(chemistry is always a little more complicated than you hope it will be)

I have long felt a need for a decent leather medieval shoulder bag. They are useful for smuggling books and other modern purchases through crowds of modern folk at events. I have a linen bag, which is alright but not waterproof and I just haven’t seen so many like them in pictures.
Plus if you get a leather satchel you can use it on pilgrimage, as seen in illustrations from the 13th to the 16th century.
Only in most of these pictures they are black. Black leather? How did they make that then? Obviously you take your leather, and dye it with something.
The idea being to look something like this picture:

The question being, how to make the leather black?

The problem being that I couldn’t find what to dye it with. Nothing in my usual sources, or the Mappae Clavicula. I don’t have much information on dyeing in general, so I turned to the internet. Many sites recommended using iron scourer pads and vinegar, smearing the rust formed onto veg tanned leather, which would react with the tannins in the leather forming a black coating. (vegetable tanned leather means it was tanned with vegetable tannins, in medieval times this would be oak bark or parts of oak tree)
I rejected this method as having no provenance and being a little too easy.
Instead I redoubled my search and found mention of leather dyeing in various 16th century texts.
including an Italian one.
Unfortunately it calls for plants that I don’t have access to right now, being in the frozen north, although one recipe does mix plant juices with iron from a grindstone.

It turned out that that oak galls were used to dye cloth black,
as in the recipes here.

So I put some ink on veg tanned leather and got this:

Black alright, but the leather curled up a little and felt a bit harder
With the addition of neatsfoot oil it came up lovely, becoming more supple and with a lovely shiny black look to it.
“Great,” I thought, “but I don’t have many oak galls and I can’t find a decent source for them online”
You can see my dilemma. Until I remembered about oak bark being used for tanning, and therefore a source of tannins. Boiling some up produced a brown liquid, which resolutely refused to turn very black, even with bits of iron added:

With iron:

There was not much effect, and it would take a long time for the relatively large iron particles to be dissolved into the acid liquid.
So I knew I was missing something. Meanwhile someone somewhere else pointed out that black leather was regarded as being weaker in the 19th century, suggesting the dye did something bad to it. Clearly if the dye used iron sulphate like in the ink (Which did after all work, but left the leather hard and less supple), then that would explain that. Sulphuric acid being bad for any organic substance. It will eat into your skin and is very unpleasant, hence used for acid attacks on people.

I found a paper online which showed that sulphuric acid did attack and damage leather.

But according to this paper, the tannins in veg tanned leather protected it better than the chrome tanned leather which was greatly weakened. At this point I realised I had a tannin rich liquid from the oak bark and wood, so that wouldn’t matter so much. Yet since it’s colour was so poor I was still missing something.
Finally, a closer reading wikipedia and other sources such as
this page.

Indicated that I was really after the gallic acid, which would be released from the tannins by the hydrolysing effects of sulphuric acid. So the medieval scribes were using exactly the right substances to maximise the amount of black ink they produced, because some of the tannins would break apart to form more gallic acid molecules which would combine with the iron to make blackness. Even better, the resulting molecules would be small and dispersed in the solution, making it good for penetrating leather.
So tannin solution before:

After adding iron sulphate:

That’s better. Much more like ink.

But hold on, that might mean we have excess sulphuric acid present, which will rot the leather (and if used as ink, the paper you wrote it on). I checked the pH and indeed it was about 3.7, which is quite low, neutral being 7, although not as bad as lemon juice which is apparently about 2.

Now the Ian’s page mentioned above did suggest using egg shells to neutralise the acid, being somewhat alkaline from the calcium hydroxides and suchlike. Having burnt my eggshells a couple of weeks ago I tried using calcium oxide, quicklime, instead. However that still left the question of how well such a more alkaline liquid would work. After all aqueous chemistry is complex and I think that if you added too much neutralising agent you would take the iron out of solution by removing the gallic acids altogether.
So I only added a little quicklime, pH 3.88 to 5.82, and compared it with un-neutralised liquid (on the right, slightly less black due to less dye added):

They are both black enough, with the more acidic liquid seeming to make the leather harder and less flexible, as expected, but the neutralised liquid not having such a bad effect. This photo is after oiling, and both look good to me.
Therefore, success! I have a concentrated solution of 150ml of it now, which should be enough to do maybe half the leather, maybe more.

Now all I need to do is cut the leather out for the bag, which will be a little tricky, and then dye it. In fact that will require a post of its own, because of the complexities of bag making. So stay tuned!