My previous attempts at the vinegar black didn't really follow a recipe, I just let some nails sit in vinegar for a few weeks and then painted it on the leather. It worked, but I wanted to compare how it would work with an actual period recipe.
The concept and overall function was the same - the iron in the vinegar solution reacts with tannins and turns the leather black - but the period recipe calls for pre-treating the leather with gallnut solution to introduce the tannins. This is largely unnecessary with modern leather because it already contains tannins from the vegetable tanning process. But if, as is suggested in the Plictho, most of the recipes were intended for use with alum tawed leather, perhaps the tannins needed to be added in period?
In any case, I wanted to see if the introduction of additional tannins would affect the colour, even for veg tan leather. The theory being that more tannins might make a darker black.
I had previously made a new batch of the vinegar solution by letting some rusting nails sit in a jar of vinegar for about three weeks. The recipe called for boiling the nails in vinegar for four hours, but I figured letting it sit for that long would have the same general effect without stinking up the house. Call it a modern adjustment. :)
The next step was to make a solution using the gallnuts to extract the tannins into the liquid, so that I could apply it to the leather. I had gotten some gallnut extract from a dying supply store when I did my last order. This is the pure extract rather than the actual galls, so I modified the recipe a bit here too. The original recipe called for grinding up the gallnuts and then boiling it in water for an hour. Since I have the extract already processed, I simply added it to the boiling water (as much as I could dissolve into it) and boiled it for about 15 minutes. I figured I didn't need longer since I wasn't trying to extract the tannins but just to dissolve the powder into the liquid.
I let it cool a bit and then painted it on the leather as per the recipe instructions. Once it dried, I applied the vinegar black to both the gallnut-treated leather and an untreated piece as a control. I did only one coating, since I wanted to have a good comparison of the darkness of the colour. Once it dried I then applied a coat of olive oil.
Results:
As you can see by the picture below, there's very little difference in colour between the two pieces. The gallnut-treated leather is a bit darker but that could be fixed by applying extra coats of the vinegar I suspect. It also has a bit more of a shiny finish for some reason.
The one big difference I found is that the gallnut-treated version made a mess when I was doing it. Basically the vinegar was reacting to the liquid I had already applied to the leather, which meant it turned black as I was applying it on the surface, rather than reacting to the leather itself. As a result, my hands were coated in black that didn't wash off.
I also noticed that it left a black residue on the surface of the leather, which would need to be polished and rubbed off. I think most of it came off when I was applying the oil finish, but I'd still be concerned about it rubbing off on someone wearing a leather item. The non gallnut-treated leather was very well fixed and the black was not rubbing off the leather at all.
It could be that I didn't let the leather dry long enough between the application of the gallnut solution and the vinegar. But, given that both methods seem to produce an equivalent quality of black, I think I'm more likely to use the non-gallnut version for day-to-day use since I have more confidence in it not rubbing off on people's clothes.
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