Tosfos established that the Gemara's validation of using clay documents for Kidushin is in accordance with Rebbi Elazar who maintains that the witnesses seeing the Kidushin are the main witnesses and the document is proof of this. However, Rebbi Elazar did not validate regular documents, but only divorce documents, and this presents a difficulty. Tosfos answers that Rebbi Elazar validates any document which doesn't need to be kept for a long period, such as a marriage document used to perform the marriage. The problem with clay is that it doesn't last, but for the marriage document (which is not used for proof after the event) it is fine.

Tosfos then questions his premise, since purchase documents need to be kept as proof of purchase, and our Gemara allows the use of a purchase document made from clay for the purchase of a field. Tosfos answers that even though they are kept as proof of purchase, this is not their main purpose, and thus they still may be made from clay even though it is not long-lasting.

Yoel Domb