mercoledì 13 marzo 2013

Modelling the glass panes

What is the shape of a glass pane? 

The glass panes have different shapes and thickness according to the production methods used during the ages: handmade flat glass, bull's eye, plated glass, cathedral glass, float glass...

However, the most common panes for stained windows, realistically used also during the Middle Ages, are made of handmade glass, generally produced by the cylinder method. (this technique involves blowing a big glass cylinder - muff [2]- and then cutting it in order to obtain a rectangle to be shaped accordingly to the figures of the window). Therefore the aspect of an ancient glass pane is that of an almost flat plane, with a thickness that can realistically be up to 5 mm [1].

Method for modelling the glass panes 

Modelling every glass pane one by one would be very time requesting. Therefore a different method seems to be working well. It consists in creating an ideal glass pane of the dimension of the entire window, 5 mm thick, and then casting out (via proboolean tools, subtraction and cookies off) the single glass panes, substantially deleting the part occupied by the lead cames. Each single pane then to be detached and modified according to the requested colour, map (in case of grisailled glass), thickness or texture to generate single variations.

To recreate the Middle Ages supposed shapes, the lead sections used for casting are visible in figure 1, numbers 1-5 (fig 1). When using the more realistic lead sections (fig.1, numbers 4-5), the casting leds to a curious glass pane border, with a rounded shape. Obviously, this is not the appearance of a glass cut for stained windows! Actually, the glass is cut very precisely (typically a oil filled glass cutter), but in Middle Ages a very imprecise tool was used, called grozing iron [2], that caused the edge of the glass to be very irregular. However, in both cases, the border of glass will not be visible in the virtual model, because it is under the lead leaf. Therefore the simplest way should be chosen, and a special set of lead sections (called “simplified lead cames” in fig. 1, numbers 2-3) be used in order to virtually cast the glass and obtain a simple edge. In case one of such glass panes would be used to zoom in the border details, a simulated grozing iron border will be created on purpose. 

The putty problem 

Putty is commonly inserted between lead came and the glass edge, in order to prevent movements of glass and to increase the impermeability of the window. This object is however hidden in a real window, unless the lead is very deteriorated. Therefore even in this case, it will be not modelled in the virtual one, unless specially needed.  

1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed.
2. Corpus Vitrearum Medii Aevi glossary

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