ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 32 (e.g., reannealing, repressing, cracking, or modification in equipment setups which compensate for loss of ductility). Beginning in the 14th century, goldsmiths used hand driven rolls, forming thin sheets of gold and silver[13]. This technology evolved and the semi- industrial practice of using a two-high rolling mill became common practice when hot working bronze castings into cymbal blanks. Figure 12 shows a rolled finished blank (~ 46.0 cm diameter). The original casting (bun) may be processed through the two-high mill up to 8 times, rotated 90° at intervals for the purpose of maintaining a round shape. Hardness of grains and matrix increase after each rolling pass. If air cooled, the blanks are brittle; matrix hardness range is 45-47 HRC. Figure 13 shows blank microstructure post-quench; twinning grains are evident in the microstructure, a common condition of most copper alloys when annealed. Post-quench, ductility is increased, hardness is decreased; the dendritic microstructure is completely dissolved. The subsequent cold-working techniques include forming the raised bell (boss) and flattening the outer edges by hand hammering (Fig. 14), or use of industrial machines (forming press and automated hammers), and lathing for weight removal and surface profile. Part II of this series, to be published in a future issue of AM&P, will discuss the remaining processes in the manufacturing of finished bronze cymbals. ~AM&P during hot working. This understanding of shape as related to heat retention is noted in the hot working of other hightin bronze objects from antiquity[12]. During thermal treatment and forging, and most importantly, following the last thermal treatment, ancient workers performed a water quench, thus transforming the brittle bell metal into a ductile condition (β phase) necessary for cold-working techniques. The temperature of bronze metal entering the quench should be as close as possible to annealing temperature. The importance of proper annealing between hot working cannot be overstated; fluctuation or unstable furnace temperatures affect workability of bronze metal during downstream forming processes and often result in rework For more information: Joseph Paul Mitchell, director of operations, Avedis Zildjian Co. Inc., 22 Longwater Dr., Norwell, MA 02061, 617.631.7955, jmitchell@zildjian.com, www.zildjian.com. References 1. H. Tindell and D. Cooper, Castings for the Home Workshop, Crowood Metalworking Guides, p 9, 2018. 2. K.T.M. Hegde, An Introduction to Ancient Indian Metallurgy, p 21, 1991. 3. K.T.M. Hegde, An Introduction to Ancient Indian Metallurgy, p 22-23, 1991. 4. K.T.M. Hegde, An Introduction to Ancient Indian Metallurgy, p 24, 1991. 5. James Blades, Percussion Instruments and Their History, p 107-108, 1970. 6. https://meehanitemetal.com. 7. American Foundrymen’s Society, Casting Copper-Base Alloys, p 53, 1984. 8. Jack Thielke, Automatic Pouring Systems, Volume 15, Metals Handbook, Ninth Edition, ASM International, p 497, 1988. 9. American Foundrymen’s Society, Casting Copper-Base Alloys, p 143, 1984. 10. American Foundrymen’s Society, Casting Copper-Base Alloys, p 145, 1984. 11. American Foundrymen’s Society, Casting Copper-Base Alloys, p 87, 1984. 12. Chattopadhyay K. Pranab, Datta K. Prasanta, Maji Barnali, Forging Technology of High-Tin Bronzes in Ancient Bengal, Current Science, 118(11), p 1830, June 2020. 13. Siddhartha Ray, Principles and Applications of Metal Rolling, p 5, 2015. Fig. 12 — Cymbal blank, finished size, as-rolled. (15-in. diameter, 0.050 – 0.053-in. thickness) Fig. 13 — Cymbal blank microstructure; finished blank, as-rolled and quenched, 75×. Twinning, 77 HRB, matrix, 39 HRC (converted: HK 25-gram load). Courtesy of Fredrick A. Skinner, metallurgical consultant. Fig. 14 — Hand-hammered cymbal. Courtesy of Manabu Yamamoto, Art Cymbal. GET ENGAGED, GET INVOLVED, GET CONNECTED The ASM Archaeometallurgy Committee welcomes new members with interest and experience in the study and characterization of historic metals and artifacts. For more information, contact staff liaison Scott Henry, scott.henry@ asminternational.org.
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