The artist is a receptacle for emotions that come from all over the place: from the sky, from the earth, from a scrap of paper, from a passing shape, from a spider's web.

This quote is saying that the artist is like a container where they can collect emotions from different sources. Some examples of these sources of emotions include the sky, the earth, something as small as a scrap of paper or a passing shape, or something as intricate as a spider’s web. In other words, the artist can collect emotions from anywhere, and they can use those emotions to create their artwork.