Both Davids - Many thanks for asking and answering the question about seeing what one is replying to. After all these years I hadn't noticed that I can scroll down just like answering an email!!.
David W. - You have mentioned 16th century glassmaking. Now my glass technology is years out of date by most standards (I quit the industry 38 years ago and didn't have much to do with the melting side for ten years before that) but relative to the 16th century I must be fairly modern!! If, as you suggest, fritting was used back then (and I wouldn't know) then "modern" practice has certainly eliminated any need for it. Copious amounts of carbon dioxide, water vapour and (sometimes) nitrogen oxides are given off and for the last hundred years at the very least (and even I can't go back much further!) they have not represented the slightest problem.
Batch when heated starts to bubble up as the components start to decompose and, if filled on top of existing glass (always in a tank furnace: usually in a pot) it will remain floating on top. I suppose you could call it frothing, but that is a dangerous word in this thread! Eventually it will settle down to something like glass but full of bubbles.
Re pots, David, ours held about a ton. In our 12-pot furnace we would work six pots every other day, so the total time between first filling and working would be under 40 hours, but in many cases the time could be cut if it fit the shift working. The second would usualy be five or six hours after the first, in some cases followed by a third at the furnaceman's discretion. Nothing to do with frothing - simply the wish to achieve a brim full pot of good glass. Present-day single pot furnaces could be much quicker than that depending on available temperature, open or closed pots etc. etc.
Re continuous tanks, the principles are much the same whether small 10 ton ones like ours or 1000 ton plus jobs as used in the bottle and flat glass industries. In all cases the batch is fed in one end at a rate compatible with how much is being pulled out the other end. In all cases some means is provided (often a solid barrier with a submerged hole or "throat") to hold back floating, partially melted batch in the "melting end" and to allow a drop in temperature in the "working end".
Small studio glassmaking his mushroomed enormously since "my" time. The small quantities involved must mean that in many cases they would prefer to buy in batch from outside. I know nothing about this (Adam A.?) but at the risk of opening another can of worms I would not be surprised if some form of fritting took place simply to make handling less dusty.