All emulsions go bad with time. They end up totally black through fog. This can take from a few months to a few years. Any emulsion kept at or above 40 deg C will go bad in hours. Emulsions should be kept at about 3 - 7 deg C for optimum keeping, with short periods at 20 deg C for removal of samples. The samples for coating should be melted at 40 deg C and coated immediately.