The passage tells us that Estabrook uses these old techniques so that his work can "[embrace] accident and idiosyncrasy in order to foster the illusion of antiquity." He leaves the stains and imperfections which result from the use of these techniques to foster a sense of nostalgia in his work (lines 36 to 45).
We also learn that Estabrook "imagines planting the [prints] he makes in flea markets and antique shops, to be discovered as 'originals' from a bygone time that never existed" (lines 13 to 16).
Estabrook's overall goal in using these techniques is to foster a sense of nostalgia by creating prints that look like they came from a long time ago. If the same effect could be achieved using modern photographic techniques, one might ask why it would be necessary for Estabrook to use these old techniques.
However, if the use of modern techniques inevitably betrayed the truth (i.e. the real age of the photos), then Estabrook's goal would be undermined as the photos would no longer raise feelings of nostalgia; they would simply be modern prints with imperfections that he was trying, and failing, to pass off as antiques.
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