Ah, I see. It occurs to me that we're talking about different things then. I'm interested in the historical method used in particular on Greek and Roman gloss. It's well known that the effect was achieved with one kind of slip, treated in two different ways.
While I'm greatly appreciative of all the generous help all of you've offered, I did feel like there was still something missing. I spent the night last night researching the process and I think I've just about got it down. Testing to be sure. In the interest of sharing, I'll post what I've learned.
FIrstly the black/red effect of Greek pottery comes from an oxidation, reduction, reoxidation firing schedule, with the peak temp achieved in reduction. The black sinters enough that it will not return to orange in oxidation. The surface treatment process as I had already known it from Toby Schreiber's book is that the pot was smoothed with a rib, then metal, then an iron-enhanced slip called miltos was added. The potter would then burnish the pot. The design was applied with a scraper or charcoal, then the "mystery slip" that later turns black was applied. Schreiber describes a process of using 80% ash water to 20% iron bearing clay, stirring it vigorously, pouring off the finest particles of clay and then evaporating the slip with heat until it was the consistency of cream. This is what I understood to be modern terra sigillata, however, in my experience they don't behave the same. I can get a warm, shiny glow with terra sig, but I can't get the level of gloss of Greek red-figure. It is truly like a glaze. I welcome correction, but I've never seen a modern terra sigillata get that level of mirror polish, John Lowes' example above is the closest I've seen. However, the broken kylix bottom I posted above also gets a truly high polish comparable to JLowes' with nothing more than basic slip-joining technique.
So, I spent tonight feeling a little at a loss, despite my appreciation for how much information has been offered up. I did quite a lot of reading and I think I've come to some conclusions about the process that may be of interest. Firstly, I have a feeling the ash water in the slip is doing more than simply levigating the particles, I think there's a level of alkali high enough to increase the level of sintering to a nearly non-porous surface. Then, illite is repeatedly mentioned in articles as a key component of Greek gloss. It's not universally present in modern, commercially produced clays, and I think this has somethign to do with the difference between warm, shiny glow of modern terra sig. and the high polish of unburnished Greco-Roman gloss.
I welcome correction on any of the conclusions I've come to, and I will be testing my theory with a oxy-red-ox firing in late July/August. I will post results here.
Thanks again for all the help. I apologize if I've caused any confusion in what I'm after.