Stained glass today generally refers to glass that has been coloured by added metallic salts during its manufacture. It is an art and a craft that requires the artistic skill to conceive of the design and the engineering skills necessary to assemble the piece. Early stained glass artists were limited to a very few primary colours, but today almost any colour can be produced. Begun in Eastern Asia and among Muslim designers, the art of stained glass reached its height in the Middle Ages, particularly 1150-1250. In the nineteenth century, Romanticism and the Gothic revival caused renewed interest in stained glass. Important contributions to the art were made by William Morris (English, 1834-1898), Edward Burne-Jones (English, 1833-1898), John La Farge (American, 1835-1910) and Louis Comfort Tiffany (American, 1848-1933).