Responsa for Menachot 84:10
אמר ליה אביי לרב שמואל בר רב יהודה הא תכילתא היכי צבעיתו לה אמר ליה מייתינן דם חלזון וסמנין ורמינן להו ביורה [ומרתחינן ליה] ושקלינא פורתא בביעתא וטעמינן להו באודרא ושדינן ליה לההוא ביעתא וקלינן ליה לאודרא
If a man overlaid [the tefillin] with gold or covered them with the skin of an unclean animal, they are invalid;<span class="x" onmousemove="('comment',' Cf. Shab. 108a: That the law of the Lord may be in thy mouth (Ex. XIII, 9) , the tefillin should be made from that which is permissible for food.');"><sup>16</sup></span> if with the skin of a clean animal, they are valid, even though he did not prepare it for this specific purpose. Rabban Simeon B'Gamaliel says, Even if he covered them with the skin of a clean animal they are invalid, unless it had been prepared for this specific purpose.<span class="x" onmousemove="('comment',' Similarly the first Tanna and Rabban Simeon b. Gamaliel would differ as to the necessity for weaving the threads specifically for the purpose of zizith. iuzkj');"><sup>17</sup></span>