Yoreh Deah Siman 18 Seif 9

 

ט בְּדִיקַת הַסַכִּין צְרִיכָה אַבִּשְׂרָא, וְאַטּוּפְרָא, וְאַתְּלַת רוּחָתָא; דְּהַיְנוּ שֶׁמּוֹלִיכָהּ וּמְבִיאָהּ עַל בְּשַׂר אֶצְבָּעוֹ. וְאַחַר כָּךְ מוֹלִיכָהּ וּמְבִיאָהּ עַל צִפָּרְנוֹ מִשְּׁלֹשׁ רוּחוֹתֶיהָ, שֶׁהֵם פִּיהָ וּשְׁנֵי צְדָדֶיהָ, כְּדֵי שֶׁלֹּא יִהְיֶה בָּהּ פְּגָם כְּלָל. וְיִבְדֹּק לְאַט וּבְכַוָּנַת הַלֵּב, שֶׁלֹּא יַפְנֶה לִבּוֹ לִדְּבָרִים אֲחֵרִים, וְצָרִיךְ לְשַׁנּוֹת הַצִּפֹּרֶן אַחַר קְצָת בְּדִיקָתָהּ, שֶׁמָּא נִפְגַּם הַצִּפֹּרֶן בְּחֻדָּהּ שֶׁל סַכִּין, וְאוּלַי יֵשׁ פְּגִימָה בַּצְּדָדִין שֶׁלֹּא יַרְגִּישׁ בָּהּ לְפִי שֶׁעוֹבֶרֶת בְּתוֹךְ פְּגִימַת הַצִּפֹּרֶן.

 

Seif 9

The checking of a knife requires the (use of the) skin and fingernail from three angles, in other words one must pass it over and draw it back over the flesh of his finger and afterwards pass it back on one’s fingernail from three angles, (which are) its tip and both of its sides so that (to insure) it will not have a blemish at all.

(after this) We check (the knife) slowly and with full concentration not turning one’s mind to other things, and then repeat on another nail slightly to check, that damaged nail on the sharp part of the knife, and perhaps there is a flaw on both sides that he didn’t feel by passing it in a damaged nail.