Leather dyes

[Crispianus]

Black dye, Green Vitriol, Copperas = Ferrous Sulphate.

Pliny describes Vitriol manufacture though he's refering to Blue Vitriol, Copper sulphate in his description, I havent tested it yet but I've read that it produces brown.. will be interesting to find out what colour brown...

However for the last month I've been testing Ferrous sulphate as a black dye/stain, in all cases on multiple types of leather, in all about a dozen samples tanned with Tannic Acid derived from various plants, this resulted in a good black finish (comparable with ROC black dye on the same leather, my prefered Danish brand), and produced what I would call a good charcoal black.

The main difference was the amount of time it took, solutions of White wine Vinegar or Vinegar essence and iron were prepared and left from one week to a month to brew, occasionally being stirred or shook these involved 2.5, 5, 12.5, 25% acid content, this was applied with a brush.
I dont think there's any difference between this solution and pure Ferrous sulphate dissolved in distilled water* (*to prevent contamination) although it may well give quicker results...

The Key to a good black I think is how much Ferrous Sulphate the solution contains and I believe that the higher acid content in the solution produced more of this, in practice what this meant was having to apply the solution multiple times or only once and the reaction speed or how quickly it turned black, needless to say the 25%/one month solution did best..

It was obvious when viewing the newly dyed/stained samples when still wet, that all the active ingredients were being used up in producing the black.... this probably means that a good proportion of the acid was neutralised also...

The more porous leather such as goat tended to absorb the solution and needed more applications as a result.

I should point out I'm no Chemist... 

[Feinman]

Reds can be achieved with cochineal and madder, a modern kermes, it creates a cool (as opposed to warm) red color, almost purple. Blues: indigotin reduction dye bath, minerals like azurite. Greens, a blue over a yellow, or verdigris, and other copper compounds applied as a paint.

[Crispianus]

Reds can be achieved with cochineal and madder,  a modern kermes, it creates a cool (as opposed to warm) red color, almost purple.  Blues: indigotin reduction dye bath, minerals like azurite.  Greens, a blue over a yellow, or verdigris, and other copper compounds applied as a paint.

Unfortunatly two of the Known "mediteranean" reds Armenian Cochineal and Kermes are rare or impossible to find today only Kermes is available in its raw form at 29€ a gram from Kremer Pigment, so too expensive to use for most purposes..
The Alternative is the commonly available Cochineal from America which although shares some similarity is not exactly the same, but its what I intend to use at least for testing purposes.. will keep you posted when I do...
The colour depends on how you treat it... would also need Alum tawed leather for best results, luckily theres a local specialist tanner... what was known as a Weisgerber here, though its now a common name...

From my point true Purple is out (see the Southfleet shoes) unfortunatly not just because of its extreme rarity but also because it stinks something rotten....