Accelerating Rust with Muriatic Acid

The Fort Garry Tobacco tin and the composite barbed point both have rusted elements to them. I've been experimenting with muriatic acid to oxidize the metal in these reproductions. Muriatic acid is dilute (20%) hydrochloric acid and can be found in some hardware stores. I picked this bottle up at Canadian Tire, but not all places have it. Its used for cleaning concrete and changing the pH in swimming pools. This is a much more potent chemical than the vinegar and miracle gro I was using on the copper. I only work with this stuff outside and wearing rubber gloves.

I've seen recipes that call for diluting the muriatic acid anywhere from 1:5 to 1:20 with water. More acid doesn't necessarily work any better - I've been getting good results in the 1:10 to 1:20 acid to water solutions.

One source that I found said that dissolving copper in the acid solution helps the rusting process. I dropped a couple pieces of heavy copper ground wire in the solution and the acid has been eating away at them. That seemed to work, but I'm finding that too much copper in the solution is bad. Instead of creating rust, its started depositing a sheen of copper on the surface (left, you can see patches of copper). Its an interesting effect, but not what I'm going for. The solution with the copper in it has turned a noticeable green colour (top photo, in the bottle with the metal lid - which was a dumb move by the way, the inside of the lid has a rubber coating, but everytime I open and close it, acid from my gloves corrodes the outside of the lid. Glass and plastic containers only with muriatic acid!) . By the time I noticed the colour change it was too late. I'll keep the solution as it is for the future, but I'll remember that a green solution has too much copper in it.

Temperature and humidity are important as well. The best rust forms on dry sunny days. I tried soaking a paper towel in the acid solution and laying it on the tin yesterday, but that turned the metal black. My guess is that it didn't get enough oxygen to oxidize red. I removed the paper towel and re-wet it with water and the black faded and went back to orange. On other occasions I have left it out in the rain overnight and it seemed to go more black than red on those occasions as well, again, I think it wasn't getting enough oxygen. The inset photo shows a before and after shot of yesterdays treatment. The copper patches are gone and the rust came on nice and orange. A little more tweaking and this piece will be done.

On another occasion, I tried covering the tin in sawdust soaked with the diluted muriatic acid solution so that I could clump it and control where the acid contacted the metal. That worked alright, except I used yew sawdust, which is very red and it actually stained the paint a light rose colour, which I've been working at reversing ever since. It didn't hurt the rusty areas, so on a piece without paint, the extra red would probably have helped. In the future, I may try it again with white sawdust, like pine or spruce. The sawdust seemed to work alright because it was still breathable and let oxygen get in and react with the metal, but for now I'm just dabbing acid on with a paper towel and leaving it exposed to the air.

So far, the rustproof paints that I used have kept the acid at bay. The rust only grows in the spots where I've scraped through the paint and exposed the metal. I'm not sure how many acid treatments the paint will withstand. There might be some cracking developing. So far its adding to the antiquing process, but I can imagine that at some point the acid will damage the paint more than I'd like it to. In unpainted areas, like the back of the tine, (right) the rust forms within a few hours on a sunny day and immediately give the metal an antiqued look. A couple weeks ago this was the inside of a new biscuit tin.

Photo Credits: Tim Rast

Photo Captions:
Top: Muriatic Acid and smaller conatiners of diluted muriatic acid, with and without disolved copper
Second: Too much copper creates patches of coppery sheen on the metal instead of rust.
Third: Before and after 1 treatment of diluted muriatic acid on a piece that previously had copper deposits.
Fourth: Using sawdust to hold the acid. Worked ok, but the red sawdust stained the paint.
Fifth: Nice rust!