Erick,
Oh, where is whatshisname, the guy who used to occasionally post here on historical tanning-chemistry? I'm a bit out of my depth, but here goes:
As I understand it, cod (liver) oil, and certain marine mammal oils now banned, e.g. whale-oil, have the unique property for currying that they readily oxidize into a basic fatty-acid jelly in the leather, sheathing each fiber of the skin, cross-linking and bonding on the molecular level, and filling the intercesses in a way simply no other oils will. Cod-oil was/is used to produce oil-dressed leathers (e.g. buff, chamois, etc.), not "tanned" at all (no tannins), but purely converted into leather by admission and saturation of oxidizing oil (alone). Currying already tanned leathers for uppers, in this instance, was an early form of what we might call today "combination tannage"--the leather was tanned (with tannin), then oil-dressed (with cod-oil) in addition to tanning--not unlike the Medieval/Moorish "Cordwain" leather was a combination of alum-tawed and then veg-tanning, or the modern examples of "semi-chrome" and chrome-veg "re-tan".
Neats foot oil (rendered from cattle hooves, etc.) is a fine oil, too, but it will not oxidize, so it's fugitive and bleeds out. Vegetable oils, like Rape-seed oil, are extensively used for currying in the UK these days because of their relative cheapness/availability, but these go rancid over time and wash right out when you dip the leather in water. Rape-seed oil, I've been told by a retired UK currier defending its use, "has the same atomic weight as cod-oil", but owing to the draw-backs I just mentioned, I don't care for it. He also claimed that cod-oil is too expensive to use now, and is "banned" (?!) in the UK for currying.
Back in the 1970s I experimented with fresh genuine whale-oil. It behaved okay, but turned very dark and smelled foul in the leather when it oxidized. Cod-oil in the leather actually creates a strong "sweet" smell over time--not fishy. In fact, that wonderful "leather" smell is usually a combination of oak-bark (which simply smells like sour beer vomit--ever visited a pit tannery?), and cod-oil (after the fishy smell has turned sweet). I read in one of the ancient tomes that the French often used whale oil for currying historically, but the English preferred cod, because the whale-oil smelled bad. Voila! History proved itself through experimental archaeology
Also, cod-oil was often used in conjunction, or mixed with other substances for currying, i.e. tallow (rendered and filtered mutton-kidney fat, the hardest fat) and "degras" ("sheep's wool grease"=lanolin), so perhaps cod-oil simply worked best combined with these other fats and waxes too?
I guess the short shoemaker's answer (versus the long tanning-chemist's answer) is: cod-oil has remained the currying oil of choice for centuries, even when many other oils, now no longer available to us were readily available, because it achieved the desired results better than the others. It's good to question, and I admire your students' interest here, but it's also, alas, just one of those things... "that's the way it's always been done...", which believe me sounds like dumb old codgers clinging blindly to tradition. As a wild young Turk (once) myself, I was not satisfied with answers like: "just because... it's always been like that"; however, experiment and research as I could, it just seems that cod was, and remains, the best oil for this job.
On the grain: yes, just using alcohol-based Fiebing's/Kelly's dyes--Tandy's house-brand is a weaker-strength dye, and smells like it's based on common isopropyl rubbing alcohol, thus easy to thin, and easier to control the color intensity of moreso than the "professional" dyes.
On the flesh: we're usually imitating the venerable old 17thc.-19thc. "waxed-calf" AKA "French calf", so it's black or black, and occasionally black too. For this a soap-based black smutting is still the fiber-filling-coloring of choice--fine powdered carbon dust ("lampblack"=purposefully, industrially-produced, fine soot from oil fires collected on metal sheets, then shaken loose and collected). Today, try printer toner or p-copier toner for blacking? Castille [sp?] soap I found works best for the suspension--mix with water and lampblack, to taste, into a stiff paste the consistency of margarine, and apply with a stiff bristle scrub-brush in circular motions until the flesh it completely loaded. When dry it has this tenacity of the soap-scum on your bathroom fixtures, IOW, nigh impossible to get off. But wet the leather, and you get blacking all over everything, so be careful. The historical waxed-calf behaved the same way--blacking got all over everything. "Size", or "sizing" was painted over the "smutting" in an attempt to fix it and give a cleaner top-coat, and "gum dragon" (gum tragacanthe) or thin flour-paste was the choice there; however, both of these agents are also water soluble, so the problem of the blacking was, "an incurable subject".
The best, "Grade A", blue-ribbon waxed-calf was last produced by J. T. Scott & Sons, Carlisle (UK). They supplied Maxwell's, Peal's, and all the top West End bootmakers. They had one old gent who would come in from his retirement periodically and make up a batch as late as the 1970s. It had to be completely hand-curried, and as he explained to me in a letter (he was coaching me and vetting my attempts to make waxed-calf), none of the young people then coming into the trade would have anything to do with it, because it was such a messy back-breaking job.
I cut my eye-teeth on J. T. Scott waxed-calf, and have tucked away a few matched pairs of butts for a rainy day. BTW, Maxwell's still had a pile of this in the cellar in 2000, but they guarded it like gold. Maybe one of you West-enders could pry it out of them? This leather bleeds blacking all over you when wetted, but burnished back down with a little thin paste, when dry, it looks like patent leather. Just the "joy" of waxed-calf, and a very vivid reminder why men's-men and ladies'-men were separate branches of our trade, especially later 1600s to later 1800s, when most men's work was made from waxed-calf, and women's fine textiles--cleanliness!
Get a-hold of a copy of Hippolyte Dusauce [sp?] 'Leather Tanning and Currying' and try following the progressive steps for making waxed-calf, if you want some good old fashioned "dirty" fun in class doing experimental archaeology on currying.
