In a previous thread, a couple of members showed interest in this subject. I would not have thought it would have been of very wide interest but here goes. As usual when I am incautious enough to offer to write something it is a bit rambling. If I polished it to my own satisfation it would never see the light of day.
The thread which started this off is at http://www.glassmessages.com/index.php/topic,5260.0.html
The video which Chris mentioned at the beginning sounds interesting, especially re pot making and setting. Bernard has recently added a reference which also sounds interesting.
What are pots?
For new members, there are two main types of glass melting furnace - tank and pot. Tanks, except at the bottom end of the size range, are for continuous, mass production of up to 300 tons per day or more. Forget them for the present purposes. Pot furnaces, which contain from one to twelve or more individual pots, are for batch production.
Pots can be âopenâ, like a bucket or âclosedâ or âcoveredâ, like a tall igloo but with the entrance nearer the top. They can be any size but the biggest, covered ones (with which I am most familiar) hold about a ton of glass. The bottoms are 6â or more thick and the sides/tops 3â or more. They are, I believe, the largest and heaviest bits of pottery in regular production.
Parkinson-Spencer (of whom more later) have a web site with excellent pictures of covered pots. PSRglasshouse.pdf (application/pdf Object)
All clay-based materials in contact with molten glass are slowly dissolved by the glass and pots are no exception. For this and other reasons the purest clays commercially available are used. They are mixed by repeatedly passing through pug mills, although bare feet (like grape treading) is very effective and good for the feet and for the leg muscles! The objective is to get every trace of air bubbles out.
Making was done entirely by hand with each handful rubbed into place, again to avoid trapping air. The bottom was made first on a shaped board then, after a few days, it was turned upside down onto a board covered with 'grog' (crushed firebrick) to allow for future shrinkage. The sides were then built up, again rubbing on by hand, a few layers each day and finally the top. Pot-makers were friendly folk but professionally secretive; locked doors or out-of -hours work were not unknown when they were working on the clever bits like the tops. Stories about old-time pot-makers insisted that even the factory owner would be shut out of the pot loft at critical times.
Drying simply consisted of leaving the pots in the draught-free room in which they were built at a constant, pleasantly warm temperature for as long as possible. Usually low-wattage bulbs would be left on in each pot to give gentle warmth as well as to help air circulation. The main lights were, of course, normally off and the P-S pictures give an idea of the fairyland effect. I did try growing mushrooms in one corner of Sowerbys' pot loft but it didn't work. If I had changed the conditions I might have got lovely mushrooms but perhaps also big heaps of rubble instead of pots!
We would have been unhappy to use pots less than six months old, although by then they could be moved into other storage areas to make way for the next batch or, indeed, transported with care to a customer's factory in the case of commercially-made pots.
I have heard of very slow-moving tunnel dryers where very precise control of humidity at a constant temperature was the method. I've never seen one and it may only have been a design exercise.
Who Makes Them?
Parkinson & Spencer (now Parkinson-Spencer), a highly respected company who say they are now the last commercial pot makers in the UK, presumably supply most if not all pots to the UK industry today. In my time, however, when multi-pot furnaces were the norm, larger users of pots like Sowerby, Davidson, Jobling, Bagley and Lemington as well as the lead crystal makers had three options. They could buy complete pots from someone like P-S (the main UK maker even then). They could employ their own pot maker and do the entire operation themselves (Jobling, Davidson) or they could buy in (from, e.g., P-S) and mix the clay themselves and have a contract maker come in once a year to do the making (Sowerby). I don't know what the others did. This was, of course dependent on demand; Sowerby at that time only had 12 pot furnace capacity whereas Davidson had 22. In earlier times Sowerby would have employed their own full-time maker.
I personally only knew three pot makers. Cliff (?) Tootell, who made for Sowerbys and father and son Joe and Joe McCartney. When I started at Davidsons, Joe jr was the pot maker. His father, Joe sr, was the Jobling maker. When Jobling ceased production of all non-Pyrex, and therefore no longer needed pots, the two Joes took over the Davidson pot making facility as Joseph McCartney & Son. They of course continued to make for Davidson as well as building up new customers. Somewhere in the 1960s they moved to an industrial estate north of Newcastle. I called on them once and they seemed to be doing well. I enjoyed hearing tales (unrepeatable) of Davidsons during the Abrahams era. They have now vanished from the phone book, which is not surprising as Joe jr would now be well past retiring age. That, of course, confirms P-S claim to be the only remaining UK maker.
This is where glasshouse pots differ from most if not all other clay-based articles. Because they are so thick and large it would be risky in the extreme to fire them, cool them down and then re-heat them when required for use. It would also make no economic sense as they need to be hot anyway. Careful, slow heating up to 1000 deg C or more (we took eight or ten days, as I recall) does the double job of firing them and preparing them for use.
Although I have no personal experience of them, I believe in single or possibly two pot furnaces the furnace is simply cooled down and then re-heated with the new pot(s) inside. In cases like ours, where ten or twelve pot furnaces were involved, the pots were heated in a small furnace and transferred hot to the main furnace. The small furnace was called a 'pot arch' (no, I don't know why) and the transfer was called 'pot setting'. The latter could be fairly simple, if very hot, if the old pot came out in one piece. If bits of it were left behind they would be stuck to the furnace floor (the 'siege') by a toffee-like mixture of the glass which had run out of the broken pot and the layer of grog which was often spread under a new pot in the hope of avoiding such problems. If the furnace itself was old (they could run for 25 years continuously) bits of the siege might come up as well making the whole process traumatic and requiring a lot of beer. I have known extreme cases where the furnace front has had to be closed up and left for a day to allow the rubbish to soften before having another attempt.
A pot could last from zero to twelve months. If it was still going after six months we would have a new one in the arch ready, if we were fortunate enough not to have a broken one somewhere else already needing priority. It was not unknown for a pot to come out of the arch cracked and immediately dumped. Sometimes the crack would be undetected and one of the first few fills would 'go downstairs', i.e. find its way through the furnace to the dedicated glass 'pockets' for melting out later. Most pots of course survived and eventually failed due to wear, usually in the form of a 'shot-hole'. Glass often dissolved the pot in a sort of shallow honeycomb effect and eventually one hollow would go through. If it was at or near the surface the pot might be allowed to limp on, short filling if necessary, until it could be replaced.
By the way.
There are exceptions - pots need not be made from clay. Lemington Glassworks used to have two open pots, each about the size of a domestic bucket, used for very special technical glasses and made of platinum. Platinum is a wonderful material for contact with molten glass as it is entirely unaffected and is used for a number of purposes in the higher-tech parts of the glass industry. Two disadvantages - firstly it is a bit pricey for large scale use and secondly it has little mechanical strength - not much better than lead. Strength can be considerably improved by alloying, up to around 10% I think, with something like rhodium. Sadly, when I last looked, rhodium was around four times the price of platinum . . .!