Designs such as this one are created by using different types of colored glass including hand cut pieces from larger sheets as well as glass powders (sifted and/or painted on)and frit (small glass bits) Also, some pieces included a rust colored image which is made from an iron oxide transfer.  These different materials are arranged onto a second base layer of glass and the double layered piece is melted in a kiln.   Reaching the finished piece usually requires multiple firings at different temperatures and  during the firing process many colors of glass "strike" to a new color and/or intensity.  When finished, color intensity also changes as different degrees of light filters through the glass.