decentering
Was to set the supporting poles of the center’s scaffolding in sand-filled kegs and then, at the time of striking, to unplug the kegs and allow the sand to escape, thereby slowly lowering the level of the wooden framework. This operation may seem simple, but timing was a major problem. Medieval mortars remained “green” for up to a year or even eighteen months, until the water necessary for crystallization had completely evaporated. The centering for the vaults of the south tribune, for example, remained in place for thirteen months, from June 1420 until July 1421, thus tying up a large amount of timber that could have been reused elsewhere — for example, in the platform for the cupola. If centering was struck too early, the mortar would still be plastic and its strength insufficient. On the hand, long-term s create a deformation of wood known to s as “creep”: if the centering was left in place for too long, the timber would warp beneath the weight of the vault it supported, causing the masonry to shift. This phenomenon was known to the ancient Greeks, who would remove the wheels of their chariots at night, or else prop the chariots vertically against a wall (as Telemachus does in Book IV of the Odyssey) in order to prevent the wheels from warping under the weight of the stationary vehicles (King, 2000). (King, 2000)