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Sand clock is an instrument for measuring time, consisting of two glass chambers connected by a narrow neck and containing a quantity of sand that trickles from the upper chamber to the lower in a fixed amount of time, often one hour.Sand clock is also referred to as hourglass or sand glass. 

Like other timepieces, it needs to be carefully calibrated. The hourglass maker must test the instrument and fine tune it to measure the correct length of time.There are many factors that contribute to the ability of an hourglass to accurately measure time. The type and quality of sand is key. It must have a rate of flow that does not fluctuate. Sand that is too coarse will wear away the glass, eventually making the neck too large. Most important is the ratio of the neck (the hole, or tube) width to the diameter of the sand particles.The frames or housings for hourglasses are open to the designer's whims.Raw materials most often consist of pieces of fine wood that can be crafted or carved to suit a particular style, decor, design, or theme. Bamboo, resin, and various metals like brass, bronze, and pewter are also beautiful framing materials.