ALESSANDRO PACINI

The Niello of Naturalis Historia

Niello is a technical decoration that is made by filling engravings, usually of gold or silver with black sulphide (also called niello) made up of an alloy of copper, silver and lead. After it has been ground the sulphide is cast in the engraving, that has been prepared in advance. The work is then flattened and polished so as to obtain a good chromatic contrast between the black of the niello and the colour of the base metal.

The Document Sources

The word niello is derived from the Latin adjective nigellus, a, um that can be translated as "blackened", an adjective used very little in Latin literature and does not refer to a goldsmith's technique.

The oldest literary source of niello is usually considered from the Naturalis Historia of Caius Plinius Secundus (23-79 A.D.) book XXXIII, 131:

"The Egyptians coloured silver, as seen in the vases of their God Anubi (1), coloured it, but the silver wasn't chaste. Afterwards this material was also used for triumphant statues; it is strange that the price increased when the shine is darkened. It is done thus: mixed with silver with a third of pure copper from Cyprus, that is called coronarium, and sulpuris (sulphur) vivi (2) with the same quantity of silver; put on the fire in a terracotta vase covered in clay; the time of cooking is till the cover opens by itself. It also becomes darker like the yolk of a hard boiled egg, this black however disappears with a mixture of vinegar and clay".
(translated from Latin by A. Pacini).

After Plinio we must wait for the old antique formulas in order to find the compositions that would correspond to the niello, such as the Papyrus X of Leida, found at Thebes in Egypt and dates back to the end of the III century A.D.:

"Asem (3), 2 parts; lead, 4 parts. Put in an empty vase of terracotta. Pour over triple the quantity of sulphur apyre, and after put in the oven to melt. When taken out of the oven flatten it and make whatever you would like. If you want to make an object with designs, with hammered or cast metal, then file and cut: it won't rust" (formula 36).
(translated by M. Berthelot).

Or the Mappae Clavicula, another chemical fromula dating back to the IX-X century A.D, in which the formula 195 was found, that is Composition of nigelli and aurum:

"Take two parts of silver and one third of copper and the same quantity or a little more of sulphur. Cast the first two in a crucible in an oven, then progressively add sulphur. When it is calcined and mixed, put into a mould or in something similar and beat while still hot to make it thinner. Then leave it to cool".
(translated from Latin by Schweizer). (4)

Book III of The colours and the arts of the Romans by Eraclio attributed to Garzya Romano (5) in the XI century A.D. contains a formula for the niello with lead in which the title compares to the word nigellus:

"How do we make the niello.
When you want to make the niello, take argento vivo (6), copper and lead in equal quantities, and put them in a crucible like a cup, to cook together. Then take 6 denari of sulphur and mix with the other ingredients, and agitate. Remove from the fire, and without cooling, put in a vase; then take the atramento (7) and dilute with wine and make your form with the silver and the atramento , immediately pour it over the argento vivo powder, copper and lead, until it is all fused, and it will make a beautiful niello". (48)

The other fundamental test is Books III of Diversarum artium schedula by Teophilus, given to Garzaya Romano in the first half of the XII century. The formula used by us of Teophilus corresponds to the classic niello containing lead, the same as the beautiful description by Benvenuto Cellini in his Trattati. The proceedings of the Trattati correspond to that still used by modern craftsmen.

 

 

The interpretation of the text

Many Roman Silvers have beautiful decorations inlaid with niello, those manufactured up to the, IV-III century B.C. and come principally from treasures found in Roman castra in Gaul, Britain and Germany. Also the Roman rings with niello engravings are usually attributed to a period of the Late Roman Empire. Objects however made before the introduction of lead in the composition of the niello, according to Schweizer were up to the X century A.D. But how then can the niello work without lead? On the other hand the use of a bimetallic niello (copper-silver) without lead has been proved to have been used in the Roman times, the analyses having been made on original pieces.

Before reconstructing the experiment of Plinio's fromula I had to resolve the problem of the interpretation of the text. The step in fact was not very clear, given that the Latin historian seemed to describe two different techniques : the patination of silver and the production of a niello without lead.

The black patination of silver is well known by every goldsmith, and today it has become known as the so-called liver of sulphur that is potassium sulphide diluted in water and applied when hot, on an object to be blackened.

The use of a hard boiled egg yolk definitely refers to a tint of silver, in fact it is possible to obtain a uniform and lucid black patina on silver by rubbing the very hot surface with an egg yolk until this carbonises. This doesn't keep a true and proper patina, as the black obtained goes away easily with a light abrasive, for example clay dissolved in vinegar.

 

The interpretation of the second part is a little more difficult. As I didn't manage to understand the meaning of conflantur. I had to look at all the cases in which Plinio, in the XXXIII book, wrote regarding the thermal treatment of precious metals. The Latin writer was precise and used various verbs to indicate various operations, such as :

Fundere

"Bronze and iron melt well with pine wood and also with Egyptian papyrus, gold with straw". (XXXIII-98).

The meaning of the verb is - that which renders liquid, literally is to fuse.

Torrere

Bake also (gold) with double the weight of salt, triple of misy, and another time with two portions of salt and a stone called sciston. (XXXIII-88)

Bake also (gold) with double the weight of salt, triple of misy, and another time with two portions of salt and a stone called sciston. (XXXIII-88)

Ignescere

Until itself (the gold) becomes red in colour similar (as fire) and incandescent. (XXXIII-60)

The meaning of the verb is - to become incandescent, without fusing. Regarding the annealing of metal

Coquere

(Gold) to refine, is alloyed with lead. (XXXIII-63 and 136)

The precise meaning of the verb is - to cook, but in the case of metals to decompose, dissolve by fire. Plinio is describing the refining for cupellation. In this case it is necessary to use an oven and here there is the similarity between cooking and food.

Conflare

Putting them on the fire in vases covered with clay. (XXXIII-136)

The meaning of the verb is - to put together two or more things on the fire to unite by heat, fuse together.

I had therefore translated conflantur as to put on the fire instead of to fuse and coquendi to cook instead of fusing being the translations closest to the meaning to the root of the verb and therefore I believe that Plinio was describing the calcining of the alloy and not the fusing of it. To consider opening the terracotta lids, even though they were sealed with clay, dose not mean the end of the fusion. In fact the fusion of the metal alone doesn't open the lids of the pots. However, perhaps the fusion gases produced by the slow combustion of the sulphur could open the lids.

The reconstruction of the ancient techniques

For the experiments I used pure silver and copper (electrolytic) and pure sulphur in powder, that is sulphum vivum of Plinius. The oven and the crucibles were made by Manuela Petti with soil from Murlo (near Siena, Italy).

Manuela perfected, and made functional various types of ovens and ceramic crucibles using as a base reference Etruscan fragments found in Murlo from the VII century B.C. After numerous attempts she was able to reach a point when the clayey paste became extremely refractory then it was possible to proceed with the preparation of the crucibles (miniscule for the niello with a 'bowl' for the fusion of the bronze) and for the ovens. Those specifically for making the niello were very simple, in reality to handle a cylindrical container with a lid. On the other hand that used to melt the niello onto the jewel required a kiln (a small room with a uniform temperature, isolated from the combustion) with a system of ventilation (with leather bellows, also made by Manuela) these parts now allowed us to made the niello without problems, and also to fuse glass and to carry out various solders on gold and silver.

To prepare the niello I used natural charcoal lit with the leather bellows. I did not use fluxes to make the niello nor when making the tests, these last ones were made in silver (95%Ag - 5%Cu).

In the first construction of the formula of Plinius I used an alloy of 50% Ag - 50% Cu reduced to a thin layer of around 0.4 millimetre. I prepared the thin layers, cutting them into pieces, into a ceramic crucible covered and sealed with clay. After I had calcined it for a night, inside a ceramic oven full of embers, thin fragile and well blackened layers of sulphide were formed. These thin layers were easy to grind, then I filled the test engravings with the sulphide and a little water. Subjected to the fusion in the oven, the sulphide melted easily in the engravings and maintained the colour.

The Latin writer however, did not tell us how to prepare an alloy of copper and silver, thus in a second experiment I used thin layers of copper and silver separately, cut in very small pieces, mixed with the sulphur and put in an oven inside a crucible similar to the previous one. The sulphide obtained could be ground, mixed and applied together in the test, this niello functioned much better than that realized with the alloy.

 

I also tried to use the niello called Type 1 by Schweizer, that is sulphide of only silver or monometallic. The sulphide obtained by the calcination in a charcoal oven showed a definite black colour, fragile but with a certain plasticity. This sulphur immediately reduced to a silver metal once applied onto the test piece and taken to a temperature of fusion. However, scraping the surface of the ring, where the sulphide had been reduced, I became aware that the reduction was only superficial and that underneath, the black colour remained. This sulphide had a point of fusion very near to that of silver, but it was possible to use it like that of a bimetallic. Excluding that the sulphide would become a soft paste at around 600° C. In fact I didn't manage to make it soft at that temperature, much less apply and press it into the engraving. Also at 600° C the sulphide reduced to a silver metal.

There exists another possibility for the application of silver sulphide for example on a buckle as described by Herman Born (8): that is a buckle with cold inserts of Domeykite (Cu3As, a rare arsenical copper) that comes from Caucasus from the end of the II millennium B.C.

I tried to insert the cold pieces of silver sulphide into a design that had been previously engraved on silver: it was possible to inlay it with the sulphide, the colour remained unchanged, but the work was too complicated, besides the fragility of the sulphide it was difficult to prepare pieces suitable for the design, thus deterring me from this procedure.

 

Conclusion

Francois Schweizer (9) analysed the niello of some Byzantine objects dating from the VI and XI centuries. From the analyses there resulted three types of sulphide: silver sulphide (monometallic), sulphide mixed with silver and copper (bimetallic) and sulphide mixed with silver, copper and lead (trimetallic), called by the writer respectively Type I, II and III. Schweizer sustained that by adding the lead it was probably a Byzantine invention of the X century, also he specified that only the niello with lead flows well into the engravings at a low temperature maintaining the colour, while the other sulphides decompose between 500°C and 600°C. He assumed that through the calcination of the sulphide Types I and II could be used. Or immediately use the natural silver sulphide (acantite for example) however, it was not specified the way in which it was to be applied.

I saw that the application of the silver-copper sulphide to calcine did not work. In fact I tried to calcine in the oven for about two hours at various temperatures (200°C, 300°C, 400°C, 500°C) the sulphide ground finely and mixed with water in the engravings of the silver pieces. In the best experiment results the sulphides were only partly fused, giving a less dark colour and having formed a grey halo all around the engraving. And nor did the other hypothetical use on the soft state function. These sulphides in fact, must be applied in powder on the engravings and taken to a temperature of fusion, eventually adding a flux to facilitate the flow into the designs.

The correct interpretation of the test of Plino and the repeated experimental proofs have demonstrated that the niello obtained by the sulphides of copper and silver is the function perfectly, as does the sulphide of only silver.

The adding later of lead probably served to lower the point of the fusion, making the application easier.

BIBLIOGRAPHY

Hermann Born, Patined and painted bronzes, P. Getty Museum, Malibu, CA, 1990.
Marcelin Berthelot, Introduction à la chimie des anciennes, II, Steinheil, Paris, 1889.
Caius Plinius Secundus, Naturalis Historia, edited by Giardini, Pisa, 1984.
Eraclio, De coloribus et de artibus romanorum, edited by Istituto Italiano per gli Studi Storici, Napoli, 1996.
Benvenuto Cellini, I Trattati dell'oreficeria e della scultura, edited by Paoletti, Firenze, 1994.
Francois Schweizer, Nielle byzantin, in: Outils et ateliers d'orfevres des temps anciennes, Société Des Amis du Musée Nationales Saint Germaine-en-Laye, 1993.

 

Alessandro Pacini was born and lives in Montepulciano, Tuscany, where he works in the Aliseda workshop.
He continues to study antique techniques of the jewellery making.
The article The Niello of Naturalis Historia is from his book Studi ed esperimenti su preziosi policromi antichi, published by Madonna della Quercia, Montepulciano, 2004.

Alessandro Pacini
Via dell'Opio nel Corso 8
I-53045 Montepulciano (SI)
e-mail: ale.pacini@gmail.com
http://www.aliseda.it/alessandro/alessandro.htm