Artistic Barometer — "The Apothecary Shop" Postcard
A decorative cobalt hygrometer postcard that indicates changes in atmospheric humidity by changing colour.
The large apothecary bottle in the centre of the illustration acts as the humidity-sensitive element. Its colour changes gradually according to the moisture content of the air, from blue (dry) to pink (humid).
There is a pleasing bit of honesty here: the bottle carries a real label — COBALTI CHLORIDUM, ad usum meteorologicum — which is precisely what makes the card work. Cobalt chloride "for meteorological use" is not a decorative flourish; it is the substance itself, named on the jar that contains it. The chemist grinding away behind the counter would have recognised his own stock.
The hygrometric coating contains cobalt salts, whose colour responds reversibly to changes in humidity. In drier air the bottle becomes blue, while increasing humidity shifts the colour through violet towards pink.
The postcard therefore works as a simple traditional chemical hygrometer, while also serving as an unusual decorative reproduction inspired by historical weather-indicating cards. Best kept out of direct sunlight and away from steam; the colour change is fully reversible.