hi Mike - at a very quick glance your essay with pix looks very interesting and should be a big help to those who, perhaps, don't have access to the books, but want some form of id for Sowerby colours.
Just a few words of idle comment ....................... Although I no longer have them, I did for a while own a couple of rubine bowls, and still have my pix of these pieces. It's a shame that Slack's book didn't include any pictures of rubine - it really is quite a stunning colour, but it isn't just another red - when held to a good light it shows an almost gold tone within the body, perhaps a tad more where the red it less thick. From memory my bowls are shown in the 1927 catalogue and I believe are patterns 2334 and 2411, and date wise this would tie in with your comments "and in most cases probably a lot later." - there seems to be a lack of evidence they were made earlier than the 1920s.
I was never aware of seeing rubine pieces that could be dated earlier than this, and unless I'm completely mis-reading the various sources, then I agree with you that there seems to be a distinct lack of reference by any of these authors - including the Potter Gazette - that guides us to a C19 source of the word rubine - but of course I'm more than happy to be proven wrong. So it looks like we remain ignorant still of the origin of this descriptive word.
My rubine bowls were certainly not any shade of pink, but are a deep rich ruby, and I dislike Cottle's comment describing Sowerby's red as 'transparent, which in my opinion is not an accurate description.
For what it's worth my opinion on the mottle pink and clear Gladstone bag is that it's neither rubine nor rose opalescent, and may well have been a 'seconds' that got through the net - it most definitely isn't rubine and looks way too patchy in colour to be rose opalescent. If you read Cottle's comments he looks to be suggesting that R.O. is simply a pink version of ruby but with some opalescent - not a patchy looking colour as shown in your pix.
Nonetheless, Sowerby seem to have been producing a deep red in the late 1870 - early 1880s which they described as a superior ruby pressed glass - although what they used as the colourant for this I don't know - was it copper, selenium, or something else - surely it wasn't colloidal gold, which would have been far to expensive for pressed glass. I say this in view of Raymond Notley's comments in his book - words which I've posted previously when we've had chats on pressed red glass. If you read Notley, he is very adamant that up until the 1920s gold was needed to produce red glass, after which selenium and copper were used. He says ............. "most pressed red glass around today is post 1924".
Would be of interest is someone knows how Sowerby coloured their ruby type glass - also what was the source of the word rubine, and does anyone have any pieces from the early 1880s.