https://blog.nationalmuseum.ch/en/2021/08/colours-revealed-by-light/
The colour palette of the glass artist: sample frames with around 800 colour nuances.
Painted glass windows have been bringing light and colour into dark church rooms since the 13th century. But how is a glass painting actually made?
Alexander Rechsteiner works at the PR department of the Swiss national museum and holds an M A in modern English literature and political science.
The art of glass painting is already over 1000 years old. It experienced its first heyday in the Gothic period, in the 13th century, when the churches stretched to unprecedent heights and the sunlight streamed into the interior through magnificent glass windows. In addition to the so-called coat of arms or cabinet disks, for which the Swiss Confederation has been known since the late Middle Ages, it is mainly church motifs that can be seen on glass paintings. They tell stories from the Bible in bright colours. Contemporary stained glass can still be found today in sacred or public buildings. The basic idea of glass painting is simple: different coloured pieces of glass are put together to form an overall picture. The work process, which is divided into many steps and carried out by different people, is all the more complex. The manufacturing process has changed little over the centuries. The glass painter does not mix colours, but uses a selection of different coloured glasses that are put together like a mosaic. Special, hand-made glass is used. It is not completely transparent like modern glass, but has a structure of bubbles and folds. The light shining through is refracted several times and the characteristic effect is created. The glass is coloured by adding iron, copper, nickel and other metal compounds.The glass painter uses his own creations as a template or implements designs by other artists. The template is transferred to thick stencil paper and all fields from which the individual glass parts are later created are numbered so that they are not mixed up later. To cut out the stencils, the glass painter uses special scissors, which remove a 1.5-millimeter-wide strip between the individual parts. The lead framework, which is so characteristic of glass paintings, is later placed in this gap.Once the stencils have been cut out, the glass painter is spoiled for choice from over 5000 possible colour nuances. As if from a paint box, he selects the right glass for each part. He does not manufacture the glass himself, but buys it from a glasswork. The stencils and glasses are handed over by the glass painter to the craft glazier, because cutting out the glass parts requires experience and great skill - depending on the work, there are several hundred parts in all possible shapes. A diamond serves as a knife. There is always the risk that a glass will break. Once the pieces of glass have been cut out, they go back to the glass painter, who temporarily fixes them on a glass easel with beeswax. Below is the design drawing. It is visible to the glass painter while he is using a paint made from iron oxide and glass powder (called black solder) to paint subtleties such as facial features, shades and other details on the coloured glasses.The glass parts are now heated to 560 degrees for two and a half hours in the ceramic kiln. In this way, the glass parts do not break and the black solder paint is firmly burned into the surface. In order to achieve additional colour effects, enamel colours are applied to individual parts after firing. The glass painter carries out this process almost blindly, because depending on the composition of the enamel colours and the nature of the glass, the result will be different after firing again.The craft glazier is responsible for the assembly. He places the lead frame around each piece of glass, to which he then soldered the parts together. He fills in empty spaces with putty. The whole plant is then cleaned with sawdust and polished with petroleum. Now the work can be mounted at its destination and the coloured glass window unfolds its full effect in the sunlight - sometimes for centuries.