Hey Zuma, probably wont get there until Saturday morning. Tom is bringing some items he made and I have one drying from his clay also. Hope to do a pit firing Saturday evening or Sunday with what folks bring. If Keith is okay with it.
I cooked and crushed some shell and added it to some local clay early this year. Fired the pots and they came out looking great. After about a month or so I noticed some cracking and noticed the shell was getting soft and appeared to be expanding. The pot eventually fell apart from the shell expanding. I believe it started pulling in moisture. I also believe I did not crush the shell enough. It may need to be close to sand in size. The pot that cracked is now pounded and crushed into grog and may add it to the clay this weekend. I will also bring the other one for you to look at. It is still intact but has pieces popping off. Again, worst in wet humid weather. Maybe if you cooked in it regularly it may not happen but I do not know.
One thing I read is after you cook the shells, wash them before crushing. Not sure why unless it is to remove ash and dirt from the surface.
See ya soon!