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- Mid 17th century
- Walnut, blue silk, glass, sand, twine
- 22.5 x 18.5 x 8.5 cm
A. Delalande & E. Delalande & D. Delalande, Sabliers d’autrefois - Hourglasses of the past - Clessidre d’ altri tempi, Parijs 2015, pp. 84-87, 97-99, 200, 201.
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This double hourglass in a walnut frame indicates the time in half hours and vifteen minutes. The two rectangular end pieces are connected by six turned columns, decorated with two balusters, a ball and several rings. The shape of the columns imitates the shape of the hourglasses, which makes it an elegant piece. The frame stands on four ball feet, continuing from the columns. The frame is symmetrical: the hourglass had to be able to be turned upside down.
The hourglasses are filled with light sand. Given its colour and grain size, this sand is probably made of eggshells. These were boiled, pulverised and sieved several times, in order to create a homogeneous mixture without organic matter. In addition to eggshells, hourglasses were filled with finely ground minerals, marble, lead, tin, and naturally occurring sand.
Both hourglasses consist of two pear-shaped glasses joined together in the middle. Between the two parts of the hourglass is a metal disc, in which a small opening has been made: just big enough to let the sand pass through at the right speed. The running time of the hourglass, and therefore the time it indicates, is determined by the amount of sand in combination with the size of the gap in the disc between the two glasses: the larger the opening, the shorter the time indicated by the hourglass. Over time, the drilled hole expands a little: the sand that keeps running past it slightly scrapes the opening. Thus, the older the hourglass, the shorter the hour becomes. These two hourglasses now show the time in half-hours and ten minutes, but it is possible that they were originally half-hours and quarters. Similar hourglasses, with several hourglasses in one frame, often show the time in quarters: ¼ - ½ - ¾ to an hour. The glasses, with the disc in between, are connected with wax, which is covered with blue silk and tightly wrapped with string. This connection had to be completely sealed, to prevent the loss of sand or the clumping of the sand by air or water. A slightly thicker rope is used for one of the hourglasses, and the connection between the glasses is thicker in this hourglass. The hourglasses are clamped at the top and bottom in the wood, with a piece of fabric between glass and wood.