Preparing the Hides
The high-quality batim - the hard black boxes that house the parchments - are made from the skin of an ox. Only one pair of Tefillin is manufactured from each individual hide.
True, one is allowed to make the batim from the skin of a sheep; however, the skin from the neck of an ox is stronger, more durable, and less likely to suffer damage over the years.
After slaughtering the ox, the soft and pliant skin is stripped off the neck. We can now begin processing it with lime in a large drum.
The object of the lime is to seal the perforations in the skin and to dry it so that it will not decay. The lime reduces the smell, removes the hairs, the fat, and any remaining flesh, and eases the manufacturing process.
After this, the leather is cut into the shape of the letter T.