Task

The reproduction of the Cologne Toen Hall prophets

Since around 1430, the eight town hall prophets have adorned the seat of the Cologne council. With their banners, the wooden figures were intended to remind the authorities of their obligations towards the common good. The figures were originally created for the prophets’ chamber in the historic town hall, named after them, which every councillor had to pass through on his way to the council. Until 2012, they found their place in the Hansa Hall, but then they had to leave the City Hall and move to the Schnütgen Museum. The reason: the fluctuations in temperature and humidity of the centrally heated rooms in the town hall would have endangered the preservation of the sculptures.

However, the figures were soon missed in their original place. To ensure that the Hansa Hall would no longer be without its ensemble of prophets, the City Hall prophets were to be digitised on the initiative of Mayor Henriette Reker and find their way back into Cologne City Hall as true-to-detail replicas. The company fröbus was commissioned with this task. The WDR reported:

The 3D model which formed the basis for the print can of course also be viewed. Simply click on the image and you will be taken to the 3D viewer with the town hall prophet.

Digitisation

3D-Digitalisierung der Figuren

In order to preserve the medieval figures during digitisation and at the same time achieve the highest possible geometric resolution, the non-contact photogrammetry method was used. With this method, sufficient photographic images of the real object are taken from different angles, which are then interpreted as a pool of images by special programmes. Based on this data, a three-dimensional point cloud is then calculated, the digital master. From this, the 3D geometry – the so-called mesh – is finally reconstructed in almost any resolution and simultaneously provided with a true-colour surface texture based on the image material.

The mesh can then be used as the basis for 3D printing. At the same time, the digital copy can be viewed from any angle and at any scale. This enables an exact comparison of the replica with the original.

3D-Druck

Production of the original sculpture

The plastic replica of the prophets was made using the 3D printing process. For this purpose, the digital geometry data was transferred to an appropriately large-format filament printer, which built up the figures layer by layer from plastic. A honeycomb-like structure inside, also printed, stabilises the outer shell of the figure’s raw body.

The extreme detail of the digitised figures resulted in very large amounts of data and made the project a technical challenge in the field of three-dimensional replica production. Up to 130 hours of printing time were needed to produce a sculpture with a height of 1.20 metres. After the printing process is completed, the sculpture is combined with a certified epoxy resin to form a composite material – a perfect combination in terms of strength, surface finish and safety.

Painting

Precise colour reconstruction of the 3D sculpture by master hand

Absolute colour authenticity cannot yet be achieved in 3D printing. Hans Wäckerlin, a painter from the fröbus network, is ahead of the technology in this case. He knows the working methods of the old masters and has enough experience to transform the unpainted raw bodies into replicas true to the original. In this artisanal process, different layers of paint and glaze are applied to reproduce the colouring of the original as closely as possible.

To ensure that the original sculptures do not have to leave their safe place in the museum, large-format and colour-accurate proofs serve as a reference when matching the colour. These printouts can be created directly from the digital master. Experienced image editors from fröbus digitally match the colour of the printouts to the original on site.

Exhibition

The town hall prophets return to their place

The first prophets from the printer have already taken the place of their predecessors in the town hall one month after the start of the project. In spring 2019, the group of eight prophets will then be reunited in the Hansa Hall. While the approx. 600-year-old oak sculptures can remain safely in the Museum Schnütgen under optimal climatic conditions, thanks to digitisation their true-to-the-original plastic counterparts will once again fulfil their original task in the Hansa Hall: to exhort good governance in the Cologne Council with wise advice on their banners, such as “… not to be too loquacious”.

Here, once again, the entire project in an overblock:

This article is also available in: German French

Zitat Frank Bayerl: "Die digitale Erfassung des kulturellen Erbes ist bei uns deshalb genau in den richtigen Händen, weil wir, aus der „schwarzen Kunst“ kommend, mit Streifenlichtscanverfahren, Photogrammetrie, Farbmanagement, Datenbanken und der Präsentation der Assets professionell seit Jahrzehnten umgehen."