Sometimes the most interesting things are the things you learn by accident.
So, I did my normal Spymaster program at Princeton on September 12, 2021 and in this program, I demonstrate invisible inks. My invisible ink (for cost and safety constraints) is a Na2HCO3 (baking soda) and turmeric reaction. Unfortunately, when the people who graciously assisted me in packing up put the bicarbonate solution away in my portmanteau, they failed to seal the bottle properly. It was a very busy week and this solution spilled into the bag and dried leaving a big, ugly black stain. What to do?
Well, invisible inks are made by taking the reagents of ordinary ink and applying them sequentially onto the paper. In my demo, I apply bicarbonate to the paper, let it dry, then brush on the turmeric and voila, red letters. A similar reaction took place in my bag. It was “veg tanned” with tannic acid. Gall ink, like that used to write on parchment is made by reacting oak galls (tannic acid) with soda. My bag was an example of this reaction. Unfortunately, its permanent. What to do?
Well, if it worked for part of the bag, perhaps it will work for the whole bag. Into a sink full of water and bicarbonate the whole bag goes.
It didn’t turn black, but it did turn a nice brown (see picture of unstained and stained leather). So at least I now have a really nice way to make beautiful brown leather without messing with stains. By the way, this isn’t “stained” leather, its brown all the way through, the bicarbonate reacts with the tannin in the leather so the LEATHER is brown, not just the top surface. Unlike pigment-based dyes, which have a bad habit of rubbing off and staining everything they touch, this changed the color of the leather via chemical reaction. Pretty cool.
Of course, I still have a bag with an ugly black stain. Which I am now weighing the options on how to salvage. Should I cut it up and make something else? Should I get some stain and try to stain it (that never works as well as advertised)? Perhaps I should just strip off the buckles and toss it. What to do?
So, I start writing this article and as I am researching the reaction mechanisms (I won’t belabor that here, this is a history blog not organic chemistry), I find another historical stain – vinegaroon – and it stains (actually dyes) BLACK. The plot thickens…
So, what is vinegarroon? Well, its basically iron acetate (FeC6H12O6). Both iron acetate and iron sulfate were common ingredients in 18th Century inks! Hum… I wonder. What have I got to lose? Let’s make some up and try.
Now, this is not something to do in the kitchen, in fact, make this outside on a windy day. Preparing vineragoon will liberate a lot of heat and a bit of hydrogen (think Hindenburg) so you want this somewhere where there is no chance of the gases collecting. Shred some steel wool into a glass bottle, I used a mason jar, and then fill the jar with full strength white vinegar (the cheap stuff). Set it aside and allow it to react for 2-3 days (remember, this gives of hydrogen so keep the jar open and in ventilated space!). After a couple of days, the liquid will have turned a murky brown and some or perhaps all of the steel wool will have dissolved into the vinegar. Strain this through cheesecloth to remove any particulates.
Now sponge this onto the leather liberally and allow it to soak all the way through the leather. Ideally, I would have made a basin of this and submerged the bag (like I did with the baking soda) but who wants a huge barrel of vinegar and iron (it doesn’t smell nice) around and how do you dispose of that? Brush it on and allow it to dry. The leather turns JET BLACK.
Once it is fully stained, fix the stain (the leather is still quite acidic now) by immersing the bag in a bath of baking soda and water. This will neutralize the acid in the vinegaroon and also take away most of the smell. If you listen closely, you can actually hear a fizzing sound coming from the leather as the vinegar and baking soda react. Finally, rinse copiously with clear water and allow the leather to dry.
When the leather was dry, it was somewhat stiff. Now I could have just worked the leather to soften it up but making it waterproof also helps so I softened it with petroleum jelly (18th Century cordwainers would have used lard, tallow, or schmaltz but that has a smell so I used petroleum jelly). Start by wetting the surfaces fully with alcohol. Work in discrete surfaces as the alcohol evaporates quickly. Once the surface is wetted rub in a generous amount of petroleum jelly. This softens the leather and makes it water resistant.
WOW, the bag is not only saved, it’s been upgraded and not a drop of modern dye or stain was used! I treated the bottom with the vinegaroon (black), the top I left with the bicarbonate, and the shoulder strap, not shown, untreated so that I could tell this story with props. Pretty cool!