R. A. Shulchan Aruch, (O.H. 340: 4) rules that even though the melacha or biblically prohibited labor of kosev (writing) is violated only with permanent script, our sages prohibited also temporary writing. Therefore, it is forbidden to write in sand, in spilled liquid that is lying on a table or in the frost on a window
The Talmud Yerushalmi, (Shabbos ch. 12) mentions that people would send secret messages by writing with "Mei milin" (an ink that is not initially legible). Subsequently, they would pour specially formulated “mei afotzim” liquid on the paper, and the message would become legible.
The Yerushalmi then rules that pouring the ink violates a Torah prohibition on Shabbos because it makes the writing legible. Whether the first step violates Shabbos is disputed by Poskim. Har Tzvi (Yoreh Deah 230) rules that it is a Torah violation, while Pri Megodim (Mishbetzos Zahav 340:3) in regards to liquids that only became visible when heated, rules that it is prohibited only rabbinically. (See also Avnei Nezer 203, Machaze Eliahu 65, Minchas Yitzchok 7: 22, Shmiras Shabbos Kehichoso 40: 3, and also different opinions in regards to thermometer-strips that show written degrees of body temperature as it changes when placed on the skin, Nishmas Shabbos 7: 170, Nishal Dovid O.H. 9 etc.)
Horav Shlomo Miller’s Shlit”a opinion is that writing with ultraviolet sensitive ink is similar and would be prohibited at least rabbinically.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a
Important Numbers
