As mentioned in a previous post, certain parts of tefilin batim must be perfectly square. Batim that are made correctly and handled properly should remain square for the lifetime of the tefilin – with one caveat: On rare occasions, the four sections of the ketzitza of the shel rosh (the box that houses the parshi’os) can spread apart. If the sections spread apart significantly, the ketzitza will no longer be square, and the tefilin will become pasul.
note: Even if the bayis warps considerably, it is usually possible for a professional batim maker to mold it back into the correct shape and return it to a kosher state.
Some batim makers glue the sections of the ketzitza together, but this may invalidate the tefilin according to some poskim. Higher-quality batim, called prudos, have no glue (or very little glue) between the sections of the ketzitza. Prudos are more mehudar, but have an increased risk of warping over time.
Keeping it Square
Visual inspection: It is a good idea to periodically examine the shel rosh, looking out for noticeable distortion. This is particularly important for handmade batim, because the sections of the ketzitza and the spaces between them are less uniform than those of machine made batim. Regular scrutiny makes it easier to catch subtle changes in the shape over time.
Proper storage and handling: Since the environmental factors that are most likely to affect the shape of the batim are extreme heat and moisture, tefilin should always be stored in a cool and dry place.1 Additionally, tefilin should not be worn over wet hair.
Modifying the cover: As extra protection against warping, the plastic cover of the shel rosh can be modified by adding three small protrusions2 – two in the inside front corners of the bottom of the cover and one in the center of the upper lip of the cover. When the cover is closed over the tefilin and the retzu’os are wrapped around it, the protrusions put a small amount of downward pressure on the middle of the titura (the base of the tefilin) while also pushing upwards on the sides of the titura. This slight compression is usually enough to help prevent the ketzitza from spreading open.
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As mentioned in the post on shipping & storing tefilin, tefilin should never be stored for extended periods of time in an airtight container. ↩
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Made from any semirigid material (e.g. felt, foam, leather, etc.). ↩