Pale Grey Until the Iron Remembered Black

Walnut hulls from last October were still sitting in a jar. The green husks had dried to black, brittle shells—Juglans nigra, same compound (juglone) that made aluminum mordant produce pale tan instead of brown two weeks ago. I knew iron was the correct mordant for walnut. I’d learned that the hard way, after printing test blocks onto cotton and getting beige.

So when the fountain pen nib came back from cleaning—stained purple-brown from mushroom dye experiments, the feed clogged, the tines misaligned from trying to force dried pigment through capillary passages—I spent an hour with brass shims and citric acid, the kind of restoration work I’d done forty hobbies ago. Dipped the nib in water afterward to check flow. It wrote grey-brown, semi-transparent lines that darkened over minutes. Oxidation.

That’s when I recognized it. Ferrous tannate converting to ferric. The mushroom dye must’ve left iron contamination in the feed. Or the pen itself had corroded during storage—old nibs are steel under the gold plating. Either way, I’d accidentally made iron gall ink in a fountain pen, and it was destroying the nib.

Wikipedia said iron gall ink was Europe’s writing standard for 1,400 years. Also said it causes “flash corrosion” of metal pen parts. The mushroom-stained nib wasn’t salvageable, but the curiosity was cheaper than ordering a replacement. I had walnut hulls. The rain barrel behind the garage holds rust-stained water—decades of corroded eavestroughs leaching ferrous ions into runoff. That’s two-thirds of an ink recipe.

What I Assembled

Oak galls would’ve been traditional—wasp-induced tumours on oak leaves, 2–4 cm diameter, rich in gallotannic acid. I don’t have any. Black walnut contains the same tannin family. I crushed fifteen hulls, steeped them in 200 mL of rainwater for six hours, strained the debris. The liquid came out dark brown with an oily film on top—probably residual nut oils. Set that aside.

Gum arabic as binder. I had a bottle left over from cyanotype work—acacia tree resin, same stuff that makes watercolour paint adhere instead of just staining. Mixed 5 grams into the walnut extraction while it was still warm. The solution thickened immediately. Too much gum and it won’t flow through a pen; too little and it won’t stick to paper. This felt close.

Then I added rusty water. About 30 mL, filtered through coffee paper to remove particulates. The ink turned grey-green, then deepened to something close to graphite. Ferrous ions complexing with tannin. Poured it into a glass bottle with a ground-glass stopper—the oxidation needs to happen on paper, not in storage. Exposure to air converts ferrous (Fe²⁺) to ferric (Fe³⁺) and makes the ink unusable before you write with it.

Dipped a bamboo skewer into the ink and wrote a test line on newsprint. It went on pale grey. Thirty seconds later it was charcoal. Five minutes later, nearly black. The colour materializes as you watch—time-lapse chemistry at room temperature.

Why This Shouldn’t Go in a Fountain Pen

Traditional iron gall ink clogs capillary passages and corrodes metal. The acidity eats brass, steel, even gold alloys over time. Modern “iron gall” formulas for fountain pens are compromises—less iron, buffered pH, synthetic chelating agents. They’re called iron gall ink the way tipping material on nibs is called iridium: historical naming that no longer describes the chemistry.

I tested mine with pH paper. Came back around 4—acidic enough to etch copper. The mushroom dye that killed the first nib was probably in this range. Putting this into a pen would be deliberate destruction.

But it works brilliantly with a dip pen or brush. The delayed darkening gives you no feedback while writing, which is disorienting—you’re composing blind, then the text appears afterward like a darkroom print under safelight. The lines are sharp, water-resistant once dry, and permanent in a way that ballpoint ink isn’t. This is the chemistry that wrote the Codex Sinaiticus in 350 CE, Leonardo’s notebooks, centuries of legal documents. Great Britain and France had laws specifying iron content for royal records. The US Postal Service issued an official recipe. Jewish Halakha still requires it for Torah scrolls.

Iron gall ink was permanence before we had better options. It’s also slow destruction—acidic enough that manuscripts survive despite the ink, not because of it. High humidity accelerates the corrosion. Ghost writing bleeds through to the reverse side. Eventually it eats holes.

The bottle is sitting next to the pen nib I wrecked. I’m not putting this ink into another fountain pen, but I’m keeping both. The chemistry that ruins one tool makes another work. Ferrous sulfate mordant would’ve given me the dark brown I wanted on fabric. Instead I got pale tan and a lesson about coordination complexes being dye-specific. Then the stained nib taught me what iron does to tannin under oxidation. Then the rain barrel turned out to be an unintentional reagent stockpile.

Walnut, rust, acacia resin. Ink appears in stages, writes illegibly until it doesn’t. Traditional enough that there’s Talmudic commentary on the recipe, modern enough that I can look up the redox chemistry on my phone while waiting for text to darken. The threads don’t always synthesize cleanly, but sometimes they connect backward through repair work and stained metal.