Modern photoengraving techniques - Encyclopedia Britannica

Basic production processes ... The essential operations for the production of all types of photoengravings are similar. They include photography, photomechanical ... photoengraving TableofContents photoengraving IntroductionHistoryofphotoengravingEarlyetchedplatesWet-collodionphotographyThehalftoneprocessBasisforselectionofscreenrulingContactscreensThebendayprocessSpecialeffectsProcessdevelopmentsChemicaletching—traditionalandpowderlessprocessesElectromechanicalplatemakingColourscannersModernphotoengravingtechniquesBasicproductionprocessesCameraanddarkroomequipmentPlatecoatingandprintingEtchingandfinishingBlockingandproofingColourplateproductionColourseparationEliminationofmoiréElectromechanicalengravingmachines—colourscannersProductionspecificationsLineplatesHalftonespecificationsCombinationline-and-halftoneplatesEngravingtechniquesappliedtointaglioprocessesLineintaglioGravureandrotogravureOthermethods FastFacts 2-MinSummary RelatedContent Media Images More AdditionalReading MoreArticlesOnThisTopic Contributors ArticleHistory Modernphotoengravingtechniques Intermsofcost,engravingmethodsrangeinascendingorderasfollows:lineengravings;halftoneengravings;combinationline-and-halftoneengravings;single-colour,two-colour,andduotoneengravings;andprocesscolourplates.Eachofthetypesmaybeproducedinanyofthecustomarymetalsorplastics.ProcesscolourplatesareusuallymadeofcopperintheUnitedStatesandUnitedKingdomandofzincelsewhere. Basicproductionprocesses Theessentialoperationsfortheproductionofalltypesofphotoengravingsaresimilar.Theyincludephotography,photomechanicaloperations,etching,finishing,routing,blocking,andproofing. Cameraanddarkroomequipment Theengravers’camera,calledaprocesscamera,isarigidlybuiltmachinedesignedtoallowprecisepositioningofthelensandcopyboardsoastoprovidecontrolovertheenlargementorreductioninsizeofthecopy.Ithasacolour-correctedlensdesignedtogivethesharpestpossibleoverallimagewhenfocussedonaplanesurface,withoutthedistortionscommon(thoughusuallyunnoticed)intheaverageportraitoramateurcameralens.Processcamerasaredesignatedasgalleryordarkroomtypes.Thegallerycameraisfreestandingandmaybeinstalledinanyconvenientlocation,butfilmmustberemovedinalight-tightcassetteandprocessedinaseparatedarkroom.Thedarkroomcameraisinstalledwithitsfilmholderasanintegralpartofthedarkroomwall,givingeasyaccesstothedarkroomfacilities. Thematerialtobereproduced,calledcopy,ismountedonaboardorglass-coveredcopyholder,carriedonthebedofthecamera.Illuminationforexposureisprovidedbyarclampsorhigh-intensitygas-dischargelamps.Themostcommoncameralampsystemsinlateyearshaveinvolvedpulsedxenonlamps,inwhichahigh-voltagealternatingcurrent,passingthroughaglasstubecontainingtheraregasxenon,causestheemissionofalightrichintheultravioletwavelengths. Virtuallyallphotographicworkisdoneonfilmcoatedwithhigh-contrastemulsionsespeciallydevelopedforgraphicartswork.Theintroductionofdimensionallystablefilmbaseshasnearlyeliminatedtheuseofglassplates.Filmemulsionsusedforhalftonesyieldtheextremelyhighcontrastneededforhalftoneorlinereproduction.Strippingfilm,alaminatedfilmwithasoftadhesivelayerbetweenthebaseandtheemulsionlayer,iswidelyusedtopermitimagestoberemovedfromthebaseandproperlyorientedontheglassorfilmflatthroughwhichthemetalplatewillbeexposed. Intheearlydaysofphotoengraving,withwet-plateimagesonaglasssupport,itwasimpossibletoprocessphotographicimagesbyanymeansotherthanimmersioninsolutionscontainedinashallowpanortrayorbydippingintoatankofsolution.Suchtankandtrayprocessingremainsimportantbutisnowbeingsupplantedbytheuseofautomaticfilm-processingmachines.Derivedfromequipmentoriginallydesignedforprocessingofmotion-picturefilmorphotostatprints,theseconsistofbelt-orroller-drivenapparatusthatcarriesthefilmthroughdeveloper,fixing,andwashingsolutions,and,inmostcases,throughadrier,permittingdeliveryofaprocessed,driedfilmwithinthreetofiveminutesafterinsertionintothemachine.Suchmachines,withdifferentprocessingsolutions,maybeusedforcontinuous-toneorlith-typefilms. Platecoatingandprinting Photomechanicaloperationsincludecleaningthemetalplatesurfaces,coatingwithalight-sensitivesolution,dryingthecoating(knownasthetoporenamel),andmakingtheexposureonthiscoatingthroughthenegativepreparedinthephotographicstep.Throughouttheseoperationscareisrequiredtopreventimperfectionssuchasbubbles,dirt,orscratchesinthelight-sensitivecoating.Thezinc,magnesium,orcopperispreparedbycarefulcleaningwithpumiceandwater.Thelight-sensitivecoatingsareusuallypouredoverthesurface,andtheplate,heldflat,iswhirledtoensureuniformcoveragebythesolution. Light-sensitivecoatingsareusuallyadichromatedcolloidmaterial,butlight-sensitiveresinsarealsoused.“Coldtop”enamelsareusedonzincandmagnesium,whichcannotbeheated;theseareusuallyslightlyalkalinesolutionsofshellacorpolyvinylalcoholtowhichadichromateisadded.“Hottop”enamelsnearlyalwayscontainfishglueaswellassomeeggalbumin,towhichisaddedadichromatesensitizer.Mixturesofglueandalbuminareusedwhenitisnecessarytocontroltheetchresistanceandtheeasewithwhichtheedgesoftheenamelbreakawayduringtheetchingprocess.Hottopenamelsmustbesetattemperaturesof550–650°F(285–345°C)andareusedmainlyoncopper,thecrystalstructureofwhichisnotalteredatthesetemperatures.Polyvinylalcoholandshellacresistantsaresetattemperaturesof350and220°F(175and105°C)respectively;thereforetheyareusedonzincandmagnesium. Thetopsarehigh-contrastmaterialsthat,whenexposedtostrongultravioletlight,hardenwherethelighthasstruckthemandlosetheirsolubilityinwater.Developmentinwaterthenremovesthecoatingfromtheunwantedareasofmetal,exposingthemetalfortheetchingprocess.Photosensitiveresinousmaterialsfindwideapplicationinelectroniccircuitprinting,anoperationanalogoustophotoengraving.Theyhavemorelimitedapplicationsinthemakingofphotoengravedletterpressplates,wheretheyareusedespeciallyonzincandmagnesiumandwheretheirexcellentstoragepropertiespermittheirapplicationinthemetal-finishingplant,obviatingthenecessityforcoatingoftheresistontothemetalinthephotoengravingshop.Theseresinousmaterialsaredevelopedinorganicsolvents. LoadNextPage