| Summary: | To identify printing techniques for medieval Korean books, ink tone analysis of printed characters is proposed. Ink tones of printed character images in two ancient books, <i>The Song of Enlightenment with Commentaries by Buddhist Monk Nammyeong Cheon</i> (<i>南明泉和尙頌證道歌</i>), designated as Korean treasures in 1984 and 2012, were compared and analyzed. Both books have been misidentified and disclosed by the Cultural Heritage Administration of Korea as woodblock-printed versions from the Goryeo dynasty of Korea in the 13th century. Ink tone analysis showed clear differences in brightness histograms between printed characters on the two books, suggesting printing technique differences. Statistical ink tone analysis of printed characters in the two books revealed totally different brightness (or darkness) histograms of pixels, within inked areas, suggesting differences in printing techniques and materials used for the two books. Ink tone analysis was performed for the <i>Jikji</i> (直指: metal type printed in Korea in 1337) and the Gutenberg Bible (metal type printed in Europe around 1455) for comparisons. As additional references, the ink tone analysis was conducted for two sets of old Korean books titled <i>Myeongeuirok (明義錄),</i> printed in 1777, and <i>Sok-Myeongeuirok (續明義錄),</i> printed in 1778, using both metal type and re-carved woodblocks. The Gongin version of <i>The Song of Enlightenment,</i> designated as a Korean treasure in 2012, showed very similar distribution and average brightness of ink with the metal-type-printed books from Korea and Europe from the 14th to 18th centuries. All metal-type-printed books from Korea and the Gutenberg Bible showed spotty prints with lighter ink tones and more symmetrical histograms compared with woodblock-printed Korean books from the 14th to 18th centuries. Ink tone analysis of printed character images can provide additional insights into a printing technique identification method. It is additional evidence for metal type printing of the Gongin version (空印本) of <i>The Song of Enlightenment,</i> designated as a Korean treasure in 2012. The version of interest is the world’s oldest extant book, printed using metal type in Korea in September 1239, as indicated in the imprint. This predates <i>Jikji</i> (1377) by 138 years and the 42-line Gutenberg Bible (~1455) by 216 years.
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