Introduction to Precious Metals

Transcription

1 1 / introduction Introduction to Precious Metls Metllurgy for Jewelers & smiths Mrk Grimwde

2 Introduction to precious metls Chpter One Physicl Properties of the Precious Metls The eight individul pure precious metls re chemicl elements, i.e., they cnnot e split up into simpler sustnces. All the toms of n element hve similr structure ut re different to toms of nother element. There re 92 elements nturlly occurring in the erth s crust nd tmosphere rnging from the gs hydrogen (H), with tomic no. 1, to the metl urnium (tomic no. 92). In ddition, there re numer of mn-mde elements produced in tomic rectors hving tomic numers greter thn 92 ut they need not concern us here. The Periodic Tle of Elements In 1869, the Russin chemist Dmitri Mendeleev pulished the Periodic Lw tht sttes the properties of elements re in periodic dependence on their tomic weights. Mny physicl nd chemicl properties of elements oey this lw nd it ws possile for Mendeleev to rrnge the known elements in the form of tle, modern version of which is shown in Figure 1.1. Verticl columns, known s Groups contin elements tht show strong fmily resemlnce with regrds to their physicl nd chemicl chrcteristics. For exmple, Group 1 consists of the elements copper (Cu), silver (Ag), nd gold (Au), nd lthough copper is regrded s se metl, s distinct from the precious metls silver nd gold, its ehvior is similr in mny respects nd, s will e seen in Chpter 5, it only just misses out on eing precious or nole metl. Horizontl rows, known s Periods, lso hve importnt fetures in common. Note, prticulrly tht the six pltinumgroup metls (PGMs) form lock in the center of the tle together with iron (Fe), colt (Co), nd nickel (Ni). This lock is referred to Group VIII. Ech element is numered in scending order from top to ottom nd from left to right. This is the tomic numer, mentioned ove, nd it tells us something out the nture of the toms of ech element. An tom consists of nucleus mde up of electricllypositive chrged prticles clled protons nd electriclly-neutrl prticles clled neutrons, together, with electriclly-negtive chrged prticles clled electrons. For simplicity, the electrons cn e considered to orit the nucleus in series of lyers or shells. Since the tom is electriclly neutrl overll, the numer of protons is equl to the numer of electrons, nd this numer is the tomic numer. Hence, for exmple, gold with tomic numer 79 will hve toms with 79 protons in their nuclei nd 79 electrons in [ 16 ]

3 1 physicl properties of the precious metls Group Period 1 I II III IV V VI VII VIII I II III IV V VI VII B 0 1 H 2 He 2 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne 3 11 N 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 A 4 19 K 20 C 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Ni 28 Co 29 Cu 30 Zn 31 G 32 Ge 33 As 34 Se 35 Br 36 Kr 5 37 R 38 Sr 39 Y 40 Zr 41 N 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 S 52 Te 53 I 54 Xe Cs 87 Fr 56 B 88 R *R.E. 89 Ac 72 Hf 90 Th 73 T 91 P 74 W 92 U 75 Re 76 Os 77 Ir 78 Pt 79 Au Trnsurnic Elements 80 Hg 81 Ti 82 P 83 Bi 84 Po 85 At 86 Rn Figure The Periodic Tle of Elements series of shells round ech nucleus. The toms of one element, therefore, differ from those of nother nd, ecuse mny physicl nd chemicl properties such s color, chemicl rectivity, nd reflectivity depend on the tomic structure, these chrcteristics vry from one element to nother. The periodicity discovered y Mendeleev nd others rises from the fct tht similrities in electron orit configurtion periodiclly reoccur with scending tomic numer, hence, the similrities in certin properties in elements within group. The elements re ssigned chemicl symols. Metls tht hve een known efore Romn times hve symols relting to their Ltin nmes. For exmple, the symol for copper is Cu from the Ltin word cuprum, gold is shortened to Au from urum, nd silver is shortened to Ag from rgentum. The sme is true for iron (Fe) nd led (P). Aout ¾ of the 92 elements re metls nd they re chrcterized y their ility to give up their outermost electrons nd the wy their toms ond together. The remining elements re gses nd non-metls. * Rre Erth Metls It is interesting to note tht when Mendeleev constructed his Periodic Tle there were numer of gps due to undiscovered elements. The puliction of the Tle nd Lw enled reserchers to predict nd find mny of these elements, germnium (Ge) eing cse in point. It will e pprent tht there should e n increse in the weight of ech tom (tomic weight) with tomic numer since the numer of protons, electrons, nd incidentlly, neutrons re incresing lmost without exception. The tomic weight of n tom is lmost entirely mde up from the nucleus, i.e., the weights of the protons nd the neutrons. This is ecuse the weight of n electron is only out 1/1800 of tht of the electriclly equivlent proton. A neutron hs out the sme weight s proton. Atomic weight is on comprtive scle using the stndrd of 12 s the weight of cron tom, which hs 6 protons nd 6 neutrons in its nucleus. Tle 1.1 gives the tomic numers nd weights of the precious metls with their tomic sizes ssuming the toms re spheres of certin rdius. The [ 17 ]

4 Introduction to precious metls Element Chemicl Atomic Atomic Atomic Symol Numer Weight Rdius (nm)* Ag Au Ruthenium Ru Rh Plldium Pd Osmium Os Iridium Ir Pltinum Pt Tle 1.1 Atomic chrcteristics of the precious metls. *One nnometer = 10-9 meters. lloying ehvior of metllic elements is prtly determined y the tomic rdii nd the tomic size mismtch of the elements present in the lloy. These fctors lso influence the strength of the lloy. Crystl Structure For our purposes, we cn consider toms to e very smll spheres, the rdii of which depend on the individul element. Oviously, to mke lump of metl tht cn e hndled, very lrge numer of toms must e rought together nd onded in some wy. Before going on to discuss onding nd crystl structure it is importnt to distinguish etween toms nd molecules. Too often, I her jewelers nd metlsmiths refer to the molecules of metl when they should e tlking out toms. Metls do not exist in moleculr form in the wy tht gses such s oxygen nd hydrogen or sustnces like wter nd certin chemicl compounds do. Molecules re generlly specific comintions of toms e.g., oxygen gs consists of molecules ech contining two toms of oxygen; ozone gs hs molecules of three toms of oxygen; the wter molecule is two toms of hydrogen to one tom of oxygen. Wter coming out of tp will consist of very lrge collection of those molecules. A solid chemicl compound cn e considered to e moleculr in the sense tht they exist in certin tom comintions. For exmple, the compound clcium sulphte hemihydrte (CSO₄ ½H₂O), etter known s gypsum (n importnt constituent of lost wx investment powder) hs the comintion of one tom of clcium to one tom of sulfur nd four toms of oxygen with hlf molecule of wter. Even so, it will hve crystl structure s ll true solids do. On the other hnd, pure metls nd most lloys exist s collection of toms nd not s specific comintions of toms. The only exception is in the cse of intermetllic compounds (see lter in Chpter 6) where it is true tht the compound hs fixed, or nerly-fixed, rtio of toms of one metl to nother. Let us now return to the suject of crystl structure of metls nd lloys. At very high tempertures, metl will vporize. Its toms will e fr prt nd will not tke up ny structurl form. As the [ 18 ]

5 1 physicl properties of the precious metls () () (c) Figure 1.2 Atomic rrngement of ) Gses, ) Liquids, nd c) Solids; the three sttes of mtter. temperture is lowered, the vpor condenses to liquid in the sme wy tht wter vpor condenses to stem (minute wter droplets) nd liquid wter. In quntity of liquid, the toms re loosely held together in rndom fshion. There is rudimentry structure of smll clusters of toms tht re continully reking up nd the toms move reltively esily to form new clusters. As the temperture drops still further, the liquid freezes to form solid nd ll the toms tke up positions in regulr geometric rry known s lttice to form crystl structure chrcteristic of tht prticulr sustnce. A two-dimensionl representtion is shown in Figure 1.2. All true solids hve regulr crystl structure whether they re metls or nonmetls. The min fctor tht distinguishes metls from non-metls is the mnner in which their toms re onded in the lttice. Metls hve specil form of onding known s metllic onding in which the toms re not so tightly held s in chemicl compounds (ionic onding) or s in cron in the form of dimonds (covlent onding). With metllic onding, the outermost electrons of the toms flot throughout the lttice s n electron cloud. It is this type of onding tht is responsile for the chrcteristics of ductility (mlleility), toughness nd good electricl nd therml conductivity. Solids tht re ionic- or covlent-onded my e strong nd hrd ut they re not ductile nd will shtter if sujected to sufficiently high force s in hmmer low. There re fourteen wys in which spheres cn e pcked together to form regulr geometric rry. The vst mjority of metls fll into one of three ctegories. The precious metls, with the exceptions of osmium nd ruthenium, hve lttice in which the unit cell or sic uilding lock is known s fce-centered cuic (). This structure hs n tom t ech corner of the cue nd one in the center of ech fce (Figure 1.3). If other similr cells re dded on in ll three directions d infinitum, single metl crystl cn e uilt up tht is sufficiently lrge enough to hndle (Figure 1.4). It will e noticed tht the corner nd fce toms re shred etween djcent cells. Aprt from gold (Au), silver (Ag), pltinum (Pt), plldium (Pd), iridium (Ir) nd rhodium (Rh), other metls include luminium (Al), copper (Cu), nickel (Ni) nd led (P). For resons tht re too complex to discuss in this ook, the metls hve very good mlleility nd ductility s will e recognized from the ove list. Osmium (Os) nd ruthenium (Ru) re exceptions in tht their lttice is close-pcked hexgonl (c.p.h.). The c.p.h. [ 19 ]

6 Introduction to precious metls lttice (Figure 1.5) often displys limited ductility. Indeed, osmium nd ruthenium re notoriously difficult to fricte nd, consequently, their industril uses re somewht limited. Other exmples of c.p.h. metls re zinc (Zn), cdmium (Cd) nd mgnesium (Mg). A third common occurrence ctegory is the ody-centered cuic lttice (.c.c.). Agin, the unit cell is cue with n tom t ech corner ut with n tom lso t the center of the cue insted of in ech fce (Figure 1.6). Exmples of.c.c. metls re chromium (Cr), tungsten (W) nd molydenum (Mo). A few metls cn disply more thn one type of crystl structure. The most notle exmples re iron (Fe) nd titnium (Ti). Iron is.c.c. (ferritic) up to 910 C (1670 F) where it chnges to (ustenitic) simply y the toms moving positions in the solid stte. At pproximtely 1400 C (2552 F) it chnges ck gin to the.c.c. form nd remins so up to the melting point of 1535ºC (2795 F). It is the ferrite to ustenite chnge tht is responsile for the fct tht steels, which re lloys of iron, cn e het treted nd hrdened. Titnium is c.p.h. up to 885 C (1625 F) where it chnges to.c.c.. These metls re sid to undergo n llotropic trnsformtion. However, it is the precious metls nd their crystl structures tht minly concern us. Wht then is the difference etween their lttices? Becuse the sizes of their toms vry from one metl to nother, the dimensions of the unit cell will lso vry. The length of cue edge for gold for instnce, will e different from tht of pltinum or silver. Crystllogrphers cn mesure the cell dimensions y X- Ry diffrction nlysis nd refer to the dimensions s the lttice prmeters for the prticulr metl. It is only necessry to quote the cue edge prmeter for the nd.c.c. metls wheres the c.p.h. metls hve the prmeter for the length of the side of the hexgon nd the c prmeter for the height of the cell (Tle 1.2). Corner Atoms Fce-Centered Atoms Figure 1.3 The fce-centered cuic () cell represented y ) points nd ) spheres where ech fce- centered tom touches its nerest corner toms. Figure 1.4 A lock of eight fce-centered cuic cells illustrting the wy crystl is uilt up. [ 20 ]

7 1 physicl properties of the precious metls Figure 1.5 The close pcked hexgonl (c.p.h.) cell. All toms on the se nd the top plnes re in contct with ech other on tht plne nd the toms on the intermedite plne nest on three of the six niches so formed. Figure 1.6 The Body Centered Cuic (.c.c.) Cell. The center tom touches ech corner tom, ut these do not touch ech other. Density The density of sustnce is defined s its weight per unit volume nd it is usully expressed in grms per cuic centimeter (g cm 3 ) lthough the preferred Interntionl System of Units (SI unit) is kg m 3. Density is function of oth tomic weight nd crystl structure. Therefore, elements with high tomic weights usully hve high densities, prticulrly if their toms re closely pcked together in the lttice. Both the nd the c.p.h. structures represent the closest form of tomic pcking of ll types of lttice. Consequently, it is not surprising to find tht gold, osmium, iridium, nd pltinum re regrded s hevy metls (Tle 1.3). Temperture lso ffects the density vlue ecuse the crystl structure expnds on heting. Density vlues re usully quoted for mesurements tken t 0 C or 2o C (32 F to 68 F) on fully nneled metl. To put the importnce of density into context, consider n item of jewelry mde in either sterling silver or 22 krt gold. Ech contins lmost identicl mounts of silver or gold t ~92.5%. The density of gold is nerly twice tht of silver, hence the item [ 21 ]

8 Introduction to precious metls Metl Structure Lttice prmeter (nm) Pltinum Plldium Iridium Osmium c.p.h. Ruthenium c.p.h. will weigh lmost twice s much in 22k gold s in silver. Similrly, pltinum ring of 950 fineness will e hevier thn n 18k gold ring of the sme design nd dimensions. It is lso n importnt considertion when costing items for mnufcture nd sle s precious metl content, density, nd price per troy ounce hve to e tken into ccount. Metl Density (g cm -³) Pltinum Plldium Iridium Osmium Ruthenium Tle 1.2 Crystl structure nd lttice prmeters * The close pcked hexgonl (c.p.h.) cell hs two chrcteristic dimensions nd hence two prmeters must e stted. The prmeter for the cell is the length of the cue edge c Melting nd Boiling Points The melting (freezing) tempertures of silver, gold, plldium, nd pltinum re of specil interest nd of gret prcticl importnce ecuse they re used s primry nd secondry fixed points on the Interntionl Prcticl Temperture Scle from which ll temperture-mesuring devices re clirted. This scle is periodiclly revised, the ltest eing in 1990, s stndrds of purity nd mesuring techniques re improved. The precious metls re chosen s reference points ecuse they re produced to high degree of purity nd re free from contmintion y oxidtion. Boiling points re more difficult to determine nd wide vrition in vlues hs een reported in the technicl literture over the yers. The vlues quoted in Tle 1.4 re the currently ccepted vlues. However, oiling points re of little interest to metlsmiths. Metl Melting Boiling Point ( C) Point ( C) Pltinum Plldium Iridium Osmium Ruthenium Tle 1.4 Melting nd oiling points of the precious metls. Tle 1.3 Density vlues for the precious metls t 20 C. [ 22 ]

9 1 physicl properties of the precious metls Opticl Properties Much of the fscintion of gold throughout the ges hs een due to its eutiful yellow color nd the high luster or reflectivity tht cn e retined indefinitely ecuse of its resistnce to corrosive ttck y the environment. Copper is the only metl esides gold with chrcteristic color, ut unfortuntely its lcks the corrosion resistnce of gold. Similrly, the esthetic ppel of silver lrgely rises from its rillint white color nd very high reflectivity. It hs the highest reflectivity of ll metls. Unfortuntely, the trnishing ehvior of silver limits its usefulness s reflector. The colors of gold nd copper result from shrp increse in reflectivity within nrrow nd of incresing wvelength of incident light flling on the surfce of the metl. Reflectivity (R) is low t the lue end of the visile light spectrum, (wvelength less thn 500 nnometers (nm)) ut rises drmticlly t the yellow-red-infrred end of the spectrum ( nm). This chnge in reflectivity is ssocited with the nture of the electron configurtions in the tomic structures of gold nd copper ut detiled discussion is eyond the scope of this ook. Figure 1.7 shows the reltionship etween R (on scle 0 to 1, where one equls 100%, nd wvelength for gold, silver, rhodium, nd pltinum. hs high R vlue over the entire visile light spectrum which ccounts for its rillint white luster. Of the PGMs, rhodium hs the highest reflectivity, with men vlue of out 80%. Tht, coupled with lck of trnishing ehvior, mkes it eminently suitle s n electroplted finish for silver nd the soclled white golds. The opticl properties of gold re profoundly ltered y lloying nd this is of considerle importnce to the jeweler Reflectivity, per cent Violet Indigo Blue Green Pltinum Wvelength nm nd goldsmith. The effect of lloying on the color of the krt gold lloys will e discussed more fully in Chpter 9. Much interest hs een shown in stndrdizing colors for gold lloys. The Europen wtch industry dopted coding system prticulrly for electroplted wtchcses lthough it hs lso een pplied for wrought lloys (Tle 1.5). In recent yers, quntittive system of color mesurement hs een developed tht enles lloy mnufcturers nd jewelers to give etter definition of the color of their products. It is the CIELAB system which mesures color in terms of spce coordintes L*, * nd * using color spectrophotometer. L* mesures the lck (0) white (100) coordinte. Metls with high reflectivity or whiteness will hve vlues pproching 100. The * coordinte goes from * vlues representing green Yellow Red Figure 1.7 Reflectivity in precious metls. [ 23 ]