Explain to a production manager the joint cost problems raised by the following description of a process producing glass bottles.

A typical mixture might be sand 60 per cent; limestone 8 per cent; soda ash 18 per cent; mineral additives 5 per cent. Up to 25 per cent of the mix can comprise cullet (recycled glass). From one mixture the molten glass would be run off into moulds for narrow bottles (such as wine bottles) and wide-mouth containers (such as jam jars). Bottles which have flaws or chips are broken up and recycled.