Specific Seashells for Self-defense

Question

Mermaids in my setting have the power to manipulate shells. In more specific terms, they can alter a shell's shape as if they are made of putty, as well as fuse them seamlessly. This enables them to make weapons and armor out of seashells, but then this brings up an interesting exploit.

You see, thanks to this question, I know limpet teeth is the hardest known biological substance, and is also among the strongest. I also see that abalone shell would be good for armor.

Given that mermaids use seashells to form structures, as well as to create weapons for self-defense, it makes sense they'd discover limpet teeth (which I'll handwave as counting as 'seashell' for the purposes of being manipulated by mermaid powers) and abalone shell and utilize them.

For example, a mermaid with abalone shells could slap one on her shoulder, smooth it down, and have a pauldron, then continue to make herself abalone plate. She could also make a spearhead out of limpet teeth to stab hungry sharks with.

My question is, how useful would these specific seashells be for self-defense? Specifically, if a mermaid were to use these seashells to form armor or weaponry in order to defend herself, how useful would they be?

These mermaids would be defending themselves against sharks, crazed knights, pirates, other mermaids, giant octopi, and the occasional demented and/or obsessive sailor. As for tech level, think mid-medieval.

Answer 1

It depends

It depends on how the armor will be structured. Even though they have the power to manipulate the shell's shape, if we respect conservation of mass, they'll stretch it thin if they try to make a full armor out of a couple of shells, so my best guess is that they're going to try and work with the shell's basic shape as much as possible and use their powers for minor fitting / uniformity

As the shell can shatter and break, one of the most important things they have to think of is padding. I'm expecting them to wear the armor over a thick layer of something (seaweed fabric??) to protect the skin. Also remember that hard materials transmit a lot of impact - that's the reason why modern cars become a complete wreck on a crash when compared to older cars: they're designed to get destroyed and absorb the impact to protect the occupants

If the shells are used as scale armor, they'd offer fair levels of protection against blades and blunt weapons, less so against projectiles

Enter the giant octopi - the type of attack they typically perform (grab and squeeze to death) would make the design of the armor tend on the spiky side, so it will deter this particular enemy. Caution is advised - while it looks very cool on a drawing, they have to be moderately sized and cautiously placed in order to not restrict mobility (spikes in a pauldron? Big no-no)