Ask A Scientist Chemistry Archive

 
name         Joan M. B.
status       other
age          40s

Question -   What can I use to clean hard water scale and stain from the inside of a 
marble basin?  The Holy Water Fonts in my church have quite the build up. How do I keep 
this from building up again once (IF) the scale and stain is removed?
----------------------
Joan,

Gently swabbing the surface with cotton balls saturated with white vinegar will remove 
some of the stains. Bear in mind, vinegar (a weak acid) will attack the marble basin 
after it spends itself attacking the stains. After swabbing, be sure to rinse the basin 
several times with clean water to remove all traces of vinegar.

The easiest way to stop the stains from reappearing is to use distilled (mineral-free) 
water in the basins and to empty them often and wipe them dry. If the fonts are fed 
water from the municipal supply or a well, you may be faced with installing a water 
softener in the basin feed line.

Regards,
ProfHoff 832
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There are a number of scale removers commercially available. Electric
coffee pot scale removers. However some of these may contain acids, either
hydrochloric or sulfamic acid that could attack the marble if you are not
careful. I would think that the best course of action would be to contact a
"Bed and Bath" store, or a bathroom fixture supplier even a place like Home
Depot and ask for scale treatment product recommendations. It is easier
and more reliable than "playing chemist".

Vince Calder
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I would find something specifically designed to clean marble.  Marble can be
etched with acids like vinegar, and you may ultimately end up with a bigger
problem.  Sand paper would probably scratch it, but there may be milder
abrasives.  With so many marble counters going into peoples' homes now, there
must be something in the cleaning section at Home Depot, Ace Hardware, a home
design center or another major store--ask them for advice.  They may also be
able to help you out with preventing hard water deposits.  Since you have
people interacting with the Holy Water Fonts, dipping fingers in, make sure
you find something that is safe for people.  I wonder if there is a way to
seal the marble surface once you have it cleaned?  Good luck!

Pat Rowe
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It is noticeably difficult to separate hard-water deposits from marble.
They are almost the same chemical, in some ways.
They are both alkaline mineral oxides, and both attacked by acids.
So you might not want to use "CLR" or "Lime-Away" or anything else that includes acids.
If you do, be careful to rise it away quick and put water with baking soda on it to stop 
the acid.
"Test in an inconspicous spot" like they say on strong laundry products.

I don't know if your marble is resin-filled and polished, or rough and holy like Swiss 
cheese.
Rough or porous is a real problem.

Let's hope your marble is stronger and less porous than your scale.
Because the other possible "discriminators" to hopefully affect scale more than your 
marble are:
   scraping at the edge of a patch, applying penetrating oil like "Liquid Wrench" after 
   it's dried of water,
   pounding lightly with wood or plastic or a small blunt metal thing, alternate wetting 
   and dryng,
   making temperature changes with a hair-drier, or sprinkling on liquid nitrogen.
All these depend on how blatantly thick and flaky the scale is.  Thicker is easier! 
because you're trying to convince it to crumble.
All except Liquid Wrench are dis-recomended for fine marble, so you'd need to use your 
judgement.
A friend at work has used a powerful laser that repeatedly zaps the surface, to clean 
statues in Italy.
Might work, but it's "big science", not available locally.
There is a shower scale cleaner/preventer which you use by spritzing the walls at the 
end of each shower.
The soap-like chemical in it dissolves a little scale each time.

I'd use patience in finding the best way.
Merely using purified water and allowing months will slowly improve the situation, at 
little risk.

If it ever does get clean, try to keep an organic film separating the marble from the 
water and scale above it.
"Bathroom Duck" and a new DuPont's Teflon (TM)-containing similar product I just saw on TV both do 
that, clean and deposit some waxy film.
Chandelier-cleaning solution sold by the gallon might be good to add to your holy water, 
in small amounts.
Don't worry, the chemical in it is about like soap.  Just use little enough so people 
don't make suds.
Same for "Quick-N-Brite" and pink pasty soap in a bucket that resembles Crisco cooking 
grease.
And of course any kind of waxing, but that's real work.
Is marble supposed to be water-sealed with oil every few years?  It could be painted on 
underneath.
If the porous marble has tiny amounts of oil constantly creeping out of it, that can help 
the scale not to stick.

I think it would be worth your trouble to use any purified water instead of tap-water.
Tap water has maybe 200 parts-per-million, or 0.02% by weight, of rock dissolved in it.
Not a small amount, if you imagine the water evaporating all year 'round.
Reverse Osmosis water from all those vending machines has about 10 times less.
It's good enough to be my windshield-washer fluid, and not make water-spots all over.
Bottled distilled water has 100 times less.  But probably not worth the money and bother.
I think Brita filtered water is partially softened, if you replace the filter as often as 
they say to.
That way, the 200ppm is salt instead of rock.  Easier to wash away with water, and less 
ready to precipitate.

Last idea:  don't just add water, empty it and fill with new (pure) water. (Can you do 
that with holy water?)
As it evaporates it looses water but keeps all it's dissolved solids.
If you only add water, in the long run the solution must reach saturation and start 
depositing scale.
If you empty the remainder, that takes away the old concentrated solution.
Actually, it might be a bad idea to do this for years, because the water would be so pure, 
so "empty",
that it might start slowly eroding the finish of the marble.

I have a vain hope that that's not too much perspective.

Jim Swenson
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