Reference for Bava Batra 40:1
דלית בה שיעורא כדתנן חרס כדי ליתן בין פצים לחבירו
We speak of a piece which has no size to speak of, [and may even be carried on Sabbath]. as we have learnt: A piece of earthenware [which must not be carried on Sabbath] must be big enough to put between one window post and another.<span class="x" onmousemove="('comment',' Shab. 82a. It was usual to place potsherds between the posts of a window-space at the top and the bottom and to plaster them with mud so as to support the wall. ');"><sup>1</sup></span> 'An earthenware vessel' is it not useful? — We suppose it to be dirty. It is still useful for a blood-letter [to collect the blood]? — We suppose it has a hole in it. 'A scroll of the Law' can serve for reading the Law? — We suppose the scroll to be worn out.<span class="x" onmousemove="('comment',' And therefore it cannot be used for the synagogue reading. ');"><sup>2</sup></span>