In his 1842 memorandum proposing the Egypt expedition to the Prussian king, Karl Richard Lepsius described the paper-impression method itself as 'still almost unknown' (eine noch fast unbekannte... Art)

epigraphyarchaeologyegyptologykarl-richard-lepsiussqueeze-methodprimary-source

Proposing the Prussian royal expedition to Egypt in 1842, Karl Richard Lepsius wrote to the king: "Flache Wandskulpturen und namentlich hieroglyphische Inschriften an erreichbaren Orten und von mäßigen Dimensionen sind in Papierabdrücken zu nehmen, eine noch fast unbekannte und für diese Zwecke unschätzbare Art, die treuesten Faksimile auf eine schnelle mechanische Weise zu gewinnen." (Flat wall sculptures, and especially hieroglyphic inscriptions of moderate size in reachable locations, are to be taken in paper impressions — a still almost unknown method, invaluable for this purpose, of obtaining the most faithful facsimiles in a quick, mechanical way.) The line is preserved and quoted by the Berlin-Brandenburg Academy of Sciences directly from Lepsius's expedition memorandum ("Denkschrift").

This is Lepsius's own characterization of the technique's obscurity at the exact moment he proposed deploying it systematically — primary evidence that he saw himself as introducing a little-used method into large-scale, disciplined use, not as inventing it outright. It does not settle how little-used the method actually was outside Egyptology — see claim-squeeze-technique-predates-lepsius-learned-not-invented, which independently confirms that the technique had a roughly 150-year prior history in Mediterranean epigraphy that Lepsius himself had studied before the expedition.