Slag glass is made by mixing a variety of colors together and then blowing and forming the piece.  The trick, which requires a lot of skill, is to avoid mixing the colors to much and producing a "muddy" look.

These two piece show how difficult it is to work with slag glass.  The artist was able to blend more colors on the top piece.

You can see the bottom part is different than the top.  There is less white and there is a purple shading which the top half doesn't have.  I assume this happened when the glass blower had a little trouble with the red and blue glass mixing to much.

An attempt at a more "artistic" picture of the rooster..