From the first three close ups I'm guessing that what you have is a LOW firing natural clay.....an earthenware. It is looking overfired on all the XC images. The part with the RR glaze is looking SERIOUSLY fluxed and seriously outgassing (hence the holes).
You likely either need to shift to a lower cone for the firings, or decrease the percentage of the local clay in the body forumlation.
And a reminder....... this is sort of like doing surgery by telling someone how to hold the scalpel over the phone and were to cut. NOT the most accurate way to diagnose this.
I really appreciate your taking time to share your insights. I knew there had to be something in the RR glaze reacting with my clay - but didn't know enough of the chemistry to figure out what. About half of the glazes they use at this studio share the same base as the RR, with different oxides for color, so that lets me know I can probably expect similar results from those.
I'll have to talk to my friends about possibiity of firing lower. Nearly everything they fire is ^05-^06 bisque and ^6 glazed. I'm going to get a copy of Cardew's book from the library - but in the mean-time: When decreasing % of local, would you recommend keeping the EPK & OM4 at the same proportions relative to each other, and just reduce the local - or is there something else you would suggest adding to help raise the maturing temp ?