Modelling a rondel - "rullo veneziano" type

What is a rondel? 

In general, a roundel is a glass pane with a round shape. The simplest method to create it is to cut a bigger glass pane to a round shape. However, this method is not very interesting from the point of view of 3d modelling. 

The second and most interesting method is to create a new glass pane after a proper modelling, each piece one by one. This second production technique [8] is used in the typical Venetian "rulli" or "rui" and it is a very ancient technique. Just as an example, a picture of this kind of glass pane refers it as dated to the sixth or seventh century [1]. 

The method is very similar to the one used to produce crown glass [9], but it is used to create singular small panes (about 15 cm diameter) instead of a big sheet of glass to be cut in regular pieces with a grozing iron. 

How to model a "rullo veneziano"?

A simple method consists to create a line drawing the half section of the rondel and then use the "lethe" modifier to create the 3D object. To have a general idea of the shape it is important to have a picture as reference. The ideal situation would be to have also a proper section of the rondel, but, to obtain it, a broken glass should be necessary! Since in my project I have no access to such material, the thickness I will choose is an hypothetical one. 

However, some features can be reproduced properly. For example, it is important to give a "broken aspect" to the pontil mark (i.e the exterior part of the bull's eye, where the glass was detached from the pontil). On the opposite side, where the glass is smoothed, the model should be in accordance. 

The edge should also be thicker then the surrounding glass, because the method involves a particular attention to it. The image [1] suggests that a U bended sheet edge is present, even if the picture is not very clear. It is likely the the edge is not bended, but just thicker then the rest. 

From the video, it is evident a certain attention to the edge, but its cross section is obviously invisible. If the edge bends, it is supposed to bend in the direction of the pontil mark, because it would be the most comfortable direction according to the position of the glass-maker when creating the rondel. 

However, in case a proper section of such kind of glass becomes available, and if a close up is needed (remember that the edge is covered by the lead!) the cross section should be shaped accordingly.
 

 

fig. 4
half section of the genuine rondel (grid = 1 mm)

fig. 5 
3D model of the rondel with pontil mark in evidence  (grid = 1 mm)

fig. 6 
3D model of the rondel on the opposite side  (grid = 1 mm)

References

1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed.
8. production of a rondel at Lamberts - 
http://youtu.be/TuDl27MZDnY
9. production of a crown glass, as shown in Alan Macfarlane Youtube channel -
http://youtu.be/AYzrBIv0VbU