What are the conditions for a prayer barrier (sutrah), and must it be upright and pure?

The summary of the answer is: The sutrah (barrier) for one praying does not have to be ritually pure (ṭāhir); it can be a ritually impure (najis) object, though this is disliked. It is sunnah for the sutrah to be a wall, a pillar, or something similar, and for the worshipper to draw near to it, with the distance between them not exceeding three cubits. If a wall is not available, one may stick a stick or something similar into the ground, its height being two-thirds of a cubit or more. As for the condition that the sutrah must be straight, the scholars have not stated this.