U.S. General Services Administration
Historic Preservation Technical Procedures
04500-03
REMOVING SOLUBLE SALTS FROM WITHIN BRICK AND STONE MASONRY
THE CLEANING OR REMOVAL OF STAINS FROM MASONRY 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 POROUS MASONRY. USE THE PRODUCTS AND TECHNIQUES DESCRIBED
HERE ONLY FOR THE COMBINATIONS OF DIRT/STAIN AND MASONRY SPECIFIED.
PART 1---GENERAL
1.01 SUMMARY
A. This procedure includes guidance on removing soluble
salts from within brick and stone masonry. Two methods
of salt removal - or desalination - are described and
include the application of a clay poultice and the
application of a sand:lime render.
B. Reducing the salt content of masonry may be necessary if
effloresence (salt deposits) on the surface becomes a
persistent problem.
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
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).
PART 2---PRODUCTS
2.01 MATERIALS
A. Clean natural fiber rags
B. Poulticing Materials:
1. Filler material for poultice: Use absorbent clay
or diatomaceous earth (attapulgite or sepiolite
clay).
a. Available from chemical supply house (both
commercial and scientific), or swimming pool
supply distributors.
2. Distilled water
C. Sacrificial Render Materials:
1. Lime putty
2. Sand
D. Clean, potable water
2.02 EQUIPMENT
A. Plastic sheeting
B. Poultice Equipment:
1. Plasterer's float or broad trowel
2. Galvanized wire mesh
3. Galvanized staples
4. Pliers
5. Plastic sacks
C. Sacrificial Render Equipment:
1. Mesh screens
2. Plasterer's float or broad trowel
3. Hacksaw blade
PART 3---EXECUTION
3.01 ERECTION, INSTALLATION, APPLICATION
A. Applying a Clay Poultice:
NOTE: THIS METHOD OF DESALINATION IS SUITED FOR USE ON
LARGE AREAS OF MASONRY WITH MINIMAL DETAILING, OR ON
SIMPLE MASONRY DETAILS. FOR DELICATE DETAILS OR HIGHLY
DAMAGED AREAS, THIS METHOD SHOULD BE PERFORMED BY A
TRAINED CONSERVATOR.
CAUTION: THIS METHOD IS NOT SUITABLE FOR USE WHERE PRE-
WETTING THE SURFACE CAN ADVERSELY AFFECT PLASTER, PAINT,
OR EMBEDDED WOOD OR METAL.
1. Saturate the wall for several days by spraying with
mists of clean, clear water.
a. Mist the water evenly over the masonry surface
at 18 gallons per hour from fine spray heads
mounted to a boom.
b. The length of time it takes to fully saturate
the wall will depend on the type of masonry,
and its porosity (typically 72 hours).
CAUTION: BE SURE TO COLLECT WATER RUN-OFF AND
DRAIN SAFELY AWAY FROM THE BUILDING DURING THE
CLEANING PROCESS.
2. When the wall is thoroughly saturated, mix filler
material with enough distilled water to form a
soft, sticky paste. Mix by hand or use a small
mechanical mixer. BE SURE TO ADD THE CLAY TO THE
WATER, NOT THE REVERSE. ADDING WATER TO THE CLAY
WILL RESULT IN A LUMPY, UNWORKABLE MIX.
3. Apply the poultice to the saturated wall in a
single layer (no more than 1 inch thick) using a
plasterer's float or broad trowel.
NOTE: MAKE SURE THE WALL IS THOROUGHLY SATURATED
BEFORE APPLYING THE POULTICE TO AVOID
REDISTRIBUTING THE SALTS BACK INTO THE MASONRY.
4. To help bond the clay to the wall surface, press a
light-gauge galvanized wire mesh into the poultice
and tack it carefully into joints with galvanized
staples.
5. Protect the treated area from direct sun or rain
with tarpaulins as needed.
6. Allow the poultice to remain in place about 1
month. As the poultice dries, the clay will
eventually lighten in color, crack, shrink and
detach from the wall.
7. Remove the staples using pliers and roll up the
clay and wire mesh.
8. Discard materials safely from the site.
9. Thoroughly rinse the treated area with clean, clear
water and allow to dry.
10. Repeat the cycle of wetting and poulticing as
needed to reduce the salts to an acceptable level;
Repointing may be required at the end of the
desalination treatment.
-OR-
B. Applying a Sand:Lime Sacrificial Render:
NOTE: THIS IS A FAIRLY SLOW METHOD OF MASONRY
DESALINATION AND MAY TAKE AS LONG AS SEVERAL MONTHS TO
PRODUCE EFFECTIVE RESULTS.
NOTE: THIS METHOD IS SUITED FOR USE ON LARGE AREAS OF
MASONRY WITH MINIMAL DETAILING, OR ON SIMPLE MASONRY
DETAILS. FOR DELICATE DETAILS OR HIGHLY DAMAGED AREAS,
THIS METHOD SHOULD BE PERFORMED BY A TRAINED CONSERVATOR.
CAUTION: THIS METHOD IS NOT SUITABLE FOR USE WHERE PRE-
WETTING THE SURFACE CAN ADVERSELY AFFECT PLASTER, PAINT,
OR EMBEDDED WOOD OR METAL.
1. Thoroughly saturate the wall with clean, clear
water.
2. Mix 1 part slaked and screened lime putty with 4
parts fine sand.
3. Apply the sacrificial render at least 1/2 inch
thick to both sides of the wall (if possible) to a
height of 2 inches above the salt
crystallization/evaporation zone.
4. As the render surface begins to stiffen, roughen it
with the fine-toothed edge of a hacksaw blade. The
salts from the wall will migrate into the render.
As the salts crystalize, the render will begin to
crack and crumble.
5. Completely remove the dried render from the surface
using a stiff bristle (non-metallic) brush.
6. Thoroughly rinse the surface with clean, clear
water and allow to dry.
7. Repeat the entire application process as necessary
to achieve the desired level of cleanliness.
3.02 ADJUSTING/CLEANING
A. Upon completion of the masonry cleaning work, clean
window glass and spattered adjacent surfaces.
END OF SECTION