Is an amorphous solid a single phase?
It is difficult to give a general answer. A glass is an amorphous solid that may change its composition in a continuous way from one point to another. When melting brown bottles and colorless bottles in the oven of a glass factory, you may obtain a non homogeneous mixture, with regions more or less colored. It is obviously one and the same phase all over the mixture. But as the mixture is not homogeneous, its composition is not the same everywhere.