Graal is a Blown technique (originally from Sweden) where a small (say 40 mm) coloured bubble is blown with one blow pipe, then another coloured bubble is blown on another blowpipe and overturned on to the first in orange hot condition. This process may be repeated 3 or four times depending on how many colours are required. This bubble is cooled in an annealer. Engraving is done on this to a depth for which ever colour is required to be visible. Then the bubble is put back into the annealer and the temperature taken to about 900 °F (approx.500 °C). Clear crystal glass is then gathered on this and blown into the required shape.
Arial is similar but the difference is that the engraving is much deeper and when again glass is gathered there is air entrapment in the engraved voids which form a very interesting illusionary pattern.