I say, and with God's success: Tanning is the removal of the foul smell and impure moisture from the skin. Tanning can be real with chemicals, or it can be symbolic through drying, sunning, or exposure to air. There is no difference between them in ruling; both are considered purifiers for the skins that can be tanned, and it is permissible to pray on them. Refer to: Sharh Al-Wiqayah p. 100-101, Fath Al-Qadeer 1: 18, Al-Hidayah 1: 20, Al-Bada'i 1: 63, Al-Mukhtar 1: 24, Al-Kanz p. 8, and God knows best.