When glass was first discovered and used within windows a ball of molten glass was spun on the end of a pole until it resembled a huge dinner plate. The flat areas were cut into shapes and joined with lead to make larger areas. The centre ‘bullion’ wasn’t wasted either! So the big houses and castles had wonderful leaded lights. Coloured stained glass was achieved with heating and pigments. We have all seen what wonderful designs can be achieved with Church stained glass panels.
The replacement window industry has modified these effects so that they can be used on full sheet flat glass; because that is what a double glazed, sealed glass unit is made from.
The lead itself comes on a roll and has a peelable glue back. It has widths of 6mm, 9mm, 12mm and I have recently used 15mm. It can have a flat or oval top, it can be treated to look dark and the cross-over joints can have solder melted on them to make the whole project look more authentic. You are of course laying this lead on a single piece of glass, it is put on both sides of the outer double glazed face. When it is done well and put in an appropiate design of window the new installation can be enhanced.