martedì 12 marzo 2013

Starting to model a 3d glass window: the lead cames

What is a stained glass window made of?

 
A stained window is a complex object, made of different materials. Strictly speaking: the glass panes, the lead cames (that join the glass panes together) and the ferramenta (the last one providing support to sustain the glass plane, that otherwise would be very weak, due to the weight of glass and lack of strength of the lead). Each feature needs to be modelled separately. 

What is the structure of a lead came? 

 

A came is a grooved strip of lead. Its section is generally shaped as a "H" letter, with two glass panes supposed to fit in the apertures (see a selection in fig. 1, grid = 1 mm). The "leaf" is the part that is always visible in a stained window and it is in direct contact with the faces of the glass panes. The "heart" is the part that is commonly invisible. It is in direct contact with the borders of the glass panes. The shape of the leaf can be round, in the case of panes used for round lines, or flat, if used for straight lines.  

Method for modelling the lead cames?


The more efficient method for modelling the cames of a stained window seems to be to create a shape (with the cross section of the lead came) and then loft it through a spline with the appropriate shape of the lead cames. To avoid intersection of lines, many different separated splines are supposed to be created. When sharp angles are obtained, they are supposed to be sliced, like in the real word, in order to avoid unrealistic intersections. 

Which lead section should be chosen for my model? 

The section of the came is variable. In particular it changes according to:
- technical problems (round cames are good for round lines, because they bend accordingly to the shape without wrinkling, while flat lines are used for straight lines, wideheart cames are used for plated glass, etc etc);
- the historical period of the window (for example, during the Middle Ages, very thin lead cames were in use);
- restoration needs (releading is a common restoration method);
- artistic needs (it means changing the thickness of the “black line” of the edges of panes according to the final aesthetic result)

So, which lead section shall I chose for my model? To answer this question is important to decide what is the aim of the project itself. It could be to create a model of an existing window in order to keep knowledge of its actual conservation state, and then visualise an hypothetical model of how the window should have look like when it was built. In this case, at least two different kinds of cames would be needed: the actual cames (generally introduced during a releading in previous restorations) and the original (or supposed to be original) ones. In my opinion, since only small Middle Ages windows are nowadays visible in their original appearance, due to corrosion and multiple releadings, this special period could be the most interesting to visualise.

 

Middle Ages versus Actual lead cames


Reference [1] states that during Middle Ages (fig 1, numbers 1-5) very thin leads were in use, up to 3 mm in leafwidth, while the standard leaf used in nowadays restoration is 6 mm (fig. 1, numbers 8-9). Middle Age used even 1.5 mm narrow lead cames for mending leads (leads used to compose a broken pane - it is the old way ancient restorators used to link broken glass, when silicone or other resins did not exist yet). Nowadays, if the aim of the restoration is to maintain this special feature, mending leads are 3 mm (fig. 1, numbers 6-7). 

The other dimensions (thickness of the leaf, of the hearth, depth of the heart) were stated because, so far, no other references were found on the subject. In particular, thickness of the leaf was stated to 1 mm for everything. Hearth thickness was stated to 1.2 mm when used for nowadays cames and decreased to 1 mm when used for the Middle Ages cames. The depth of the hearth was maintained at 6 mm for every section, in order to allocate glass panes that, in Middle Ages, could have been up to 5 mm thick, according to [1]. However, if visualising plated glass (i.e. two glass panes coupled in the same slot), at least 12 mm will be necessary.


fig. 1 Sections used in the modelling of the lead cames, 1 mm grid.

In details:
1- Middle Age mending lead.
2- Middle Age round came, simplified for modelling.
3- Middle Age flat came, simplified for modelling  
4- Middle Age round came, original.
5- Middle Age flat came, original.
6- Nowadays mending lead, round.
7- Nowadays mending lead, flat.
8- Nowadays round lead.
9- Nowadays flat lead.

Reference
1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed.

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