Non toxic/caustic prints made with iron oxide (rust) on paper or fabric: The technique is simple but does take time. Generally I allow a couple of days for each print to develop.  Warning:  Adding iron oxide to fabric or paper is not considered archival practice.  Though iron oxide (rust) is a highly permanent pigment, it is also moderately acidic. 

Gather your materials: 
Polyethylene sheeting or large plastic bag, preferably see through.
Rusty metal with flat surfaces, low relief, nails with no heads, steel wool, car parts, bottle caps, iron pans, iron tools, chain, wire, etc.
Printmaking paper or fabric
2 Spritz bottles, 1 for water & 1 for white vinegar
White vinegar can darken/blacken the rust and speed things along. Photo below I have the vinegar in a small cosmetic spritz bottle and the water in a larger Garden/pump type sprayer. 
Optional: Kosher Salt, rolling pin or paint brush
FrogTape by Shurtape™ to mask areas to prevent rust from leaching into those areas, this is not a perfect mask but it’s the best I’ve found if you want to control the pattern. Mask the paper before wetting it.

Cover worktable with clear polyethylene sheeting, large enough to protect the table and fold over the paper you rusting. You will fold the plastic over the metal while it is sitting on the paper to keep everything damp. Soak your paper as you would for an etching. Large plastic bag works great for this if you don't have a soaking sink/tank.

When the paper is evenly wet place it on your protected table. Arrange the metal pieces on top of the paper in a pleasing arrangement. Use a spritz bottles with water and vinegar to add more moisture and to wet the metal bits. Wherever the metal contacts the paper it will transfer iron oxide.  Sprinkle the kosher salt in areas on the rusty metal to increase the corrosive effects and to create speckled areas where the rust sticks to the salt.  

To keep everything from drying out fold the plastic sheet over the wet paper and rusty bits and then wait.  Inspect every 4 - 8 hours or more and re arrange the bits.  Turn the paper over and continue to work the paper.  Re wet with spritz bottle if you feel areas are drying.  Use enough water to create puddles and use the rusty puddles to evenly tone the whole sheet.  Use rolling pin or paint brush to work the puddles into the paper and even out the prints.

When you are satisfied with the print, remove the rusty metal and hang paper to dry or continue with other wet paper techniques.  When the paper is dry the rust must be fixed or it will move like pastels pigments do.   Use a spray on fixative /spray on Krylon™ Crystal Clear Acrylic or PYMII. The acrylic spray will minimal change the color and surface a bit that is part of the process.  The rust will move if you don’t fix it to the surface of the paper.
The image below shows the rust technique with no salt.  Other techniques could be introduced at any point in time.  Liquid inks and dyes could also be washed onto the papers for more color.  These large sheets were torn down to create pages for the artist’s book "Life Cycles" (center photo).

This photo is of a small print with added salt