An hourglass - also known as a sand clock or sandglass - may seem to be a simple enough timekeeping instrument, but if it is to tell the time with anything approaching accuracy it has to be manufactured to some quite exacting standards.

Firstly, the glass from which it is made must be properly formed. The surfaces that slope down to the central hole, through which the sand pours, must be completely smooth so that sand does not stick as it descends. For the same reason, they must slope at a sufficiently steep angle. The hole through which the grains of sand pass must be made in the correct ratio to the diameter of the individual particles of sand. And the glass must be completely impervious to water, since the slightest moisture will affect the flow of the sand or even clog up the neck completely.

The sand itself must be of the right kind. It must be dry and fine enough to flow easily, and it must also be relatively smooth in texture: if it is too rough or coarse its constant abrasive effect may wear the glass away, making the hole in the neck of the hourglass too big to keep accurate time.

Various substances have been used to provide a sufficiently high-quality sand for accurate timekeeping, including powdered marble and finely crushed eggshells.

The origins of the hourglass are lost in the mists of time, although it has been claimed that the Greeks were the first to use them as long ago as the 3rd century BC. Probably the earliest confirmed record of their use, however, dates from the 14th century, when Sienese artist Ambrogio Lorenzetti depicted one in his celebrated frescoes illustrating the effects of good and bad government. The frescoes were painted in the Sala dei Nove in Siena's Palazzo Pubblico in 1338. One of the Virtues associated with good government - Temperance - is depicted as the figure of a woman holding an hourglass in her right hand. The hourglass later came to be used as a symbol of an altogether less wholesome kind: pirates of the 16th and 17th centuries sometimes eschewed the more familiar practice of displaying a skull and crossbones to warn victims of their impending fate: many displayed flags depicting an hourglass, to indicate that time was running out for the crew of the ship they were attacking.

Hourglasses came to play an important role in naval timekeeping. A glass with a timespan of 30 minutes was used to time watches. At sea, as a result, they became known as watchglasses. They were also used in measuring the speed of a vessel under way. A crewmember would pay out a knotted line into the water for the duration of one sand fall. When the last grains fell into the base of the hourglass the number of knots that had passed through the crewman's hands would indicate the speed of the vessel - hence the modern use of the term 'knots' to indicate speed on water and in the air.