I'm using Insight and the Level II online North American database with many of the materials then adjusted to my local materials supplier typical analysis. Since this is to be electric fired, I kept the RIO in the ampoteric oxide form.... but MnO is a flux in both oxidation and reduction... so that is set to go into the flux unity. You are correct...... I made the mistake of simply omiting the small amount of CuO in the flux unity... so if included that will drop the other flux numbers just a tiny bit.
Yes... the manganese at that percentage level is of GREAT concern in this particular case... and will account for a huge proportion of the surface microcrystaline materials.
An important point I make in my materials classes is that it is always possiblke that while the person who has the original recipe knows enough not to put certain formulations onto food contact (or even POTENTIAL food contact) surfaces... the people that get the information shared to them (via workshops, books, or things like this forum) may not necessarily realize this fact. Too often the "non-food use" message is left out or assumed to be obvious.,....... and for many people, particularly the less experienced, it is NOT obvious.
Additionally another related point I make is that just because someone is a "famous ceramic artist" and is seen absolutely everywhere and in every significant exhibition and other venue ....... and their work is absolutely stunning........ that does NOT guarantee that they have a solid technical understanding of things like glaze chemistry or firing theory or the like. SO it is possible that the technical information they share is not all that accurate. That does not take away from the visual power of claywork they produce.