Cleaning Limestone and Muriatic Acid
Date: August 2007
Peter Faletra suggests using muriatic acid to clean
limestone. I thought that this acid can discolor limestone. Can you
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
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
- 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
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
That is not a problem, but other impurities might be.
I am wondering if a cleaner reagent-grade hydrochloric is less like to
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.)
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
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
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
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
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
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
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.
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!
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.
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Update: June 2012