The cost of shipping commodities fell to a record, amid signs that Chinese demand for iron ore and coal is slowing, hurting the industry’s biggest source of cargoes.  The Baltic Dry Index, a measure of shipping rates for everything from coal to ore to grains, fell to 504 points on Thursday, the lowest data from the London-based Baltic Exchange going back to 1985. Among the causes of shipowners’ pain is slowing economic growth in China, which is translating into weakening demand for imported iron ore that’s used to make the steel.