What are the necessary conditions for materials used in tanning (dabbgh)?

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The material used for tanning must possess the qualities of absorbing moisture and purifying impurity. Examples include alum (ash-shabb) and acacia bark (al-qirz). Furthermore, the tanning agent itself must be pure. If the tanning agent is impure, the hide will not be purified because purification from an impurity cannot be achieved by something impure, similar to purification by istijmar (dry ablution) or washing.