Strops, or "stopping sticks" do work much better
The leathers Garsault specifically mentioned from the Levant were the exotics of his day, real Morocco, "cordovan", shagreen, etc. Bear in mind that the mysterious near-east was the original source of "cordwain" leather--whence we get our moniker during the Crusades--a combination-tannage with alum-tawing involved, on at first goatskins, then by Garsault's time that mustard seed-imprinted asshide. Whether leathers from the near-east, Hungary, Russia and the Baltic, the level of international commerce was hardly new by 1767--remember the Romans invaded Britain not only for her tin mines, but for her oak tanned leathers.
While preparing the book I grilled Roy Thompson, former tanner himself, and the go-to historic tanning guy at Leather Conservation Centre, Northampton about the Irish "lime and barley" sole leather. He'd never heard of it, but suggested that a bath in fermented barely could have been done for raising or drenching the hides (opening the pores) before the actual tanning. The 1807 directions for making Russia leather Janne shared around the other week also mentioned working flour-paste into the wet hides before tanning. I don't know enough about tanning chemistry to say more, sorry.
Bear in mind always with Garsault, it was he or his interviewers going into cloistered trade shops and interviewing tradesmen on specialized and secret stuff--field research. Imagine a grad-student from W&M coming to the Shoe Shop at Colonial Williamsburg and interviewing us on how to make a shoe? What were they told? What were they not told? What did they gloss, interpret/misinterpret? At least Roland de la Platiere says he sent Garsault's text to his personal shoemaker for his professional comments and input.That's all part of the fascination I think, and it will take a generation of researchers to chase all these little things to ground. Some shoemaker in Paris might have extolled the sole leather from Ireland, the so-called "lime and barley leather". Was it so superior because of the Irish cattle hides? The "lime & barley" processing? Importer/exporter marketing? Or the final tanning or preparation? I can't say, but it does illustrate the depth and complexity of these people's industrial world, and their savvy about the materials they used from around the world--this hopefully will dispel the popular myth that shoemakers worked all alone like a Keebler elf, using leather tanned locally from last Sunday's roast beef (especially considering it could take nigh on to two years to tan sole bends) to make their shoes.