my two penn'th as usual...........
When something has become part of acceptable standard language, then whether right or wrong, we have to go with the flow it seems.
However, we've been down this road before, and seem to recall coming to the conclusion that those swirl coloured pieces, whatever the colours (but which include white, usually) might be more correctly termed marbled Vitro-Porcelain.
Sowerby initially created a product which they called Malachite, and this was offered in blue, brown, and the obvious green (the alternative name for Malachite may well have been Sorbini, but not entirely sure).
Malachite then seems to have been superseded by New Marble Glass - they all have this general look of marbling.
I like the States take on the purple and white effect which they seem to call blackberries and cream.
But coming back to the word slag - now so well embedded in the glass world that pieces showing a marbled effect will doubtless never be called anything else - the following may be of interest...........
Somewhere around the beginning of the 1880's, Sowerby's advertised a specific factory product which they called 'slag' glass. According to Slack............ "This was a type of glass that was black by reflected light and either a bottle-green or dark purple colour by transmitted light. As far as is ascertainable this was the only coloured glass that was termed 'slag' glass and it always appeared semi-opaque, being made in the same manner as cheap bottle glass. Apparently it was a short-lived product and was superseded by a dense black opaque glass which was advertised throughout the 1880's as 'Jet'."
Whether or not slag actually ever found its way into the pots (deliberately or accidentally), may not be known........Lattimore believed it was added deliberately, but can't imagine that scum from the smelting works would have improved anything. It may just have been a comment that was repeated down the years without question.
The other less than technical name was 'end of day glass', and this does now seem to have been a notion dispelled completely.
Anyway, nice piece of 'slag' glass Stew