Last Updated: Oct 19, 2016 Views: 48
Great question! I contacted experts on Roman glassmaking Mark Taylor and David Hill (http://www.romanglassmakers.co.uk/), who sent this response to your question:
"We have not seen this bottle, except in photographs, but it does look as though it was made by three glassblowers placing three equally-sized parisons, each consisting of an elongated bubble on a long neck, into a mould (probably a two-part wooden mould) and blowing at equal rates. I think there has to be a mould involved as otherwise it would be very difficult to achieve the shape and to make the joins on the outer surface so good.
It then needs to be turned round (puntied? and cracked off from the blowing irons) and finished by one glassblower by working for a short time on each neck in turn until they are all of equal length and circular.
There is a good attempt at this by some Japanese glassblowers in this book:
'Okayama Orient Museum - Glass Art - From History,To Future'
I've copied the relevant photos. The one embellishment is that the glassblower is re-attaching the initial bubbles to the blowing iron so he is able to blow the three segments at the same time - which makes sense.
The other interesting thing is the X-ray photo, which clearly shows the three segments in cross-section."
I am copying the photos sent and attaching them to this response. I hope this helps!