Talmud for Menachot 84:16

ותכלת אין לה בדיקה והא רב יצחק בריה דרב יהודה בדיק ליה (סי' בגשם) מייתי מגביא גילא ומיא דשבלילתא ומימי רגלים

nevertheless they should only be bought from an expert.<span class="x" onmousemove="('comment',' One who knows that it is essential to prepare the leather specifically for the tefillin.');">²⁷</span> Scrolls of the Law and mezuzoth can be tested, and may be bought from anyone.<span class="x" onmousemove="('comment',' Since it is not necessary that the parchment upon which they are written be prepared specially for the purpose. oad d tkhd a ackhk,t n');">²⁸</span> Is there then no manner of testing the blue thread? But R'Isaac the son of R'Judah used to test it (mnemonic sign: with Ge Shem)<span class="x" onmousemove="('comment',' being the initial letters of the ingredients used in the mixture for testing: for (alum) , for (fenugreek) and for ohkdr hn');">²⁹</span> thus: He used to mix together liquid alum, juice of fenugreek, and urine

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