Tannic acid patination is an ancient technique for forming a dark protective coating on iron objects. It may have been discovered accidentally, after iron objects were lost or buried in peat bogs, then recovered later in an unrusted condition. Iron Age bog bodies have been found with intact iron blades, more than a thousand years old. (reference for iron preservation in bogs)
The process itself is simple and non-toxic. The object is placed in a container, and covered with bark pieces. Water is added to cover the object and the bark. This should then be allowed to sit for several weeks, to develop the coating. Ideally, the water should be free of chloride, so water that has passed through most water softeners is not acceptable. Distilled or de-ionized water will work well. If you have a municipal water supply, they may be able to tell you the chloride content of your water.analysis of Grand Rapids, Michigan water
As with any chemical process, heat will speed up the reaction. Small objects can be boiled with bark, or the container can be put outdoors in a sunny location, depending on the climate and weather. click here to see a small iron sculpture, which was processed by heating with bark and water in a slow cooker
Lowering the pH of the solution with an acid will also speed up the process, but do not use muriatic acid, which is hydrochloric acid, as this adds undesirable chloride ions. Chloride will lead to destructive corrosion of the iron. An organic acid, like vinegar, will work. Phosphoric acid is found in some rust removers. This may add a blueish cast to the patina.
I have fermented the residue from jelly making, which contains some sugars and organic acids. This will form some alcohol, which can be further fermented to acetic acid. Strain out the liquid, and add it to the bark. The resulting solution was quite dark, probably due to some water soluble plant pigments.
Large objects can be painted with a solution, as recommended by the Canadian Conservation Institute. CCI bulletin