布 and 絹: cloth as a form and silk as a material
Separate cloth in general from its material, reading means and composition.
Using cloth
布 (ぬの) means cloth. 「布で机を拭きました」 means wiping a desk with a cloth.
「布を切って袋を作りました」 describes cutting cloth to make a bag. The word cloth alone does not specify its fiber.
Silk as a material
絹 (きぬ) means silk. 「絹の布を買いました」 means buying silk cloth.
「この服は絹でできています」 says the garment is made of silk. で marks its material.
Reading form and material together
「絹ではなく、綿の布です」 says it is cotton cloth, not silk. 布 gives the form category, while the materials are contrasted separately.
「柔らかい布」 does not establish silk. 柔らかい describes a tactile quality rather than naming the material.
Must soft cloth be 絹?
Show the answer and why
No. Softness is a property, not the material information silk.