The gold is reduced to a liquid with organic solvents. Then applied to the piece and fired at the annealing point. You are correct marcel it is a luster. Haven't you see it on expensive wine glasses before? Thermal Chemical plating. It can also be used to fume with. A lot of ceramic shops sell it. I think it is call Liquid Brite Gold made by Duncan Ceramics Co. Yes, It will work on your soft glass too. Ask me how I Know. Ventilate and don't breath the initial fumes. On this piece I have fumed and fired the gold.
Have fun.