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Concrete Acid Stain Using Iron

There are a couple of approaches one can take to making an iron-based acid stain which will give you a concrete stain that produces various rusty shades. Depending on your patience and pocket-book, here are some ideas:
  1. more time than money approach to making iron-based acid stain
  2. instant gratification approach to making iron-based acid stain
  3. some notes on using stain once you have it

 

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The Slow Approach to Iron-base Acid Stain

  1. Obtain some muratic acid (any hardware store should have it).
  2. Obtain some old bent nails (a couple pounds should be more than plenty), or anything made out of iron for that matter.
  3. Drop the iron into the acid.
  4. There will be some bubbling and fizzing as the materials react, so if you drop the iron directly into a gallon container, don't screw the lid back on tightly.
  5. Once the acid has taken on a deep, dark rust color, you will have some usable stain.

    It will likely take at least a couple of weeks for this approach to yield a usable stain. I don't use this approach to making iron stain very often, so I've not experimented with it enough to develop a reliable feel for outcomes. But you should acquire a usable stain on a much quicker time scale than when taking the slow approach to making a copper-base stain.

NOTE: Be sure to keep the container covered...acid fumes will badly corrode any exposed metal nearby.

 

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The Fast Approach to Iron-base Acid Stain Making Acid Stain

  1. Obtain some muratic acid (any hardware store should have it).
  2. Obtain some BLACK iron oxide...check with your local ceramic supply. Iron oxides are frequently used as a component in glaze formulas. It is generally inexpensive (around $2-$3/lb), so this is a great way to make some cheap stain.

    Note that you can buy yellow, red, and black iron oxides...so let me stress, be sure to get the BLACK oxide, even if the others are cheaper. My experience is that red and yellow don't react well with acid. If instant gratification is what you're after....get black iron oxide.

    Finally, note also that red, yellow and black iron oxides form the basis of virtually all earth-toned, integral concrete pigments, so if you also want to experiment with colored concrete, your trip to the neighborhood ceramic supply company would be a good time to stock up, although you can also buy integral pigments for concrete at hardware stores.

  3. Sprinkle about one-eight-cup of the iron oxide into about two cups of acid. The reaction between the acid and oxide is not nearly as exothermic as the copper-oxide-acid reaction, but I think you'll find the container warm up as you add the oxide.
    Make sure you exercise proper precautions: Use goggles and rubber gloves and perform the operation outside or somewhere an acid spill would be relatively inconsequential if things go horribly awry.
  4. The photo to the right shows the result of adding about one-eight-cup of black iron oxide to about two cups of muratic acid. It is ready to use in only a few minutes.

    For tips on using it, see below...

NOTE: Again, always be sure to keep the container covered...acid fumes will badly corrode any exposed metal nearby.

 

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Using Home-made Acid Stain
I have virtually no experience with commercially made stains, so the approach outlined here is probably not transferrable to "off-the-shelf" stains. Nor is the reverse likely to be true. But who knows?....I simply don't know.
  • Full strength muratic acid is very corrosive and will quickly eat away cement paste if applied too liberally. When using the stains you make yourself using the approaches outlined here (both for copper and iron), I suggest watering them down.
  • Take a small amount of the stain, which is dark and opaque (or should be) to begin with and add water until you achieve a light straw color (see the images to the right).
  • The light color is deceptive. It is capable of producing a nice, deep brown color.

    NOTE: After adding water to dilute the pH of the straight acid I sometimes will add some additional oxide...not much...just sprinkle a bit more into the mixture and stir some. Operating on the premise that the greater the concentration of metal atoms the deeper will be the color of the stain, adding enough until one obtains good saturation seems like a good approach. If you find some oxide not dissolving completely and subsequently collecting at the bottom of the container, not to worry, just recycle it for later use.

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  • Most everything I work on has vertical surfaces, so if you are wanting to stain a horizontal surface, my suggestion is...experiment, experiment, experiment on test pieces yourself to develop a feel for results before you commit to staining what you really want to work on.
  • Using the watered-down acid solution, take an old brush and begin applying it in relatively small amounts. I just barely dip the brush and repeatedly go over the same area. I generally try to prevent "runners"...i.e. rivulets of stain that roll down the side of the piece below the area I'm currently applying stain to. Since "runners" are somewhat inevitable (acid is basically just water after all), I at least "catch them", stroking upward toward the area I'm working on.

    Note that if you're partial, you might find the effect of "runners" appealing and simply let them go. Different strokes, as the saying goes (no pun intended).

  • The final two pictures shows a piece before and immediately after applying the iron-based acid stain made by adding one-eight cup black iron oxide to two cups of muratic acid.
  • The piece is still wet in the final picture, but the final appearance wouldn't be much different if one applied a high-gloss acrylic sealer at the end of the process. (NOTE: this piece was eventually used as the pedestal for a birdbath.)
  • Even though color develops relatively quickly with iron stain, I leave it overnight as a general rule to allow whatever reaction is taking place to have plenty of time to run its course.
  • The staining process leaves a slight residue which will need to be washed and scrubbed off.
  • After cleaning the surface and allowing it to dry completely (usually another day or so), one can apply sealer if one so chooses. But its entirely up to you....you're in charge at this point.
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