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Historic Preservation - Technical Procedures
Surface Repair Of Limestone By Consolidation And Use Of Lime Mortar
Developed For Hspg (Nps - Sero)
Surface Repair Of Limestone By Consolidation And Use Of Lime Mortar
SURFACE REPAIR OF LIMESTONE BY CONSO= LIDATION AND USE OF LIME MORTAR
EXECUTION OF THIS PROCEDURE REQUIRES A HIGH DEGREE OF EXPERTISE.
CONSIDERABLE SKILL IS REQUIRED IN COLOR MATCHING AND IN MATCHING
A. This procedure includes guidance on repairing damaged
limestone by flooding the surface with limewater 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.
1. 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
2. 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:
1. Safety Precautions
2. Historic Structures Precaution= s
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.
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 adequa= te
for stain removal work, and these products should be purchas= ed
when available, as they tend to be less expensive. Com= mon
names are indicated below by an asterisk (*).
A. Calcium Hydroxide:
1. Other chemical or common names include Calcium
hydrate*; Hydrated lime*; Lime hydrate*; Slaked
2. Potential Hazards: SKIN IRRITANT, AVOID INHALATION
OF THE DRY POWDER.
3. Available from chemical supply house, construction
materials yard, construct= ion specialties
distributor, garden and lawn supply center, or
1. Other chemical or common names include
Formaldehyde; Formic aldehyde; Methanal; Methyl
aldehyde; Oxomethane; Oxymethylene.
2. Potential Hazards: TOXIC AND MODERATELY FLAMMABLE;
SKIN IRRITANT; MAY DISCOL= OR THE SKIN, MAKING IT
DARKER DUE TO THE TANNING ACTION OF THE CHEMICAL.
3. 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-hydrau= lic 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
NOTE: 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
B. Adhesive mortar (spalls): Mix 1 part lime with 1 part
aggregate. Aggregate often includes a 10% HTI powder
C. Grouting mortar (cracks): Mix 1 part lime with 1-1/2
parts aggregate. Aggregate often includes a 10% HTI
D. Shelter coating: Mix 1 part lime with 2-3 parts
aggregate. Aggregate often includes LESS THAN 10% HTI
A. Preparing the Limewater:
1. Mix 0.14g of calcium hydroxide in 100ml of water at
60=F8F and let stand until the water is clear.
2. Cover the surface of the limew= ater in its container
with a float of polystyre= ne 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.
3. Draw off the limewater as needed and hand pump it
into spray bottles; Make sure that the water is
clear and not cloudy.
B. Consolidation by Limewater:
1. Carefully remove any cementiti= ous filling to spalls
2. Cut out cavities to be consoli= dated where new
spalls and splits have developed and where previous
spalls and cracks were found.
3. Remove all loose dust and debris by flushing with
4. Flood the limestone surface with a biocide if
necessary to provide a clean, sterile surface for
the new mortar filling.
5. Treat the cleaned surfaces and open cavities with
clear limewater 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.
6. Flood the limestone surface with the limewater for
several days or as long as the surface will absorb
(up to 40 applications may be necessary).
7. Remove any excess limewater 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.
1. Flush out cavities and cracks again with water from
the hand sprays to avoid de-watering the repair;
Make sure the surface remains damp.
2. Fill deep cavities with a slurry of repair mortar
(see Section 2.03 A. above).
3. 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.
4. Brush a thin slurry of repair mortar containing
"HTI" powder into the cavity or fracture to provide
an additional key for the repair.
5. 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.
6. Protect the treated area from direct sunlight or
strong drafts to avoid rapid drying out.
7. Apply wet cotton-wool packs over the repair mortar
to avoid rapid drying.
8. When the cavity is dry, wet it again and repeat
steps 5-7 until the cavity has been filled
9. Using a spatula, trim off excess mortar to the
10. 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
D. To protect the limestone and slow down the weather= ing
process, apply a shelter coating (or sacrificial
coating): This is similar in compo= sition 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.
1. Mix 1 part lime with 2-3 parts aggregate; Add water
and mix to achieve a consistency of thin cream;
continue to mix for anoth= er 20 to 30 minutes.
2. Add Casein and Formalin to the mixture.
3. Thoroughly wet the stone with water using hand
spray bottles. Wet the stone until it can no
longer absorb any more water.
4. Apply the coating to the stone surface using a
short-haired bristle brush.
5. Using a second short-haired (worn) bristle brush,
work the coating into the texture of the stone.
6. 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
7. 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
END OF SECTION