Analysis of bell materials: Tin bronzes

Overseas FoundryAugust 2008Analysis of bell materials: Tin bronzes*Jaromir Audy and Katarina Audy(Edith Cowan University, South West Campus Bunbury, Robertson Drive, 6230, Western Australia)Abstract: The present study was set up to examine the effect of alloying elements (including harmful elements)on metallurgical features (material properties and qualitative parameters) of tin bronzes, with particular referenceto church bells from Middle Ages to Current times. A driving force of this study was to identify and demonstratefeatures related to the quality of church bells made in different centuries. The findings have been derived viametallographic and chemical analysis of specimens of bells from various parts of Australasia and Europe. The bellmaterials consisted of a mixture of the a phase and the (a+3) eutectoid essenilly, in proportions determined by tincontent and mould materials during casting. The samples from the 15th century to the one from the 20th centuryshowed a progressive increase in hardness, ranging from the minimum of ~280 VHM20g to a maximum of ~470VHM20g for the (a+B) eutectoid, and ~160 VHM2og to ~230 VHM20g for the a phase. The investigation also showsthat the sound decay of the bell decreased with lowering the wt.% of tin and increasing the wt.% of lead and silver.This information is expected to provide an aditional interesting knowledge into manufacturing practices and theirsignificance in the quality of church bells over past centuries.Key words: tin bronzes; bell material; church bells; microstructure; chemical composition; material properties; qualityCLC number: TG146.1*1Document code: AArticle ID: 1672-6421(2008)03-0199-06he success of bell materials in bell making applications bell material compositions, their properties and effects on」L has to date largely been based on tin bronzes (1-61. It hasvarious qualitative/performance measures. The most importantbeen recognised that tin bronzes (Cu-Sn) vary widely infindings are summarised in Table 1 with references to differentcomposition. Traditionally, bell materials consist of a wideliterature sources [2-17].variety of other elements such as Pb, Zn, Bi, Ag, Sb, As, Ni,From Table 1 it is evident that the most appropriate bellFe, P, S and Si 12357. Some of these elements (e.g. Pb, Zn,material characteristics (such as cast ability, tensile strength,Ni, Fe, Ag, Sb) were used for alloying purposes, whereashardness, wear resistance, cast quality, sound, cost etc.) canothers such as S and P were impurities entering the bronzeprobably be obtained by having a type bronze composition as:liquid through the process of melting with coke and charcoal.~20wt.%Sn, < 2wt.%Ni, < 1.5wt%Pb, ~0.01wt.%P, < 1wt.%Sb,Bell materials differ in both number of alloying elements andtheir amount. This is because through the centuries the bellwith balance of Cu.makers have been searching for 'best' tin-bronze compositionTable 2 from ref. [5] shows the composition of a standardthat would satisfy: (a) melting/casting requirements, (b)‘modern and current' bell material produced in France,expectations in high quality of cast bell, (C) good bell materialSwitzerland, former Czechoslovakia and Germany.properties, (d) long service life, and (d) nice bell sound. It isTable 2 indicates that the quantitative recommendationstherefore not surprising that the knowledge associated withof weight percentage of the main elements (as a whole) in tinpreparation of a bell material mixture was closely guardedbronze materials are incomplete and vary in detail for eachsecret in different bell foundries.reference. Incomplete quantitative recommendations contributeIn the present study, the effects of individual elements onto the lack of knowledge about this popular and commondifferent type bell materials was investigated qualitatively -bell material. Furthermore, it is also anticipated that largeassessed by material properties, casting properties, servicelife, and bell sound. Different bell materials produced from thebecause of different bell manufacturers' knowledge/experience/Middle Ages to Current times were included for comparison.preference as reported in the reference [-11]. Therefore, it is1 Effects of bell material compositionnecessary to determine the quality of different bell materialson qualitative measures of bells:by metallurgical and chemical analyses, including themeasurement of mechanical properties (e.g. microhardness)reported datawhenever possible due to small size of specimens.An extensive literature survey has been carried out on different*Jaromir Audy2 Experiment VLT中国煤化工F1ethodsMale, born in 1962, Ph.D. Research interests: castingProperties and charaiCNMHGoforiginaltechnology and archaeo meallurgy.bell materials rangiTHui w " uiivit Eras wereE-mail: j.audy@ecu.edu.auReceived: 2008-01-22; Accepted: 2008-04-25investigated using chemical and microstructural analyses prior99CHINA FOUNDRYVol.5 No.3Table 1 Number and amount of alloying elements in tin bronze bell materials and their possible effects on qualitativeparameters of church bellsAlloying element(s), wt.%Variables effectedReference(s)SnPtiAgSbAsSNiFeP10- -20~Tensile strength increases[2, 3, 6-10]10-20yeyesTensile strength decreases[2,3,6, 8, 10]Max. 23Max.5~1esHardness increases[2,3,6, 10]23>1 yeBritleness increases[2,3, 6, 10]yes yesWear resistance increases[1upto5Dutilily increases[2,3,7, 10]<5Ductility decreases[2,5]20Elastic limit increases[2,5,6,7]Elastic limit deceases[2,5,6,.7]Abrasion resistance increases[11Toughness increases[2, 5, 6]Toughness decreasesFatigue resistance increases[12]Fluidity increases[5,7, 10]>2Fluidity decreases0.01Acts as a deoxider[5]Reduces deformation[13Rust resistance decreases[2,3, 14]15Crack resistance decreasesChanges the colour of castings[2,5,7,15]Crsallisation interval is reduced[2, 3, 5, 10, 15]Cast-ability decreases[2,3, 5-7, 10, 15]Machinability improves[2,3,5,6, 10, 15]Machinability decreases[2,3,5-7]10Porosity increases[2,3, 5.7, 15]0.4-5[16, 17]0.5-2Structure softens[7, 16, 17]2-4Structure stabilises20-231Sound quality improves[2,3, 5-8]>1.5 >1.5Sound quality decreases[2,3,5-8]Melting temperature decreasesCost increasesCost decreasesTable 2 Composition of a standard 'modern and current bell material produced in France, Switzerland, formerCzechoslovakia and GermanyCountrySAFenCFrance261.5中国煤化工Switzerland2:0.5Former Czechoslovakia20.25.5MYHCNMH G BalanceGermany22-26).3200Overseas FoundryAugust 2008to the microhardness tests and sound measurements.to identify inclusions, porosity, and other casting defects, aswell as cracks. The specimens were etched in a 2% acid ferric2.1 Chemical analysischloride solution for about 10 s to reveal internal microstructure.A Perkin Elmer 2380 atomic absorption spectrometer wasIn addition, a scanning electron microscope (SEM) equippedused to determine the metal elements present in the bellswith an energy dispersive spectrometer (EDS) was also used toinvestigated. The experimental conditions used for atomicquantify and map individual phase in the microstructure. Theabsorption tests were selected from the recommendationsEDS system only provides quantitative measurement of elementdescribed in a manual of the Perkin Elmer Device 18. Theonce its concentration is greater than 0.1wt.%. Therefore, Perkinprocess was as follows: .atomic absorption spectrometer was necessary for analysingFirstly the debris was obtained from the bell specimens,tracing elements in the bronze debris.then they were dissolved in a solvent mixture of H2O2 andHCl and sprayed as an aerosol towards the air-acetyleneVicker microhardness tester under an indentation load of 20 g.flame. Because of the burning solution in a cathodic lamp,The investigated areas were the a phase, the (a+8) eutectoid,the light source created by the free atoms of an element beingand inclusions.determined was carried through the absorption environmentto a monochromator. The later was used to isolate the2.3 Sound measurements and analysisselected narrow spectral band from the spectrum of the lightThe sound of bells was recorded on a tape recorder and thesource. The light signal was transformed into the electricsound frequencies were analysed using a computer equippedsignal by a special photodiode situated in the output of thewith Types Snap Master and Math-Cad softwares. The patternsmonochromator. The absorption level of the element being were plotted in a way to allow recognising one whole completeinvestigated was determined from the level of this electricsound signal and the beginning part of the next beating.signal.element in the bell material was calculated.outputime graphs to give (1) overall information about theIt needs to be noted that sulphur levels could not beone decay (sound duration) and (2) to plot the real width ofdetermined by the Perkin Elmer apparatus so it was done by an frequency sound charts.iodometric method. The iodometric method involves burningthe bronze debris in an oxygen environment. After burning,3 Features of the experimental bellsulphur and oxygen reacts to form SO2 gas that can be easilymaterialsabsorbed in water. This solution is then mixed with excessiveiodine for quantitative analysis of sulphur after back filtration.3.1 Chemical composition of specimensThe results from chemical analyses are given in Table 3. In2.2 Microscopical observations andthe‘Gothic' bell materials investigated the amount of tinmicrohardness testsvaried from 7 to 12 wt.%. This sort of bell-material has aAn optical microscope was employed to carry out thesolidification range of about 180C 51, so such bronzes aremetallographic analyses on the selected specimens. Thespecimens taken had a prismatic shape of approximately 3 mm'.For the‘Empire' bell materials the amount of tin varied inThey were mounted with epoxy resin and polished to a 1 micronthe range of 12wt.% to 15.5wt.%. The 'Modern' bell materialfinish. Optical examination was performed prior to etchingshowed tin levels in the range of 18 wt.% to 20wt.%, while theTable 3 Experimental data on chemical analysis of world-wide bell materials produced in different centurieswt.%，SnPIZnBSbAsNiFeClEra date ;min.020.20.40.090.270.Gothicmax.0.170.0380.93.940.791.44 0.39balance1150-15601.50.140.0170.15 0.320.130.08 0.050.11Renaissance1.0.470.0210.161.340.540.321420-16200.0180.281.10.31Baroque0.230.021.20.450.420.151620- -17500.50.0120.03 1.250.49Rococomax1.74 0.640.033 .3.620.410.761720- 18000.30.0250.12max.15.5 0.81 0.62 0.026 0.028 0.270.840.8 0.096 0.821800-187018.570.25 0.060.008 0.004 0.210.050.028Moderrhalance,20.86.10.0110.0070.350.63中国煤化工00-19503.2.8YH| CNMHGCurrent25.20.061950-Note:Simiotis arbouLo.ut% wa5bmasbs seved roimeperaerare tnleninleles earty Midle Ages. In adin phosphous.in201Overseas FoundryAugust 2008Table 4 The wt.% of individual elements in the bell specimens produced from the 15th century to the 20th century beingchosen for microhardness measurementsBell materialEraSnPZrBAgAsNFe15th Century7.92.0.0.030.1572.511.30.4116th Century11.52.10.020.040.163.80.90.080.250.212.10.520.311.10.060.3217th Century14.71.4.60.630.070.418th Century15.21.90.19.07! .50.119th Century10.21.0.920.090.450.1218.40.007.0030.220.5620th Century19.90.370.0080.000.230.3920.80.290.0030.321.52.4).3500450alpha phase■eutectoid■ oxides400350Fig. 3 Histogramsummarising themeasured variations至250in microhardness of200various experimental15(bell materialsinvestigated0叶°oL15th16th 1 6th 17th 1 8th 19th 19th 20th20th 20thDate (century) of origin162.6 VHM20g and standard deviation of 2.1 VHM202), and、 Sn52.3 VHM202) when comparing sample by sample as well asbell-material by bell material. Following the agelorigins of10the bell-materials there was a progressive increase in averagehardness, sample from the 15th Century to the one from the3i,Sb,S,P,As20th Century, ranging from the minimum of ~280 VHM20g;“| Ag.Pbto a maximum of ~470 VHM20g for the (a+8) eutectoid, thelevels which probably reflected the higher amounts of tin andantimony in bells cast after the 18th century.3.4 Sound of bells16701870187Cchemical composition/microstructure on the quality of bellsound considered from frequency and duration perspectives.Fig. 4 A plot showing concentrations of elements found inReferring to Table 1 it appears that in Cu Sn bronze alloys theifferent bell bronze materials producedtin element (up to 23wt.%) improves the bell sound quality,16th to the 20th centurywhereas the Pb (over 1.5wt.%), Zn (over 1.5wt.%) and Ag inany quantity have a negative effect on the sound in terms of0. 0959Strike 2time existence. Figure 4 adopted from reference [6] indicatesthat there was an increasing trend in the wt.% of Sn, anddecreasing trend in the wt.% of potentially harmful elementsparticularly Pb and Ag in different bell materials producedfrom the 16th century to modern times.中国煤化工Figure 5 shows the frequency-sound charts of twoYHCNMHGgeometrically similar bells made of dffrent Cu-Sn alloys. The-0.09585 L2.13Sound duration, s20.75bell (a), contained ~15.2wt.% of Sn, ~1 .57wt.% of (Pb+Ag),203CHINA FOUNDRYVol.5 No.30.0907Strike 1type of topic in 1983. Secondly to Mgr. Dobrova, and Dr.三Kejlova (from East Slovak Museum in Kosice City) andIng Sarudyova (from Technical Museum in Kosice City),as well as Slovakian government for permission to selectand evaluate such valuable specimens from bells. Thirdly toProfessor Strafford for his friendship, advice, help and support0.08549 L0..Sound duration,s16.76associated with topics relevant to bells, and for his assistanceand supervisory work on the Ph.D project. Finally, to allFig.5 The out-put frequency/sound charts of twoAustralian organisations/churches, and their representatives, .geometrically similar bells cast of different bell-for allowing us to take the bell material specimens formaterialsanalysing that enabled us to continue this research.~2.93wt.% of (Zn+Bi+Sb+As+Ni+Fe), with balance of Cu.The bell (b), contained ~20.8wt.% of Sn, ~0.293wt.% ofReferences(Pb+Ag), ~1.2wt.% of (Zn+Bi+Sb+As+Ni+Fe), with balanceofCu.1] Derry T K, Williams T J. A Short History of Technology: Fromthe Earliest Times to AD 1900. Oxford, Clarendon Press,The patterns are plotted in a way that one can recognise onecomplete sound signal and discrete the end of first beating andthe beginning of the next beating. The frequency charts areCasting Point of View. Engineering Thesis, The Technicalpresented in a pattern of voltage out-pu/time graphs to give (1)University in Kosice City, Czechoslovakia, June 1988.overall information about the tone decay (sound duration) and (2)[3] Audy J, Cech J, Beno J. Composition and microstructure ofmedieval bells. Practische Metallography, 1992, 29(2): 74-84.the real width of frequency sound charts. By comparing plots 5 (a)[4] Hua J. The sound of chime bells of 2400 years ago.and 5 (b), one might conclude that the sound decay decreasedEndeavour, 1993, 17:32- 37.with lowering the wt % of tin and increasing the wt.% of lead[5] Audy K. The Metallurgy of Ancient Artefacts. Ph.D Thesis,and silver.University of South Australia,1999.[6] Audy K, Audy J. Analysis of bl-making process from themiddle ages to recent times. Practische Metallography, 2006,4 ConclusionsTraditionally bells have been manufactured by casting, and[7] Strafford K N, Newell R, Audy K, Audy J. Analysis of BellMaterial from the Middle Ages to the Recent Time. Endeavour,several hundred years ago skill and know-how were suchElsevier Science, 1996, 20(1): 22-27.that bells, often with complex bell material composition, .[8] Buchner A. Workshop Diary and Recipes (1884-1975).could be created. Through metallurgical analysis of the alloysCollection of Slovak Technical Museum (STM), ltem Numbermost commonly used, namely tin bronzes, the high level ofHU 1384/193/84, Kosice City, Slovakia, 1984.'metallurgical' skill (in melting, alloying and casting) becomes[9] Sarudyova M. Work of A. Buchner - the Bell-maker and Artblack-Smith (1884-1975). Collection of Slovak Technicalmore evident, to be able to produce intricate castings freeMuseum (STM), Brochure of Permanent Museum Exhibition,from defects such as cracks and porosity. Because of theKosice, 1987.lack of this metallurgical understanding the compositions of[10] Grigerova T. Metallurgy of Casting Alloys of Nonferrousthe alloys used in earlier centuries are substantially differentMaterials. Technical University (VST) Kosice, Slovakia, 1986.from those in use today. Metallographic examination of bells[11] Vilcko J, Slovak S. Casting Technology. SNTL Bratislava,fabricated through the century reveals significant differences[12] American Society for Metals: Casting Design Handbook,Slovakia, 1987in microstructure, which brought about by compositionalMcGraw Hill Inc, USA, 1958.variations in the alloys, as well as through differing melting13] American Foundrymen's Society: Patern-makers' Manual,and casting techniques. Also, the use of purer constituentMcGraw Hill Inc, USA, 1953.elements, and improved melting practices, have reduced14] Schubert R, D' Egidio s M. The surface composition of copperthe levels of undesirable impurities such as phosphorus andwith indoor exposures ranging from 3 to 47 years. CorrosionScience, 1990,30(10): 999- -1008.sulphur, rendering the bell less susceptible to cracking andfatigue when struck by the clapper. Also the bells of better[15] Borland D W. The Structure of Materials. The University ofMelbourne, Australia, 1989:24- -25.sound quality can be obtained by proper selection of alloying[16] Heine R W, Loper C R and Rosenthal P C. Principles of Metalelements and optimum balance of their individual weightCasting. McGraw Hill Inc, USA, 1967.percentages in Cu-Sn alloys.[17] Zeuner F. Dating the Past. Methuen & Co. Ltd, London, USA,19701970.，[18] Analytical Methods for Atomic Absorption Spectrophotometry.AcknowledgementsBrochure, Perkin EImer, Norwalk. 1982.Firstly to Ing Grigerova T and Doc. Cech J from TU in Kosice中国煤化工CNMHGThis work was supported by the University of South Australia Postgraduateivyiain uliiy d period of1996 to 1999.204