The Orvietan archives are singularly silent with regard to a monument of so large extent and vast importance, which must have taxed to the uttermost the resources of the ablest stone-carvers in Italy.[66] Meanwhile, what Vasari says is valuable only as a witness to the fame of Niccola Pisano.

His manner, as continued and developed by his school, is unmistakable at Orvieto: but in the absence of direct information, we are left to conjecture the conditions under which this, the closing if not the crowning achievement of thirteenth-century sculpture, was produced.
When the great founder of Italian art visited Siena in 1266 for the completion of his pulpit in the Duomo, he found a guild of sculptors, or _taglia-pietri_, in that city, numbering some sixty members, and governed by a rector and three chamberlains.

Instead of regarding Niccola with jealousy, these craftsmen only sought to learn his method.

Accordingly it seems that a new impulse was given to sculpture in Siena; and famous workmen arose who combined this art with that of building.

The chief of these was Lorenzo Maitani, who died in 1330, having designed and carried to completion the Duomo of Orvieto during his lifetime.[67] While engaged in this great undertaking, Maitani directed a body of architects, stone-carvers, bronze-founders, mosaists, and painters, gathered together into a guild from the chief cities of Tuscany.