Some thoughts upon reading the piece.
I think Ms. Cameron is far more knowledgeable about archeaology and research methods than about leatherwork. This creates a number of background assumptions in her work that are suspect.
For one, she makes the assumption that cour boulli means one thing, and that the same process was used to produce various different pieces, from armour and sheaths to boots to cases to bottels and jacks. There are reasons to believe that this is not a valid assumption.
I am currently unable to locate the source of a relatively modern reference I came across to cour boulli as a kind of leather used in bookbinding. In context it was completely obvious that the leather could not possibly be anything of the sort we think of when referring to cour boulli. I apologize for having misplaced the source and hopefully will either be able to find it myself again or perhaps someone here may recognize it and rescue me.
Modern Scandinavian sheaths are made using a "half tanned" leather, and the resulting forms are both hard and resilient without any more treatment than being wet molded and allowed to dry naturally.
Cameron tells us that her experiments in hardening leather using heat produced unsatisfactory results - even after heating at temperatures of 85 C her test pieces were not hard. This runs contrary to my experience and that of Dan Houchins as well.
I believe Chris Dobson's article post-dates Cameron's (and wonder if her article was where Dobson learned of the boot hardening instructions). His experiments with wet molding, nailing to a last and then heating and brushing on hide glue produced results which he found sufficiently hard to consider viable for armour.
Cameron mentions Waterer's observation that a wax hardening process would add considerable weight to a piece of leather and that such a result might be undesireable for armour. Although appearing to consider this a valid point, Cameron later omits it from consideration when discussing the resin and wax treated leather cuirass.
I think it is entirely likely that there was more than one method used to harden leather, that the method used was likely related to the object's intended end use and that hardening may well have been incidental to waterproofing in certain applications.
A leather drinking vessel (bottel, jack, costrel, bombard...) does not particularly need to be hard, but must be waterproof to function. That a waterproofing method coincidentally hardened the leather might have led to the development of the great bombards of Elizabethan England, but hardening is not critical to smaller vessels.
A leather case for a beaker, on the other hand, arguably needs to be hard more than it needs to be waterproof. The halftanned leathers used today for making Scandinavian style sheaths might well provide sufficient hardness for such cases - with no treatment at all.
Leather armour needs to be hard, impact resistant (not brittle) and would benefit from being light in weight. It also does not need to hold water. If a method for producing adequate hardness and resilience could be found that did not call for adding weight to the piece, that method would likely be optimal for the purpose.
I have not experimented with a formulation of rosin and wax such as described in the process for waterproofing/hardening the boot. I have some volume of experience both with forming and wax hardening bottels and jacks, and with forming and "bake hardening" leather armour.
I cannot speak to the end result using the rosin and wax approach.
I can say that my bottels will soften quite noticeably during the course of a day wearing them (and drinking from them) out in the sun on a (not especially warm) June day in Pennsylvania, where as my unwaxed hardened leather armour does not soften at all in the same conditions.
I think there is a need to address the question of what kind of leather, how it was tanned and to what degree it was tanned.
I think there is a need to consider the distinct possibility that there is not a single answer that explains all hardened leather artifacts.
I think attempting to find a single solution for all examples quite possibly prevents a proper understanding of what was actually done.
Cameron's reporting of Volken's project constructing the cuirass treated with rosin and wax after the boot recipe is interesting and I will undertake some experiments along these lines and compare them to my results with other methods. Such comparison should be informative.
Title edited to reflect Klaus' correction of my spelling
I've seen it so many different ways I'm lucky I can write it so anyone knows what I intended 