Answer (by @DWFII , a bespoke shoemaker): Back in the '70's Sydney Brinkerhoff wrote a monograph called "Boots and Shoes of the Frontier Soldier," published by the Arizona Historical Society, in which he explored the various construction methods of Union shoe and boot procurements during and after the Am. Civil War.
In the aftermath of the war the government had over a million pairs of surplus boots and shoes that were issued to soldiers on the frontier. The government commissioned a study to determine which was the best method of construction.
The long and short of it is/was that welting was the only method of making that stood up to the contrasting environments and abuses that the soldier subjected their footwear to.
One of the methods that was very popular at the time and, hence, subject to study, was the use of "rivets"--essentially brass nails...in the terminology of the time.
Nails on modern shoes...used to hold the outsole on...can be brass or iron. Both...in order to be an effective and secure means of attachment...have to be driven in such a way that they "clinch" (turn) on a metal plate mounted on the bottom of the last..
The studies Brinkerhoff cites--done by the military--showed that nails tend to continue to work through the leather, eventually protruding into the interior of the shoe enough to cut soldiers feet. In point of fact, part of this is that the leather of the insole itself, tends to shrink and compress around the nails, leaving them "proud."
Additionally, iron nails rust when they come into contact with the moisture and salt of perspiration. Rust is an oxidative process--a "slow fire"--and it will carbonize vegetable tanned insole leather to the point
of making it brittle.
Interestingly enough, one of the other common methods of attachment (almost ubiquitous for common, RTW footwear)--wooden pegs--was also shown to be inadequate in extreme conditions, esp. dry heat. Pegs fell out of shoes in the barren landscapes of the Southwest.
This question was first posed and answered in our Official Shoe Care Thread.
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