Liquid metal, metal liquor -- whatever you call it, it's a pretty effective way to make marks on fabric. When you add willow to the mix, the results can be even more exciting.
To make iron liquor, stuff one or two pads of steel wool into a bottle and add vinegar. Don't fill the bottle too full, because it will fizz up as the acid starts to work on the metal. (It's lovely to have studio flooring that isn't precious!) Don't cap the brew while it's working. I rubber band a piece of fabric over the opening to keep bugs out. The longer it brews, the stronger the liquor gets, but you can use it after just a few days.
To use the liquor, strain it through a piece of fabric to catch any undissolved metal bits. Then apply to fabric. I generally apply it to fabric that has been dyed with willow, sumac or rhubarb leaves. I've had good success with fabrics that were cold-bundled, steamed and simmered in bundles as well as immersion dyed with willow.
This test piece is a silk scarf that was very briefly simmered in a willow immersion dyebath. My iron liquor had only been working for a few days, so the color was a very light gray. I applied the liquor with a Tsukineko Fantastix applicator, but have done brush and sponge applications, too.
Older (more aged?) iron liquor and more willowy fabric can give darker (as in black) marks.
I'm cheating here (trying to get caught up on my willow dye posts for the SDA group) and using a photo from last winter. This shows iron liquor used with the tannin liquor I make from staghorn sumac leaves and vinegar. I make the same kind of tannin liquor using willow leaves and vinegar, but these are the pictures I can find at the moment. I soaked the folded/clamped fabric (a recycled cotton percale pillowcase) with the tannin liquor, then mopped up a puddle of very old iron liquor with it.
Here's the resist pattern on that pillowcase.
For most of my natural dye projects, I like to let a piece oxidize for a couple of weeks before I wash it (if I can hold out that long). For mineral-printed pieces, especially ones made with mature (vintage?) iron liquor and especially on silk, I usually rinse the fabric within a few hours.
I'm not worried about metal-made marks washing out. Have you ever left a load of wet clothes in a dryer with chipped enamel on the tumbler?