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.
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.
EXECUTION OF THIS PROCEDURE REQUIRES A HIGH DEGREE OF EXPERTISE. CONSIDERABLE SKILL IS REQUIRED IN COLOR MATCHING AND IN MATCHING MORTAR REPAIRS.
A. This procedure includes guidance on repairing damaged limestone by flooding the surface with lime water for several days followed by surface repair with lime mortar.
B. This procedure describes three stages in what is known as Baker's Lime Method.
- Developed by Robert Baker in the 1950's, Baker's Lime Method is a 4-stage process of limestone cleaning and repair which includes cleaning with a hot lime poultice, repair, consolidation and preservation.
- This procedure describes the repair, consolidation and preservation stages. The cleaning stage of the process is described in 04460-01-R.
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
- 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.
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. Calcium Hydroxide:
- Other chemical or common names include Calcium hydrate*; Hydrated lime*; Lime hydrate*; Slaked lime*.
- Potential Hazards: SKIN IRRITANT, AVOID INHALATION OF THE DRY POWDER.
- Available from chemical supply house, construction materials yard, construction specialties distributor, garden and lawn supply center, or hardware store.
- Other chemical or common names include Formaldehyde; Formic aldehyde; Methanal; Methyl aldehyde; Oxomethane; Oxymethylene.
- Potential Hazards: TOXIC AND MODERATELY FLAMMABLE; SKIN IRRITANT; MAY DISCOL= OR THE SKIN, MAKING IT DARKER DUE TO THE TANNING ACTION OF THE CHEMICAL.
- Available from chemical supply house, dairy supply distributor, drugstore or pharmaceutical supply distributor, photographic supply distributor (not camera shop), or printer's supply distributor.
D. Lime mortar - "High Calcium", Non-hydraulic Type (see mixes in Section 2.03 below)
E. HTI Powder (white refractory brick powder - "High Temporary Insulation") - may be used as a pozzolanic additive to make a weak hydraulic mortar.
F. Aggregate - size, grade and color to be determined by RHPO (see mixes in Section 2.03 below)
G. Cotton-wool packs
H. Polyethylene sheets
I. Clean potable water
A. Hand spray bottle
B. Mechanical spray equipment
C. Dental picks
D. Small, soft bristle brushes (short-haired)
E. Trowels and spatulas
F. Hand pump
H. Rubber gloves
PROPORTIONS MAY VARY BASED ON SIZE, GRADE, COLOR AND FUNCTION OF AGGREGATE
A. Repair mortar: Mix 1 part lime with 2 parts aggregate. Aggregate often includes LESS THAN 10% HTI powder additive.
B. Adhesive mortar (spalls): Mix 1 part lime with 1 part aggregate. Aggregate often includes a 10% HTI powder additive.
C. Grouting mortar (cracks): Mix 1 part lime with 1-1/2 parts aggregate. Aggregate often includes a 10% HTI powder additive.
D. Shelter coating: Mix 1 part lime with 2-3 parts aggregate. Aggregate often includes LESS THAN 10% HTI powder additive.
A. Preparing the Lime water:
- Mix 0.14g of calcium hydroxide in 100ml of water at 60=F8F and let stand until the water is clear.
- Cover the surface of the lime water in its container with a float of polystyrene sheetrock pierced by a siphon tube with a filter. NOTE: IT IS IMPORTANT TO PROTECT THE LIMEWATER FROM AIR SO THAT IT DOES NOT CARBONATE AND BECOME INEFFECTIVE.
- Draw off the lime water as needed and hand pump it into spray bottles; Make sure that the water is clear and not cloudy.
B. Consolidation by Lime water:
- Carefully remove any cementitious filling to spalls and cracks.
- Cut out cavities to be consolidated where new spalls and splits have developed and where previous spalls and cracks were found.
- Remove all loose dust and debris by flushing with clean water.
- Flood the limestone surface with a biocide if necessary to provide a clean, sterile surface for the new mortar filling.
- Treat the cleaned surfaces and open cavities with clear lime water to attempt to consolidate the more friable areas. If water becomes cloudy, stop work and allow lime to settle to the bottom and water to again become clear. = ;Proceed when water is clear.
- Flood the limestone surface with the lime water for several days or as long as the surface will absorb (up to 40 applications may be necessary).
- Remove any excess lime water on the surface with sponges; Squeeze them out in clean water.
C. Follow Consolidation with Surface Repairs Using Mortar:
NOTE: All mortar repairs are based on lime - NO PORTLAND CEMENT OF AN KIND IS USED.
- Flush out cavities and cracks again with water from the hand sprays to avoid de-watering the repair; Make sure the surface remains damp.
- Fill deep cavities with a slurry of repair mortar (see Section 2.03 A. above).
- Follow this by inserting small pieces of limestone into the cavity to reduce the thickness which needs to be built up in fine repair mortar.
- Brush a thin slurry of repair mortar containing "HTI" powder into the cavity or fracture to provide an additional key for the repair.
- After the slurry has dried, wet the slurry again, knead the first repair mortar and push it into place with fingers, dental plugging tools or spatulas; Not more than 5-6 mm should be pressed in at one time.
- Protect the treated area from direct sunlight or strong drafts to avoid rapid drying out.
- Apply wet cotton-wool packs over the repair mortar to avoid rapid drying.
- When the cavity is dry, wet it again and repeat steps 5-7 until the cavity has been filled completely.
- Using a spatula, trim off excess mortar to the desired profile.
- To achieve a texture matching the stone, take a dry sponge (hessian pads, stencil brushes or purpose-made plastic scrapers will also work) and press it against the repaired surface (be careful not to press too hard as it may absorb moisture from the repair).
D. To protect the limestone and slow down the weathering process, apply a shelter coating (or sacrificial coating): This is similar in composition to the repair mortar except the proportion of aggregate to lime is slightly higher and the sand and stone dusts of aggregate are more finely crushed.
- Mix 1 part lime with 2-3 parts aggregate; Add water and mix to achieve a consistency of thin cream; continue to mix for another 20 to 30 minutes.
- Add Casein and Formalin to the mixture.
- Thoroughly wet the stone with water using hand spray bottles. Wet the stone until it can no longer absorb any more water.
- Apply the coating to the stone surface using a short-haired bristle brush.
- Using a second short-haired (worn) bristle brush, work the coating into the texture of the stone.
- Cover the treated area with polyethylene sheets and intermittently spray mist the area with water during the first few hours to avoid rapid drying of the coating. NOTE: RAPID DRYING MAY RESULT IN A POWDERY APPEARANCE.
- During the drying stages, additives of finely ground stone dust may be dusted on to the surface to achieve subtle color variations in the stone appearance.