getting/slaking lime from ...mussel shells

Howdy!
I"m building a rocket sauna in my backyard (pix to follow), and I had a question.
I was hoping to do a lime coat on the outside of the earthern exterior (pretty white coating, strong against the elements, etc.), but
everytime I try to slake the shells (cook them in my fireplace kiln), they come out dark black. I broke the shells down, added water to it, no reaction.
I've watched the 'pet kiln' video, watched other slaking videos, and I'm thinking mussel shells may be the wrong shell to use.
Has anyone used them, slaked them, had them turn white, then used them as a lime putty?
Thank you ahead of time! -V

Hi Vince,   Welcome to rocket central.

Oh boy! A rocket sauna sounds awesome. Definitely on my pre-bucket list so you can bet I'll be your shadow to watch your progress.

I've seen the pet kiln video many times (another thing to try). The only thing I can think of is are you getting your fire hot enough?

EDIT: A link to the Rocket Sauna brainstorming at Wheaton Labs:  Wheaton-Labs-Rocket-Sauna-Build

Someone else is sure to chime in with experience. In the meantime, I'll add your question to the rocket stove forum to get you a bit more exposure to the folks over there also.