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Name: Chris
Status: other
Grade: other
Location: WI
Country: N/A
Date: August 2007

Peter Faletra suggests using muriatic acid to clean limestone. I thought that this acid can discolor limestone. Can you please explain?

Unless the muriatic acid contains some impurities, or unless the limestone contains impurities, it should not discolor limestone. Muriatic acid is just hydrochloric acid. The acid dissolves the surface layer of the limestone, which thus releases the dirt on the stone's surface.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming

It certainly eats into limestone. Maybe more than you want, and I think I have had sea-shells discolored by pool-use muriatic acid.

But you may want something with such positive erosive power, if you:

- "test on an inconspicuous spot" and actively manage its action by:

- learning how much, if any, to dilute it.

- brushing while applied (so it attacks surface evenly instead of following seams and pores) (Wear gloves, goggles, and dust mask because of the splatter that will cause)

- diluting it away with water flush soon after applying, and applying again if you want more done, (Weakened acid and colored reaction-products should not be allowed to hang around.)

- neutralizing with dilute (2%?) baking soda solution before you walk away.

I think proper use would actually be quite attention-intensive, the opposite of "walk-away-and-let-it-work".

Muriatic acid might not be especially pure HCl. I do not know what the impurities are. Some Sodium (i.e., salt), from cheap simple manufacturing. That is not a problem, but other impurities might be. I am wondering if a cleaner reagent-grade hydrochloric is less like to discolor.

I am wondering if a diluted nitric acid would be a little safer too. I would avoid HCl and nitric together. I get the impression that HCl + oxidizer causes the discoloration. But I could have it 180 degrees wrong. Maybe 1% peroxide would help HCl. (I think peroxide might arouse less fumes than nitric.)

A hypothesis:
Your limestone may well have an 0.01%-0.1% fraction of copper oxides in it, and perhaps some reducible organic material in the voids between mineral grains.

The copper dissolved in HCL will make a faintly bluish copper solution, which could precipitate dark copper if the organics reduce it, or precipitate blue-green copper oxides if the acid is allowed to sit and get weak.

It does not take much of either kind of copper to discolor the subtle off-white shades of limestone.

Even Iron, which definitely should be in your limestone at a higher level than copper, might re-precipitate as a darker surface discoloration if initially strong acid is allowed to be stand and get weak

Acid reaching way into cracks and leaching out metals would create some kind of lightening, bleaching effect too. It might change natural yellowish to sick or boring grayish.

So frequent rinsing and applying more makes some sense. I have some thought that the rinses should last longer than the acid-attacks, so the diffusion-averaged attack rate is low deep in narrow cracks. I wonder if including some soap would help keep the colored metals flushable off the eroding stone surface. I wonder if a little EDTA to chelate dissolved metals would help. I wonder about a powder mixed into the acid such as starch or colloidal silica, to catch some precipitates instead of the stone's porous new surface. Unfortunately I do not know what commercial acidic products exist particularly for this.

But you are kind of on your own in the last analysis; so use your own judgment and be very cautious, even skeptical, if bad results are not acceptable. One stone could react differently than another, and one application could get longer than your practice application, and your real use might well have more re-applications than your practice run.

Difficult to ever get scientific repeatability on your side, to ever know what it is really going to do next. Someone with lots of experience might begin to know.

Jim Swenson


Muriatic acid is also known as hydrochloric acid. It is a very strong acid and can corrode a good number of household surfaces.

The major component in limestone is calcium carbonate, which is a weak base. Acids such as hydrochloric or sulfuric (acid rain) will damage and etch the surface of limestone, so do not use acid to clean limestone. You should try basic (alkyline) or neutral pH cleaners to clean limestone. There is a product called stone soap that should be available at any home supply store. Also, if there are really bad stains, then you might try bleach on a small, out the way area first to see if you get a suitable result. Bleach can contain small amounts of hydrochloric acid, so be careful!

Matt Voss

Hydrochloric acid (common name "muriatic acid") is commonly used to "clean" limestone (largely calcium carbonate). The mechanism is to dissolve the calcium carbonate producing carbon dioxide and calcium chloride. However, the natural mineral may contain other minerals, especially iron, that may discolor the limestone when the calcium carbonate is dissolved. So what is one to do? First, use dilute acid. Second, test the treatment out on an area that is not visible so that any mishaps are minimized. Third, use a commercial cleaner. The commercial cleaners will be formulated with agents that will complex the possible staining substances. In addition, you did not mention whether the objects are exterior or interior, which can have a bearing on the choice of cleaning agents.

Unless the object is large, or there are some other considerations, it is usually economical in the long run, to use a formulated commercial cleaner so that you do not have to make all the mistakes that others have made.

Vince Calder

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