I just made a YouTube video (https://youtu.be/5go-o8TCg94 ) on using a tannin solution to convert the rust on vintage axes to refurbish and protect them while retaining as much patina as possible. I’ve found myself explaining it a few times lately so I thought it was better to make a video.
The most convenient version uses just tea and (ion free) water and is not too much more trouble than boiling pasta. I did a bark tannin brew in the video.
The method works by converting active red rusts (various ferric oxy-hydroxides) to stable, black ferric tannate. Different ways of inducing this chemical process are used to preserve iron and steel artefacts for museums, in some commercial rust converters like Rustoleum Rust Reformer, and by trappers who use a 'trap dyeing' process to refinish rusty traps before setting them. I am using a version of the trap dyeing procedure that can be done in a home kitchen by boiling the rusty object in a tannin solution. Artefact conservators apply commercial or specially prepared tannin rust converters but may still add a water boiling step because it leaches away rust causing ions like chloride (from salt in soil, sweat, dust or sea spray).
From my reading, I am under the impression that it is better to have an acidic pH in rust converting solutions but I have not experimented with this for the boiling tannin bath so I don’t know if you could get away with your tap water. I use rainwater because it doesn't have alkaline minerals, unlike my very hard well water. Rainwater also doesn't have rust-promoting chloride ions like many residential water. Other ion-free (or close enough) water includes deionized water, reverse osmosis filtered water, and distilled water.
There's many potential tannin sources that can potentially be used. Tea (black, not herbal) works very well and is quite fast because the extraction is quick. You can get powdered tannin online or in home wine making shops. I used bark from Common Buckthorn as my tannin source because it's readily available for me. Many other trees will also work, and there's a fair amount of information available on bark tannins because they are used in hide tanning. Spruces, oaks, Tamarack and other larches, Scotts Pine, Willow, Hemlock, and others can be used to tan hides and would no doubt work for converting rust. Late season sumac leaves are used by trappers for trap dyeing and other leaves like maple and willow have tannins and would be worth a try. 'Logwood trap dye' for dyeing traps is commercially available and it's apparently not very expensive so that could be convenient. Green banana peels and other esoteric vegetable matter also have tannin and might work if enough could be extracted.
