November_December_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 1 2 6 Stratification was developed as early as 1200 B.C. and was widely used for millennia, first in the Near and Middle East, India, Persia, the south- ern Mediterranean region, and later reached Europe [3,5,6] . These early metal- lurgists have found that slags facilitate the forging process. Indeed, given their lower melting point, they would melt during heating of the wrought iron and solidify during the shaping operation, allowing the blacksmith to make more hammer blows before having to reheat the part. Therefore, slags will take the shape imposed by the hammering. This cycle is made repeatedly until the part is completely shaped. This study did not go as far as to pinpoint the source of the slags based on their composition. They could corre- spond to residue that the blacksmiths collect from the bottom of the forging furnace during the forging process, or additives they use to improve the form- ability of wrought iron. SURFACE OF THE ARTIFACT Iron-base artifacts are generally difficult to study due to the advanced state of corrosion. Parts are generally found in a state of 70 to 90% “mineral- ization” (or corrosion). Even though the nail was found in a coastal site of the Mediterranean Sea, no elements such as chlorine and sodium, were found at the surface, suggesting no corrosion re- action with a marine environment. When iron pieces are found in a condition that allows for a metallurgical study, the focus is to understand what could have kept them in that condition. In this study, the state of mineralization of the nail is only 20% if we assume that the layer measured in Fig. 3 is a corro- sion layer. However, chemical and mi- croscopic analysis did not support this theory. The product on the surface is a composite layer of silica (SiO 2 ) aggre- gates embedded in a heavily oxidized low-carbon iron-base matrix. Several studies pointed out that blacksmiths were familiar with the ox- idation of metal parts during the forg- ing process and knew that success of the operation lies in the protection of the surface. Therefore, in some cases, they used sand (consisting mainly of sil- ica) as a cooling agent that would also protect against oxidation and maintain the hammering temperature higher for a longer time to reduce the overall cy- cles necessary to form the part [7] . This could be an explanation of the com- posite layer building up at the surface. Thus, at some point, blacksmiths real- ized that this operation also made their part more resistant to the environment, and they reserved this practice for the last hammering passes. Moreover, studies [8] have shown that they could have discovered a pro- tocol to prevent the formation of the least protective iron oxide (hematite, Fe 2 O 3 ) at the surface of iron-base mate- rials by a combination of heating, me- chanical cleaning, and water rinsing to produce a more stable and protective magnetite (Fe 3 O 4 ). The heavily oxidized matrix in which the silica aggregates are embedded could then be magnetite, which would explain how the compos- ite microstructure of the surface layer prevented the progression of corrosion. CONCLUSION It is certain that these blacksmiths did not have the level of scientific knowledge and technological capabil- ities that are available today, but they surely had a sense of observation and a dexterity which made them find win- ning manufacturing processes. Bridging their work to current studies could cer- tainly be very inspiring. ~AM&P For more information: Nihad Ben Salah, DSC, founder and president, NBS-M&P Consulting, 69 Saint-Charles, Quebec, Canada, nihad.bensalah@ nbsmpconsult.com, nbsmpconsult.com . References 1. J.R.L. Allen, Chemical Composi- tional Patterns in Roman-British Bloomery Slags from the Wetlands of the Severn Estuary, Journal of the Historical Metallurgy Society, Vol 22, p 81-85, 1988. 2. A. Kamaraj, et al., Characterization and Evolution of Non-metallic Inclu- sions in Fe-Al-Si-O System, Trans. Indian Inst. Met., Vol 70, p 1887-1899, 2017. 3. O.P. Agrawal, et al., Lamination Technique in Iron Artifacts in Ancient India, Journal of the Historical Metal- lurgy Society, Vol 24, p 12-26, 1990. 4. R.F. Tylecote, A History of Metal- lurgy, The Metals Society, 1976. 5. J.P. Mohen, Métallurgie Préhistor- ique – Introduction à la Paléométal- lurgie, Masson, 1990. 6. J.D. Muhly, An Iron Adze of the Fifth–Fourth Centuries B.C. from Al Mina, Levant, Vol 9, p 156-161, 1977. 7. G. Renoux, et al., Contribution à L’Histoire des Techniques de L’Armement : Essais de Restitution du Forgeage de Pointes de Flèche à Partir de Barres de fer d’Époque Antique, Gladius, Vol 24, p 39-70, 2009. 8. J. Ahlström, et al., Electrochemical Properties of Oxide Scale on Steel Exposed in Saturated Calcium Hy- droxide Solutions with or without Chlorides, International Journal of Corrosion, Vol 2018, Article ID 5623504, 10 pages, 2018.