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D143-25 · Miscellaneous
early 18th century
n/a
brass, glass
Object Overview
A richly ornamented Hanekam-decorated donderglas first half of the 18th century — an early Dutch weather instrument responsive to both atmospheric pressure and temperature. It takes the form of a pear-shaped glass reservoir standing 27 cm tall, drawn out into a slender spout that curves upward. The piece combines genuine decorative artistry with a functional design, and the rare pairing of blown glass with a fitted brass mount marks it as a work of exceptional craftsmanship rather than an ordinary household object.
The body is free-blown in colourless glass. Its most distinctive feature is the hanekam (cockscomb) ornament: a rhythmic row of applied glass droplets running along both sides of the reservoir. The glassblower formed these by gathering a drop of molten glass on a metal rod and laying it onto the surface, building up a decorative relief bead by bead.
Beneath the reservoir is a flattened decorative finial — one of the characteristic marks of eighteenth-century donderglazen. Later examples generally show a smooth, spherical terminal, which makes this flattened form a useful indicator of the earlier date.
The upper end of the glass is fitted with a brass top that is at once ornamental and functional. Its rim is pierced with decorative cut-outs in the form of stylised Gothic tracery, lending the piece particular elegance, while a heavy suspension loop shows that the instrument was meant to hang within an interior. The combination of glass and metal is exceptionally rare in these instruments and reinforces the impression of a masterpiece rather than a utilitarian object.
An empirical scale is engraved along the spout: straight divisions numbered from 1 to 12, read from bottom to top, against which the movement of the liquid could be followed. The scale made the instrument more practical to read, though the numbers are purely relative and correspond to no conventional unit of measurement.
The reservoir is filled with water — of any chosen colour — and, being hermetically sealed except at the spout, also encloses a fixed volume of air. The instrument responds to the balance between that trapped air and the outside atmosphere. When atmospheric pressure rises, it presses the water down and slightly compresses the enclosed air, and the level in the spout falls. When pressure drops, the trapped air expands and drives the water up the spout. Before an approaching storm, when pressure falls sharply, the level may rise so far that the water spills dramatically over the lip of the spout.
That same body of trapped air makes the instrument sensitive to temperature. A rise in temperature expands the enclosed air and lifts the water in the spout even when pressure is unchanged; cooling lowers it. A donderglas therefore reads most faithfully in surroundings of relatively stable temperature, where the movement of the water reflects changes in pressure rather than heat.
The donderglas belongs to an early family of thermo-barometers whose origins reach back to the first decades of the seventeenth century, and which is regarded as a distinctly Dutch invention. Two names attach to its prehistory: Cornelis Drebbel and Gijsbrecht de Donckere.
Drebbel was a Renaissance polymath — glassblower and lens-grinder, musician, engraver and draughtsman — for whom craft was a route to experiment. He worked in an age when seafaring was pressing for new ways to read the world: the compass was still barely mastered, there were no accurate clocks at sea but only sandglasses, latitude was often badly miscalculated, and an instrument that could warn of a coming storm was little more than a wish. Moving with equal ease among furnaces and workshops, Drebbel is remembered for his perpetual-motion device, his optical lenses, fountains and automata, and his experiments with the "new spirit" (oxygen). His reputation carried him to London and the court of James I, with demonstrations at Whitehall, and then to Prague under Rudolf II.
It is along this line — from the perpetual-motion glass to the "weather glass" — that the prehistory of the donderglas begins. Working with glass and sealed volumes of air, Drebbel observed how a column of liquid in a narrow tube answered not only to heat and cold but to disturbances in the air: a gust of wind, a slammed door, the approach of bad weather. His contemporaries already credited such glasses with the power to foretell storms, long before Torricelli's mercury barometer. From this tradition the "bottle" form took shape, in which pressure, temperature and the liquid column interact — a principle later brought to a more coherent and reproducible form by another Dutchman, Gijsbrecht de Donckere, at whose initiative such weather glasses were made and sold in quantity, valued especially by those living on the coast and by fishermen.
Under long-wave ultraviolet light (365 nm) the glass gives a moderate green-yellow glow, strongest where the glass is thickest — in the applied droplets and at the root of the spout. Yet in ordinary daylight the glass is colourless, and a handheld Geiger counter shows no elevation above natural background. Both observations exclude uranium: this is not uranium (vaseline) glass but glass decolourised with manganese, whose fluorescence arises by a different mechanism.
Manganese as a decolouriser. Silica sand inevitably carries traces of iron, which tints glass: ferrous iron (Fe²⁺) gives a blue-green cast, ferric iron (Fe³⁺) a faint yellow. To obtain colourless glass, glassmakers for centuries added manganese in the form of pyrolusite (manganese dioxide, MnO₂), known in the trade as "glassmaker's soap." It works in two ways: chemically, as an oxidiser, converting strongly colouring Fe²⁺ into weakly colouring Fe³⁺; and optically, since the trivalent manganese (Mn³⁺) thereby produced lends a pale violet complementary to the residual yellow of the iron, the two cancelling to a neutral, visually colourless tone. The practice is known from Antiquity and remained standard until the early twentieth century, when selenium replaced it. Manganese-decolourised colourless glass is therefore fully consistent with an early-eighteenth-century date.
The cause of the glow. The fluorescing species is the divalent manganese ion (Mn²⁺) dispersed in the silicate network. The emission arises from an internal d–d transition of the ion (⁴T₁ → ⁶A₁), and its colour is set not by the ion alone but by its surroundings in the glass — the strength of the local crystal field: in tetrahedral coordination Mn²⁺ emits green (around 520 nm), in octahedral coordination orange-red. In silicate glasses the green to yellow-green emission prevails, as observed here. Because the transition is doubly forbidden — by both spin and parity — the emission is soft and of low intensity.
Distinction from uranium glass. The green fluorescence of manganese and of uranium look outwardly alike — hence the common confusion — but their mechanisms and diagnostic signs diverge. In the nature of the emission: in uranium glass the uranyl ion (UO₂²⁺) emits by charge transfer with vibrational structure, giving a bright, saturated glow, whereas in manganese glass a forbidden d–d transition of Mn²⁺ gives a paler, yellower one. In role: uranium was added as a colourant, to impart colour, while manganese was added as a decolouriser, to remove it. In daylight appearance: uranium glass shows a distinct yellow-green (vaseline) cast in transmitted light, whereas manganese-decolourised glass is colourless, acquiring at most a faint amethyst tone through prolonged exposure to light. In radioactivity: uranium glass, in which uranium serves as a bulk colourant, registers clearly above background on a counter, whereas manganese glass is inert. And in dating: deliberate uranium (vaseline) glass is a development of about 1830 (Bohemia, Josef Riedel), so genuine glass of the first half of the eighteenth century necessarily predates it — the silent counter and the colourless body agree with manganese and confirm the early attribution.
In short, the green fluorescence of this glass is caused not by uranium but by traces of manganese — a relic of an old decolourising recipe, visible only under ultraviolet light.