... Incidently, the black patternation & the bubbles etc, are produce using chalk and is not controllable. It is quite literally, luck-by chance! The black glass inclusions are however controlled to a certain degree, in so much as they are produced using Manganese in high volumes, which must be adjusted for compatibilty to the other glass "ingredients". one has to be a type of chemist to get to grips with this (particulualry when working with any glass of iridescent qualities) and it is not until the item has cooled (after anealing) that one can truly see what one has tried to achieve! That is, as a glass maker-blower! It is the fractions of HEAT, other than the colourants used, which shall achieve an end result. SUCH IS GLASS!