Myths, folklore & symbolism
Witch balls: Large, heavy glass balls coated with glossy reflecting silver, gold, or brightly colored paint, first made about 1690, and popular in the 19th century; or a smaller type from the late 18th century, with swirling multi-colored patterns. Both were meant to hang in cottage windows; their purpose was to attract and neutralize the evil eye of a passing witch, either by reflecting it back upon her or by puzzling her with the pattern. Plain glass balls filled with bright tangled threads were thought to have the same effect. Alternative names are "wish ball" and "watch ball"; the latter is sometimes explained by saying that the ball grows dull if there is infection "in the air", and so should be watched for a warning. (Oxford Dictionary of English Folklore)
What other traditions/customs do you know of that use physical objects to protect against witches?