, ▁in ▁turn , ▁was ▁broken ? ▁ ▁Man ufact ure ▁C ame o - glass ▁vessels ▁were ▁probably ▁all ▁made ▁within ▁about ▁two ▁gener ations , ▁as ▁experiments ▁when ▁the ▁blow ing ▁technique ▁( dis cover ed ▁in ▁about ▁ 5 0 ▁BC ) ▁was ▁still ▁in ▁its ▁inf ancy . ▁Rec ent ▁research ▁suggests ▁that ▁the ▁Port land ▁v ase , ▁like ▁the ▁majority ▁of ▁came o - glass ▁vessels , ▁was ▁made ▁by ▁the ▁dip - over lay ▁method , ▁where by ▁an ▁el ong ated ▁b ubble ▁of ▁glass ▁was ▁partially ▁di pped ▁into ▁a ▁cru cible ▁of ▁white ▁glass , ▁before ▁the ▁two ▁were ▁blow n ▁together . ▁After ▁cool ing ▁the ▁white ▁layer ▁was ▁cut ▁away ▁to ▁form ▁the ▁design . ▁ ▁The ▁work ▁in ▁making ▁a ▁ 1 9 th - century ▁copy ▁proved ▁to ▁be ▁incred ibly ▁pain st aking , ▁and ▁based ▁on ▁this ▁it ▁is ▁believed ▁that ▁the ▁Port land ▁V ase ▁must ▁have ▁taken ▁its ▁original ▁art isan ▁no ▁less ▁than ▁two ▁years ▁to ▁produce . ▁ ▁The ▁cutting ▁was ▁probably ▁performed ▁by ▁a ▁sk illed ▁gem - c utter . ▁It ▁is ▁believed ▁that ▁the ▁cut ter ▁may ▁have ▁been ▁Dios k our ides , ▁as ▁eng ra ved ▁g ems ▁thought ▁to ▁be ▁cut ▁by ▁him ▁of ▁a ▁similar ▁period ▁and ▁signed ▁by ▁him ▁( Vol len we ider ▁ 1 9 6 6 , ▁see ▁Gem ▁in ▁the ▁collection ▁of ▁the ▁Duke ▁of ▁Dev ons hire ▁" Di om