Remember that leather is skin (from a cow, horse, pig, or goat and etc.) that has undergone a tanning process to make it "decay resistant" along with imparting qualities of strength, flexibility and overall appearance. This is done by the tanner to ensure years of good use from the leather. The long term care of leather will extend the life cycle of the tanning process. There’s been a long debate on whether there is a formula to treat leather, whether to oil it on occasion or simply do nothing. So, what is the best solution?
Moisture is the most important part of leather care. If the leather becomes dried, it will also become hard and brittle. While plain water is the best lubricant, it evaporates too quickly and oil is used to maintain a soft supple grain surface on the leather. Leather tanners of the past would oil the leather with cod oil for a shorter term lubricant and tallow for the longer period. On the opposite end of that spectrum, if the leather is kept in a damp climate, the leather can mold.
Oiling leather seems to be the choice in caring for leather and yet oil can also cause damage. It is a necessary lubricant, allowing the leather at the fiber level to move with out friction and resulting damage. Unfortunately, too much oil will prevent the leather from “breathing” and as humidity rises, the leather will absorb the moisture. With too much oil, the moisture then does not evaporate and the leather will begin to rot and the leather fiber loses its tensile strength. A small amount of oil goes a long way. Oil penetrates the leather through oxidation. All oils will oxidize with time.
Oil penetrates the leather fibers causing distention and swelling; it actually changes the shape and placement of the fibers. This is what makes the leather soft and supple. Too much oil will create physical changes that irreparably weaken the leather fiber causing swelling to the point of cracking the surface. Remember, a little oil is all that’s necessary.
Oils vary and those with mineral oils or indicated as petroleum distillates will do even more damage when they break down into their solvent basis. If you need to oil, use a natural oil as in “pure neatsfoot” oil or a good grade of tallow. Read the labels of any oil you select to apply to your leather!