הלכה על גיטין 42:19

We don't write a divorce document on [something] attached, even if he gives to her the attached thing as it is, since we need "and he puts it in her hand" (Deuteronomy 24:1). And even if the witnesses signed after it was detached, and then he gives it to her, it's not a [valid] divorce document. And if he wrote it on a leaf that was planted in a flowerpot (explanation: like a half potter pot that you plant in it) that was holed, even though he gave her the entire flowerpot, it's invalid, lest it be cut off. But he [can] write it on the pottery of a flowerpot and give it to her, and provided there are witnesses of the exchange in front of us, since pottery is something that can be forged. Ram"a: The same law applies to a flowerpot that is not holed; and some say that when it's not holed we don't decree perhaps it will be cut off (you will find the two opinions explained in the Beit Yosef).