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Normally when I build a prop, I use steel, or cover it with glue and iron dust, and then apply various oxidizing compounds, acids (and sometimes salts and bases) to make a real rust. Then, I gently wash it and seal, or not.

This is great for a photoshoot, because nothing can beat real rust in "look like rust" contest. Sadly, removing 100% of the chemicals is not possible and I end up with stuff that can irritate gentler skin.

How can I imitate rust on props for LARP or "accessorized RPG", that would look rather real, but would be safe and non-allergic even with prolonged handling?

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    \$\begingroup\$ Commenting as its not a complete answer - Look up plastic modelers (like model airplanes and model railroads) and their techniques on "weathering". These techniques are applied to plastic models using paint, colored chalk, and sealants to make them look like real life metal objects that have been outdoors and picked up some corrosion. \$\endgroup\$
    – Freiheit
    Jun 10 at 17:37
  • \$\begingroup\$ A note from a blacksmith you can achieve rust with chemicals that will not irritate it takes longer but not that much longer. it is also a lot easier to neutralize. Here is a method I have used successfully. bobvila.com/articles/how-to-rust-metal \$\endgroup\$
    – John
    Jun 13 at 3:34
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Cinnamon.

Use adhesive to paint the parts where you want rust, pour on cinnamon and seal it off with a clear coat. It will not smell like rust, but it will look good enough for most practical cases and is safe. Experiment with a bit of color in the cinnamon to get variations.

This was used a lot in the swedish post-ap LARP "Blodsband", to the point where the title was jokingly referred to as "Blodsband: A smell of cinnamon".

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    \$\begingroup\$ Do you have any photographs of this application? \$\endgroup\$ Jun 9 at 11:13
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    \$\begingroup\$ Here, a tutorial for the thing. \$\endgroup\$
    – T. Sar
    Jun 9 at 19:04
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    \$\begingroup\$ Szandor, if it matches what you've done in the past please could you edit into your post the relevant pictures? \$\endgroup\$ Jun 9 at 19:09
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    \$\begingroup\$ Just be aware - there are a non-trivial number of people who are allergic to cinnamon. Prevalence is unknown but it’s probably in the same order as those who are irritated by rust. \$\endgroup\$
    – Dale M
    Jun 10 at 3:40
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    \$\begingroup\$ @DaleM I'm not afraid of rust itself. I'm mostly afraid of the rust accelerators I use. They can be hard to keep under sealant as they're quite reactive. \$\endgroup\$
    – Mołot
    Jun 10 at 10:15
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Use rust

It might sound mad, but you can use actual rust! But not the type made on the item, but you can buy rust as a pigment. To apply it, I prefer to gently powder and brush the dry pigment into a thin layer of glue called "Anlegemilch" (Gilding Dispersion), the same stuff one would use for adding leaf metal. That stuff takes about 20 minutes to pre-cure, then you add the leaf metal or pigment. For pigments and glue, use a broad brush and poke the surface with it instead of brushing. Note that the gilding dispersion should not be touched with bare skin while it is not yet fully cured.

Other people swear on applying their pigments with a mixture of water, turpentine, or alcohol to get very light adhesion of the rust pigments to the item. Other people use acrylic paints as a binder before applying pigments in a similar fashion to me using gilding dispersion. In my opinion, it's better to use actual glue as that keeps the pigment where it belongs just a tad better than turpentine, though paints allow giving a little color character into the rust. Adding some clear lacquer afterward seals it in, so even if you go with the adhesion-only way, you can make it stay.

If you want to get the texture up, you might mix wood glue with baking powder before adding the pigment, creating a rough texture.

The result is actual rust with the real rust color in various shades, but you will require quite some skill to add the glue and the lacquer nicely. Also, Rust pigment is quite fine and you will need to wear a mask to work with it.

Some hints on working with pigments can be gleaned from here using a turpentine/alcohol base, the Isopropyl base is explained in this tutorial, and you can make your own rust by just soaking cheap steel wool in water for a few days to weeks, like in this tutorial - No chemicals but water and iron involved. Adding a little water again can alter the shade of the rust.

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  • \$\begingroup\$ Iron oxide has been used as a pigment for basically as long as humans have been using pigments. \$\endgroup\$
    – John
    Jun 13 at 3:27
  • \$\begingroup\$ @John including in the shape of blood, yes. \$\endgroup\$
    – Trish
    Jun 13 at 7:50
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Originally a comment, but posting as an answer as requested.

Change how you treat/clean the pieces after rusting them.

The exact changes depend on the exact mix of chemicals you use to produce the rust, but in general most of the common options have some way you can get rid of a significant percentage of them.

As a specific example, one of my friends who is a rather active LARPer typically uses a blend of hydrogen peroxide, acetic acid (distilled white vinegar), and iodized table salt. In general with such a mix, the primary potential sources of irritation are any residual salt, any metal halide salts that formed as a result of interaction between the salt and the metal, and any residual hydrogen peroxide.

The salt and metal halides can be gotten rid of by simply rinsing thoroughly a couple of times with a lot of hot (almost boiling) distilled water (he typically uses a few liters even for a small piece). The stuff you care about (such as the iron oxides) is insoluble in even boiling water, so this should not remove any of the newly formed rust unless you also try to scrub the piece. Be careful with the water from this though, as it will probably be mildly acidic. It should be safe to pour down a regular kitchen drain, but might cause mild irritation if you get it on your skin, and you do not want to get it in your eyes.

The hydrogen peroxide is a bit trickier. It will eventually dissipate by itself by decomposing into water and atomic oxygen (and probably cause a bit more rusting in the process). You can accelerate this by either baking the piece at a 100-200 degrees Celsius for a few hours (preferably in an area with good ventilation), or washing with a solution of potassium iodide (also in a well ventilated area). The potassium iodide will in theory do a better job because it chemically catalyzes the decomposition, but it may discolor non-metallic parts because it produces elemental iodine, and should still be rinsed thoroughly with distilled water afterwards (ironically, this catalysis is actually why iodized table salt is usually included in mixes that use hydrogen peroxide). The heating approach obviously won’t work in some cases, but is a bit easier and a bit safer when it is an option.


On a side note, the skin irritation may actually be caused by two other aspects of the piece that have nothing to do with how it was treated to produce rust:

  • The rust itself may actually be a source of irritation, especially if it's a particularly rough and uneven layer. A lot of people with sensitive skin will react to abrasive materials much more readily than most others, and this can be a contributing factor.
  • The metal may have a high nickel content. Metallic nickel actually causes contact dermatitis for some people with prolonged exposure (among many other nasty things it can do to your body). This is actually why ‘hypoallergenic’ metal watch bands exist, they have no nickel in them. Good quality steel usually has a nickel content of about 2.25%, which is high enough to cause such a reaction. Corrosion makes this worse though, because many of the salts of nickel (including the oxide) also cause similar reactions.
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I have never attempted this, but between people I trust who have (though not for a LARP), my knowledge of the physics involved, and some double-checking I’ve done on the internet, I think you could achieve real rust, formed from the item itself, much as you are doing now, but with no harsh chemicals introduced.

Specifically, there is a notorious statue at my alma mater known as “Old Rusty,” since it is very thoroughly rusted. The story of how it got to be that way is told to every freshman: when it was installed, the artist boasted that it was “rust proof”—a very unwise assertion to make to an irreverent and skeptical engineering program. Story goes, the night after its installation, several students slathered it in ketchup and hooked it up to a car battery—rusting it very thoroughly almost instantly.

Searching online certainly demonstrates numerous incidents of even tiny electrical currents accelerating oxidation by incredible amounts. It also makes sense from a chemical/physical perspective—and the school I attended was very much the sort of place that would not have gotten away with repeating that story if it was impossible, since too much of the student body would know.

You would have to do more research to figure out the right voltage, and if ketchup is really the best choice for corroding agent, and if the process might create chemicals more harsh than any you are introducing. There are probably better ideas here. But if it works as advertised, it might be the most natural rusting effect available without any kind of harsh chemical.

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  • \$\begingroup\$ I know for a fact that Dijon Mustard causes formation of black iron oxide. Use it on knives regularly because it's non stick and prevent "red" rust. So, I can totally believe the ketchup story :D \$\endgroup\$
    – Mołot
    Jun 11 at 18:05
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    \$\begingroup\$ The process you're referring to is called electrolysis - most commonly used to remove rust, but can just as easily be used to induce it by swapping the poles. Don't ever attempt this with stainless or galvanized steels, though - it produces some aggressively nasty byproducts that are illegal to dispose of in many jurisdictions. \$\endgroup\$ Jun 12 at 9:57
  • \$\begingroup\$ @Mołot Black iron oxide also can be made by boiling metal in water - this is used to conserve guns. \$\endgroup\$
    – Trish
    Jun 12 at 21:16
  • \$\begingroup\$ @Trish boiling is most effective at converting red rust to black :) for creating brand new, mustard is, in my experience, faster. \$\endgroup\$
    – Mołot
    Jun 13 at 9:20

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