Tube and Yoke -How \"white\" was white?

[PMBardunias]

The Romans called tawed leather aluta. It seems to have been used occasionally for sails in later periods.Or so it says on our website!

I think some key information is missing in our discussion. From all I have read, and as you suggest, alum is unstable when wetted, but Julius Ceasar Bell Gall 3.13 wrote that the Gauls used aluta for their sails and alum tawed leather was evidently used to make uppers for shoes which must have been frequently exposed to water.

3.13.
For their ships were built and equipped after this manner. The keels were somewhat flatter than those of our ships, whereby they could more easily encounter the shallows and the ebbing of the tide: the prows were raised very high, and, in like manner the sterns were adapted to the force of the waves and storms [which they were formed to sustain]. The ships were built wholly of oak, and designed to endure any force and violence whatever; the benches which were made of planks a foot in breadth, were fastened by iron spikes of the thickness of a man's thumb; the anchors were secured fast by iron chains instead of cables, and for sails they used skins and thin dressed leather (pelles pro velis alutaeque tenuiter confectae). These [were used] either through their want of canvas and their ignorance of its application, or for this reason, which is more probable, that they thought that such storms of the ocean, and such violent gales of wind could not be resisted by sails, nor ships of such great burden be conveniently enough managed by them.

Some sort of tanning would seem to have been done in addition to the alum treatment. Maybe it is a matter of getting the process, and tanning agent, correct. Something that can tan but not darken the final product. Maybe a fat tan?