Glass Message Board
Glass Identification - Post here for all ID requests => Glass => Topic started by: Frank on March 05, 2005, 05:29:14 PM
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As mentioned in a earlier thread here is the link to how diatreta or cage cups were made. For years it has been accepted that these were made by carving a solid thick walled blank.
http://www.rosemarie-lierke.de/English/Cage_Cups/cage_cups.html
see also http://www.pressglas-korrespondenz.de/aktuelles/pdf/pk-2004-1w-lierke-diatrete.pdf for lots of images with German text
For the myth (Or is it :? ) here is a link at Corning Museum of Glass
http://www.cmog.org/index.asp?pageId=756 note that they state this was decided in the 1960's.
There was also a reproduction made in the 70's or 80's by a Scottish glass artist I will try and find that link too. He used the casting method as too, have several others who have made cage cups since. I suppose the next stage will be to scan an original in 3D produce a metal or plastic copy on a 3D printer and use that to create a mould. The you will get an almost indistinguishable copy of the real thing :wink:
The methods used by Barry Sautner http://selman.com/sautner.html are very different but he uses more recent sand-blasting technology,
July 23, 2004
Roman Glass Bowl Sets World Record in London
An extraordinary and fragile Roman glass bowl, dating from circa A.D. 300, broke the world record price for a piece of glass sold at auction July 14th in London. Selling to a telephone bidder at Bonhams' Sale of Highly Important Antiquities, The Constable-Maxwell Cage-Cup fetched £2,646,650. It was once used as a lamp and had been carved from a solid piece of glass.
Following the sale, Joanna van der Lande, head of antiquities at Bonhams, said: "The Cage-Cup is exceptionally fragile, cut from a single block of glass. It would have been clear but has become iridescent due to a reaction between the earth it was buried in and the glass. Its probable use was as an oil lamp suspended by a collar around the rim. It's really a very highly prized piece."
The cup was last sold in 1997 as part of a collection formed for investment purposes by the British Rail Pension Fund. At that time, the cup sold for £2,311,500.
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Thank's for the details and the links, Frank.
I am now having recollections of reading something before about the "questions put to a proponent of the cutting theory". I had forgotten about that. Most of what is said in Rosemarie Lierke's 2004 webpage article makes good sense to me. I can therefore accept the "two-shelled blank" explanation. The parts that I don't quite follow are probably because I don't properly appreciate how the outer wall network was formed in order for parts to be removed at the correct positions.
But even if the two-shelled approach was the method used by the Romans, it still seems clear that the outer cage and the support struts were finished by cutting, including some possible undercutting.
In the thread on "Paperweights" my analogy to the Chinese "balls-in-balls" can now be said "not to hold water" (both figuratively and physically :) ). And yes, the actual technique used for Barry Sautners's work is somewhat different.
What I still find intriguing, though, is the (apparent or real) lack of acceptance by some authorities of anything but the "cut thick-wall" process. I assume this is the reason for Bonhams blurb to stick to the long-standing line. And it perhaps also explains why the Glass Cone article, which was based on much of the Bonhams commentary, and focussing on the "cup vs lamp" idea, gave no alternative views on the procedure.
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If anyone gets a chance to visit the Glass Musem at Jelenia Gora in Poland, they have a very nice example of a modern reproduction of a "cage cup", but unfortunately I did not get any information on it when I was there.
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What I still find intriguing, though, is the (apparent or real) lack of acceptance by some authorities of anything but the "cut thick-wall" process.
What is missing here is the discussion that set the cut thick-wall process. If anyone can track that down at least we can see details of both arguments.
It is not uncommon in Academia for theories to persist despite later eveidence. This is often caused by conservatism rejecting the new kid on the block versus the aged and respected professor of the original concept (general terms, not specific to this case) particularly in anything 'scientific'.
At another level, they are both theories and could be completely wrong.
Certainly for an auction house there is more more marketing value in some guy spending twenty years scratching away the the glass with a small diamond on the end of a stick, than knocked out in a mould. It is a question of awe :shock:
If you read all of the material you will find that these cage cups were expensive but not rare items in Roman times. Probably much cheaper than the equivalents today in relative terms. Some fifty whole or partial cage cups have survived.
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Incomplete discussion :?: :?: :?: Polish data, German info
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http://www.cmog.org/collection/detail.php?t=objects&type=browse&f=maker&s=Scott%2C+George+D.&record=0
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Dear Frank,
there's no doubt whatsoever that late Roman cage cups WERE carved from thick glass blanks. Lierke's preposterous idea that they are made by squeezing hot glass through a colander-like mould of plaster is absolutely risible, like most of her impractical theories. If you can, do try and examine one of the Roman originals up close, and you will clearly see the marks of grinding and polishing wheels where they have carefully carved away the walls, leaving the cage supposrted on the glass pegs. I'm afraid it's definitely all one piece of glass. I have examined many originals, and there are several of us in the world who actually MAKE cage cups for a living - we'll all tell you the same thing!
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Who are you ?
I don't usually look in this forum, it is Angela's Scottish forum, my forums moved to Scotland's Glass - not sure why this Roman topic is in here though the Scots kept the Romans out.
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Probably here because of this:
There was also a reproduction made in the 70's or 80's by a Scottish glass artist I will try and find that link too. He used the casting method as too, have several others who have made cage cups since. I suppose the next stage will be to scan an original in 3D produce a metal or plastic copy on a 3D printer and use that to create a mould. The you will get an almost indistinguishable copy of the real thing :wink:
George D Scott. see link in to Corning above.
:)
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Dear Frank,
I'm afraid I don't know why this topic's on the Scottish Glass page either! I found the reference by typing in 'cage cup', to see if anyone was discussing diatretus work, and replied to you post!
I'm David Hill - one half of Roman Glassmakers (www.romanglasmakers.co.uk) We specialise in researching and reproducing glass from early periods, and we've recently begun working on cage cups, and the related series of 'diatretus' vessels. Cage cups have always excited folks' imagination because it's hard to accept that anyone can have the patience to create a vessel with an attached 'exo-skeleton' by carving from a thick blank, but believe me, there are very few shortcuts, and they're certainly not made by the 'plunger and colander mould' method that has been suggested elsewhere! The late George Scott (of Edinburgh) made his excellent reproductions using small copper wheels and different grades of abrasives, with a direct drive machine, whereas I use a Merker glass lathe, copper, stone and many other wheels, as well as other tools, including much hand finishing. Attached is a picture of me working on a reproduction of the Munich cage cup. As you can see from the piccie, these vessels are not as big as the impression one often gets from pictures in books. Apart from two notable whoppers, the Hohenzuelzen bowl (destroyed 1945) and the so-called 'Constable Maxwell cup', most of the surviving examples are not much bigger than large coffee cups. Although they do take a time to carve, they are very satisfying to create - precisely because of, not despite, their difficulty! Best wishes,
David
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As this is more general GLASS than Scottish Glass I'll move it over to the proper place.
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Here's the correct link http://www.romanglassmakers.co.uk/ the other one is missing an "s".
Dave, I've seen your amazing work at one of the glass fairs. I was recently explaining the annealing process to students on one of my courses and the subject of how the Romans worked out how to anneal glass came up. One of those not so rare moments when I didn't have an answer! Any ideas? I looked at your site and saw your furnace and annnealing oven. Did you just allow the glass to cool naturally within the oven or did you try to control it to any degree?
Btw Cage cups are unbelievable, whichever way you look at it and the skill, patience and dedication required only confirms my theory that you have to be slightly mad to be a glassmaker! ;D
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Dear aa,
thanks for supplying the correct website address for www.romanglassmakers.co.uk ! What a clot - fancy getting our own web address wrong.
Trying to judge the correct annealing temperature in the primitive lehr 'by eye' was undoubtedly one of the most difficult tasks we set ourselves during the wood-fired furnace project. Naturally we had an array of thermocouples and temperature readers (and we recorded temperatures at points all over the furnaces and lehr every half hour, day and night), so although we could easily maintain the temperatures we knew that we needed for annealing with these modern aids, we were flying fairly blind. We suspect that they used something like a thin rod of glass suspended between two points in the lehr, and kept an eye on whether or not it sagged at all, rather like the little cones potters use nowadays. We tried annealing without checking the temperatures, but it was hard to be sure whether it was accurate or not. I suspect they made sure their lehr slaves (probably children) just kept the lehr ticking over, and that at the end of the working day, they just closed the doors on it and allowed the fire below to go out naturally overnight. We checked every piece of the glass that was made (hundreds of items, in fact), with a strain viewer, and most items were annealed properly, in fact. We re-annealed most items in the modern lehr in the workshop just to be sure anyway!
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Vasart's annealing oven was primitive with the glass being piled on milk crates which were just pushed in, as they reached the other end they were removed. They also achieved better annealing than in Monart days.
What tools did the Roman use to carve the glass?
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I believe that the chaps who engraved the glass in Roman times were probably not glassmakers themselves, though they must have worked closely with them. These 'diatretarii' were probably the same folk as the gem-cutters, and were certainly using similar mechanical cutting devices. The tool marks are all over the vessels themselves. There are countless surviving small oval gemstones, usually signet rings, which have clearly been engraved using tiny wheels, almost certainly of stone, possibly with a gritty abrasive to perform harder work. George Scott cut his cage cups using hand-made copper wheels, and Josef Welzel in Germany uses copper, as well as diamond ones, but on a glass lathe, as I do. The Roman cutters certainly had a system that was at least as good as we have today, but they must have been even more patient (and careful) than we tend to be. Scott noticed that the areas surrounding the supporting posts of the cage show the profiles of the wheels used, and this can be seen more clearly on the broken areas of some of the fragments. I'll post below a photo I took holding a fragment in the British Museum, showing the profile of one of the 'raised' letters of the inscription. You can see the main 'letter-box' cut of a larger sized wheel (about a centimeter in diameter - not exactly huge!), and some finer cuts of a different, smaller wheel with a rounder profile at the sides, used just to get into the tricky areas and tidy it up a bit. It is just possible that they could have used diamonds for their cutting, since they certainly used single diamonds for the 'point-engraved' glass vessel work at this time, but whether they could have mounted lots of fine diamonds into a wheel, as we do today, I can't say, though I think it's unlikely. It is a big leap technologically, and they were very innovative chaps, but I wouldn't like to go that far! I'm sure that they used small, very hard stone wheels on a good, well-balanced lathe, fed with plenty of water at all times to prevent the glass overheating and cracking. The important thing is that the vessel is offered/held to the wheels, not the wheel brought to the vessel.
As well as wheels, they would have used hard wooden sticks with grades of abrasives (such as pumice) to polish back the shine of the glass where it had been cut and abraded. Again, Scott noticed and recorded the fine scratches caused by this process, and was able to repeat them with his own polishing technique.
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Interesting, thank you. I had not been aware that they had such sophisticated mechanical tools then.
... they must have been even more patient (and careful) than we tend to be...
How does that show?
The important thing is that the vessel is offered/held to the wheels, not the wheel brought to the vessel.
Is that because it is less likely to 'move'?
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Hi Frank,
sorry not to attach the picture of cage cup detail last night - message board wouldn't let me for some reason! I'll try again tonight.
To try to answer your questions: "... they must have been even more patient (and careful) than we tend to be... How does that show?"
Well, I think that it shows in the fact that the appearance of the 'scratch marks' left by the wheels that they were using show that they were small stone wheels - the abrasive lines are consistent with stone (Much cleverer folk than us have looked at these marks under microscopes, and done tests to replicate what they might have been done with!). Stone, when water-fed, will abrade the glass easily (we use a soda-lime recipe based directly on analyses of glass recipes used by the Romans - it's not exactly the same as modern soda lime), BUT - it takes absolutely ages! This is why some have suggested that they might have used diamonds, because diamonds certainly make light work of cutting away areas. As to the machines used - yes, they certainly had something sophisticated, perhaps like a 'modern' glass lathe. We know this from the vast amount of Roman glass with wheel-cut work. Some of the wheels used are small - about the size of a modern penny, but others were about five or six inches in diameter, since the deep (12mm) cut ring on a cage cup (below the frieze with the inscription) is one of the first things to carve away, using a stone wheel.
We've never tried to work out what an 'authentic' glass lathe would have looked like, but the most important thing is that the shaft that holds the wheel is true and rigid. They were probably rather like the foot-operated 18th/19th century intaglio lathes - small table-top machines.