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Historic Preservation - Technical Procedures
Removing Mildew Stains From Marble
Stain Removal Guide For Stone - F. M. Hueston
Removing Mildew Stains From Marble
REMOVING MILDEW STAINS FROM MARBLE
THE CLEANING OR REMOVAL OF STAINS FROM STONE MAY INVOLVE THE USE OF
LIQUIDS, DETERGENTS OR SOLVENTS WHICH MAY RUN OFF ON ADJACENT
MATERIAL, DISCOLOR THE STONE OR DRIVE THE STAINS DEEPER INTO POROUS
STONES. USE THE PRODUCTS AND TECHNIQUES DESCRIBED HERE ONLY FOR
THE COMBINATIONS OF DIRT/STAIN AND STONE SPECIFIED.
A. This procedure includes guidance on removing mildew from
marble surfaces. Mildew stains from fungus, algae or
other living plants are typically black, green, blue,
orange or blotchy white in color and tend to disappear
when the offending object has been removed. This
procedure may be used to remove those stains which
B. Biological growths such as lichens, algae, moss and fungi
growing on stone masonry walls is usually an indication
that there is excess moisture in or around the stone
masonry. These growths should be removed, as they
attract moisture to the masonry surface and hold it
there, which can lead to more serious problems. Lichens
and mosses in particular, produce oxalic acid which can
damage certain types of historic masonry.
C. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO).
E. For information on removing other organic stains from
marble, see 04455-14-R.
F. For general information on the characteristics, uses and
problems associated with marble, see 04455-01-S.
NOTE: Chemical products are sometimes sold under a common
name. This usually means that the substance is not as pure as
the same chemical sold under its chemical name. The grade of
purity of common name substances, however, is usually adequate
for stain removal work, and these products should be purchased
when available, as they tend to be less expensive. Common
names are indicated below by an asterisk (*).
A. Household Bleach:
1. Other chemical or common names include Bleaching
solution*; Laundry bleach*; Sodium Hypochlorite
(NaOCl); Solution of chlorinated soda*.
2. Potential Hazards: CORROSIVE TO FLESH.
3. Available from chemical supply house, grocery store
or supermarket, hardware store or janitorial supply
B. Mild dishwashing detergent such as "Ivory Liquid", "Joy",
or approved equal.
C. Clean, potable water
D. Clean dry towels for blotting the area after treatment
A. Plastic spray bottle
A. Verification of Conditions: Examine the marble surface
CAREFULLY to determine the cause of staining before
proceeding with any cleaning operation.
B. Test cleaning procedure in a small inconspicuous area to
determine the effectiveness of the treatment. If the
stain/mildew is not removed, contact a stone specialist
or consult RHPO. Some biological stains are very
difficult to identify and require laboratory analysis.
A. Protection: Provide adequate wash solutions (i.e. water,
soap and towels) before starting the job.
3.03 ERECTION, INSTALLATION, APPLICATION
NOTE: DO NOT USE BLEACH ON DARK COLORED STONES AS THIS WILL
CAUSE THE STONE TO LIGHTEN.
A. Thoroughly rinse the area to be treated with clean, clear
B. Mix a solution of 3 parts household bleach with 1 part
water and a dash of dishwashing detergent in a spray
C. Thoroughly moisten the stained surface with this liquid
by misting the surface using the spray bottle.
D. Mist the surface continuously until the stain disappears.
E. Rinse the surface with clean, clear water and allow to
F. Once the surface has dried completely, check for
remaining residue and repeat the treatment if necessary.
G. For alternative guidance in removing biological growth
from masonry, see 04200-02-R.
END OF SECTION