In traditional (painted) stained glass you have three stages: contour (= linework), grisaille (= grey areas and shadows) and emaille (= coloured areas).
I did the linework on one side of the glass panel, on the other side I covered the total area with grey grisaille (which is very fine earth diluted with vinegar and de-mineralised water). Then you have to use brushes and other scratching equipment to take away the greys, thus creating shaded areas. In the end it will be heated and burnt into the glass.
For this experiment I used a part of Picasso's Guernica painting, because of the varied use of greys.