In the late 1800s some pen makers produced ink filled glass tubes with cork seals for use with early versions of the fountain pen. Later, when rubber was developed it was used to make rubber ink sacs that contained the ink within the barrel of the fountain pen, but in the 1950s with the plentiful supply of plastic and injection moulding, ink cartridges became used more widely due to their convenience and practicality. Most fountain pens now use ink cartridges despite the meagre amount of writing ink that they contain. The standard cartridge that is widely used is referred to as the 'International Standard' and will fit many makes of pen. Amongst the exceptions however are Lamy, Cross, Namiki, Pilot, Sheaffer and Sailor whose cartridges are designed to only be compatible with their fountain pens.