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Glass Identification - Post here for all ID requests => Glass => Topic started by: Adam on July 19, 2006, 10:28:15 AM

Title: Glasshouse Pots
Post by: Adam on July 19, 2006, 10:28:15 AM
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,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 “open”, like a bucket or “closed” or “covered”, 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 6” or more thick and the sides/tops 3” 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)

Clay Preparation

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.

Life etc.

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 . . .!

Adam D.
Title: Glasshouse Pots
Post by: Anne E.B. on July 19, 2006, 10:56:22 AM
Wonderful article Adam 8)  I really enjoyed reading it. Thank you so much for sharing.
Much respect (
Title: Glasshouse Pots
Post by: Glen on July 19, 2006, 10:57:29 AM
Fascinating. Amazing. I don't know quite what to say, but THANK YOU, ADAM, is possibly going to have to suffice. To have an insight into this little known aspect of the glass industry (well, it certainly was little known to me) is wonderful.


Title: Glasshouse Pots
Post by: Lustrousstone on July 19, 2006, 11:41:37 AM
Thirded  :D Thank you
Title: Archive Candidate
Post by: Frank on July 19, 2006, 12:05:31 PM

I was right to push you on this :D  Most of the accounts I have read are in Glass Tech books and of course written in a technical manner as well at at least ten times as long. Your from the hip account is accessable to collectors and full of vitality, first class job.

Thank you.
Title: Glasshouse Pots
Post by: pamela on July 19, 2006, 02:12:27 PM
Impressive and a pleasure to read - THANK YOU, Adam  :D
Title: Glasshouse Pots
Post by: chopin-liszt on July 19, 2006, 03:53:36 PM

Thank you so much, Adam!
An absolute joy!
Title: Re: Glasshouse Pots
Post by: flying free on June 29, 2012, 08:50:35 PM
I'm bumping this thread because it is a brilliant and very interesting description of the process.  I'd not seen this before as it was posted long before I became a member and  I'm sure there must be other newer members also, who will find this incredibly interesting to read.
Thank you  :)
Title: Re: Glasshouse Pots
Post by: Anne on June 29, 2012, 09:15:25 PM
I've set it sticky so it will stay at the top of the forum.
Title: Re: Glasshouse Pots
Post by: flying free on June 29, 2012, 09:17:15 PM
Thanks Anne  :)
Title: Re: Glasshouse Pots
Post by: aa on July 01, 2012, 08:30:26 PM
Adam, I have a feeling it was Joe McCarthy rather than McCartney. Anyway, this link looks interesting.
Title: Re: Glasshouse Pots
Post by: David E on September 28, 2012, 08:52:59 AM
A brilliant article that I have only just come across - thanks Adam D!  :D 8) Missed this first time around in 2006, so thanks for "stickering" it.

Pot making is quite an art, but the very last para on platinum coated pots is of particular interest and something Chance Brothers also experimented with to achieve a better quality optical glass. I'm going to have to read this through carefully as it might be in my interest to extract certain sections for a future book. Acknowledged as always.
Title: Re: Glasshouse Pots
Post by: David E on September 28, 2012, 09:03:49 AM
To take Adam's article a little further, I found the link to PSR Parkinson Spencer Refactories here ( - several downloads but, frustratingly, the one on glasshouse pots is a broken link. I have asked the company to reinstate this, so do check later.
Title: Re: Glasshouse Pots
Post by: ian.macky on November 27, 2012, 11:29:12 PM
FYI y'all, I transcribed Apsley Pellat's 1849 book "Curiosities of Glass Making" recently.  The chapter on glass-house pots ( is an interesting read.  Actually, all of it is interesting.  --ian
Title: Re: Glasshouse Pots
Post by: Lustrousstone on November 28, 2012, 07:08:55 AM
Thanks Ian!  ;D
Title: Re: Glasshouse Pots
Post by: flying free on November 28, 2012, 08:48:39 AM
I had a look yesterday and didn' t realise I could click on the page numbers and read it all!  That's brilliant, thanks so much.
Reading about the pots, I found it quite scary when the article discusses if the pot gets a crack or fissure in it and how they patched and repaired them.  The thought of all that boiling metal pouring out if it burst  :o
Title: Re: Glasshouse Pots
Post by: Frank on November 28, 2012, 09:12:21 AM
Did you not download the video on SG?     Portobello - The Last Tapping (Video)   you can only download if registered, but still images in article may be enough for you. Video is a very large download.
Title: Re: Glasshouse Pots
Post by: Frank on November 28, 2012, 09:26:43 AM
Pot setting at Caithness Glass is shown in a series of pictures in Reflections 3 which can be seen on Scotlands Glass

LOCATION: Artists & Makers >> Caithness Glass >> Reflections 3 - 1987
Title: Re: Glasshouse Pots
Post by: David E on November 28, 2012, 10:38:57 AM
Actually, all of it is interesting.  --ian
And don't I know it! Wished I had a hardcopy myself, but this is definitely the next best thing. Many thanks.
Title: Re: Glasshouse Pots
Post by: Lustrousstone on November 28, 2012, 10:59:18 AM
I think I may know where there is one...
Title: Re: Glasshouse Pots
Post by: chopin-liszt on November 28, 2012, 11:31:57 AM
And we also know that Caithness has been having dreadful problems with their pots recently, Christine - they told me so when I was there just before we saw you in Crieff.
They got through more than they should have, (was it 5 rather than 3 last year?) they've been patching things up and doing their level best to carry on, but it's a really, really serious problem - and contributing an awful lot extra to the already appalling expense of glassmaking.
Title: Re: Glasshouse Pots
Post by: David E on November 28, 2012, 01:47:40 PM
It seems to intimate that they make their own pots? Otherwise, I wonder why they are having so much trouble. It must be such a palaver and a kerfuffle changing the pots - there are enough videos and images to show what was involved. Not for the fainthearted! Pot-making was something each of the larger glass factories used to make themselves and a very skilled job it was too, but nowadays I imagine they are bought in.
Title: Re: Glasshouse Pots
Post by: chopin-liszt on November 28, 2012, 02:08:01 PM
I don't think so, David. The facilities are not that big in Crieff!
I was trying to glean as much info as possible in the 30 seconds before the shop shut, from a very knowledgable and helpful member of staff. I think I'll have to go back and get it again, when I'm not so much under pressure to remember everything that my brain did its collander thing...
What I do remember is that it is causing truly serious financial stress on Caithness and their ability to stay viable. The pots they've had simply haven't lasted nearly as long as they should have.
Title: Re: Glasshouse Pots
Post by: ian.macky on November 28, 2012, 03:30:15 PM
And don't I know it! Wished I had a hardcopy myself, but this is definitely the next best thing. Many thanks.

David, I have the 1968 Ceramic Book Company reprint, which was relatively affordable (US$50).  Wish I had an original, but nyet.  If looking for a copy (or any book), I recommend (, the best meta book-search engine I know of.

Title: Re: Glasshouse Pots
Post by: David E on November 28, 2012, 04:15:08 PM
Yes, I do use bookfinder occasionally, and abebooks is also a very good source. One day all these rare books will be on the net but, until then, we'll have to sit around with our legs crossed.
Title: Re: Glasshouse Pots
Post by: Frank on November 29, 2012, 04:24:22 PM
I have the original, wonder what that fetches now!
Title: Re: Glasshouse Pots
Post by: sph@ngw on January 07, 2013, 05:05:30 PM
can I just correctone small factual error in Adam's otherwise excellent post about pots, as someone inbolved in pot buying and use.
You do not "allow the furnace to cool, remove the old pot and heat it up again"!
A furnace has to be cooled slowly so as not to damage the refractories ( bricks, etc), so it would take three weeks to cool down and three weeks to heat up!
So you do a 24 hour pot set, by removing the bricked up front of the furnace taking out the old hot pot ( on a gun carriage or shielded fork lift truck) and replace a pre heated new pot and the bricking up again.
About 3 to 5 hours for a team of 5 guys who loose about 1/2 a stone (7lbs in weight0 through sweat and exercise.
At Nazeing Glass we got up to 18 furnaces ( single for of 750kgs a day). We now have four and two 100kg studio pots, but not all are in use!
Title: Re: Glasshouse Pots
Post by: Fuhrman Glass on January 08, 2013, 02:27:09 AM
I think several of the factories are getting their pots from a French manufaturer. I know my friend Tim Mosser at Mosser Glass here in the US told me last year that was the only place he could get them. and they are extremely expensive. there are several manufacturers here in the US but do make the larger sizes as they are very difficult to produce.
I also know that the Chinese are making their own but have a hard time getting the quality materials they need to make them with.
Title: Re: Glasshouse Pots
Post by: sph@ngw on April 10, 2013, 02:51:55 PM
For those of you who visit Northern France, there is an excellent museum "Glass vallee Museum at Blangy sur bres;e, the heart of the French bottle glass industry ( over 80% of the world's perfume bottles are made there by about four companies( or their overseas subsidiaries). La Glass Valle show pot making in detail and several jockey pots for small colour runs.
You can also do a "concave" that means blowing cold air on the crack in the pot ( if accessible ) to cool the glass, like stopping a wound with congealed blood! This can sometimes last a few weeks....
Not pot makers do not gurantee their pots, so if it cracks on coming out of teh pot arch, they assume it must be your fault for poor preheating!
Some pots last 10 melts others over 100 melts. At over !1,00 each it makes a big difference to your costs!
Title: Re: Glasshouse Pots
Post by: sph@ngw on April 10, 2013, 03:48:31 PM
"I think several of the factories are getting their pots from a French manufaturer"
This is probably Rieux Refractaires in the Oise region, who claim to supply Lalique, Daum, St Louis etc, in France.They are expensive but good I believe.
I cannot find their website!
Title: Re: Glasshouse Pots
Post by: cagney on January 31, 2015, 07:00:51 PM
old stereo view of glasspots being made circa 1903
Title: Re: Glasshouse Pots
Post by: sph@ngw on February 27, 2020, 04:59:19 PM
Wonderful article Adam 8)  I really enjoyed reading it. Thank you so much for sharing.
Much respect (

Sadly Parkinson Spencer no longer make pots, so they have to be bought in France or Germany.T hey are made to order, take 6 months to make by hand and deliver and need 3 months to  dry and each glass manufacturer has his own design and capacity. WE use 750 kg and fill them to 700 kg max,
Title: Re: Glasshouse Pots
Post by: chopin-liszt on February 27, 2020, 06:03:34 PM
This is really interesting - I've been after "the capacity of a pot" for some time now!
Do you know if your 700kg (in a 750kg pot - it's very sensible not to overfill, I reckon!) is a fairly normal sort of capacity?
It is specifically the size of one of the original pots used at Mdina I'd really like to know about, I can't imagine you'd know that, but might you have a rough idea? :)