THE ROLE OF MARINE OILS IN TANNING
You may have heard me say before that we can tan any animal hide in any method - and this includes marine animals. Fishskin leather and sealskin leather are the two most prominent and widely-created leathers, historically and now. They are often said to “self tan." This is actually a form of oil-tanning, a method traditionally beloved in high latitudes and coastal climes around the world. Applying marine oils to terrestrial hides works its own magic as well.
Read on.
OIL-TANNING x MARINE FATS
There are 3-slash-4 natural hide tanning methods: smoke (brain) tanning; bark (veg) tanning; mineral tanning (alum-tawing). The fourth? Oil-tanning.
It's a quasi-tanning method because it's like, is it really tanning? We go through the same preparatory motions as the first three methods - we clean and scrape the hide - and then we rub in oil and…we wait. Instead of then smoking the hide, applying tannins, or adding mineral, we just wait; and do some softening in the meantime.
The magic of oil-tanning is that over time, the oils we've absorbed into the hide begin to oxidize. Oils are comprised of lipid molecules. When lipids oxidize, they decompose into several different compounds. One of these compounds is aldehyde.
What else contains aldehyde? Smoke. When we smoke-tan, we are applying aldehyde to a hide with immediacy. When we oil-tan, we apply them in a long-game.
It takes months or year for terrestrial (above-water) oils like animal fat or olive oil to fully oxidize in a hide. This is fine: some passive tanning techniques do indeed rely on oiling a hide and then packing it away, aging the hide in a dried state until the tanner is ready to soften it. But bring marine oils into the mix and we speed up the oxidation time.
Marine animals live in a much colder environment than land animals, even in the Arctic zones. Fish are not warm-blooded; marine mammals are, but they are housed inside an impressive layer of blubber (oil) that shelters them from the outside frigidity. In both types of marine animals, their fat is used to the cold. When we bring them up onto land, that fat encounters never-before-felt warm temperatures. It oxidizes in a hurry.
This is why we can tan fishskin and sealskin in their own fats, rubbing the oil into the skin as we soften it and watching a golden hue emerge from the depths of the skin. That goldenness is the oxidation process in action - much the same as butter on a countertop turning deep yellow after a few forgotten weeks.
Apply these same marine oils to terrestrial hides and now we've got a fast-tracked approach to oil-tanning. This, most famously, is the leather cloth chamois. Chamois is a grain-off mineral-tanned leather. From Wikipedia:
"The term chamois as used to refer to specially-prepared leather originated sometime before 1709,[5] referring to the prepared skin of any goat-like animal,[6] specifically the European antelope—commonly called the "chamois"—and exclusively used by the glovemakingindustry of southwest France. It was discovered that when tanned in the local cod oil of nearby Biarritz, the result was a material of unprecedented absorbency. This leather was fashioned into soft white gloves designed for carriage footmen, who were responsible for the care and polishing of carriages. This industry usage later transferred to the chauffeurs of the "horseless carriages" invented in the early 1900s. The popularity of chamois leather greatly increased with the advent of mass-produced automobile windshields, which needed to be washed frequently for visibility purposes."
Because it relies on both mineral and oil, chamois is a combination-tan: the result of two tanning methods in the same hide. We make a variant of it here using terrestrial oils; the chamois I made back in February is still pearly-white with a very faint sign of oxidation slowly emerging.
To work with marine oils, render the oil as you would any animal fat, by heating on very low heat, ideally using a double-boiler on a predictable stovetop element. Separate the cracklins from the oil through straining. Add the freshly-rendered, still-warm oil to a jar and tighten the lid before it cools. This will seal and protect the fat - though, you don't need to worry about the fat going rancid if your only goal is to tan hides with it.
When applying the oil to hide, I still recommend adding an emulsifier to make the most of the lipids and get them into every inner part of the skin. Powdered sunflower lecithin absorbs well; one tablespoon lecithin + ⅓ cups oil + 2 cups water is my class fat emulsion recipe.