Barometers Realm

One of the key features of this piece is the hanekam (cockscomb) ornament—rhythmically arranged glass droplets decorating both sides of the body. Glassblowers applied these droplets using a metal rod, picking up a drop of molten glass and carefully placing it along the surface, creating a decorative relief.

Take note of the flattened decorative finial beneath the reservoir. This is one of the characteristic features of 18th-century donderglazen. Later examples usually have a smooth, spherical shape.

Another intriguing detail of this thunder glass is the empirical scale engraved on the spout. To observe changes in liquid levels, straight divisions numbered from 1 to 12 are marked from bottom to top along the spout. This scale likely made the instrument more practical, though it does not represent a conventional measurement system with defined units.

A truly unique feature of this piece is its brass top. Not only does it enhance the glass’s aesthetic appeal, giving it a distinctive character, but it also serves a practical fastening function. The brass rim with decorative cutouts in the form of stylized Gothic elements lends the piece particular elegance, while the heavy suspension loop suggests that this thunder glass was designed to be hung in an interior setting. Such a combination of glass and metal is exceptionally rare, indicating that this is not merely a household object but a masterpiece of craftsmanship.

Typically, thunder glasses are filled with water, the color of which can be chosen at will. The reservoir also contains a fixed amount of air, as it is hermetically sealed except for the spout. When atmospheric pressure rises, the water level in the spout drops, and the air inside the reservoir compresses slightly. Conversely, when pressure falls, the air in the reservoir expands, pushing the water level in the spout upward. In the case of a sudden drop in pressure, such as before an approaching storm, the water level in the spout may rise so high that it dramatically spills over the edge.

This is an early type of thermo-barometer, dating back to the first decades of the 17th century, and it is considered a distinctly Dutch invention. It is known that by 1640, the region around Liège (Luik) was home to a flourishing glassblowing industry, from which donderglazen emerged.

Unfortunately, thunder glasses are also affected by temperature fluctuations. If the temperature rises, the air inside the reservoir expands, causing the water in the spout to rise even if atmospheric pressure remains unchanged. Conversely, cooling causes the water level to drop. Thus, a donderglas functions best in environments with relatively stable temperatures, where changes in water level are primarily influenced by fluctuations in atmospheric pressure.