Coloring the batim black is a lengthy process, the quality of which greatly influences the quality of the Tefillin. The color must be slowly absorbed by the leather. To achieve this end, the leather is dyed and polished at least four times. Each time four coats of dye are applied, so that the color will be well absorbed by the leather. This repeated action is in accordance with the halakhic requirement of "hazuta be-alma" (solely in appearance), that is, that the dye cannot be an external layer, but must be fully absorbed by the leather and become an integral part of the bayit, thereby giving the appearance of blackness. Spray-painting the batim is therefore totally forbidden, because the sprayed layer of paint is liable to peel off. The process of coloring takes about three weeks.