Thanks for the nudge, David E. I would have missed this due to the well-known fact that very long threads do sometimes find the subject matter changing part way through. One or two points I might make.
First, briefly a word of warning about the word "frit" when coupled with reference to Pilkingtons. For some reason, Pilks used the word to mean the same as what all the rest of the industry called "batch", i.e. mixed raw materials which were not, so far as I know, "fritted" in the heat-processed sense used by most people.
David E., I think I can understand why people find the concept of glass "frothing" hard to visualise. Remember we would be talking about glass at the temperature at which it is melted, which is a lot hotter than its working temperature which is how most people see it. Think of warm or even hot golden syrup (and no, I haven't seen that froth!). About five years ago, buried deep in a thread called "Glass Moulds" I said, following talk about stirring glass, I think, I said:-
"This sort of treatment was highly respectable and was (is?) used all over the world. It was also called blocking, but we peasants always called it tatying. It could be used for at least two different reasons. The first was to use the large bubbles of steam produced to sweep away any fine bubbles (seed) remaining from the melting process. The second reason, and the one for which I tatied most frequently, was to mix the melt thoroughly, usually if I was changing colours. Anything which produces huge glugs of gas and generally stirs up the melt can be used. The textbooks talked about wet banana stalks and the Germans reputedly used lump vitreous arsenic (arsenious oxide) which was very dense and therefore could be just chucked into the pot. Big wads of wet newspaper have been talked about, but I'm not sure how that was used. We used big potatoes, one at a time, cutting a hole in so that the sharpened end of the heavy rod (maybe 3/4" dia at least) could be forced in. After maybe a minute of glugging, thumping and minor earthquakes the remains of the spud would break free and bob quickly to the top. Molten glass does not wet carbon, so the black thing was easily fished out. Visitors were always surprised that the bit of spud left in the middle was still raw. We would only need a second go if the spud came off the rod prematurely.
I vividly remember being called in once during the night because a pot of cadmium yellow (still a bit experimental) was refusing to clear of seed. It must have somehow (I haven't a clue how) become supersaturated with gas because when we tatyed it half the pot contents came out on to the floor looking like a cross between candy floss and something out of Quatermass! That was the exception - tatying was normally a benign if spectacular process."
With hindsight, the excess gas might have had something to do with lack of cullet. Any new composition obviously couldn't use cullet to start with because there wasn't any! I cannot remember whether this was a first attept (zero cullet) or just early production (very little cullet).
Re cullet in general, I have made glass with cullet content varying from zero to 90% or higher. At the extreme ends odd things might happen as described, but in general the amount used depended on what was available on the day. I have little or no experience with bottle or flat glass production where "foreign" (i.e. not produced in-house) cullet is used and where tight economics may dictate a close watch on the effect of cullet ratio on melting rate.
Re sovereigns thrown in the pot I too have heard this one from trusted sources within the factories concerned. I'm still not convinced and it was much too expensive to have a go myself!
Since typing the above, I have seen David W.'s posting, which also covers frothing. Re fritting, David, was this actually done in the non-Pilkington sense of the word?