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Removing Copper/Bronze Stains From Limestone And Marble
REMOVING COPPER/BRONZE STAINS FROM LIMESTONE AND 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 MASONRY OR DRIVE THE STAINS DEEPER INTO THE
POROUS STONE. USE THE PRODUCTS AND TECHNIQUES DESCRIBED HERE ONLY
FOR THE COMBINATIONS OF DIRT/STAIN AND STONE SPECIFIED.
PART 1---GENERAL
1.01 SUMMARY
A. This procedure includes guidance on removing copper and
bronze stains from limestone and marble. Several methods
are described.
B. Copper stains appear as green or muddy-brown
discolorations and result from the action of moisture on
nearby or embedded bronze, copper and/or brass items.
C. Safety Precautions:
1. DO NOT save unused portions of stain-removal
materials.
2. DO NOT store any chemicals in unmarked containers.
3. EXCELLENT VENTILATION MUST BE PROVIDED WHEREVER ANY
SOLVENT IS USED. USE RESPIRATORS WITH SOLVENT
FILTERS.
4. No use of organic solvents indoors should be
allowed without substantial air movement. Use only
spark-proof fans near operations involving
flammable liquids.
5. Provide adequate clothing and protective gear where
the chemicals are indicated to be dangerous.
6. Have available antidote and accident treatment
chemicals where noted.
D. 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
3. Submittals
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 general information on the characteristics, uses and
problems associated with limestone, see 04460-01-S; for
marble, see 04455-01-S.
PART 2---PRODUCTS
2.01 MATERIALS
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. For Light-colored Stains:
1. Sodium Hydrogen Citrate (NaC6O7H7) - (appears like
enlarged salt granules):
a. Other chemical or common names include Citrate
of soda*.
b. Available from chemical supply house, drug
store or pharmaceutical supply distributor.
2. Glycerine:
a. A sweet syrupy hygroscopic trihydroxy alcohol
usually obtained by the saponification of fats
and used especially as a solvent and
plasticizer.
b. Other chemical or common names include
Glycerol; Glyceryl hydroxide; Glycyl alcohol;
1,2,3-propanetriol; Propenyl alcohol.
c. Potential Hazards: FLAMMABLE.
d. Available from chemical supply house, drug
store or hardware store.
3. Cotton-wool pads
B. For Sharply-outlined Stains:
1. Sodium Hydrogen Citrate (See 2.01 A.1. above)
2. Crystalline Sodium Thiosulfate - white sal or
"hypo" of photographic fixing agent (NA2S2O3):
a. A hygroscopic crystalline salt used especially
as a photographic fixing agent and a reducing
or bleaching agent.
b. Other chemical or common names include Sodium
hydrosulfite; Sodium Hyposulfite; Sodium
subsulfite; Antichlor*; Hypo*; Hyposulfite of
soda*.
c. Potential Hazards: TOXIC; CORROSIVE TO
CONCRETE, STEEL, WOOD OR GLASS.
d. Available from chemical supply house, dry
cleaning supply distributor, drugstore or
pharmaceutical supply distributor,
photographic supply distributor (not camera
shop), or water and sanitation supply
distributor.
3. Cotton wadding
C. For Poulticing:
1. Ammonium Chloride - salt-like substance (NH4Cl):
a. A white crystalline volatile salt that is used
in dry cells and as an expectorant.:
b. Other chemical or common names include
Ammonium hydrochloride; Chloride of Ammonia*;
Hydrochloride of Ammonia*; Muriate of
Ammonia*; Sal Ammoniac*.
c. Potential hazards: TOXIC; CORROSIVE TO FLESH;
CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
d. Available from chemical supply house, dry
cleaning supply distributor, drugstore or
pharmaceutical supply distributor, or hardware
store.
2. Ammonium Hydroxide (NH5O):
CAUTION: DO NOT MIX AMMONIA WITH CHLORINE
BLEACHES, A POISONOUS GAS WILL RESULT! DO NOT USE
BLEACH ON BIRD DROPPINGS.
a. A weakly basic compound that is formed when
ammonia dissolves in water and that exists
only in solution.
b. Other chemical or common names include Ammonia
water*; Aqua ammonia*; Household ammonia*.
c. Potential hazards: TOXIC; MAY IRRITATE THE
EYES.
d. Available from chemical supply house, grocery
store or pharmaceutical supply distributor, or
hardware store.
NOTE: TRADITIONAL USAGE OF AMMONIA DIRECT OR IN A
PASTE WITH WHITING IS ONLY SUCCESSFUL ON LIGHT
STAINING.
3. Ethylene diamine-tetra-acetic acid (EDTA):
a. EDTA is chelating agent used to remove
metallic stains from masonry.
b. Chelating agents are molecules that coordinate
metal ions together. The metal ions in the
stain attach themselves to the chelating agent
to form either a soluble or insoluble metal
complex that can then be removed.
c. Available from chemical supply house.
d. It is available as an acid or in the form of
its more soluble sodium salts. It is known to
be used as a preservative. Its most important
use, however, is in the removal of unwanted
metal ions from water, as in the manufacturing
of agricultural chemical sprays.
4. Appropriate filler such as Attapulgite clay, talc,
diatomite, or ground or powdered chalk
D. Mineral water
E. Distilled water
F. Accessible source of clean, potable water, soap and
towels for washing and rinsing in case of emergencies
associated with the use of chemicals
2.02 EQUIPMENT
A. Glass or ceramic container for mixing the solution
B. Bowl for mixing chalk paste
C. Wooden utensil for stirring the ingredients
D. Wood scrapers or plastic spatula
E. Plastic sheets
F. Clean dry towels for blotting the area after treatment
G. Masking tape
H. Rubber gloves to prevent skin irritation
I. Glass plates to cover pads
J. Weights
PART 3---EXECUTION
3.01 EXAMINATION
A. Examine the masonry surface CAREFULLY to determine the
cause of staining to ensure the required treatment is determined before proceeding with any cleaning operation.
3.02 PREPARATION
A. Protection: Provide adequate wash solutions (i.e. water,
soap and towels) before starting the job.
B. Surface Preparation: If possible, remove the sources of
moisture to prevent further oxidation of the metal.
Where the source of the stain is an embedded
anchor, tie, or other device, the only non-destructive remedy is to
eliminate the moisture at its source.
3.03 ERECTION/INSTALLATION/APPLICATION
NOTE: DO NOT TRY MORE THAN ONE TREATMENT ON A GIVEN AREA
UNLESS THE CHEMICALS USED FROM PRIOR TREATMENT HAVE BEEN
WASHED AWAY.
A. For Light-Colored Stains: Apply an active agent with
cotton-wool pads:
1. Thoroughly cleanse the area to be treated with
mineral water.
2. Put on the rubber gloves to avoid contact with the
chemicals to be used and to avoid unnecessary skin
irritation.
3. Prepare a 15% aqueous solution of sodium hydrogen
citrate with some glycerine (to deter evaporation)
in the appropriate container.
4. Dampen the cotton-wool pads in the solution and
apply them to the area to be treated.
5. Cover the pads with the glass plates.
6. Apply weights on top of the glass plates to insure
close contact between the pads and the stained
stone.
7. Allow the pads to set 3-4 days, before removing
them.
8. Remove all the equipment including the pads,
thoroughly rinse with mineral water and allow to
dry.
9. If there is residual staining, repeat the process
to achieve the desired level of cleanliness.
B. For Sharply-outlined Stains: Apply an active agent with
cotton wadding:
1. Rinse the area to be treated with distilled water.
2. Put on the rubber gloves to avoid unnecessary skin
irritation.
3. Mix a 15% aqueous solution of sodium hydrogen
citrate in the glass or ceramic container.
4. Form a paste of ground chalk and distilled water.
5. Soak the cotton wadding in the sodium hydrogen
citrate solution and apply them to the stained
area.
6. Spread the chalk paste over the cotton wadding.
7. Sprinkle the crystalline sodium thiosulfate over
the paste.
8. Allow the chemicals to act on the stain for one
hour.
9. Remove all of the materials with a wood or plastic
spatula.
10. Thoroughly cleanse the area with mineral water and
allow to dry.
11. If there is residual staining, repeat the process
to achieve the desired level of cleanliness.
-OR-
C. Apply a poultice of ammonium chloride, ammonium
hydroxide, EDTA, and attapulgite clay (see also 04455-02-R
for additional guidance on poulticing).
1. Prepare the poultice mixture:
a. Add 70 g of ammonium chloride to 570 ml of
ammonium hydroxide; Add water to make the
volume 1 liter.
b. Add 37 g of EDTA to the ammonia water.
c. Add attapulgite clay to form a soft paste.
2. Pre-wet the stain with clean water to prevent too
deep a penetration of the chemical cleaning agent.
3. Apply the poultice mixture to the stained surface
in layers no more than 1/4 inch thick.
4. Cover the poultice with sheets of plastic taped
against the wall in order to prevent too quick of
an evaporation.
5. Re-wet the poultice as needed and leave to dry for
several days.
6. When the paste has dried, remove it with a wood or
plastic spatula.
7. Thoroughly rinse the area with clean, clear water
in order to remove any chemical residue and allow
to dry.
8. Repeat this process as often as necessary to
satisfactorily lift or lighten the stain.
END OF SECTION
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