An empty clock cabinet

Astronomical art clock under revision

Anyone who has wanted to visit the astronomical art clock in the German Watch Museum in Glashütte in the past few weeks had to discover that the monumental clock case is currently empty. The dial with many interesting displays and the clockwork behind it cannot be seen, nor can the distinctive gong, which normally sounds every 15 minutes, be heard. A small sign tells you why. The movement of the art clock is currently undergoing a general overhaul. This is done every 15 to 20 years and takes several weeks to complete. The last revision took place in 2004. This regular process is necessary to ensure the reliable and error-free running of the movement with all its functions over the long term. The fact that the clock has a multifunctional calendar mechanism ensures that the clock does not stop at any time during operation. Subsequent setting of each individual display would be very time-consuming. The clock has been running without major interruptions for 17 years now. This long running time brings with it wear and tear, soiling or sometimes even malfunctions, which should be eliminated with the overhaul.

Removal of the movement of the astronomical art clock at the beginning of the revision © German Watch Museum Glashütte, photo: René Gaens

Creation of the art clock
The astronomical art clock is one of the main attractions in the German Watch Museum Glashütte. It was created between 1892 and 1925 by the master watchmaker Hermann Goertz, who worked in Glashütte from 1918 and also completed his “Great Clock”, as he called it, here. In 1929, the Free State of Saxony bought the valuable clock for 15,000 Reichsmark with funds from the Association for the Support of Artists and donated it to the German School of Watchmaking Glashütte. Since then it has had its place in the foyer of the former school building, which has housed the German Watch Museum Glashütte since 2008.

What happens during the revision?
Supported by the findings of the previous revisions, a plan of the work to be carried out was drawn up before the start. This time it was again determined to document all work steps in detail so that they can be made available for future revisions. At the beginning of the current revision, the movement and the dial were removed from the mount in the wooden clock case. For the first time in 17 years, the movement was visible again. After the movement had been taken to the watchmaker’s workshop and after an intensive study of the overall construction and the functional processes, dismantling began. The removed parts are now examined and checked for signs of wear or damage. If defects are found (e.g. broken jewels, damaged teeth on pinions or wheels), the necessary measures are taken. It is always crucial to proceed as carefully and as true to the original as possible. All parts of the movement are cleaned. If oxidation is found, appropriate preservation measures are taken. In further steps, functional surfaces, such as the running surfaces of the shafts, are elaborately polished. The parts are precisely measured for documentation. The results obtained from this also allow conclusions to be drawn about the functional relationships in the movement. During the subsequent reassembly of the movement, it is now important to perfectly coordinate the interplay of the parts. This ensures the functional processes until the next revision.

Master watchmaker Jürgen Franke mounts the movement on a mounting rack for the examination and the later dismantling © German Watch Museum Glashütte, photo: René Gaens

Previous revisions
The current revision is the fifth since the clock was completed in 1925. The previous overhauls took place in 1938, 1956, 1984 and 2004. In 1956 it was carried out by the Glashütte master watchmaker Alfred Helwig, who also redesigned the display of the moon phase. Instead of the original lunar disc, he used a rotating sphere that now presents the phases of the moon even more clearly. This representation was already favored by Goertz after the clock was completed, but could no longer be realized by himself. In order to design the watch for a very long service life with the greatest possible revision intervals, Hermann Goertz had taken appropriate precautions when designing the movement. The bearings and pivots of the watch were designed in such a way that they ensured that the oil would hold for a long time. The running surfaces are extremely hard and have an excellent polish, which minimizes signs of wear and tear. In addition, the majority of the wheels and levers were designed so that they could be dismantled to allow cleaning and re-gilding of the wheels.

For the first time for 17 years a view on the movement is possible again © German Watch Museum Glashütte

Technical data of the art clock
The watch is a superlative in all respects. The monumental case made of mahogany wood measures 250 x 150 x 50 cm in height, width and depth. The drive mechanism weights for the movement and striking mechanism weigh 12.5 kg and 20 kg, the second pendulum weighs a total of 12 kg. The striking mechanism strikes the number of hours with a double stroke every quarter of an hour and every full hour with a single stroke. The movement and the dial together have 1,756 individual parts; including 122 wheels and pinions, 54 levers and springs, 13 jewels (bearing stones) and two jewel pallets of the lever, 424 screws and nuts, 17 hands, two display discs, three number rings and one display sphere. At that time, around 700 parts were required for the manufacture of the dial alone. A total of 20 pieces of information can be read on the dial, which has eight individual dials for the time, calendar and astronomical displays. This includes hours, minutes, seconds, a perpetual calendar with details of the day, day of the week, month, year, leap year, the equation (equation of time) and the power reserve displays for the movement and striking mechanism. The astronomical displays show the age of the moon, the course of the moon, the moon phase, the times of sunrise and sunset with the mean Glashütte local time. Also the starry sky, which shows the currently visible part of the starry sky over Glashütte and the real position of the sun in relation to the constellations and in the zodiac maps.