The tannic acid in logwood bark reacts with the iron within the rust, and this produces iron tannate – which is much more stable than rust.
American trappers use logwood bark in a hot dye bath to chemically convert the rust on their traps. The logwood is loaded with tannin – tannic acid, which reacts with rust and produces a different substance. Logwood trap dye not only converts any rust on their traps into a more stable protective layer, but logwood also darkens the metal, and makes it smell different from the human who sets the trap.
From Rust to Rust
Rust is formed when iron reacts with oxygen in the presence of water, or to put it another way: Wherever there’s a railing. The chemical formula for rust is approximately Fe2O3, with the exact amount of water being variable. Rust consists of hydrous iron, and iron oxide-hydroxide. The tannic acid in rust converter, and logwood bark, reacts with iron within the rust and produces iron tannate – which is much more stable than rust.
Not all tannins are the same
Tannic acid is a complex organic acid found in plants. Different types of tannic acid are usually identified by the species of plant from which they come.
When applied to iron, tannic acid reacts with the iron ions to form ferric tannate, a somewhat porous blue-black film whose degree of protection can be controlled to some extent by the method of application.
It produces a uniform finish that enhances the appearance of an object.
Any tannin will work, more or less. Oak bark and many others are worth a shot, with varying levels of success to be expected. We have selected Logwood as a traditional source of tannic acid, it’s this material which has the strongest historical association with rust conversion. Not all tannin is created equal and it’s best to start with the best.
All rust particles must have contact with the tannic acid in order to convert – the rust converting logwood will not ‘soak in’. Any deep pockets of rust must be taken back to the last molecular layer or the untreated rust will be spreading, hidden underneath the surface. In practice most rust can go straight in the dye bath (or get painted) without any surface prep. Where some preparation is needed, it is a minimal brushing away of any unsound material.
Logwood is mixed into large volumes of water in the dye bath
This has two effects:
- firstly the rust converter is really watery and paints on as if water.
- Secondly, the quantity of water is somewhat elastic – for consistency you will need rudimentary quality control.
If you are using an open pan over a bonfire this level of control will be harder, but we’re not talking utter precicioon – a simple “100 gram logwood in 5 litre water” note to yourself will help you repeat the shame shade of black with the iron tannate the next time you make up a dye bath for a spot of rust converting.
Make a note of heat and time – although neither are critical, and both can be adjusted to suit.
As with most chemical reactions the addition of heat speeds things up. An American hunting website recommends a pan be left on the open fire for an hour, whereas a tray left in an unheated garage would take a week or more.
It also says the fire will keep the pan at about 80 degrees – good luck with that.
The good news is the reaction happens even if you boil it right over.
I find it amusing that we now stock rust itself (being the colour of Brixham Trawler sails), and this logwood rust-converter!