The process of building up a sculpture with soft materials is known as:

Modelling is the sculptural process of building up a form by manipulating soft, pliable materials such as clay, wax, plaster, or papier-mache. The artist shapes the material by hand or with tools, adding and pressing material to gradually construct the desired three-dimensional form. Modelling is classified as an additive process because material is built up rather than removed.

Casting involves pouring molten material into a mould and waiting for it to solidify - the form is reproduced, not built up by hand. Subtracting (subtractive sculpture) is the opposite of building up; it involves removing material from a solid block, as in carving. Assembling (assemblage) involves joining pre-existing found objects together rather than shaping a soft material into form.