The Chemistry of Minerals: Revamping historical techniques for educational purposes at the National Museum of Natural History in Lisbon

Fernando J.A.S. Barriga

The history of the discovery of the chemical composition of minerals isfascinating.Thetopiciscloselyrelatedtometallurgicalarchaeologywhendealingwiththeverybeginningsoftheuseofmetalsbymankindanditsbearingonthedevelopmentofhumansocieties. From the beginnings of chemistry as a true science, in the seventeenthcentury,toneartheendofthenineteenthcentury,themostpowerfulwayofstudyingmineralsconsistedofhightemperaturetestingofsamplesofmineralstofindevidenceoftheircomposition.Thisisknownasblowpipetestingbecause,beforetheinventionoftheBunsenburner,hightemperaturesweregenerallyobtainedbyblowingairintoawax,oilorspiritflamewithasimpleinstrumentcalledtheblowpipe(Figure1;seeBurchard1foranexcellentessayontheblowpipeandblowpipetesting;alsoNiinistö2).Atfirst,thiswasdonequalitativelyandthenslowlyevolvedintoquantitativeassaying.Wetchemicalmethodswereintroducedtowardstheendofthenineteenthcentury,withthedevelopmentofArrhenius’ionictheory. Modernmethodsofstudyhaverenderedbenchchemicalmineralidentificationobsolete.Flametestingled toopticalspectroscopicmethods; thediscoveryofX-raydiffractionbycrystals3anditsuseintheidentificationofmineralswasthefinalblowto qualitative testing ofminerals.Yet, before physicalmethods ofmineral analysisbecametrulycommon(aslateasthe1960s),4skilledchemists,geologistsandmineralprospectorswereabletoanalysequalitativelymostminerals,generallyinamatterofminutesoruptoacoupleofhoursatthemost.Inthecaseofpreciousmetals,quantitativeassayingwascommonpractice,withenoughprecisionformostpurposes,scientificoreconomic.

Figure 1.Eighteenth-centuryillustrationofablowpipe(C)actingonacandle(D)flametoproduceareducingflame (E) heating a crystal (G) deposited on charcoal (F) (T.O. Bergman, Manuel du minéralogiste, ou Sciagraphie du règne minéral, distribuée d’après l’analyse chimique,1784). Thetechniqueswereinexpensiveandportable.Procedureswerecentredonheating themineralbeing tested tohigh temperatures,up to1200 ºCor evenmore.Themineralcanbemixedwithanappropriatefluxtoallowfusion,ordecomposition.Results include simple fusion, flame coloration, dissolution in, and coloration of, afluxbead,evolvingofvapoursandgases,sublimates,metalglobulesandthelike.Theamount ofmineral necessary can be as low as a fewmilligrams.Many proceduresincludesolutionchemistry,whichwasaddedtothearsenalaschemistryevolved. Theeducationalvalueof chemical testsonminerals isveryhigh,becausestudentscanperceivedirectly thechemicalcomponentsofminerals; thephenomenaobservedcanbeexplainedwiththeappropriatechemicalreactions;inmanyinstancesmetallicglobulesareproducedbyprocessesidentical,orcomparable,tothoseoffullscalemetallurgy;and, lastbutnot least, thewholeprocedure isverypleasurableforcuriouspeople,especiallychildrenandteenagers. Forthereasonsoutlinedabove,bothhistoricalandeducational,theNationalMuseumofNaturalHistory(MNHN)inLisbonisreintroducingthebenchchemicalstudy ofminerals. In this paper some of the results of thiswork are presented anddiscussed.

Blowpipe testing of minerals: Historical background

Formorethan100years,uptothelatenineteenthcentury,hightemperaturedissolution and decomposition ofminerals,with orwithout the help of appropriate

Revamping historical techniques

257

fluxes,wastheleadingtechnique,notonly–asmentionedabove–toidentifyminerals,butalsofortheadvancementofchemistryitself.5Duringthisperiod,inorganicchemistryandchemicalmineralogywerelargelythesamediscipline.Someofthemorerespectedfigures of the history of chemistrywere also eminentmineralogists and vice versa.Twoschoolssetthestandards,consecutively.ThefirstmajorpushwasproducedbytheSwedishschoolintheseventeenthcentury.Tonamebutafew,GeorgeBrandt(1694-1768),AxelFredricCronstedt(1722-1765),KarlWilhelmScheele(1742-1786),JohannGottliebGahn(1745-1818),PeterJacobHjelm(1746-1813),andJönsJakobBerzelius(1779-1848)allproducedmajoradvancements,includingthediscoveryoftheelementscobalt,nickel,manganeseandmolybdenum.ThecentreformineralchemicalstudiessubsequentlyshiftedtoFreiberg,inSaxony,inthefoothillsoftheErzgebirge(literally‘theOreMountains’),homeoftheBergakademieFreiberg,theschooloffoundersofmineralogyandgeologysuchasGeorgiusAgricola(1495-1555)andAbrahamGottlobWerner (1749-1818). Elements such as indium, germanium (and arguably thallium)werediscoveredinFreiberg.However,theimportanceoftheFreibergschooltomineralchemicalstudies is largelyattributable to theworkofKarlFriedrichPlattner(1800-1858),whodevelopedanddescribedinutmostdetailandaccuracythetechniquesofquantitativehightemperaturestudyofminerals,particularlywiththeblowpipe.6 Theinstrumentationandreagentsnecessaryformostmineralidentificationsandevenforquantitativeassayswererelativelysmallinnumberandsimpleinnature.Manyschoolsandmanufacturersproduced‘blowpipekits’foruseinthefield(Figure2).Mineralogists couldperformmany studieson location,whilevisitingmines andothermineral localities.Apart from various levels of completeness and practicality,someof these setswere trueworksof art.Manynotonly included instruments andreagents,butalsosmallmineralfragmentsforuseasstandards.Theseareessentialtoinexperiencedusers,asitisoftennecessarytorepeatthetestsseveraltimestoobtainthecorrectresults,becauseslightdeviationsfromthecorrectproceduresmayleadtopoorresults.

Figure2.MaintoollayersoftheFreibergKit.Thesetraysfitneatlyinsideawoodenbox,togetherwithabalanceandreagentsandstandards.(MuseuGeológicodaBahia,Brasil,photosbytheauthor/ImagensMNHN).

Barriga

258

Figure3.Thefamous‘guineaset’blowpipekit(‘best’version),immenselypopularbecauseofitslowpriceandhighquality(CollectionMNHN,photobytheauthor/ImagensMNHN).

Teaching the chemistry of minerals

Benchchemicalexaminationofminerals,afterlosingitsscientificimportance,continuedtobetaughtfordecadesinuniversities,especiallythosewithaninterestinmineralexploration.InNorthAmerica,particularly,manyprospectorscontinuedtouseandmasterblowpipetechniqueswellintothetwentiethcentury.Mostofthebooksonthesubject,stilluseful today,areNorthAmerican.7Manymineraldiscoveries in theRockyMountainsandelsewherewerebasedonmineralidentificationsandquantitativeassays by blowpipemethods. However, the relatively high expertise level requiredtoproducegoodresults,thedisappearanceofexperiencedinstructorsandthegreateraccesstomoremodernmethods,reducedthepracticalinterestofthebenchchemicalexaminationofmineralsessentiallytoalostart.Modernmineralogytextbookscontainlessandlessinformationonbenchchemicalexaminationofminerals. Inmostuniversities,undergraduatelaboratoriesondeterminativemineralogyaretodayrestrictedtotheteachingof thephysicalpropertiesofminerals(including,inmanyuniversities,opticalmineralogy).MineralidentificationviaX-raydiffractionis taught inmany schools, in geology and chemistry departments (especially usingcomputer programs to treat XRD data). The chemistry of minerals has become anessentiallytheoreticalsubjectforundergraduatestudents.Graduatestudentslearnthemodernresearchtechniquesofanalysis,withgreatemphasisontheelectronmicroprobeandX-rayfluorescencespectrometry,inducedcoupledplasmamassspectrometry(ICP-MS)andotherphysicalmethodsofchemicalanalysis.Recently,manyofthesemethodshave becomeportable, partly as a result of space exploration. It is nowpossible toacquire a portable X-ray diffractometer. Russian, North American and Europeanresearchers and space agencies haveproduced a variety of portable tools, includingXRFandRamanspectrometers.However,thesearestillquiteexpensiveanddifficultto justify outside well funded scientific research projects and mineral explorationprogrammes. Wehavetestedthehypothesisthat,inspiteoftheabove,‘blowpipetesting’still has a role inmuseumand even university activities inmineralogy. It is highly

Revamping historical techniques

259

educational(andgreatfuntoallinclinedtobenchchemistry)towatchthetransformationsthatmineralsundergounder theblowpipe.Manyof thesetransformationscaneasilybe describedwith proper chemical reactions.Thuswehavedeveloped a number ofprocedures and exposed them to the public, originally as demonstrations entitled‘AlchemywithMinerals’duringsessionsof‘opendays’intheframeworkof‘GeologyintheSummer’proposedbytheNationalAgencyCiencia Viva.Thesesessionswereagreatsuccess,verywellattended(generallywithdemandabovethepossiblenumberofparticipants)andwithenthusiasticratingsbytheparticipants,manyofwhombecamemembersoftheMineralogyClub.

Bench mineral chemistry at the National Museum of Natural History in Lisbon AttheNationalMuseumofNaturalHistory,membersofthepublicrequestidentificationofspecimensfromaroundtheworld,oftenwithlittleornoinformationconcerningtheexactlocationwheretheywerecollected,norontheirgeologicalmodeofoccurrence.ManyusersoftheMuseum’sexpertiseareamateurmineralcollectors.TheMuseumhostsaMineralogyClub,themainactivitiesofwhicharefieldvisitsandlaboratorymineralidentification. TheNationalMuseumofNaturalHistoryderivesitsrootsfromtheperiodofnationalreconstructionfollowingthegreatearthquakeof1755.SinceitsbeginningstheMuseumhasincludedimportantcollectionsofminerals,fossilsandrocks,frommanyworldlocalitiesbutwithemphasisonspecimensobtainedintheeighteenthcenturyinscholarlyexpeditions through thePortuguesecolonies inAfricaandSouthAmerica.Nowadays,afteralongandinterestinghistory,whichincludestwomajorfiresandthreemoves around Lisbon, theMuseum is composed of three departments (MineralogyandGeology,ZoologyandAnthropology,andtheBotanicalGarden).TheMuseumisintegratedwiththeUniversityofLisbon,whichfacilitatesaccesstoscientificexpertise,butinkeyareas,suchasVertebratePalaeontologyandMineralogy,theMuseumhasitsownresearchers. Itsmineralcollection isconsidered thebest in thecountry.TheMuseumofferspermanentandtemporaryexhibitions,manyinmineralogy,andhostsanannualInternationalMinerals,GemsandFossilsFair.Educationalactivitiesareamajorcomponent,withwell-attendedsessionsofpractical,hands-onstudyofminerals. TheMuseum’s approach is largely based on takingmineral identificationas faraspossiblewith simple, inexpensive testing.Tobeginwith, it isnecessary toobserve carefully the physical properties of the specimens under study. This is anessential prerequisite, on both educational and efficiency grounds. There are about4,200recognizedmineralspecies.Ofthese,probablymorethan3,000areextremelyrareand/oroccurincrystalstoosmalltobevisibletothenakedeyeorhandlens.Amoderatelyskilledmineralogist, justbydeterminingsimplepropertiessuchaslustre(farmore important than colour), crystallographic shape, cleavage, hardness, streakandafewothers,cannarrowthepossibleidentificationofmanymineralstoadozenorsolikelypossibilities.Benchchemicaltestingisusedinthiscontext,todistinguishphysicallysimilarminerals. Asimple laboratoryhasbeenassembledat theMuseum(called the ‘OpenLaboratory’).Insteadoftheold-fashionedmouthblowpipe(whichwecanneverthelessstill use,Figure 4)we use a butane-propane jeweller’smicro-torch (Figure 5).Themainequipment,consumablesandreagentslistarepresentedinTable1.

Barriga

260

Equipment Consumables Reagents

Flameandblowpipeorgastorch Platinumwire Borax(Na2[B4O5(OH)4]•8H2O)

Forceps Charcoal Na2CO3

Mortars(agateandAbichtypeinsteel) Glasstubes Saltofphosphorous(HNaNH4PO4•4H2O)

Hammer,pliers,magnet,steelfileSilverspoonorplate

GlasstesttubesGlazedpaper

Hydrochloric,nitricandsulphuricacidsHCl,HNO3,H2SO4

Porcelaindishes Acetatefilm PotassiumhydrogensulphateKHSO4

Handlensorbinocularmicroscope Cobaltnitrate,Co(NO3)2

Ammoniumheptamolybdate,(NH4)6Mo7O244H2O

Table1.Resourcesrequiredforavarietyof‘blow-pipe’testsonminerals.

Figure 4.Techniqueof themouthblowpipeproducinga reducingflame froma spirit lamp.Notecheeksofoperator,producingthepropulsionfortheair,whilebreathingisthroughthenose.Manypeoplehavedifficultymaintaining a steady flame thisway.Nowadays small, very convenientwelding torches can be used as analternative(seeFigure5)(photoJ.Vicente/ImagesMNHN).

Inperformingdemonstrations in theMuseum’s‘OpenLaboratory,’weuseavideocameratoproduceclose-upsofthemanipulationsprojectedonalargescreen

Revamping historical techniques

261

withavideoprojectorinrealtime.Thishasprovedtobehighlyefficientinkeepinganaudienceofabout15peopleinterestedinwhatishappening,asmanyofthephenomenatakeplaceonaverysmallscaleindeed.

Figure5. Jeweller’sbutane-propanemicro-torchproducinga reducingflame to fuseaboraxbead.Noteair-accessorificesandregulationring(photoJ.Vicente/ImagesMNHN).

Main Tests

We briefly outline here the main classical tests of the ‘blowpipe type’developed at the edge of the mineralogical and chemical sciences throughout theeighteenth and nineteenth centuries, as the best means of studying minerals andmanychemicalelements.Thesecanbegroupedaccordingtothetypeofsupportused(forceps,openorclosedtube,oncharcoal,orplatinumwirebeads).Withtheforcepsthefusibilityofsmallsplintersofmineralscanbetested,accordingtotheKobellscale,8andflamecolorations,oftendiagnostic,canbeobserved.Openandclosed tubesareused to heatmineral fragments or powders, to observe the formation of sublimatesandvapours(e.g.wateroracidvapours),bothwithandwithoutafluxofair.Charcoalprisms (12x3x2 cm) are used as disposable ‘crucibles’ and also as reducing agents.Observationsoncharcoal include smell (sulphur, arsenic etc.), sublimates,magneticresidues, andmetal globules.Themost emblematic tests are possibly the formationofcolouredbeadsofanappropriatefluxsustainedbyaloopofplatinumwireabout3mmindiameter.Theingeniousarrangementallowsfusionoftheflux(usuallyboraxorsaltofphosphorous)toatransparent,moltenbead.Subsequently,theoperatorbringsthebeadintocontactwiththepowderedmineralandbacktotheblowpipeflame.Themineraldissolvesinthefluxproducingacolourdependentonthecation(s)presentandwhethertheflameisoxidizingorreducing.Tothisthereisavarietyofadditionsfromionicchemistry,manyofwhichareimportanttoidentifycertaincomponents,includingphosphatesandsilicatesandmanyothers. Itisnotwithinthescopeofthisarticletopresentamanualofbenchmineralchemistry, to this end the reader is directed to existingmanuals.9Also, the famousandwidelyknownDana’sTextbook of Mineralogy(1946)containsveryusefuldata.10Pough’s(1960)manualisoneofthemorerecentbookswithsizablebenchchemical

Barriga

262

dataonminerals.11In theappendixafewexamplesof testsofveryhigheducationalvalueareprovided,withillustrations.

Future plans

TheNationalMuseum of Natural History is preparing a new educationalprogramme,with the participation ofmineralogists and chemists, as a second levelactivityinmineralogy.Also,theMuseumisconsideringpublishingasupportbookletandassemblingakitforbenchmineralchemicaltesting.TheMuseumisinvolved11insettingupthescientificcontentsofoneoftheforthcomingsciencecentres(Mina de Ciência,literally‘ScienceMine’)ofthePortugueseCiência Vivanetwork12.CentresintheCiência Vivanetworkpresentmostlyhands-onscientificexhibits.Mina de CiênciaislocatedatanancientmineinsouthernPortugal(LousalMine).Thesiteisalreadyopen,withaminingmuseum,hotel,restaurant,handicraftshopsanddemonstrations,etc.ThemainattractionsatMina de Ciênciawill includescienceandvirtualreality.Sciencewillcomprisenotonlygeologyandmineralogy,butalsobiology,physicsandchemistry.VirtualrealitywillbeacuttingedgeinstallationcalledtheCAVE.13Weareassemblingamodulededicatedtochemistryandmineralogy,bywayofusingmineralsandmetallurgytoillustratechemicalreactionsandprinciples.Benchmineralchemicalexamination of minerals will be a well developed topic. If the experience provespositive, wemay be able towiden the scope ofmultidisciplinary projects betweenmineralogyandchemistrytootherinstitutions,perhapseventohighschoolsatlarge.

Concluding remarks

At the National Museum of Natural History in Lisbon we are revivingnineteenth-centuryblow-pipemineralanalysesandtestingasahighlyeducationalandfunmeans of teachingmineralogy, chemistry and history of science. The activitiesinclude demonstrations and hands-on activities with chemical descriptions andinterpretations,aswellasthehistoryofeachtestanditspastimportance,whenpossible.Someexperimentsarealsoelegantdemonstrationsofpresentandpastmetallurgicalmethods(archaeologicalmetallurgy).Alargepartoftheparticipantsindemonstrationsand activities have returned to a second session and/or became connected to theMineralogyClub.TheMuseumconsiders this activity a significant success andhasplansforadditionaldevelopments,includingpublicationofamanualandpreparationofatoolboxformineralchemicalidentificationtobemadeavailablecommercially.

Acknowledgements

TheauthorexpressessincerethankstoGuyLemeire,AdelaideNeves,BrunoRibeiro,CesarLopes,AlvaroPintoandLilianaPovoasforvariousformsofsupportto the mineral chemistry project and activities at MNHN. Thanks also for usefulcommentsfromJudithEverard.ThisisacontributionoftheMNHNandCreminerLA/ISR(FacultyofSciences,UniversityofLisbon).

Revamping historical techniques

263

Notes

1.U.Burchard, ‘The history and apparatus of blowpipe analysis,’MineralogicalRecord (1994), 25,251-277.2. L. Niinistö, ‘Analytical instrumentation in the 18th century,’ Fresenius’ Journal of Analytical Chemistry(1990),337,213-217.3. Laue,M. von, 1912. Eine quantitative Prüfung der Theorie für die Interferenzerscheinungen beiRöntgenstrahlen.Proceedings Bavarian Academy of Sciences,July1912:363-373.SeealsoFriedrichW., P. Knipping and M. Laue, 1912. Interferenz-Erscheinungen bei Röntgenstrahlen. Proceedings Bavarian Academy of Sciences,June1912:303-3224. See a reference in the indexed literature to techniques of ‘blowpipe analysis’ as late as 1958:R.Belcher,R.HarrisonandW.I.Stephen, ‘Studies inqualitative inorganicanalysis.VI:Charcoalblockanalysis,’Microchimica Acta(1958),46(2):201-203,DOI10.1007/BF01224786.5. B. Dolan, ‘Embodied skills and travelling savants: Experimental chemistry in eighteenth-centurySwedenandEngland,’inTravels of learning. A geography of science in Europe,(ed.AnaSimões,AnaCarneiroandMariaPaulaDiogo),KluwerAcademic,2007,1-27.http://dahsm.medschool.ucsf.edu/faculty/images/Dolangallery/Dolan_Embodied_Skills2003.pdf.6.C.F.Plattner,Manual of qualitative and quantitative analysis with the blowpipe,VanNostrand,NewYork,1892(firstedition1835).7.G.J.BrushandS.L.Penfield,Manual of Determinative Mineralogy with an Introduction on Blow-Pipe Analysis,16thedition,JohnWiley,NewYork,1898(firstedition1874);O.C.Smith,Identification and qualitative chemical analysis of minerals,VanNostrand,NewYork,1953(firstedition1946).8.Fusibility(Kobell)scale:1.stibnite,2.natrolite,3.almandine,4.actinolite,5.orthoclase,6.bronzite.9.Seenote710.E.S.DanaandW.E.Ford,A Textbook of Mineralogy,JohnWileyandSons,NewYork,1946(firstedition1898).11.InpartnershipwithMNHN’ssisterinstitution,theFacultyofSciencesoftheUniversityofLisbon,co-ordinatorJorgeRelvas,CreminerLA/ISR.12.http://www.cienciaviva.pt/home/.13.CAVEstandsfor‘CaveAutomaticVirtualEnvironment.’Undertheco-ordinationofMiguelDias,ISCTE/ADETTI.

Barriga

264

Appendix Examples of tests

A.Decompositionofarsenopyrite(FeAsS)inaclosedtube

Figure6.Decompositionofarsenopyriteintheclosedtube.Notemetallicmirror-likedepositofarsenic,blackringofarsenic/AssulphideandyellowringofAsSandsulphur.Bottomphotoshowsmagneticcharacterofresidue(photosbyF.Barriga/ImagensMNHN).

1.Prepareaclosedtubeinthefollowingmanner:cutalength(15-20cm)ofglasstubeabout6mmindiameterwiththeaidofatriangularsteelfile(makeasuperficialincisionononesideofthetube,andbreakitbyhandwithasnappressure(halfbend,halfpull);closeoneendofthetubebyheatingituntilmoltenandusingapairofsteelpliers.2.Withtheaidofanarrowpieceof(glazed)foldedv-shapedpaper,putabout50mgofpowderedarsenopyriteinsidethetube,carefullyavoidingmineralpowderadheringtothewalls.3.Heatthepowdergraduallywiththeblowpipe.4. Observations: in a matter of seconds the mineral will decompose into a yellowsublimate(sulphurwitharsenicsulphide),elementalarsenic(metallic,mirror-likeandblackfartherawayfromtheheatingflame)andamagnetic,iron-richresidue.Thisistrulyspectacular,astheaudienceseesclearevidenceforeachofthecomponentsofthemineralappearingbeforetheireyes.

Revamping historical techniques

265

B.Decompositionofgalenaovercharcoal

Figure7.Decompositionofgalenaovercharcoal.Notecoatingsonthecharcoal,yellowneartheassay(PbO)andwhiterimmedbybluetotheoutside(mainlyPbSO3).Notealsotheglobuleofmetallicleadinthelowerphoto(photosbyJ.Vicente/ImagensMNHN).

1.Placea3mmfragmentofgalenainasmallcavityduginacharcoalblock.2.Heatthegalenawiththeblowpipe.3.Observations:themineralwillfuseeasilyanddecompose,emittingsulphurousfumesandproducingcoatingsonthecharcoal,yellowneartheassay(PbO)andwhiterimmedbybluetowardstheoutside(mainlyPbSO3).Thereappearsaglobuleofmetalliclead.

C.Heparreactionforsulphates

Figure8.Silversulphideblackstainsonmetallicsilver,producedbytheheparreaction.Thestainsaredifficulttoremove,hencethescratchesfrompreviouscleaningoperationswithsteelwool(photo©F.Barriga/ImagensMNHN).Thiseleganttestwasfirstreportedin1819bymineralogistJamesSmithsoni(laterhonouredwiththenameofthezinccarbonate,smithsonite,andtheSmithsonianInstitution,foundedthroughalegacybySmithson)ii.

Barriga

266

1. A little (40mg) of a sulphate (barite, anhydrite, gypsum, etc.) is mixed with acomparableamountofNa2CO3andfusedovercharcoal(reducingmixture).2.Theresultingbrownishmassistransferredtoawetsilversurfaceandcrushedagainstthesilverifnecessary.3.Observation:thesilversurfacetarnishesblack.Thesulphatereacted,underreducingconditions,withNa2CO3 to produceNa2S (brownmaterial).The latter is soluble inwater;thesulphateionincontactwithsilverformsinsolublesilversulphide,black.Itcanberemovedfromthesilversurfacebutwithdifficulty(steelwoolworksfine).Themainreasonwhysilverwaretarnishesissulphurpollutionintheatmosphere.Thetestisverysensitiveandeasytoperform.ThemaindrawbackisthatthetarnishwillalsobeproducedbySeorTe.However,thesearemuchrarerthanS.

D.AluminiumrefractorymineralsdetectedwithCo(NO3)2.6H2O

Figure 9.Kyanite(Al-silicate)powderafter treatmentwithCo(NO3)2.6H2Oand ignitionovercharcoal.BluecolourisduetoformationofCo-Alspinel(seetext)(photobyF.Barriga/ImagensMNHN). This testwill identify infusibleAlminerals (silicates, oxides, phosphates,etc).Fusiblemineralsmaybecomeblueby incorporationof thecobaltnitrate in themelt.Calamine (refractory zinc silicate)will becomeblue aswell.Notwithstandingtheselimitations,thetestisextremelyusefulandelegant.1.Ignite40mgofthefinelypowderedmineralovercharcoal;2.Treatwithadropof10%solutionofCo(NO3)2.6H2O;3.Igniteagain;4.Observation:InthepresenceofAl,thepowderedmineralwillturnintenseblue. TheinitialignitiondecomposestheAlmineralandspeedsupthesubsequentreaction with Co nitrate. The blue colour results from formation of Co-Al spinel,CoAl2O4.Thiscompoundiswellknownasapigmentusedforblue,resistantpaintsforoutdooruse(CobaltAluminumBlue).

Revamping historical techniques

267

Appendix Notes

iJ.Smithson,‘OnanativecompoundofSulphuretofLeadandArsenic,’Thomson’sAnnalsofPhilosophy(1819), XIV, 96, cited byMartin, http://www.rockhounds.com/rocknet/archive/messages/17816.shtml(2002),accessed2October2009.iiSeeMartin,op.cit.(12)foradetailedaccountoftheheparreaction.

Barriga

268

269