Fully saponified soap itself does not expire, but any additives in the bar might. This is especially true if any milks, butters, or extra oils were used to “superfat” the bar. The same applies to scents that might have been added or botanicals. Butters and oils can go rancid and develop undesired blemishes and odors in the bar. Again, if all the oils have been used in the saponification process, it will not go rancid. However, most soap makers choose to superfat their soaps by at least 3% to 5%