In the 1970s, an entirely new way of packaging wine was developed, expressly to provide a significant volume of wine in a package that is not as breakable or heavy as a bottle, and is better able to preserve any wine left in the container. It comprises a collapsible laminated bag inside a strong cardboard carton, and wine is drawn out of a tap specially designed to minimize the ingress of potentially harmful oxygen. The difficulty of making the wine container completely airtight restricts the potential shelf life of bag-in-box wines. but there have been considerable improvements in this respect since the mid 1990s. There are two main types of bag: in the silver-coloured ones, the oxygen barrier is provided by a thin layer of aluminium, or aluminium-coated polyester, between layers of high-density polyethylene; in the clear bags, the oxygen barrier is made from polyvinyl alcohol. The main disadvantage of the former is ‘flex cracking’, which tends to occur during transportation and weakens the oxygen barrier. The oxygen barrier in the clear bag is less effective and becomes even less so as relative humidity increases.