 |
THE CARE AND PRESERVATION OF
Historic Tabby
Lauren Sickels-Taves, Eastern Michigan University
Historic tabby buildings can be maintained for years of use and
enjoyment provided that some basic attention is given to their
care and preservation. The conservation staff at The Henry Ford
have compiled the information in this fact sheet to assist in
helping individuals to care for their tabby buildings. The first
step in the care of buildings is to understand and minimize or
eliminate factors that cause damage. The second step is to develop
and follow a basic maintenance plan for care and longevity.
Contents:
Identifying Tabby Buildings
Tabby Preparation
Identifying Features
Pours
Pin holes
Stucco
Other
Wedges
and Bricks
Roofs
Floors
Plugs
Architectural
Designs
Causes of Deterioration
Environment
Water
Vegetation,
Wildlife, and Insects
Structural
Damage
Incompatible
Materials
Portland
Cement
Silicone
Sealants
Asphalt
Chemicals
Lack of
Maintenance
Repair
Tabby
Waterproofing
Removal
of Earlier Repairs
Patching
Structural
Repairs
Stucco
Maintenance Plans
Bibliography
Suppliers
References
IDENTIFYING TABBY BUILDINGS
Tabby refers to a unique, centuries old, southern U.S. coastal building
material purportedly composed of equal proportions of homemade lime,
sand, oyster shells and water. A surface layer of stucco originally
protected the finished product. After the introduction of Portland
cement in the 1870's, the tabby recipe was modified to include cement
and substitute pre-made bag lime for homemade lime, and the stucco
was omitted. Various modern forms of tabby, employing only cement
are still used today. Modern imitations often expose the shell and
forego the stucco in an inaccurate attempt to recreate the appearance
of this historic material.
Terminology has been established to clarify the different construction
techniques and their time of use. Original is the adjective used
to define tabby constructed before 1875. This material employed
only the fore-mentioned ingredients of lime, sand, oyster shells,
and water. Tabby Revival is the term given to tabby made between
1880 and 1925; although additional ingredients were added, the construction
method remained the same. (The five-year span between two types
covers a period of no documented construction of either form of
tabby.) Tabby falling into either of these categories is considered
to be historic or traditional tabby. The present surge of tabby-like
construction, termed Pseudo-Tabby, visually represents a link with
the past, but is basically a thin cement panel with shells shot
into it. Its characteristics do not represent historic tabby, and
therefore fall outside the realm of this fact sheet.
TABBY PREPARATION
Original tabby was made from a mix of slurry of water, homemade
lime, local sand, and oyster shells. Occasionally, aggregates of
broken glass, brick or other similar products were added. The mixture
was poured into a wooden form or rectangular bottomless cradle made
of finished boards approximately two inches thick. The length of
the cradle varied; the height was 20 to 22 inches in the eighteenth
century but was reduced to 10 to 12 inches in the nineteenth century
to minimize collapse and provide greater strength. The sides were
held in place by dovetailed braces. The tabby was tamped and leveled
by hand.
Round pins set at regular intervals held the cradle in place during
the entire process. The tabby air-dried in its cradle for two to
three days. After it hardened, the form and pins were removed and
placed atop the first pour or “round” for subsequent
rounds, thus building a wall in a layer-like fashion. The finished
wall was then brushed with a broom before stucco or whitewash was
applied.
Minute modifications to this procedure distinguish Tabby Revival
tabby from its copied original. In addition to the commercially
available Portland cement and lime altering the mix after 1880,
the cradle was modified to eliminate the use of pins: huge clamps
now held the sides. The qualities of the cement enabled builders
to discard the previously essential stucco.
The sand was always from a local body of water. Sand from salt water
was washed to remove as much salt as possible, since salt causes
decay and deterioration, such as spalling. Oyster shells, procured
from Indian middens or trash piles, provided a well-washed aggregate
and, when burned, produced the third ingredient, lime. A "rick",
or bonfire, was built, starting with a frame of hardwood tree trunks
surrounding a deep pit filled with pine knots. Layers of logs and
oyster shells were then piled on top and set afire. The intense
fire burned the shells, turning them into a white powder called
quicklime, which reacted with water to set or cure the mixture.
After 1880, this burn was omitted in favor of purchasing pre-bagged
lime and Portland cement.
Tabby making was labor-intensive and dependent on weather. February
through September were the recommended months for manufacture. This
allowed builders to avoid winter freeze-thaw cycles and rainy, hurricane-prone
autumn months, and take advantage of months of high humidity, which
shortened the setting time. Drying tabby was protected from rain
by palmetto branches. With the introduction of Portland cement –
imparting increased setting time among other properties; tabby builders
heeded this traditional advice, but may not have always needed to
follow it.
IDENTIFYING FEATURES
POURS
Pours, when visible, are a key characteristic of historic
tabby. By locating the lines demarcating individual pours, distinct
differences in pour layers are notable which aid in separating
eighteenth century military fabrication from nineteenth century
plantation manufacture. The eighteenth century was distinguished
by pour heights of 20" or higher; irregular, vertically spaced
pinholes; and obvious mold marks with distinct jogs, that showed
the forms were not secured at the ends. The late 1790’s
onward were characterized by 10 to 12" pour heights; uniformly
spaced and aligned pinholes; and no mold marks.
PIN HOLES
In original tabby manufacture, round pins were used to hold the
bottomless cradle in place during construction and prevent it
from sliding down onto the previously poured layer of tabby. These
pins (along with dovetail bracing) also kept the two sides of
the cradle from spreading. They were periodically placed but generally
not aligned vertically from layer to layer until after the late
1790’s.
At a few unrelated locations, surviving pinholes indicate the
use of square pins. Although the shape is an anomaly, their placement
and alignment were similar to that for round pins. Intriguingly,
the sites are separated by hundreds of miles and approximately
100 years.
STUCCO
Original tabby was meant to be covered by stucco or lime wash
for protection. The pitted and brushed sides of tabby provided
an excellent surface for bonding with the stucco. Generally limited
to a single coat, 1/8 – ¼" thick, it required
constant renewal. The quality varied with the use of the structure
and the wealth of the owner. The mix consisted of quicklime, sand
and water and was applied with a trowel. The surface coating also
covered the pinholes made during construction. Early twentieth-century
coating repairs to historic tabby have often incorrectly employed
neat cement or asphalt caps.
OTHER
Although poured walls were the norm, wedges, bricks, roofs, floors
and plugs were just a few of the additional features fashioned
from tabby. The flexibility of tabby allowed builders to be creative.
- Wedges and bricks: By the altering the recipe slightly
to employ crushed oyster shells, smaller shapes could be constructed
with tabby by suing special cradles or moulds. Wedge-shaped
moulds were used for columns: four wedges of dried tabby were
placed together to create a circular unit. One unit was set
atop another to the desired height. Due to the fireproof qualities
of tabby, chimneys were often constructed of rectangular bricks
when fired – clay ones were unavailable. Tabby mortar,
incorporating either finely crushed oyster shells or other similar
aggregate, such as coquina shells, were used to cement the wedges
or bricks together.
- Roofs: Tabby roofs tended to be flat (pitched roofs
were rare due to their massiveness). They were created by pouring
a three-to-four inch tabby mix over roof-bearing timbers covered
by wooden lath or layers of twigs, in a key-like fashion similar
to adobe roofs. To prevent leaks from standing water, a coating
of tar and sand was applied.
- Floors: Tabby floors were also popular along the coast.
Depending on the wealth of the owner, tabby was poured directly
on top of tamped earth or, in better buildings, on top of a
sub-grade of shell rubble or boards. Laid three to six inches
deep, the tabby was tamped, then coated with linseed oil to
form a seal. Slab tabby could not withstand concentrated weight
and easily became rough and pitted. Tabby floors had a short
life span, but could be renewed by pouring subsequent layers
over the old.
- Plugs: Tabby plugs were merely chunks of tabby mortar.
They were created to ingeniously solve a problem when stucco
or plaster was unavailable. Holes left by pins that held the
cradle in place while a wall was being poured produced a structure
that was neither weatherproof nor critter-proof. A quantity
of fresh, or wet, tabby mortar was simply inserted into the
pinholes and allowed to dry. These plugs effectively filled
the holes and did so permanently.
ARCHITECTURAL DESIGNS
If wood could be bent to create a mould, architectural
designs in tabby were limitless. Walls, rarely exceeding two stories,
could take on unique shapes. One plantation house, now in ruins,
near Woodbine, Georgia, was poured in the shape of a large anchor;
sugar mills had octagonal walls; military fortifications had bastions.
Foundation walls were constructed by merely pouring tabby –
minus the pins – several feet below ground level. Where time
prohibited construction of an entire tabby structure, tabby foundation
piers were built, with upper stories of wood.
Across the centuries of its use, tabby was most commonly employed
in the erection of homes, exemplifying its ability to reflect
architectural styles. A few of the styles uniquely utilizing historic
tabby were:
- Georgian (1700-1870, rural to 1830): Although tabby was not
uniquely manipulated beyond the expected norm, simple domestic
plans reflected Georgian characteristics: large, two rooms deep,
two-story. Tooth-like dentils made of tabby bricks on the cornice
have also been employed as decoration. Dentils were first introduced
as a Georgian feature, but were retained during the subsequent
Federal style (1780-1820).
- Greek Revival (1825-1860): This style best reflected the ingenuity
of the builders of tabby. The key characteristic of Greek Revival
was a full-façade porch comprised of wide, simple cornices
supported by rounded columns. Columns were constructed of tabby
wedges mortared into circular units which were then stacked
atop one another to achieve the desired height.
- Queen Anne (1860-1890): Eclectic and asymmetry characterize
Queen Anne, one of the many Victorian styles. Tabby suited this
style due to the flexibility of the mould or brick. Chimneys
were massive and patterned. Another feature is to use wall surfaces
as primary decorative elements. By modifying the typical cradle,
arches over all door and window openings could become segmental
arches, even Palladian windows.
- Spanish Mission (1890-1920): The key feature of this style
is the mission-shaped or Baroque curve. Walls were usually smooth
stucco. Tabby once again lent itself well to this style. The
curves were easily produced with the malleable material, and
the smooth walls created by the cradle produced the desired
surface finish without employing stucco.
CAUSES OF DETERIORATION
Water, incompatible materials, and lack of maintenance
are the major causes of damage to tabby buildings. The introduction
of water is usually the result of the incorrect use of materials,
particularly non-compatible repair materials, and the lack of maintenance;
95% of all deterioration can be linked to water. Once introduced
and allowed to remain, water can weaken the chemical structure of
tabby and encourage wildlife and insect infestation. Allowed to
continue, a building will eventually become unstable and collapse.
NOTE: The bulk of deteriorating tabby was built before 1875. Of
the documented Tabby Revival sites, only one is in ruins and the
denseness of the cement walls are by far more stable than original
tabby’s lime-based, porous walls. Unless otherwise noted,
the rest of this fact sheet is directed at the preservation of original
tabby.
ENVIRONMENT
Stucco forms a protective barrier over original tabby. This does
not prevent deterioration of tabby under adverse conditions; however,
it greatly reduces it, minimizing the risk for infestation by
wildlife and insects, and instability by water.
WATER
The moisture content of building materials varies in response
to changes in the local humidity and will not usually damage
the material or induce decay. The paradox of tabby is that it
is a highly porous material and yet was built in coastal areas,
which normally have high salt water tables. Tabby is forced
to absorb high levels of salt water; however, its porous nature
in collaboration with the warm climate allows it to adequately
evaporate this same level without harmful side effects. This
cyclical feature is assured as long as the pores are not clogged
by incompatible, less porous materials and the structure is
being maintained.
Water can enter and harm structures in a variety of ways. The
path it takes in buildings from entrance to exit can be illogical.
There are three sources of water: rain penetration, rising damp
and condensation.
- Rain penetration in original tabby structures is generally
caused by the effects of structural movement, the wrong choice
of materials for repair, badly executed repairs or lack of
routine maintenance. It is the single, greatest source of
tabby deterioration.
- Rising damp occurs when tabby is in direct contact with
damp soil. Moisture is drawn into the pores by a physical
process called capillary action. The absorbed moisture will
rise in the wall to a height at which there is a balance between
the rate of evaporation and the rate at which it can be drawn
up by capillary forces. This height will vary somewhat with
the time of year and the level of the water table of the soil.
Rising damp is not a great concern to tabby unless the pores
are clogged as mentioned in the proceeding paragraph.
- Condensation is the product of cooled water vapor. When
moisture in the air is cooled at a certain temperature called
dew point, it will change to liquid water. Due to the climatic
location of tabby, this source of water rarely affects tabby.
The grounds immediately around tabby structures play an important
role in minimizing water damage. Poor drainage and shallow eaves
can allow water to build up near the foundation, permitting
excessive rising damp and coving. Excessive water can ultimately
pop stucco and break down a tabby wall's chemical and physical
bond.
VEGETATION, WILDLIFE, AND INSECTS
Vines and trees, with their extensive, creeping root systems,
are the most detrimental of all vegetation, weaving into porous
tabby walls, upheaving them and creating cracks for future growth.
Without intervention, complete failure is imminent. Maintenance
and early detection are therefore vital. Despite legislative
protection, several ruins near Woodbine and Darien, Georgia,
have recently collapsed from tree encroachment, tunneling by
gopher turtles and riverbank erosion.
All vegetation within 12 to 18 inches of a building should be
removed. This eliminates the potential for damage from roots
and permits air circulation near the stucco surface, discouraging
mildew. Roots and vines that have penetrated tabby and become
a structural support should be treated cautiously: they should
be cut to prevent further growth, yet left in place as they
are now an integral part of the wall. Birds and burrowing animals
should be discouraged.
Tabby interiors exposed to the elements invite other infestations.
Joist beams experience rot and termites, and the pockets are
often filled with spiders and birds' nests.
STRUCTURAL DAMAGE
One key link to structural problems is derived from the weak or
inadequate lime produced by burning oyster shells. The best lime
was made when bonfire temperatures exceeded 1094oC
(2000oF) and no rain penetrated the lime before it
was placed in barrels. Any deviation created a weaker lime and
a weaker block of tabby. Weaker limes can place buildings structurally
at risk. Inferior tabby, carrying the weight of a second story,
was apt to slowly compress and settle at a rain enters and further
erodes the lime, collapse is imminent. This construction flaw
may manifest itself only under certain conditions (e.g. rain and
missing stucco.)
Today quality control is expected in bag lime, and a repair can
be made easily, using a simplified cradle. Neat cement or synthetic
additives of any kind should be avoided in repair, because their
plasticity is incompatible with that of lime.
Another potential structural problem may occur in the foundations.
Tabby foundations were generally poured only two to three feet
deep and lacked spread footing or buttressing. Walls were 10 inches
thick underground. Fortunately, the nature of tabby and its construction
prevented the risk of toppling: each pour had hardened sufficiently
before a subsequent pour was added, and cradles were made to pour
around corners in a continuous form, thereby linking one wall
to the next cohesively. Joists and other beams merely added additional
support to the structure. However, high winds can put pressure
on walls. Cracks in stucco are a good indication of structural
problems.
INCOMPATIBLE MATERIALS
Traditional building materials are more porous than their modern
substitutes. They will absorb more water but have the advantage
of allowing it to evaporate freely under drier conditions. When
certain modern materials with low porosity are introduced during
maintenance and repair, incompatibility becomes an issue. There
is a decrease in natural ventilation that can cause persistent
dampness in many old buildings. The logical progression is deterioration.
PORTLAND CEMENT: Cement and its various forms
can do irreparable damage to tabby structures. It has most popularly
been used as the sole ingredient in repairs or stucco, which
in itself is historically inaccurate. Its density reduces uniform
breathability throughout a structure, effectively sealing the
porous surface from water. Clogged pores mean that the tabby
is unable to breathe.
Spot repairs with cement create saturated surrounding areas,
thus weakening the bond between the new and old materials.
Stucco should be flexible, and it should never be stronger than
the material to which it adheres. By employing cement, a rigid,
incompatible stucco is being introduced, basically sealing the
tabby structure in a plastic bag.
SILICONE SEALANTS: Sealants are soft or
liquid moisture-impervious compounds. They are designed to be
flexible in areas with anticipated movement, but have little
or no ability to breathe. These modern products are not suitable
for historic tabby structures due to their non-porous nature
as well as being historically inappropriate. Sealants, generally,
have improved lately to achieve color stability, and resist
ultraviolet rays and mildew, but their impervious nature prohibits
them from working with traditional building materials, particularly
porous ones.
ASPHALT: Asphalt caps were a popular solution
to sealing the tops of tabby ruins in the 1950’s. Unfortunately,
the asphalt did not merely coat the tabby; it also penetrated
into the honeycomb fabric of the wall. Once hardened, it effectively
sealed the top surfaces from rain penetration, but prohibited
its removal.
CHEMICALS
Water-based and toxic chemicals should never be used on or near
tabby. They can escalate decay as well as visually alter the color
and appearance of tabby. The walls draw the chemicals into its
pores by capillary action and water rarely can dissipate the effects
fully.
LACK OF MAINTENANCE
While water does the most harm to buildings, it is merely a source.
Lack of maintenance is the key catalyst to its introduction. No
building can go without maintenance. If regular maintenance is
carried out, the longevity of the structure is assured and the
financial outlay for major repairs is minimized.
Lack of maintenance can be graphed as a downward curve. Initially,
the plateau of status quo begins to sag. As maintenance is deferred,
the curve deflects more until it reaches a vertical line, indicating
that collapse is imminent and the costs of salvation will be exceedingly
high.
REPAIRS
Reversibility is a prime issue in repairing historic
buildings. It minimizes problems during maintenance and future repairs,
and helps to maintain the integrity of the structure. Replacing
deteriorated components with compatible materials ensures that the
new and old materials will work together. Repairs are best made
with materials that are traditional to the structure.
NOTE: Tabby is technically an early form of concrete, using oysters
shells as aggregate. Unfortunately, many contractors assume incorrectly
that neat cement or a pre-mix with cement in it is a suitable repair
material. Cement was not introduced in this country until around
1870, so is not a compatible material for original tabby. Cement
and its many twentieth century variations may physically harm tabby
structures, creating long-term problems, and they may not be totally
reversible.
TABBY
There are varying degrees of tabby restoration, ranging from patching
to structural repairs. Each tabby structure should be evaluated
on its own merits to determine the most appropriate repair. Every
attempt should be made to save as much historic tabby as possible.
WATERPROOFING
The simplest, most effective, and perhaps sole means of waterproofing
original tabby, is to retain and maintain its traditional stucco
coating. Stucco reduces the potential for water infiltration
and protects tabby from permeability and absorption until carbonation
has increased its strength.
REMOVAL OF EARLIER REPAIRS
The easiest form of tabby restoration involves repairing neglected
structures. In other cases, earlier repairs must be removed.
Documented examples of unacceptable, incompatible repairs include
Portland cement stucco, caps of asphalt tar, silicone sealants,
and visible alterations created by the sculpting of the stucco
to redefine the edges and basic appearance of the tabby.
Tar and cement are difficult to remove, since they tend to
be stronger than the material to which they adhere. Upon excision
of the tar or cement, substantial sections of tabby may come
away as well. Subsequent repairs, therefore, are more costly,
necessitating the employment of cradles to fill significant
voids. It is best to leave these repairs alone. Asphalt will
eventually deteriorate and actually become weaker than the tabby.
It can then be carefully removed, prior to the installation
of a more suitable cap.
Silicone sealants have also been applied to the external surface
of tabby and/or stucco. Unfortunately, they cannot be removed
or reversed, since the silicone seeps into the pores and voids
of tabby. Time alone can aid in stabilizing the imbalance of
moisture levels.
Even with the use of proper ingredients and proportions, a new
tabby mix and/or stucco can be applied inappropriately or carelessly,
dramatically altering the visual character of the original structure.
Color and application are as important as ingredients and proportions.
Historically, tabby has been light gray, from the combination
of white lime and gray-to-black wood ash (ash being included
inadvertently when lime was retrieved from the pit after burning.)
The stucco was either the same color or a soft beige, if the
sand dominated the color more than the ash. The gray in Tabby
Revival tabby was the result of the cement since ash was no
longer a component from burning oyster shells to obtain lime.
The use of boards and dovetailed corners in cradle construction
produced a flush wall with sharp corners. When stucco was applied,
masons retained this appearance. Efforts should be made to duplicate
tabby in color, texture, and application, as well as ingredients
and proportions. The basic preservation guidelines outlined
in the Secretary of the Interior’s Standards for Rehabilitation
are applicable.
PATCHING
Simple tabby repairs can be made employing a trowel and a tabby
mix including broken oyster shells. In its historic form, tabby
utilized only whole oyster shells. However, broken shell pieces
are better suited to small repairs.
Stucco (the tabby mix minus shell) may be used to repair portions
of a tabby wall, if the depth of the repair does not exceed
1 inch. (The basic thickness of the first or scratch coat of
stucco is one-quarter to one-half inch.) Extending the depth
will slow the stucco’s drying time, but will not irreparably
harm the tabby or its final appearance.
When a restoration project requires new tabby, a properly formulated
mix must be specified. All materials should conform to ASTM
standards.
- Lime should conform to ASTM C-207, Type S, hydrated lime,
or ASTM C-141 for hydraulic hydrated lime.
- Sand should conform to ASTM C-144 to ensure proper gradation
and freedom from impurities. Sand, or other type of aggregate,
should match the original as closely as possible. Research
suggests that channel sand, not pit sand, was the key source.
- Cement should conform to ASTM C-150, Type II, white, non-staining
Portland cement.
One suggested mix for original tabby is:
| 1
parts by volume |
hydraulic
lime |
| 1
parts by volume |
hydrated
lime |
| 4
parts by volume |
sand |
If Tabby Revival tabby is being repaired, Portland cement
may be included in the mix. One recommended recipe is:
| 1
parts by volume |
white
Portland cement |
| 2
parts by volume |
hydrated
lime |
| 9
parts by volume |
sand |
These recipes may vary depending on the tabby under restoration.
Additional information is given in the sources listed in the
bibliography.
Once the appropriate tabby mix has been selected, the ingredients
need to be measured out by volume, not weight. Mixing is a critical
factor in proper application; tabby can be overmixed, ultimately
causing crazing or shrinkage from a too-rapid set. Manual mixing
should be limited to 15 minutes, and machine mixing should not
exceed four minutes. All substrate surfaces, defined here as
the apparent surface layer, be they tabby or a stucco base coat,
must be thoroughly wetted to retard drying and minimize the
rate of moisture absorption from the new materials into the
old. Drying tabby should be covered with burlap and misted during
the daylight hours to prevent rapid moisture loss and cracking.
STRUCTURAL REPAIRS
Before patching or stucco repair, structural tabby should be
assessed. Tabby is a structurally sound building material. Historical
documents cite its strength and re-use value in converting ruins
to new structures. For example, the wall thickness of one tabby
structure, examined after a fire, had diminished to six inches,
a loss of two inches on each of the external and internal surfaces,
yet it is still a substantial wall. Where vines have become
a structural support within tabby walls, and the full thickness
remains, the different layers may have shifted from alignment.
In evaluating repairs, therefore, one needs to ascertain whether
the traditional 10-to-12-inch width is extant and aligned, and
if not, whether the missing portion requires repouring or just
a thicker stucco.
If repouring is required, traditional cradles or forms must
be made, utilizing wood boards of a minimum one-inch thickness.
The sturdiness of the boards helps to minimize the potential
for warping from shrinkage. The existing wall must be stabilized
with bracing as necessary. All disintegrated tabby must be removed,
leaving a clean, detritus-free surface. Simple brushing by hand
may be sufficient, but where necessary, low-pressure mechanical
air blowing is acceptable.
Once a clean surface has been exposed and wetted to improve
adhesion, a new cradle must be fashioned to fit the repair area.
Resting on substantial lower pours, if possible, or bracing,
the cradle should be made of sideboards only. The new tabby
slurry should be mixed and poured in the traditional manner
and tamped periodically to settle oyster shells and minimize
air pockets. ) See recommended mixes cited under “patching”.)
The cracks and grooves of existing tabby serve as points of
adhesion for the new tabby. No attempt should be made to fill
the void or repair areas with substances other than tabby slurry,
such as brick chips, unless clear evidence exists of its inclusion
in the original mix.
After drying in its cradle for two to three days, the boards
may be removed and set above for subsequent pours. Walls should
be roughened by brushing to improve the adhesion of newly applied
stucco.
It is difficult to define the point at which cradle restoration
is substituted with a thicker stucco. A one-inch thickness represents
a lower limit for the use of a cradle, since the oyster shells
might not be adequately seep into areas of less than one-inch
depth. If a pour shows more erosion on its lower half than its
upper, tabby may have to be repaired by hand with a trowel or
the vertical cradle board pivoted outward to allow tabby slurry
to reach the repair area. Once the deteriorated area is nearly
filled, the cradleboard can be returned to its upright position
and the pouring resumed to the desired height.
Some tabby structures have been constructed from tabby bricks
rather than from the more traditional pouring method. Structural
repairs should include reconstruction of handmade sundried tabby
bricks, held in place by a tabby mortar. However, some handmade
clay bricks in earlier repairs do not appear to have damaged
surrounding tabby bricks.
STUCCO
Traditional materials are recommended when stucco repairs or replacement
are necessary, despite the fact that they may require annual attention.
The retention of historical integrity and the lack of problems
due to compatibility far outweigh the trouble of undertaking seasonal
repairs.
Since stucco is by and large tabby without the shells or at most,
crumbled shell, the recipe for stucco is the same as for original
tabby.
| 1
parts by volume |
hydraulic
lime |
| 1
parts by volume |
hydrated
lime |
| 4
parts by volume |
sand |
Though generally not used, ground oyster shells have been noted
in some surviving examples of original stucco.
The use of hydraulic lime imparts increased strength while retaining
plasticity; Portland cement increases strength, but reduces plasticity.
White Portland cement is not as strong as gray cement. Any decision
to substitute Portland cement should be made by a professional
knowing the detrimental qualities of cement, and its use is largely
recommended only on a Tabby Revival structure with cement in its
original mix. Manmade additives should never be used. The properties,
such as freeze resistance, they impart to the recipe are generally
unnecessary and have largely been tested on modern tabby only.
Stuccoing should be completed before cold weather and freezing
temperatures, above 10oC (50oF) is best.
When replacing stucco, a maximum of two coats should be applied.
This may vary from the more common single coat (one-eighth to
one-quarter inch), but historical documents mention annual restuccoing.
A thicker, two-coat stucco will obviate the need for annual renewal.
The substrate of tabby must be thoroughly cleaned and misted –
not saturated – with water before restuccoing begins. This
will increase the bond of the stucco with the tabby, and will
prevent the stucco from drawing the water out of the tabby, shrinking,
and developing hairline cracks upon drying.
After applying the scratch coat (one-quarter to one-half inch)
with a trowel, the surface should be gently scored with a comb,
and excess stucco removed before the layer completely dries. The
wall should be draped in burlap and the latter frequently misted
during sunlit hours to prevent the stucco from drying too quickly
and to aid in reducing hairline cracks. An effective drying time
is two days, after which the final coat of one-eighth-inch thickness
may be applied smoothly and evenly. The burlap and misting should
be repeated after this coat as well.
MAINTENANCE PLANS
Historic tabby structures require annual inspections.
Traditional stucco may need annual renewal or spot repairs. Maintaining
stucco, roofs and gutters; and monitoring vegetation, burrowing
animals and other soil-eroding activities can virtually eliminate
major problems. Without this care, the above environmental causes
of deterioration will inevitably occur.
By establishing a regular maintenance plan, areas comprised of missing
materials or problems that can ultimately cause decay can be addressed
early. Repair costs will be dramatically reduced while the longevity
of the historic tabby structure will be assured.
BIBLIOGRAPHY
Secretary of the Interior's Standards for Rehabilitation and
Guidelines for Rehabilitating Historic Buildings. Washington,
D.C.: National Park Service, 1990.
Sickels-Taves, Lauren B. "Understanding Historic Tabby Structures:
Their History, Preservation, and Repair." Association for Preservation
Technology Bulletin, vol. xxviii, no. 2-3 (1997): 422-29.
Sickels-Taves, Lauren B and Michael S. Sheehan. The Lost Art
of Tabby Redefined: Preserving Oglethorpe's Architectural Legacy.
Southfield, MI: Architectural Conservation Press, 1999.
SUPPLIERS
Hydraulic Lime:
Riverton Corporation
1111 Riverton Rd.
Front Royal, VA 22630
540-635-4131
REFERENCES
Superintendent, Cumberland Island National Seashore
National Park Service
PO Box 806
St. Mary's, GA 31558
(912) 882-4336
Secretary of the Interior's Standards for Rehabilitation and
Guidelines for Rehabilitating Historic Buildings. Washington,
D.C.: National Park Service, 1990.
http://www2.cr.nps.gov/tps/secstan8.htm
|
|
