Japanese shakudō resembling the description of some Corinthian bronze varieties such as hepatizon. Image: Okami-san, CC BY-SA 3.0

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In the age of nanomaterials, plastics and designer proteins, the ancients can still proffer chemical mysteries and achievements, impressive for the fact their notions of matter were primitive at best, and despite Democritus’ prescient atomic hypothesis languishing for centuries ‘disproven’ by Aristotle’s contention that a vacuum could not exist.

Damascus steel blades, for example, were proverbial for their sharpness, flexibility and their beautiful surface pattern resembling flowing water known as damask, or damascene (not to be confused with pattern welding). It was said they were so sharp they could slice in half a silk handkerchief dropped in midair and cut through stone and blades made of lesser steel. The secret of their production was lost in medieval times to be rediscovered only in recent years.¹ A similar story can be told concerning humanity’s first artificial pigment, produced by the ancient Egyptians as early as 3200 BCE, ‘Egyptian blue’, the use of which declined after the fall of the Roman empire, possibly due to an incomplete recipe found in Vitruvius, the last known use being in the 16^th century.^2,3   

An aura of metallurgical and chemical mystery surrounds the ancient alloy known as Corinthian bronze (Latin aes Corinthium).    

The term fell out of use in the 10^th century CE. Earlier references in the literature are often imprecise, hence there are different theories as to what precisely this alloy was.⁴ Corinthian bronzes and their fabulous cost are first referred to in Roman literature by Cicero in 80 BCE in his defence of Quintus Roscius of Ameria.⁵ He had cause to lambast the proverbially corrupt and avaricious Gaius Verres ten years later for taking precious Corinthian bronzes from the temple of Cybele at Enguinum during his tenure as Governor of Sicily.⁶ Verres was ultimately obliged to go into exile in Massalia (modern Marseilles) and rather ironically met his end during the proscriptions of the second Triumvirate over a Corinthian bronze sculpture desired by Mark Antony.⁷ Nor was Mark Antony the only triumvir smitten with desire for Corinthian bronzes, as Augustus himself was said to be “a good deal too fond of expensive furniture, Corinthian bronzes and the gaming table.” While the proscriptions were in progress, someone had scrawled on the base of his statue: 

"I do not take my father’s line;
His trade was silver coin, but mine
Corinthian vases.”⁸

This fad for Corinthian bronze, so beautiful and expensive that desire for it drove men to murder, ensured such remained costly antiques throughout the first century BCE and first century CE.⁹ Corinth had been known as a source of fine bronze work since the 4^th century BCE, a reputation that would continue even after the city was burned by Mummius in 146 BCE.^10,11 An unlikely story, repeated in different forms, was that Corinthian bronze was, “produced by accident, when Corinth was burnt at the time of its capture.”¹² Pausanias (c.110 to c.180 CE), author of the Description of Greece, knew of Corinthian bronze only by repute. Plutarch (c.46 to c.119 CE) refers to the manufacture of Corinthian bronze in the past tense. Before and after the heyday of this alloy, roughly from Cicero’s day until the end of the 1^st century CE, golden surfaced bronze was made using mercury gilding that used less gold.¹³

Of the ancient authors it is Pliny the Elder (who died during the eruption of Vesuvius that buried Pompei in 79 CE) who has most to say about Corinthian bronze:¹⁴    

“The other kinds are made artificially, all of which will be described in the appropriate places, the more celebrated kinds first coming under our notice. Formerly a mixture was made of copper fused with gold and silver, and the workmanship in this metal was considered even more valuable than the material itself… we have now so entirely lost the method of making this valuable compound by fusion, that, for this long time past, not even chance itself has assumed, in this department, the privilege which formerly belonged to art. 

The only genuine Corinthian vessels, then, are those which these men of taste metamorphose, sometimes into dishes, sometimes into lamps, or even into washing-basins, without any regard to decency. They are of three kinds; the white variety, approaching very nearly to the splendour of silver, and in which that metal forms a large proportion of the compound; a second kind, in which the yellow colour of gold predominates; and a third, in which all the metals are mixed in equal proportions. Besides these, there is another mixture, the composition of which it is impossible to describe, for although it has been formed into images and statues by the hand of man, it is chance that rules in the formation of the compound. This last is highly prized for its colour, which approaches to that of liver, and it is on this account that it is called "hepatizon", it is far inferior to the Corinthian metal, but much superior to the Æginetan and Delian, which long held the first rank.”¹⁵

According to Pliny, Corinthian bronzes came in multiple varieties, most more valuable than silver and almost as valuable as gold, one resembling silver in appearance, one gold and another intermediate between them along with a lesser variety prized for its liver-like colour.  

It should be noted that even the ancients had issues identifying precisely which objects were made of Corinthian bronze.¹⁶ After sacking Corinth, Mummius’ triumphant return from Greece with booty included objects captured elsewhere, all considered by the Roman public as “Corinthian”.¹⁷ As far as Pliny was concerned, Corinthian bronze was not found in statues (despite his nephew claiming to own a small statue¹⁸) but principally in vessels and smaller utensils, furthermore he only accepted the elaborately decorated vessels as truly “Corinthian”.¹⁹ Even the items plundered from Corinthian graves after the sacking of the city could not be guaranteed to be of Corinthian manufacture or of aes Corinthium.²⁰

Electrum and other alloys of gold and silver along with copper had been known from ancient times, so what made Corinthian bronze different and so valuable?  

Other attributes and possibly manufacturing procedures can be gleaned from the literature. Cicero informs us that it was resistant to corrosion.²¹ Pausanias claims Corinthian bronze was quenched red hot in the waters of the Peirene.²² The Peshitta, a Syriac translation of the Old Testament (dated between 1^st and 3^rd centuries CE) calls the metal described in 1 Kings 7:45 ‘Corinthian bronze’ declaring it was ‘burnished’.²³ The Leiden papyrus amidst its many recipes for faking gold and other compounds or diluting the same refers to acid leaching of gold with vinegar, along with iron and copper pyrites and salt, to impart the brilliance and colour of gold to an alloy containing only a portion of the precious metal. This mixture when heated forms ferric chloride and sulfuric acid, a highly effective leaching solution for copper.²⁴ Plutarch also refers to the appearance of Corinthian bronze as a surface feature.²⁵

Taken together, these procedures intimate depletion gilding and may have been the key process in the manufacture of Corinthian bronze. Such treatments could make copper-gold alloys look like solid gold (or silver depending on which element predominated in the primarily copper mix) and the surface would benefit from the corrosion resistance of gold (a desirable attribute in vessels used for food).²⁶  

Depletion gilding was certainly known to the Romans at least by the 4^th century CE, as witnessed by the systematic debasement of the coinage silvered using various methods including surface depletion of base metals with the coins heated in air and the resultant surface oxide removed using organic acids leaving a dull, porous layer of silver that was burnished to produce a bright, silver sheen.^27,28

Such treated copper-silver-gold alloys are well known from pre-Columbian cultures in South America, with gold containing copper alloys referred to as tumbaga. The first examples were copper-silver alloys. The knowledge of depletion gilding emerged within the Moche culture (c.100 BCE to c.800 CE), spreading from Peru to Colombia and as far North as Mexico. Typically, tumbaga alloys, resembling solid gold items in appearance, contain between 15 and 50% (w/w) gold.²⁹

The products and skills of the pre-Columbian cultures may also offer a clue as to the composition of the alloy Pliny the Elder mentions briefly - the liverish appearing hepatizon - described as a lesser variety of Corinthian bronze.  

Two Roman satirists state that Corinthian bronze had a distinctive odour. Petronius, in his first century work, Satyricon, has garish, nouveau-riche Trimalchio speaking in reference to Corinthian bronze declare, “But I prefer glass, if you don’t mind my saying so; it don’t stink, and if it didn’t break, I’d rather have it than gold, but it’s cheap and common now.”³⁰ Martial, lampoons one Mamurra, observing, “He took council of his nose whether the bronzes smelt of Corinth.”³¹ Authors, such as Jacobson and Weitzman,¹⁰ dismiss these statements claiming, “since both were writing satirically, however, these references need not be taken seriously.  Metals normally have no intrinsic smell.”³² Yet Roman and Greek literature is frequently mined for just such informative incidental remarks.  

According to Spanish sources, the Tainos (indigenous to the Caribbean) used the term ‘guanín’ to refer to a low-karat gold of purplish colour whose scent and iridescence made it particularly attractive. This alloy was shown in Columbus’ time to be an alloy of gold, copper and silver. Modern analysis of the few guanín objects known indicates a copper content of > 25% in all cases (reaching as high as 57%).^33,34

Purplish coloured alloys of copper containing gold and silver could indeed smell.

Regardless, guanín still has a relatively high gold and silver content compared with the purple-black-patinated predominantly copper alloys produced around the Old World (from ancient Egypt through to Japan) that contain around 0.5-4% gold and 0.3-2% silver, all artificially produced using chemical baths of different kinds or simply holding items in sweaty hands (the most famous alloy being Japanese shakudō).³⁵ 

The composition of the Old World patinated alloys are more in keeping with the only text that gives a recipe for the Corinthian bronze alloy and description of its manufacturing process, that of Alexandrian alchemist Zosimos of Panopolis (flourished c.300 CE). A translation of the relevant recipe, “To make black metal strips or the Corinthian alloy”, summarised by Giumlia-Mair,²⁸ gives a composition of silver and gold of 6.5% total in copper. This alloy was treated in various ways with several chemicals to produce a black appearance. Zosimos, however, considered iosis (Greek: purpling) as the most important process involving treatment with various ingredients such as pimpernel, rhubarb, vinegar, verdigris, sulfur and arsenic (similar to the shakudō processes).^36,37

In a series of experiments using compounds known to the ancients on various alloys of copper, tin and lead along with a ‘Corinthian bronze’ consisting of 4% silver and 1% gold in copper, Devogelaere³⁸ noted the following:

“For Corinthian bronze and its black patina, I tested three types of solutions: a sulphur and alum solution that gives a black colour; a copper acetate solution for a dark red colour; and an application without a solution but heated with a flame. The latter test gives a dark bluish gray patina. The formation of this patina occurs only with the Corinthian bronze, it doesn’t form a patina on binary or ternary bronzes. The formation of a dark oxide layer must be produced with gold and/or silver present in small quantities in the alloy.”

The ‘hepatizon’ - the poor man’s version of Corinthian bronze - could have been produced by simply heating the alloy surface, although a treatment with sulfur and alum might explain the purported smell.  

Mystery still remains. Jacobson & Weitzman¹⁰ contend “the textual sources state clearly that the colour of Corinthian bronze, for which it was prized, was that of gold or silver, or intermediate between the two.” Giumlia-Mair²⁸ however notes “high-tin bronze and tumbaga have never been identified among many thousands of Roman objects analysed, while several black patinated pieces are known,” militating against depletion gilding of copper-silver-gold alloys being key to the making of Corinthian bronze. The more expensive versions of Corinthian bronze, compared with the black-purple patinated varieties, had relatively high concentrations of silver and gold and may have been melted down for that reason or simply have been rare compared with less expensive varieties.  

The Roman orator, Hortensius, possessed a sphinx of Corinthian bronze given to him by his client, the greedy Verres. Hortensius declared during Verres’ trial for corruption that he could not guess riddles, to which Cicero replied that he should be able to, as he kept a sphinx made of the precious alloy in his house.³⁹   

It is a shame this fabled sphinx has not come down to us, as an examination using modern techniques might have resolved a number of questions concerning the exact composition of Corinthian bronzes and their method of manufacture that remain subjects of scholarly debate and chemical experiment two thousand years after Romans lost their heads over it.  

Darren A. Saunders, BSc (Chemistry, University of Canterbury), MSc (Pharmacology, University of Otago) worked from 1988 to 1998 as a Scientific Officer in the Clinical Pharmacology Department at the University of Otago, Christchurch School of Medicine.  He was employed from 1998 to 2022 at the Institute of Environmental Science and Research Ltd (ESR) based in Christchurch, working in the Chemistry Laboratory and holding the position of Senior Scientist upon “retiring” in 2022. Darren’s major interests are chemistry in antiquity, painting, astrophotography and pottery. He is currently completing a Bachelor of Arts in Classics at the University of Canterbury.     

References

1. Verhoeven, J. D.; Scientific American, 2001, 284(1), 74-79. Damascene blades contained bands of iron carbide particles, Fe₃C, known as cementite. These particles line up parallel to the blade surface and when the blade is etched with acid, the carbides appear as white lines in a dark steel matrix. The carbide particles are extremely hard, and it is thought that the combination of these bands of hard steel within a softer matrix of springier steel gives Damascus weapons a hard cutting edge combined with a tough flexibility.
2. Vitruvius, On Architecture Penguin Classics: England, 2009.  The Roman architect, Vitruvius (1^st century BCE) gives a recipe for Egyptian blue, calcium copper silicate (CaCuSi₄O₁₀), or caeruleum as the Romans called it, in Book 7, Chapter 11 of On Architecture, “Sand is ground up with natron flowers so finely that it becomes like flour, and Cypriot copper is grated with a coarse file over it… Then it is made into balls by being rolled in the hands and compressed so that it dries.  The dry balls are put in an earthenware jar, and the jar in the furnace.”  Vitruvius however neglects to mention the proper ratio of ingredients (theoretically SiO₂:CaO:CuO – 63.93:14.92:21.16) or the fact that calcium is necessary and the right temperature range is crucial for its formation.  The calcium probably came from calcium rich sand in Vitruvius recipe.  
3. Fontana, R.; et al, Journal of Cultural Heritage, 2020, 45, 370-378.  Egyptian blue progressively disappears after the fall of the Roman empire: see also: https://www.smithsonianmag.com/smart-news/glistening-egyptian-blue-pigment-was-forgotten-then-lost-180956451/  
4. Grimes, S.; Becoming Gold Zosimos of Panapolis and the Alchemical Arts in Roman Egypt.  Rubedo Press: Auckland New Zealand, 2018
5. Cicero, M. Tullius., The Complete Works of Marcus Tullius Cicero, Delphi Classics Kindle Edition, United Kingdom 2014.  Cicero, Rosc. Am. 46  “Another comes down to you from his palace in the Palatine; he has for the purposes of relaxation to his mind a pleasant suburban villa, and many farms beside, and not one that is not beautiful and contiguous; a house filled with Corinthian and Delian vessels,… that he lately bought at so great price, that passers-by, who heard the money being counted out, thought that a farm was being sold.”
6. Cicero, Verr 2.4.98  “Are you, forsooth, the only man who delights in Corinthian vases? Are you the best judge in the world of the mixture of that celebrated bronze, and of the delicate tracery of that work?”  As quoted in: https://www.perseus.tufts.edu/ 
7. Pliny the Elder.; Complete Works, Delphi Classics Kindle Edition, United Kingdom 2015.  (Plin. HN. 34.3)
8. Suetonius.; The Twelve Caesars, Penguin Classics Kindle edition, England, 2007. (Suet. Aug. 70)
9. Emanuele, D.; Phoenix 1989, 43(4), 347-358
10. Jacobson, D. M. & Weitzman, M. P.; American Journal of Archaeology, 1992, 96(2), 237-247
11. Mattusch, C. C.; Corinth, 2003, 20, 219-232
12. Pliny the Elder, Plin. HN. 34.3 As quoted in: https://www.perseus.tufts.edu/  The story is also satirised in Petronius Satyricon 15.31.50
13. Jacobson, D. M. & Weitzman, M. P.; 1992
14. Mattusch, C.C.; 2003
15. Pliny the Elder, Plin. HN. 34.3  As quoted in: https://www.perseus.tufts.edu/  Note Pliny contradicts himself in 34.3 HN as he repeats the story about Corinthian bronze being produced by accident when Corinth was sacked and burned then informs us the method of its manufacture was lost.
16. Pliny the Elder, Plin. HN. 34.6 Indicates that when Corinth was sacked ‘bronzes’ from many different sources were dispersed, “But although it is well known that there are no lamp-stands in existence made of the Corinthian metal, yet this name is very generally attached to them, because, in consequence of the victory of Mummius, Corinth was destroyed: at the same time, however, it should be remembered that this victory dispersed a number of bronzes which originally came from many other cities of Achaia.”
17. Emanuele, D.; 1989 
18. Pliny the Younger, Complete Works, Delphi Classics Kindle Edition, United Kingdom, 2014.  Plin. Ep. 3.6 lauds a small Corinthian bronze he purchased all while disclaiming any great knowledge of art.  He claims, “The bronze itself, judging by the genuine colour, is old and of great antiquity.”  Perhaps it was composed of ‘hepatizon’ the lesser ‘Corinthian’ bronze often used in statuary.  
19. Emanuele, D.; 1989 
20. Ibid. 
21. Cicero, Tusc. Disp. 4.14
22. Pausanias.; Guide to Greece 1: Central Greece, Penguin Classics, London England, 1979. Paus. 2.3.3
23. Jacobson, D. M. & Weitzman, M. P.; 1992
24. Caley, E. R.; The Leyden and Stockholm Papyri Greco-Egyptian Chemicals Documents from the Early 4^th Century AD an English Translation, Oesper Collections in the History of Chemistry, University of Cincinnati, 2008. “Sometime around 1828 a considerable number of papyri were recovered (presumably by grave robbers) from burial sites near Thebes in central Egypt. These were not in the form of rolls written in ancient hieroglyphics but rather in the form of separate numbered sheets or codices written in Greek, indicating that the documents and burials were from the Greco-Roman period and probably dated from sometime around the late 3rd or early 4th century AD.”  Several of the Leiden papyri’s recipes stem from 1^st century compilations and reflect metallurgical knowledge of a far earlier date (Jacobson & Weitzman 1992).  Recipe 15 describes leaching with vinegar.
25. Plutarch; Complete Works, Delphi Classics Kindle Edition, United Kingdom, 2013.  Plut. De. Pyth. 2 tells of two rumoured ways Corinthian bronze was produced i.e. during the sack of Corinth from melted copper, silver and gold and a bronze smith of Corinth who came upon a hoard of gold and fearing detection diluted it with copper.  Both tales are rejected by Plutarch who adds, “However, both this story and that are fiction, but there was apparently some process of combination and preparation; for even now they alloy gold with silver and produce a peculiar and extraordinary, and, to my eyes, a sickly paleness and an unlovely perversion.”
26. Grimes, S.; 2018
27. Jacobson, D. M. & Weitzman, M. P.; 1992
28. Giumlia-Mair, A.; Advances in Archaeomaterials 2020, 1, 1-26
29. Jacobson, D. M. & Weitzman, M. P.; 1992
30. Petronius, The Complete Works, Delphi Classics Kindle Edition: United Kingdom, 2015. Petr. Sat. 50
31. Martial, The Complete Works Delphi Classics Kindle Edition: United Kingdom, 2014. Mat. 9.59
32. Jacobson, D. M. & Weitzman, M. P.; 1992
33. Martinón-Torres, M.; Rojas, R. V.; Samper, J. S.; and Guerra, M. F. Journal of Anthropological Archaeology, 31, 2012, 439-454.  The average composition of guanín objects (n=8) analysed by authors were: copper 48%, silver 12% and gold 40%.
34. Giumlia-Mair, A.; 2020
35. Ibid
36. Grimes, S.; 2018
37. Giumlia-Mair, A.; 2020
38. Devogelaere, J.; EXARC Journal of Experimental Archaeology 2, 2017  https://exarc.net/ark:/88735/10289
39. Pliny the Elder. Plin. HN. 34.48