Often referred to as the sand clock or a sand glass, the hourglass was constructed from two separate glass bulbs that were rounded and held together by a neck of narrow glass. The hourglass contained particles of sand in it. When it was turned upside down, a measured amount of sand particles dropped down from the top part of the glass to the bottom part of the glass. The type and quality of the sand are key to enable the hourglass to determine the time as accurate as possible. Experts say sand that is too coarse will wear away the glass, eventually making the neck too large.