The process of the tanning of the leather includes several stages. All of the stages of the tanning of the leather and the production of Torah Scrolls, Tefillin, Mezuzot, and Megillot must be performed with the proper intent, that this be "for the sake of the sanctity of Tefillin."
After the slaughtering of the animal, its hide is salted to preserve it.
The hide is then soaked in water, to extract the salt.
The hairs are removed from the hide, by soaking it for an hour in a sulfur solution.
After the hairs have been removed, the hide is soaked in a saturated solution of water and lime for ten days. The lime enters the pores in the hide and dries it, so that it will not rot and spoil. The lime also thickens the leather, softens it, and gives it added strength.