one of the most important things in art glass is colour, glass seems to glow with an inner light and that is just what it is, glass transmits light, even opaque glass transmits some light. It is the transmission and absorpsion of light that gives us the colour in glass, blue glass for example absorbs all the red-yellow end of the spectrum and transmits all the violet-blue end, so we see blue glass. this ability to absorb and transmit light is dependant on the composition of the glass, more precisely the types and quantities of metals in the glass. The blue glass contains cobalt, although copper will also give a blue glass it requires an oxidizing base glass to do so, a reducing base glass will give a ruby glass with copper, this is due to the copper being disolved into the glass in an oxide form in an oxidizing base glass and as a metal in a reducing base glass. The composition of the glass has a great effect on the colours that metals will produce, for example in a soda glass nickle will only give a brown while in a potash glass it gives a deep violet, manganese is also dependant on potash to produce its deep purple, in soda glass it has a brownish cast, Some colours require lead to produce the desired colour, for example gold, to produce a ruby and copper to produce a deep green like to have lead in the base glass as it assists in the absoption of the metal into the glass.  The glass used in art glass work falls into the category of "optical glass" due to it's very complex composition, it often contains things like Boron, Magnesium, Barium, Zinc, Lead and small amounts of Arsenic or Antimony to name some of the important ones while the alkali composition can be high in sodium or have mainly potassium all depending on the colours that are being made.