Reference for Gittin 90:11

תנו רבנן מעלין בדמיהן עד כדי טרפעיק מאי טרפעיק אמר רב ששת איסתירא

will he not require the writing to have been for the specific purpose? For it has been taught: If a man overlays the phylacteries with gold or covers them with the skin of an unclean animal, they are disqualified; [if with] the skin of a clean animal, they are fit for use, even though he did not tan it for the specific purpose. Rabban Simeon b. Gamaliel says: Even if covered with the skin of a clean animal they are disqualified unless it has been tanned for the specific purpose!<span class="x" onmousemove="('comment',' V. Sanh. 48b. ');">¹¹</span>

Jastrow

Jastrow