Sentence:  I need not point out the advantages of modelling in a material as durable as stone. . . . Mixed up with just enough water to form a stiff paste, it accommodates itself to the touch of the modelling tool. . . . There are two inherent difficulties in using it—one, it is not so <WSD>tractable</WSD> as clay. . . . 
Meaning of the word marked with <WSD> tags:  Capable of being shaped; malleable.