A shampoo bar is judged twice. The first judgment happens when someone opens the package. The second happens after the bar has lived in a wet shower for several uses.
Hardness Is Not the Same as Longevity
A bar can feel hard because it is dry, brittle, or packed with high-melting materials. That does not mean it will resist mush. Longevity depends on how quickly water penetrates the structure, how well the bar drains, and whether the formula re-hardens between uses.
Design for Repeated Wetting
Wet the bar, use it for a fixed time, rinse it, place it on a draining dish, and weigh it after drying. Repeat the cycle. This gives better data than a single hardness reading.
Packaging Cannot Solve a Wet Formula
Paperboard, wraps, and cartons can protect a dry bar in transit. They cannot rescue a bar that sweats, softens, or migrates fragrance oil.