Transcription of A Blueprint for Conserving Cyanotypes
1 A BS T RAC TThe technical history of the cyanotype process has beenelucidated from a study of the original experimental notesof Sir John Herschel and others, in order to replicate earlycyanotypes for experimental testing. Their chemical vul-nerability is threefold:1. They are faded by visible light, when the Pr u s s i a nblue (ferric ferrocyanide) is photochemically reduced toPrussian white (ferrous ferrocyanide): but this reaction isfound to be substantially reversible, because the latter isslowly reoxidized by molecular oxygen. The importance ofallowing Cyanotypes access to the air is stressed. Thezeolitic nature of the Prussian blue lattice can incorporateimpurities which affect its stability to light: oxidizableorganic molecules promote fading, but lead toningenhances the stability to light.
2 Because the fading of cyan-otypes does not obey the photochemical law of reciprocity,they may be safely exhibited under 50 lux illuminationwithout fading; their recovery from fading caused by mod-erate exposure to full daylight was Prussian blue is rapidly and irreversibly hydrolyzed toferrocyanide and hydrated ferric oxide, even at the mod-erately alkaline pH of , corresponding to a saturatedsolution of calcium carbonate, which effectively bleachesthe pigment color. This sensitivity to hydrolysis by alkalican be greatly diminished by treatment of the Pr u s s i a nblue with nickel(II) salts, but may itself entail some imageloss and a color shift. The importance of using unbufferedenclosures and wrappings for Cyanotypes , contrary to thenormal practices of paper conservation, is Significant amounts of Prussian blue image can beirreversibly lost from Cyanotypes during aqueous washing,which causes perceptible changes within a short time,because the colloidal pigment is peptized and dispersed asa sol.
3 Susceptibility can vary with the type of Prussian blueand water purity. It is concluded that wet treatments ofprecious specimens should only be undertaken with E F E R E N C EWare, Mike. 1999. cyanotype : the history, science and artof photographic printing in Prussian blue. London:Science Museum; Bradford [England]: NationalMuseum of Photography, Film & book is distributed in the USA by Michigan StateUniversity Press (ISBN 1900747073): <http://www. m s >.MIKE WA R EConsultantNational Museum of Photography, Film & TelevisionBradford, United Blueprint for Conserving CyanotypesM I K EWA R EThe Book and Paper Group Annual 21 (2002) 33 Presented at the Book & Paper Group/Photographic MaterialsGroup Joint Session on Reprographic Processes, AIC 30thAnnual Meeting, June 6 11, 2001, Miami, Florida.
4 Received forpublication Winter 2002.
