The most primitive incense clock is a graded incense (joss) stick; the time elapsed is registered by the speed at which the trail of incense is consumed. Powdered incense was formed into a stick of hardened paste and graded for hour intervals. There are three common types of post-16th century incense seals. All consist of a metal base, tray for the ash bed and powdered incense, perforated grid pattern (the seal), and perforated cover. The utensils include a tamper and a small shovel, stored in the base which insulates the burner. A bed of wood ash is put into the tray and tamped. The grid is placed on the ash and the sharp end of the shovel traces the path of the grid. Powdered incense is placed in the groove and smoothed. The grid is removed and small bamboo pegs, each stamped with an hour character, are placed at regular intervals along the incense track. The seal is then covered with the perforated lid to protect the incense from draughts, and the incense is ignited. The types are: the single seal, round, square or rectangular; the double square seal having two trays and two different seal grids stacked one on top of the other; and the Ju'i sceptre, in the shape of the ancient sacred mushroom. A rarer type is the dragon vessel, of bronze or lacquered wood, lined with pewter and fitted with V-shaped wire racks to hold an incense stick. Strings with metal weights at each end were suspended perpendicular to the body at regular intervals, or at one place only, depending on the specific use; when the heat from the burning incense ignited the string, the balls fell into a metal platter over which the dragon was suspended. The resulting clatter served as an alarm for the light sleeper.