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Precious Metalsmetals PRECIOUSPRECIOUS METALSMETALS MetallurgyMetallurgy andand RelatedRelated TopicsTopics By AldoAldo M.M. RetiReti Topics:Topics: MetallurgicalMetallurgical AspectsAspects GoldGold Karat System Legal Aspects, Hallmarking Color, practical example of Color Matching Depletion Gilding, Gold Leaf Gold Filled SilverSilver (Ag/Cu(Ag/Cu System),System), Reticulation CaseCase StudyStudy: Embrittlement of Ancient Silver PlatinumPlatinum (Pt(Pt Alloys)Alloys) WhyWhy “Precious”?“Precious”? Expensive,Expensive, Rare,Rare, DesirableDesirable NobleNoble Tradition,Tradition, HistoryHistory Gold Silver Platinum PGM (Platinum, Palladium, Iridium, Rhodium, Ruthenium, Osmium) UsuallyUsually Alloyed,Alloyed, inin orderorder toto enhanceenhance properties:properties: Mechanical Color “Extend Use” GoldGold AlloysAlloys WhatWhat isis Important?Important? •• HowHow muchmuch Gold?:Gold?: KaratKarat SystemSystem KaratKarat SystemSystem (Carat)(Carat) Karat:Karat: GoldGold Content,Content, expressedexpressed inin 24ths24ths Karat Fineness (ppt) 24 1000 22 916.7 18 750 14 583.3 10 416.7 9 375 8 333.3 GoldGold AlloysAlloys WhatWhat isis Important?Important? •• HowHow muchmuch Gold?:Gold?: KaratKarat SystemSystem ColorColor ColorsColors ofof GoldGold DeterminedDetermined byby AlloyingAlloying ElementsElements “Classic”“Classic” System:System: Au/Cu/AgAu/Cu/Ag (and(and Zn)Zn) isis thethe basisbasis forfor “Colored“Colored Golds”Golds” YellowYellow RedRed GreenGreen “White“White Golds”:Golds”: Au/Cu/NiAu/Cu/Ni oror Au/Ag/PdAu/Ag/Pd (Ni(Ni andand PdPd areare veryvery strongstrong goldgold “whiteners”)“whiteners”) NickelNickel AllergyAllergy Unfortunately, many people, the female population especially, are allergic to nickel in contact with the skin and this gives rise to a red skin rash or irritation. The European Union countries have enacted legislation valid from the 20th January 2000 that limits nickel release from jewellery. Thus, in Europe, nickel white golds are being phased out and being replaced by palladium white golds. The USA is taking a more relaxed approach, requiring jewellery to be labelled as nickel- containing, and much jewellery in the West is now advertised as 'non-allergenic' or 'nickel-free'. (See article in Gold Technology, No 28, Spring 2000, "Nickel gets under your skin“) GoldGold AlloysAlloys WhatWhat isis Important?Important? HowHow muchmuch Gold?:Gold?: KaratKarat SystemSystem ColorColor PropertiesProperties –– UtilizeUtilize PhasePhase Diagrams!Diagrams! Strength,Strength, HardnessHardness WorkingWorking CharacteristicsCharacteristics CastabilityCastability (melting(melting point,point, fluidity)fluidity) (lost wax process, investment casting) TernaryTernary CompositionComposition DiagramDiagram GoldGold ColorColor DiagramDiagram ColorColor cancan bebe Measured!Measured! CIELABCIELAB ColorColor SystemSystem InternationalInternational SystemSystem AdoptedAdopted inin 19761976 byby thethe InternationalInternational CommitteeCommittee onon IlluminationIllumination ASTMASTM StandardStandard ClassicClassic ExampleExample ofof Use:Use: PaintPaint IndustryIndustry CIELABCIELAB SystemSystem UsesUses 33 Coordinates:Coordinates: “L”“L” forfor BrightnessBrightness (100=white, 0=black) “a”“a” forfor RedRed--GreenGreen (100=red, -100=green) “b”“b” forfor YellowYellow--BlueBlue (100=yellow, -100=blue) Thus,Thus, ColorColor cancan bebe representedrepresented byby aa vectorvector inin 33 coordinatecoordinate spacespace HowHow toto MeasureMeasure Color?Color? SpectrophotometerSpectrophotometer Manufacturers:Manufacturers: MacbethMacbeth HunterHunter LaboratoriesLaboratories BYKBYK GardnerGardner AppliedApplied ColorColor SystemsSystems (ACS)(ACS) ExamplesExamples ofof ColorColor LL aa bb GoldGold 84.084.0 4.84.8 34.334.3 SilverSilver 95.895.8 --0.70.7 5.35.3 CopperCopper 84.084.0 11.811.8 14.314.3 70/3070/30 BrassBrass 91.191.1 --4.14.1 22.022.0 ColorColor VariationVariation inin 1414 KtKt AlloysAlloys ColorColor LimitsLimits aa LimitsLimits bb RedRed >> 3.53.5 15.015.0 toto 23.023.0 YellowYellow --1.01.0 toto 3.53.5 18.518.5 toto 23.023.0 GreenGreen --1.01.0 toto --5.55.5 17.017.0 toto 23.023.0 WhiteWhite --1.01.0 toto 1.01.0 lessless thanthan 12.012.0 ColorColor DifferenceDifference ColorColor ofof metalmetal 1:1: LL1,, aa1,, bb1 ColorColor ofof metalmetal 2:2: LL2,, aa2,, bb2 2 2 2 2 DD == (L(L2--LL1)) ++ (a(a2--aa1)) ++ (b(b2--bb1)) Human eye can differentiate D>2 Composition of 51 14Kt Au/Cu/Ag/Zn Alloys (D.P. Agarwal and G. Raykhtsaum, “The Santa Fe Symposium onon Jewelry Manufacturing Technology”, 1988, pp. 229-244) ConstantConstant “L”“L” LinesLines forfor 1414 KtKt Au/Cu/Ag/ZnAu/Cu/Ag/Zn AlloysAlloys ConstantConstant “a”“a” LinesLines forfor 1414 KtKt Au/Cu/Ag/ZnAu/Cu/Ag/Zn AlloysAlloys ConstantConstant “b”“b” LinesLines forfor 1414 KtKt Au/Cu/Ag/ZnAu/Cu/Ag/Zn AlloysAlloys ExampleExample ofof “Color“Color Matching”Matching” OldOld piecepiece ofof goldgold alloyalloy needsneeds toto bebe matchedmatched (repaired)(repaired) withwith aa newnew 14Kt14Kt alloyalloy MeasureMeasure colorcolor ofof oldold piecepiece Say,Say, LL == 9393 aa == --4.54.5 bb == 19.519.5 NowNow gogo toto ternaryternary chartscharts toto findfind compositioncomposition ofof 1414 KtKt alloyalloy withwith bestbest matchmatch ConstantConstant “L”“L” LinesLines forfor 1414 KtKt Au/Cu/Ag/ZnAu/Cu/Ag/Zn AlloysAlloys L=93.0 ConstantConstant “a”“a” LinesLines forfor 1414 KtKt Au/Cu/Ag/ZnAu/Cu/Ag/Zn AlloysAlloys a= -4.5 ConstantConstant “b”“b” LinesLines forfor 1414 KtKt Au/Cu/Ag/ZnAu/Cu/Ag/Zn AlloysAlloys b=19.5 Composition of 51 14Kt Au/Cu/Ag/Zn Alloys (D.P. Agarwal and G. Raykhtsaum, “The Santa Fe Symposium onon Jewelry Manufacturing Technology”, 1988, pp. 229-244) Desired L, a and b are within red circle: 70% Ag, 20% Cu, 10% Zn BestBest ColorColor MatchMatch 1414 KtKt AlloyAlloy Silver:Silver: 70%70% xx 41.67%41.67% == 29.17%29.17% Copper:Copper: 20%20% xx 41.67%41.67% == 8.33%8.33% Zinc:Zinc: 10%10% xx 41.67%41.67% == 4.17%4.17% Gold:Gold: == 58.33%58.33% 100.00%100.00% PurplePurple GoldGold Purple gold (also known as amethyst or violet gold) When gold and aluminium are alloyed in a certain fixed ratio, they form a gold intermetallic compound with the chemical formula AuAl2 (or Au6Al11). That is one atom of gold to two atoms of aluminium. This compound has an attractive purple color. In terms of composition, this compound is about 79% gold by weight and hence is hallmarkable as 18 carat gold. All intermetallic compounds, and purple gold is no exception, tend to be very brittle. They cannot be easily worked by conventional metal working processes. If one attempted to roll or hammer a piece of purple gold, it would shatter into pieces! “Purple Plague” when bonding gold wire to aluminum. BlueBlue GoldGold AuAu--FeFe AlloysAlloys (i.e.,(i.e., 7575 AuAu--2525 Fe)Fe) AuAu--InIn AlloysAlloys ColorColor InterferenceInterference effecteffect causedcaused byby thinthin filmfilm ofof oxideoxide onon surfacesurface PurplePurple filmsfilms foundfound inin TutankhamunTutankhamun goldgold (presence(presence ofof FeFe andand As)As) DepletionDepletion GildingGilding “Coloring“Coloring thethe Gold”.Gold”. SurfaceSurface richrich inin gold.gold. RemoveRemove basebase metalsmetals fromfrom thethe surface,surface, whichwhich isis enrichedenriched inin gold.gold. Process:Process: conversionconversion ofof basebase metalsmetals nearnear thethe alloyalloy surface:surface: Chemical attack (alum and common salt), applied to Guanin (silver-rich). Form silver and copper chlorides, which at high temperature are absorbed by the crucible (cementation) Oxidation/leaching, applied to Tumbaga (copper-rich) ModernModern Method:Method: ChemicalChemical “Bombing”“Bombing” Hot cyanide solution + hydrogen peroxide DepletionDepletion GildedGilded TumbagaTumbaga fromfrom thethe VicusVicus Period,Period, PeruPeru CrossCross--SectionSection ofof aa VicusVicus DiscDisc (x200)(x200) DepletionDepletion GildingGilding Tribes in Pre-Columbian Peru, specifically the Moche and later the Chimu who were located in northern coastal Peru from between 250 BC to 600 AD and 800 AD to 1500 AD, respectively, hammered copper alloys to make ornamental objects. Pre-Columbian metalsmiths specialized in a copper alloy called tumbaga, which is primarily gold and copper, perhaps with some silver because silver typically occurs with gold when found in Andean stream beds. Tumbaga can achieve a gold color, despite being as low as twelve percent gold through a process called depletion gilding. Depletion gilding requires pickling a sheet of tumbaga in an acidic solution. Acidic pickling solutions could be easily obtained by the Moche and Chimu by soaking certain plant roots. The pre-Columbian Peruvian's primary interest in metalworking stems from a strong sense of spirituality. They believed that a divine power gave them the ability to "magically" make colors and designs appear in materials from where they had not previously existed, such as bringing a gold color out of a copper colored alloy. One can clearly see how the pre-Columbians would see depletion gilding a copper colored alloy and making it turn gold as a very spiritual activity since they did not understand the metallurgical principles behind depletion gilding. 99 KtKt GoldGold--SilverSilver AlloyAlloy treatedtreated 77 hourshours atat 165165 CC inin alumalum pastepaste (x250)(x250) 99 KtKt GoldGold--SilverSilver--CopperCopper AlloyAlloy afterafter 99 cyclescycles ofof torchtorch heatingheating inin alumalum pastepaste
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