U.S. General Services Administration
Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
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.
PART 1–GENERAL
1.01 SUMMARY
A. This procedure includes guidance on cleaning dirt from limestone using a hot lime poultice - also known as Baker’s Lime Method. Developed by Robert Baker in the 1950’s, THIS METHOD OF DIRT REMOVAL IS LABOR INTENSIVE AND, THEREFORE, IS BEST USED ON HIGHLY SIGNIFICANT ARCHITECTURAL DETAILS.
- Baker’s Lime Method is a 4-stage process which includes cleaning, repair, consolidating and preserving.
- This procedure describes the cleaning stage. Other stages in the process are described in 04460-02-R.
B. Dirt may consist of particles of dust, sand or grit, or tarry soot. Particulate dirt or soiling may result from vehicular or human traffic or from pollutant or sulfate crusts building up in protected areas not regularly washed by rain. Pollutant crusts hasten masonry dissolution and should be removed.
C. Safety Precautions: Check manufacturer’s literature for precautions and effects of products and procedures on adjacent building materials, components, and especially vegetation. Take appropriate protective measures.
D. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Submittals
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- 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.
PART 2–PRODUCTS
2.01 MATERIALS
A. Lime putty
B. Burlap or cheesecloth
C. Plastic sheeting
D. String
E. Clean potable water
2.02 EQUIPMENT
A. Hand spray bottle
B. Dental picks
C. Small, soft bristle brushes
D. Trowels and spatulas
PART 3–EXECUTION
3.01 ERECTION/INSTALLATION/APPLICATION
A. Thoroughly wet the stone with clean water.
B. Using a trowel, press lime putty into the affected limestone ornament.
C. After a thick coat has been applied, bind it with wet burlap or cheesecloth; Secure it with string.
D. Loosely tie a heavy-duty polyethylene sheet around the ornament.
E. Keep the ornament covered for about 2-3 weeks.
F. Periodically, lift the sacking during that time and spray the poultice with water to keep the poultice damp.
G. Carefully lift off the lime in small portions using spatulas or small trowels (use water sprays to help loosen the poultice).
H. Much of the dirt will not immediately detach with the poultice material; The poultice moistens the dirt so that it can be removed more easily by washing and brushing.
I. Using hand sprays, dental picks, small tooth brushes or stencil brushes, gently remove loosened dirt left on the stone surface.
J. If, after cleaning, there is any evidence of stone damage, attempt limestone surface repair using consolidation and lime mortar (see 04460-02-R).