Brunelleschi looked to the great dome of the Pantheon in Rome for solutions. The dome of the Pantheon is a single shell of concrete, the formula for which had long since been forgotten. Soil filled with silver coins had held the Pantheon dome aloft while its concrete set. This could not be the solution in the case of a dome this size and would put the church out of use. For the height and breadth of the dome designed by Neri, starting 171ft above the floor and spanning144 ft, there was not enough timber in Tuscany to build the scaffolding and forms. Brunelleschi chose to follow such design and employed a double shell, made of sandstone and marble. Brunelleschi would have to build the dome out of brick, due to its light weight compared to stone and being easier to form, and with nothing under it during construction. To illustrate his proposed structural plan, he constructed a wooden and brick model with the help of Donatello and Nanni di Banco, a model which is still displayed in the Museo dell'Opera del Duomo. The model served as a guide for the craftsmen, but was intentionally incomplete, so as to ensure Brunelleschi's control over the construction. Brunelleschi's solutions were ingenious. The spreading problem was solved by a set of four internal horizontal stone and iron chains, serving as barrel hoops, embedded within the inner dome: one at the top, one at the bottom, with the remaining two evenly spaced between them. A fifth chain, made of wood, was placed between the first and second of the stone chains. Since the dome was octagonal rather than round, a simple chain, squeezing the dome like a barrel hoop, would have put all its pressure on the eight corners of the dome.