OXIDATION OF ELEMENTAL IRON WITHIN STONE
SURFACES(YELLOWING OF STONE SURFACES)
Frederick M. Hueston
Sitting
in my office one morning I received a call from a very upset homebuilder. He
told me he was building a
two
million-dollar home on the west coast of Florida. They installed nearly 3500
square feet of a white statuary
marble
tile. Over the weekend one of the water pipes broke in a bathroom and
completely flooded the home.
They
managed to vacuum all the water and started to access the damages. Beside
warped wood, soaked drywall
and
an irate homeowner the marble tile seemed fine except for some minor water
spotting. After several weeks
the
replacement of warped wood and drywall began and then he noticed the white
marble tile turning yellow. At
first
he thought it might be some type of residue so they tried cleaning the marble
with some bleach and waterthe
yellowing
was still there. The homeowner was getting more and more irate and was
threatening a lawsuit.
He
asked if I could get down there right away and take a look at the marble and
suggest what to do.
The
above story is not uncommon and is a frequent occurrence on white marble tiles
exposed to flooding. Can
the
yellowing be removed or does it need to be replaced? What causes the yellowing
and will it get worse. The
following
is an explanation of yellowing in white marble and some techniques that may
help.
Why
Does White Marble Turn Yellow?
The
problem of yellowed white marble is not uncommon. All over the United States I
have encountered yellow
to
brown marble. Although flooding is a common cause there are several other
reasons this color change will
occur.
1.
Improper Maintenance- As marble wears the highly polished surface begins to
disappear. The wearing of
this
polish causes the surface to become rough and is a magnet for dirt. If improper
cleaners are used, this dirt
begins
to accumulate in the pores of the stone and will turn yellow. It is surprising
how often I have seen this
condition
on marble. Upon investigation in these cases I have found dirty mops being
used. Mops used to
clean
the restrooms and/or kitchens were also used to clean the marble floors. Floors
are mopped with strong
cleaners
or wax cleaner combinations or with no cleaners at all.
Cure: If you suspect
yellowing due to improper maintenance the marble tile will have to be cleaned
with an
alkaline
marble cleaner. I would suggest a heavy duty stone cleaner. Be sure the stone
cleaner you buy is
alkaline
and not an acid since acid cleaners will dull the polish. Apply the cleaner to
the marble and scrub with
a
soft brush. Be sure to rinse the floor throughly. It may be necessary to repeat
this procedure several times to
remove
all the imbedded dirt. If after cleaning the marble is dull I would suggest
re-polishing and an
application
of a good quality penetrating sealer(impregnator). If after several cleaning’s
the yellowing is not
removed
than proceed to the next cause.
2.
Wax Build-up or Coatings- Many marble floors are coated with waxes, acrylics,
urethane and other coatings.
Many
of these coatings are not specifically designed for use on marble floors. Some
of these coatings are of
poor
quality and will begin to yellow. It is not uncommon for coating to be applied
in multiple coats. As the
coating
builds up it becomes soft and dirt is easily embedded in the soft layer. These
coatings require frequent
stripping
which is often neglected.
Another
process used for polishing marble floors is a process known as
recrystallazation. If this process is
applied
to a white marble floor that contains moisture it will turn the marble yellow.
If this process is to be
used,
it is important that the marble be dry.
CURE:
To remove yellowing due to a wax or coating buildup the marble will need to
stripped with a
commercial
wax stripper. I would strongly suggest using a stripper manufactured by the
same company as the
floor
wax or coating. This will help avoid incompatibility problems. Follow the
directions on the stripper’s
label
and be sure to rinse the floor throughly. These strippers often require the use
of abrasive pads which can
scratch
and damage the marble surface. Before undertaking the entire project perform a
small test to determine
results.
If
the marble tile has been recrystallized, it will be necessary to remove the
recrystallized layer. This layer can
often
be removed by polishing the tile with a powder marble polish containing oxalic
acid. Apply the powder to
the
tile, added water and work into a slurry with a hog hair pad and a standard
buffing machine. Continue to
work
until yellowing has disappeared. If this technique fails then the tile will
have to be re-honed. It is strongly
suggested
that the polishing and honing procedure be performed by trained individuals. If
these techniques fail
to
remove the yellowing then proceed to the next cause.
3.
Iron Staining- Many white marble tiles contain naturally occurring deposits of
iron. Iron is a mineral found
in
stone and can occur randomly throughout the stone. If iron is present in the
marble tile, it will begin to
oxidize
when exposed to water or other oxidizers such as acids and household bleach.
White marble tiles can
remain
on a floor for years without yellowing then over time may slowly turn yellow
and in severe causes may
turn
completely brown. This oxidation process is accelerated when the tile is
saturated as in the flood in the
above
example. This process of oxidation is similar to the rusting of metal. If you
expose a brand new nail to
water
and air it will turn brown and rust. The same process is occurring with the
iron in the marble. If water
and/
or air is eliminated the iron will not oxidize. This is the reason certain
white marble suddenly turn yellow.
The
process is difficult to reverse and replacement of the tile may be necessary.
The following stain removal
technique
has proved successful in several cases. Before testing this procedure it is
important to first determine
if
iron is the cause.
Testing
for Iron:
1.
Before assuming the marble is yellowed due to iron be sure to attempt cleaning
and stripping as outlined in
#1
& 2 cause above. If these procedures fail then testing for iron will be
necessary.
2.
If a flood has occurred or excessive water was used first check the water for
iron. There are several
inexpensive
test kits available that can be used to check the iron content in water. Check
with your local
plumbing
supply store or store carrying water softening supplies. If any amount of iron
is detected then it is
possible
iron has entered the stone through the water supply. To eliminate the iron
there are chelating chemicals
that
can be added to the water to prevent the iron from staining. This is very
important if the tile is cleaned withthis water.
3.
If the water contains no iron and even if it does the tile should be checked
for iron content. Remove one tile
and
contact a testing lab and have them analyzed the tile for total iron. If there
are spare tiles that have never
been
installed also have them tested for total iron. If iron is present naturally in
this stone, it will probably be
detected
in the spare tile. If the results return with iron present then the following
procedure should be tested.
4.
Check the tile for moisture. A moisture meter is a useful instrument that can
be employed to check the tile
for
moisture. If the tile contains water, it is very possible that iron is
beginning to oxidize.
Removing
Iron Staining:
1.
Prepare a solution of water and the following chemical: Sodium Hydro sulfite
and Sodium Metabisulfite.
These
chemicals are available in a product called Iron-Out(TM) from your plumbing
supply or home center.
Mix
a solution in water and apply to the effected tile. Allow solution to soak into
tile and keep wet for several
hours.
Do not allow solution to dry. After several hours pick up excess solution with
a wet vacuum and rinse
throughly
with water and a chelating agent such as EDTA. Be prepared to repolish the
marble since these
chemicals
can cause etching.
2.
If the above procedure fails than prepare a poultice with diatomaceous earth
and the Iron Out(TM). Mix the
poultice
into a thick paste and apply to a small area. Cover the poultice with plastic
and allow it to sit covered
for
24 hours. After 24 hours remove the poultice paste and rinse the area with
water and a chelating agent. If
the
stain is removed, the entire floor can be treated. If the stain still remains
then replacement is the only
solution.
3.
There are also some new chemicals that are available which contain Ammonium
Thioglycolate which look
promising
for removing iron oxidation. Check with several stone maintenance supply
companies.
Before
the above procedure can be performed, it is important that the effected tiles
be dry. If water or moisture
are
still present, oxidation of iron may continue
The
yellowing of white marble is a common problem. New installations should be
sealed with a good quality
penetrating
sealer(impregnator) Which will help prevent oxidation of the iron by
eliminating moisture.
The
above procedure has proven successful in some cases of iron staining however if
the outlined test does not
produce
the desired results I would recommend replacement of the effected tiles.
Written
by:
Frederick M. Hueston
Stone Forensics
www.stoneforensics.com
