Topic: Exploding glassmaking myths - Roman Diatreta or Cage Cups

[aa]

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!

[Div]

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!

[Frank]

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?

[Div]

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. 

[Frank]

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'?

[Div]

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.