I remember practicing my calligraphy on "water-writing paper" (with @pixljar and other friends) as a youngster. I found a few sheets lying around and got curious. The comments here https://www.reddit.com/r/chemistry/comments/pcf6g/how_does_magic_paper_work/ … suggest that some "magic paper" is made of multiple layers: 1/6https://twitter.com/telemooon/status/1167752568778280960 …
Here are photos of our sheets (sorry for the poor lighting). The texture of the gray side reminds me of a blackboard, but it still feels like paper. When the gray side is wet, it turns black. It fades to gray again as it dries. Wetting the back side doesn't have any effect. 3/6pic.twitter.com/FZi5tzZe0t
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I then dug up two patents with more details. So far as I can tell via the machine-translation and my poor Chinese, the first https://patents.google.com/patent/CN85101953B/en … from 1985 is related to the "physical change" type and covers the use of activated clay (aka fuller's earth) for the top layer. 5/6
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The second patent https://patents.google.com/patent/CN1048572A/en … from 1990 is related to the "chemical reaction" type and describes a coating made of a slurry containing carbon black, water, polyvinyl alcohol, kaolin clay and some other stuff. I sort of get it, but I'm still curious to learn more! 6/6
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As an aside, figuring out that "higher Ling soil" = kaolin clay was tricky: the patent reads 高砱土 but it should be 高岭土, after 高岭村 (Gaoling village, see https://en.wikipedia.org/wiki/Kaolinite ). A related oddity: in this screenshot from the scan, you can see 砱 written in by hand! 7/6pic.twitter.com/AkEcooWaL1
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