Mould-blown glass was a sub-category within blown glass which as a technique revolutionised the glass-making world during the first century AD. A mould would be manufactured from a hard substance such as metal, clay or wood. This allowed intricate designs to be produced across the glass as the design was already carved, ready for the hot glass to be blown into the mould. The mould itself would usually be in two parts to make it easier to open and remove the glass vessel. Once removed, the glass blower would then manipulate the piece while it was still hot with any extra desired features, for example, forming the handles. Glass makers would create several similar moulds, if they had been successful, and thus there are many vessels of these designs. As moulds were reused, some would shrink due to constantly being heated up. Consequently, the later centuries saw small shaped vessels.
The iridescence on ancient Roman glass was unintentional, and was caused by weathering on its surface. The extent to which a glass object weathers depends mainly on the burial conditions; however, the humidity, heat, and type of soil in which the glass was buried also all affect its preservation.
To find out more about Roman glass please see our relevant blog post: Ancient Glass and Collecting Roman Glass.