In a word (or three): obtaining proper materials.
Talked yesterday to the CEO of Siegel's of California (a high-end leather retailer), regarding the differences between alum-tanned leather with an oil finish or oil combination-tan, versus Indian Tan leather (alum tannage with a russet veg-tan outside, resulting in the yellow-interior, red-exterior leather one commonly sees used for lacings on boots). This is in relation to trying to find an appropriate leather to reconstruct the Cuman "farsetto" referred to by Matteo Villani in the middle of the fourteenth century, which I believe to be the direct ancestor of the buff coat. Since some of you are curious about what goes into separating out legit experimental archaeological reconstructions from that stuff you see at RenFairs...
CEO: (snipped for brevity).... Could you tell me more about the properties that you are trying to reproduce and the end use....
The leather will be used to recreate objects of material culture. Some of this will be relatively simple: bags, archery gear, equestrian equipment. Other uses will be harsh: wear-testing the leather in abusive winter conditions for extended periods, destructive testing by subjecting multiple glued-up layers to archery fire in order to determine the accuracy of chronicle references depicting soft-armor efficacy, where the chronicler's vocabulary is sufficiently odd that it must be double-checked (he uses terms reserved exclusively for clothing, rather than armor). The author posits what I believe should be interpreted as an early form of "buff coat on steroids." In combination with other known protective gear, how it is protective (as opposed to "how much" it is protective) needs to be assessed.
The leather in question needs to have the following characteristics:
1. Producible in an environment in which typical tannin materials such as oak galls are in fairly short supply, in a relatively short timeframe (The people in question were transhumant ranchers, moving significant distances between summer and winter pastures. It is not credible to believe that they engaged in primitive pit-tanning, with a timeframe-to-completion which would have involved abandoning the product for months at a time.)
2. Optimally, the leather should allow for gluing with a hide glue with little difficulty.
3. Mechanically, the same tan needs to be
3b. Sufficiently tough so that it can be put to very hard use
3c. *AND* sufficiently versatile that the same tannage can be put to use for products that require toughness plus suppleness (for example, strapping, lacing, and garments), or else put to use without work-softening, for items requiring more rigidity (quivers and bow-cases, for example).
The "India Tan" dry alum+veg-retan latigo satisfies the chemical (2) and mechanical (3) properties with no difficulty assuming that the samming/oiling is done after gluing-up. (I have rediscovered their method of waterproofing hide glue so that it forms a weather-proof bond. With the surface to be glued gone over with a lye soap, the oiled-off version would also satisfy these requirements.) Period chronicles have also compared the leather used to cordwain, which in a medieval context definitely allows for the russet color of the vegetable re-tanning. For this reason, the purchased side is plausible.
However, the difficulty with assuming its equivalence is:
1. lack of readily-available materials for the tannin re-tannage
2. the universal depiction, within those images sources that are otherwise considered to be highly reliable, of their leather garments being a yellow, not russet, color. While one can "argue away" the color use in some of these depictions based on iconographic convention (yellow = bad guy), other images sources are not subject to such an argument.
Therefore, if I'm going to drop the tons of cash required to get the leather required, obtaining samples ahead of time is an absolute must.