/ 

42 PRESERVATION 

BRIEFS 

The Maintenance, Repair and 

Replacement of Historic Cast Stone 

Richard Pieper 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

An Imitative Building Material with Many Names 

The practice of using cheaper and more common materials 

on building exteriors in imitation of more expensive natural 

materials is by no means a new one. In the eighteenth century, sand impregnated paint was applied to wood to look 

like quarried stone. Stucco scored to simulate stone ashlar 

could fool the eye as well. In the 19th century, cast iron 

was also often detailed to appear like stone. Another such 

imitative building material was "cast stone" or, more precisely, precast concrete building units (Figs. 1,2 and 3). 

Cast stone was just one name given to various concrete 

mixtures that employed molded shapes, decorative aggregates, and masonry pigments to simulate natural stone. 

The basic mixtures included water, sand, coarse aggregate, 

and cementing agents. Natural cements, portland cements, 

oxychloride cements, and sodium silicate based cements 

were all used as binding agents. The differences in the 

resulting products reflected the different stone aggregates, 

binding agents, methods of manufacture and curing, and 

systems of surface finishing that were used to produce 

them. Versatile in representing both intricately carved 

ornament and plain blocks of wall ashlar, cast stone could 

be tooled with a variety of finishes. 

During a century and a half of use in the United States, cast 

stone has been given various names. While the term "artificial stone" was commonly used in the 19th century, "concrete stone," "cast stone," and "cut cast stone" replaced it in 

the early 20th century. In addition, Coignet Stone, Frear 

Stone, and Ransome Stone were all names of proprietary 

systems for precast concrete building units, which experienced periods of popularity in different areas of the United 

Figure 1. In the early 20th century, elaborate custom building ornament was frequently produced in cast stone. Here, at the Benedict Stone Company's plant in 

Chicago, cartouches, balustrade pane/s, railings, and balusters await finishing and shipment. Photo: Courtesy, National Building Museum. 
Figure 2. The prominent Delaware and Hudson Building in Albany, New 

York (1916) made extensive use of cast stone as trim combined with a random 

ashlar facing of natural granite. The elaborate gothic trim was produced by 

the Onondaga Litholite Company of Syracuse, New York. 

States in the 19th century. These systems may be contrasted with "Artistic Concrete," decorative molded concrete 

construction, both precast and cast-in-place, which made 

little effort to simulate natural stone (Fig. 4). 

Having gained popularity in the United States in the 1860s, 

cast stone had become widely accepted as an economical 

substitute for natural stone by the early decades of the 20th 

century. Now, it is considered an important historic material in its own right with unique deterioration problems 

that require traditional, as well as innovative solutions. 

This Preservation Brief discusses in detail the maintenance 

and repair of historic cast stone--precast concrete building 

units that simulate natural stone. It also covers the conditions that warrant replacement of historic cast stone with 

appropriate contemporary concrete products and provides 

guidance on their replication. Many of the issues and techniques discussed here are relevant to the repair and 

replacement of other precast concrete products, as well. 

2 

Figure 3. Sculptural ornament was frequently produced in cast stone. 

Repetitive detail, such as these banding course panels on the Level Club in 

New York City (1926), were produced much more economically than they 

could be in natural stone. 

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Figure 4. As shown in this Sears, Roebuck and Co. concrete machinery catalog, artistic concrete used standard concrete mixes to produce precast and 

cast-in-place concrete in molded forms, but made little effort to simulate naturalstone. 
History of Use and Manufacture 

Early Patented Systems 

While some use of cast stone may be dated to the Middle 

Ages, more recent efforts to replicate stone with cementitious materials began in England and France at the end of 

the 18th century. Coade Stone, one of the best known of 

the early English manufactures, was used for architectural 

ornament and trim, and saw limited use for interior decoration in the United States as early as 1800. Significant 

advances in the artificial stone industry in the United States 

were tied to the production of natural cement or hydraulic 

lime, which began about 1820. 

A large number of patented American, English, and French 

systems were marketed immediately after the Civil War. 

One of the earliest American patents for cast stone was 

awarded to George A. Frear of Chicago in 1868. Frear 

Stone was a mixture of natural cement and sand, to which a 

solution of shellac was added to provide initial curing 

strength. Frear's system was widely licensed around the 

country, and the resultant variation in materials and manufacturing methods apparently resulted in some significant 

failures. 

Another product which utilized natural cement as its 

cementing agent was Beton Coignet (literally, "Coignet concrete," also known as "Coignet Stone"). Francois Coignet 

was a pioneer of concrete construction in France. He 

received United States patents in 1869 and 1870 for his system of precast concrete construction, which consisted of 

portland cement, hydraulic lime, and sand. In the United 

States the formula was modified to a mix of sand with 

Rosendale Cement ( a high quality natural cement manufactured in Rosendale, Ulster County, New York). In 1870 

Coignet's U.S. patent rights were sold to an American, 

John C. Goodrich, Jr., who formed the New York and Long 

Island Coignet Stone Company. This company fabricated 

the cast stone for one of the earliest extant cast stone structures in the United States, the Cleft Ridge Span in Prospect 

Park, Brooklyn, New York (Fig. 5). 

Some proprietary systems substituted other cements for the 

portland cement or hydraulic lime. The British patent process of Frederick Ransome utilized a mixture of sand and 

sodium silicate, combined with calcium chloride, to form 

blocks of calcium silicate. The sodium chloride by-product 

was intended to be removed with water washes during the 

curing process. The Sorel cement process, developed in 

1853 and later applied to the manufacture of grindstones, 

tiles, and cast stone for buildings, combined zinc oxide 

with zinc chloride, or magnesium oxide and magnesium 

chloride, to form a hydrated oxychloride cement mixture 

that bound together sand or crushed stone. The Union 

Stone Company in Boston manufactured cast stone using 

the Sorel process. Ultimately, however, alternate cementing 

systems were abandoned in favor of portland cement, 

which proved to be more dependable and less expensive. 

Figure 5. Constructed in 1868 

of Beton Coignet, the Cleft 

Ridge Span in New York City's 

Prospect Park is one of the earliest extant cast stone structures 

in the United States. 

Late 19th and 20th Century Development 

The use of cast stone grew rapidly with the extraordinary 

development of the portland cement and concrete industries at the end of the 19th century. In the early decades of 

the 20th century, cast stone became widely accepted as an 

economical substitute for natural stone. It was sometimes 

used as the only exterior facing material for a building, but 

was more often used as trim on a rock-faced natural stone 

or brick wall (Fig. 6). In most early 20th century installations, cast stone was used for exterior window and door 

surrounds or lintels, copings, parapets and balustrades, 

banding courses, cornices and friezes, and sculptural ornament. On occasion, decorative interiors were also finished 

with cast stone, although elaborate interior cornices and 

ornaments were more frequently fabricated of plaster. 

3 
Manufacture 

Manufacturers of cast stone used graded 

mixes of crushed marble, limestone, granite, and smelting slag to produce a variety 

of stone effects. A light cement matrix 

with an aggregate of crushed marble 

could replicate limestone, while a mix of 

marble and small amounts of smelting 

slag would give the effect of white granite (Fig. 7). Some manufacturers added 

masonry pigments and varied colors on 

the faces of the stone to give a somewhat 

stylized effect of variegated sandstone. 

Each manufacturer prepared a variety of 

stock mixes as well. Not surprisingly, 

aggregates varied in different localities. 

In New York State, for example, crushed 

Gouverneur and Tuckahoe marbles were 

popular facing aggregates; in other areas, 

crushed feldspar or granite and even 

silica sand were commonly used. 

The two basic cast stone production systems were "dry tamp" and "wet cast." 

The dry tamp process employed a stiff, 

low-slump concrete mix that was pressed 

and compacted into the molds. The decorative aggregate mix was frequently distributed only on the exterior facing of the 

cast units (typically 3/4" to tW' thick), 

while the cores of the units were common 

concrete. Because of the stiff mix, dry 

tamp units required a relatively short 

period of time in the molds, which could 

then be used several times a day. After 

removal from the molds, the dry tamp 

units were often cured in steam rooms to 

assure proper hydration of the cement. 

The wet cast process, on the other hand, 

used a much more plastic concrete mix 

that could be poured and vibrated into 

the molds. This system used significantly 

more water in the mix, assuring proper 

hydration of the cement mix without 

elaborate curing, but requiring that the 

units be left in the molds for at least a 

day. Because of this method of fabrication, wet cast products necessarily distributed their decorative aggregate mix 

through the entire unit, rather than simply an outer facing. 

Concrete was cast in molds of wood, 

plaster, sand and, early in the 20th century, even hide glue or gelatin, depending 

upon the production method, the intricacy of the piece to be cast, and the number 

of units to be manufactured. Metal 

molds were sometimes used for stock 

ornamental items, less frequently for custom architectural work. When the units 

4 

Figure 6. Cast stone was commonly used fo r 

molded trim in conjunction with brick or natural stone. This brick building in Rochester, 

New York, uses cast stone for the entry surround, and natural stone for unornamented 

window sills, thresholds, and water table units. 

Figure 7. A combination of crushed white 

marble and black smelting slag was frequently 

used to give the appearance of a "white granite". The hammered finish on this early twentieth century cast stone would have been produced with a pneumatic chisel. 

were adequately hard, finish surfaces 

were worked to expose the decorative 

stone aggregate. When removed from 

the mold, wet cast units exhibit a surface 

film of cement paste, which must be 

removed to expose the aggregate. 

Partially cured units could be sprayed 

with water, rubbed with natural bristle 

brushes, etched with acid, or sandblasted 

to remove the cement layer. The surface 

of dry tamp products required less finishing. 

High quality cast stone was frequently 

"cut" or tooled with pneumatic chisels 

and hammers similar to those used to cut 

natural stone (Fig. 9). In some cases, 

rows of small masonry blades were used 

to create shallow parallel grooves similar 

to lineal chisel marks. The results were 

often strikingly similar in appearance to 

natural stone. Machine and hand tooling 

was expensive, however, and simple 

molded cut cast stone was sometimes 

only slightly less costly than similar work 

in limestone. Significant savings could 

be achieved over the cost of natural stone 

when repetitive units of ornate carved 

trim were required. 

Finally, cast stone is sometimes used 

today to replace natural stone when the 

original historic stone is no longer available, or the greater strength of reinforced 

concrete is desired. Reinforced cast stone 

columns, for instance, are frequently 

used to replace natural stone columns in 

seismic retrofits of historic structures. 

Fine-grained stones, such as sandstones, 

may be very successfully replicated with 

cast stone. Coarse-grained granites and 

marbles with pronounced patterns or 

banding are, for obvious reasons, not so 

successfully matched with cast stone. 

The replacement of natural stone with 

cast stone requires careful attention to 

selection of fine aggregates and the pigmentation of the cementing matrix (Fig. 

8). Coarse aggregate, which is generally 

used in cast stone to control shrinkage 

and assure adequate compressive 

strength, can present an aesthetic problem if it is visible at the surface of cast 

stone elements which simulate sandstone. Careful control of aggregate sizes 

in the mix formulation can reduce this 

problem. 

Figure 8. For this column, the appearance of a "pink 

granite" was simulated by using a pinkish matrix 

with white and black aggregate. Erosion of a tinted 

matrix results in a significant lightening of the cast 

stone surface. 
Mechanisms and Modes of Deterioration 

The best historic cast stone can rival natural stone in longevity. Many quality cast stone installations from the first decades of 

the twentieth century are still in excellent condition, and require little repair. Like any other building material, however, cast 

stone is subject to deterioration, which may occur in several ways: 

• Separation of the facing and core layers 

• Deterioration of the aggregate 

• Deterioration or erosion of the cementing matrix 

• Deterioration of the iron or steel reinforcement 

• Deterioration of cramps and anchors used in its installation 

Separation of the Facing and Core Layers 

Separation of the facing and core layers of dry tamp units is not uncommon, and often reflects fabrication defects such as poor 

compaction, lengthy fabrication time, or improper curing. Where separation of facing and core layers is suspected, cast stone 

units may be "sounded" to establish the extent of delamination. 

5 
Fi$ure 10. Scaling of cast stone units signals problems with the cementing 

mIx and method of manufacture. Serious deterioration of cast stone, such as 

this, warrants replacement. 

Deterioration of the Aggregate 

Cast stone failure caused by deterioration of the aggregate 

is uncommon. Granites, marbles, and silica sand are generally durable, although limestone and marble aggregate are 

subject to the same dissolution problems that affect quarried units of these stones. In rare instances, a reaction 

between the alkalis in the cement matrix and the stone 

aggregate may also cause deterioration. 

Figure 11. Erosion of the cementing 

matrix on the surface of cast stone is 

common in areas exposed to water 

runoff, such as window sills and copings. If no other deterioration is present, 

this condition does not warrant repair or 

replacement, but should be considered 

normal weathering. The unweathered 

surface at the left of this photograph 

shows the appearance of the original 

hammer-dressed finish. At right, the 

matrix has eroded, and the aggregate is 

much more prominent than on the 

unweathered surface. 

6 

Deterioration or Erosion of the Cementing Matrix 

While it is relatively uncommon in twentieth century cast 

stone, serious deterioration of the cementing matrix can 

cause extensive damage to cast stone units (Fig. 10). A 

properly prepared cementing mix will be durable in most 

exterior applications, and any flaking of exterior surfaces 

signals problems in the cementing mix and in the method 

of manufacture. The use of poor quality or improperly 

stored cement, impure water, or set accelerators can cause 

cement problems to occur years after a structure is completed. Improper mixing and compaction can also result in a 

porous concrete that is susceptible to frost damage and 

scaling. Severe cement matrix problems may be impossible 

to repair properly and often necessitate replacement of the 

deteriorating cast stone units. 

More common and less serious than flaking or scaling 

caused by deterioration of the cementing matrix is the erosion of the matrix surface (Fig. 11). This usually occurs on 

surfaces of projecting features exposed to water runoff, 

such as sills, water tables, and window hoods. In these 

areas, the matrix may erode, leaving small grains of aggregate projecting from the surface. The resultant rough surface is not at all the intended original appearance. In some 

historic cast stone installations, the thin layer of cement and 

fine sand at the surface of the cast stone units was not originally tooled from the molded surface, but was finished 

with patterns of masonry pigments in a stylized imitation 

of highly figured sandstones or limestones (Fig. 12). 

Erosion of the pigmented surface layer on this type of cast 

stone results in an even more dramatic change in appearance. 
Figure 12. Elaborate pigmentation of the cementing matrix sometimes gave a rather stylized or unnatural appearance to the finished cast stone (left). Texts state that 

effects like this were sometimes produced by drawing strings dipped in masonry pigments across the surface of the uncured units. Cast stone, such as this, has its coloration only at the outer surface of the stone, and was not intended to be brushed or tooled to expose the aggregate. Erosion of the cementing matrix of this type of 

cast stone, as seen on the base of the wall (right), will result in a significant alteration of appearance. 

Deterioration of the Iron or Steel Reinforcement 

During their original manufacture, unusually long and thin 

cast stone units, such as window sills or balustrade railings, 

and units requiring structural capacity, such as lintels, were 

generally reinforced with mild steel reinforcing bars. Large 

pieces sometimes had cable loops or hooks cast into them 

to facilitate handling and attachment. On occasion, this 

reinforcement and wire may be too close (less than 2") to 

the surface of the piece and rusting will cause spalling of 

the surface (Fig. 13). This frequently happens to sills, copings, and water tables where repeated heavy wetting leads 

to loss of alkalinity in the concrete, allowing the reinforcement to rust. If damage from the deteriorating reinforcement is extensive, as for instance, the splitting of a baluster 

from the rusting of a central reinforcing rod, the cast stone 

unit may require replacement. 

Deterioration of Cramps and Anchors 

Even when reinforcement has not been added to individual 

cast stone units, mild steel cramps may have been used to 

anchor a cast stone veneer to backup masonry. Where 

spalls have occurred primarily at the tops of ashlar or frieze 

units, this is generally the cause. 

Maintenance of Cast Stone Installations 

Cleaning 

Cast stone installations with marble or limestone aggregates may sometimes be cleaned with the same alkaline 

pre-wash/ acid afterwash chemical cleaning systems used 

to clean limestone and other calcareous natural stones. If 

no marble or limestone aggregates are present, acidic cleaners, such as those used for natural granites and sandstones, 

may be used. In either case, dark particulate staining in 

protected areas may be persistent, however, and require 

experimentation with other cleaning methods. Some 

micro-abrasive cleaning techniques used under very controlled circumstances by skilled cleaning personnel can be 

appropriate for removing tenacious soiling. Ordinary sand 

blasting or wet grit blasting can seriously damage the surface of the cast stone and should not be used (Fig. 14). 

Figure 13. Rusting of reinforcement near the surface of the stone may result 

in spalling. Sections of reinforcement, such as this, may be cut out and the 

spall repaired with a matching composite mortar. 

7 
Figure 14. Sandblasting and wet grit blasting can seriously erode the surface 

and should not be used to clean cast stone surfaces. 

Repointing 

Early cast stone installations may have been constructed 

with natural cement mortars, but in late 19th century and 

20th century installations, cast stone units were generally 

bedded and pointed with mortars composed of portland 

cement, lime, and sand. When rep ointing or replacement 

of the historic mortar is required, a Type N mortar (about 

one part cement, and one part lime to six parts of sand) is 

generally appropriate. When rep ointing any historic 

masonry, it is important to match both the character and 

color of the sand and color of the cement matrix in the historic mortar. Cement matrix color can often be adjusted by 

using combinations of white, "light," and gray portland 

cement in the mortar. 

Joints in historic cast stone installations can be quite thin 

and the dense mortar thus difficult to remove. 

Unnecessary repointing can cause significant damage to 

8 

historic cast stone. Cracked and open joints will most often 

be found on exposed features, such as balustrades and copings, and, of course, require rep ointing. When a hard and 

tenacious mortar was used in the original installation or a 

later repointing, the removal of the mortar can easily chip 

the edges of the cast stone units. 

While the careless use of "grinders" to remove mortar has 

damaged countless historic masonry buildings, a skilled 

mason may sometimes use a hand held grinder fitted with 

a thin diamond blade to score the center of a joint, and then 

remove the rest of the mortar with a hand chisel. If this 

method is not done carefully, however, wandering of the 

blade can widen or alter joints and cause significant damage to the cast stone. Care must be taken to prevent damage from over cutting of vertical joints by stopping blades 

well short of adjacent units. The use of small pneumatic 

chisels, such as those used to tool stone, can also work well 

for mortar removal, but even this method can cause chipping to the edges of cast stone units if it is not done 

carefully. 

Methods of Repair 

Much historic cast stone is unnecessarily replaced when it 

could easily be repaired in situ, or left untreated. This is 

especially true of areas that exhibit isolated spalls from 

rusting reinforcement bars or anchorage, or installations 

where erosion of the matrix has left a rough surface of 

exposed aggregate. 

The weathering of cast stone, while different from that of 

natural stone, produces a patina of age and does not warrant large-scale replacement, unless severe cement matrix 

problems or rusting reinforcement bars have caused extensive scaling or spalling. Severe rusting of reinforcement 

bars on small decorative features, such as balusters, may 

signal carbonation (loss of alkalinity) of the matrix. Where 

carbonation of the matrix has occurred, untreated reinforcement will continue to rust. Replacement may be an acceptable approach for exposed and severely deteriorated features, such as hand railings, roof balustrades, or wall copings, where disassembly is unlikely to damage adjacent 

construction. Conversely, small areas of damage should 

generally be repaired with mortar "composites", or left 

alone. 

Re-securing Separated Surface Facing 

Where the decorative facing of dry tamped cast stone has 

separated from core layers, injected grouts may be used to 

re-secure the facing. Re-attachment of a separated facing 

layer may be time consuming, and should be undertaken 

by a conservator, rather than a mason (Fig. 15). This technique may be the best, most economical, approach for 

repair of figurative sculpture or unique elements that are 

not repeated elsewhere on a building. Theoretically, 

cementitious grouts are most appropriate for re-attaching 

separated facings, but hairline fissures may require the use 

of resin adhesives. Low-viscosity epoxies have been used 
for this purpose, and may be applied through small injection ports. Cracks that would allow the adhesive to leak 

must be repaired prior to injection, of course. Holes made 

for adhesive injection will require patching after re-attachment is complete. 

Repairing Spalls from Rusting Reinforcement Bars and 

Mechanical Damage 

Drilled holes, mechanically damaged comers, and occasional spalls from rusting reinforcement bars and anchorage are 

repairable conditions that do not warrant the replacement 

of cast stone. Small "composite " repairs to damaged 

masonry units can be made with mortar formulated to 

visually match the original material, and may be successfully undertaken by a competent and sensitive mason. If 

deterioration appears widespread, however, or if large surface areas are spalling or cracking and replacement appears 

necessary, the owner may wish to consult a preservation 

architect or consultant to determine the cause of deterioration and to specify necessary repairs or replacement, as 

appropriate. 

The methods of composite repair used for stone masonry 

are also generally applicable for the repair of historic cast 

stone. For repairs to damaged cast stone to be successful, 

however, both the cement matrix color and the aggregate 

size and coloration must match that of the historic unit. 

Crushed stone and slag (such as "Black Beauty" abrasive 

grit), which are similar to many common traditional aggregates, are widely available, although some additional 

crushing and / or sieving may be necessary to obtain aggregate of an appropriate size. Remember that half or more of 

a weathered surface is exposed aggregate, so careful aggregate selection and size grading is extremely important for 

patching. Even differences in aggregate angularity 

(rounded pebbles vs. crushed stone) will be noticeable in 

the final repair. If more than one aggregate was used in the 

cast stone, the ratio of the selected aggregates in the mix is, 

of course, equally important. Variation in coloring of the 

cement matrix may be achieved through the use of either 

white, "light, " or gray portland cement. If additional tinting is required, only inorganic alkali-resistant masonry pigments should be used. Because most historic cast stone 

was manufactured primarily from portland cement and 

aggregate (with a less than 15% lime / cement ratio), it is not 

necessary to add large amounts of hydrated lime to cast 

stone composite repair mixtures. Small amounts of lime 

may be added for plasticity of the working mix. 

To repair a spall caused by deterioration of a ferrous reinforcement bar or anchorage, it is necessary to remove all 

cracked concrete adjacent to the spall; grind and brush the 

reinforcement to remove all rust and scale; and paint the 

metal with a rust-inhibiting primer prior to applying the 

cast stone composite. If the reinforcement bar is much too 

close to the surface of the stone, it may be advisable to cut 

out the deteriorating section of reinforcement after consultation with a structural engineer. If deteriorating cramps 

are removed, it may be necessary to install new stainless 

steel anchorage. 

Figure 15. A delaminated layer of ornamental cast stone on the Orpheum Theater, San Francisco, California (1925), was successfully re-attached using qJoxy. The 

multiple delivenj ports for the qJ0XY are removed after treatment and the holes patched. Photo: David P Wessel/. 

9 
10 

Composite Repair of Damaged Masonry Units 

Figure 16. The (a) damaged area is cut out to create a shallow void of even depth, 1h" or more. Undercutting the sides of the void may provide better adhesion 

while the composite is being applied. A small grinder fitted with a diamond blade may be used to prepare the void. (b) A range of aggregates matching those 

in the cast stone is required. Matching aggregate size and angularity is very important. Small spatulas and trowels are used for application of the composite. 

The aggregates are mixed to a ratio approximating that in the cast stone. (c) Matching the color of the cementing matrix may require numerous tests. Only 

small amounts are required for anyone repair. Lime may be added to the mix for workability. (d) The composite mortar is applied to the void with a small 

spatula or trowel. An aggregate rich mix may not adhere properly, and it may be desirable to apply a more cement rich mix directly to the substrate. (e) The 

composite mix is pressed into the void, leaving the patch slightly proud of the surface. if) The composite is struck flush with the adjacent surfaces. (g) Patting 

the patch with a moist sponge removes matrix and exposes the aggregate. (h) It may be desirable to impress additional aggregate into the surface of the patch 

to achieve a better match to surrounding surfaces. If so, the mix aggregate should be crushed to a smaller size. Laying in larger aggregate may result in a 

mosaic appearance which does not match adjacent surfaces. (i) This completed composite repair could have been improved by brushing to remove the matrix 

residue at the edges of the repair before the surface cured. 
Where spalls have a feather edge, it will be 

necessary to cut back the repair area to a 

uniform depth (1/2" or more). As with 

natural stone composite repairs, a bonding 

agent may assist adhesion of the repair 

material to the original concrete. For 

unusually large or deep patches, mechanical anchoring of the repair with small 

nylon or stainless steel rods may be 

required. If the adjacent cast stone is 

tooled or weathered, it will be necessary to 

scribe or brush the repaired area to give it 

a matching surface texture. Adding 

enough coarse aggregate to match adjacent 

original material will sometimes interfere 

with adhesion of the composite, and it 

may be necessary to press additional 

aggregate into the applied patch prior to 

finishing. If this is not skillfully done, 

however, the surface of the patch may take 

on a mosaic appearance. For this reason, 

it is advisable to undertake test composite 

repairs in an unobtrusive location first 

(Fig. 16). Figure 17. Unlike natural stone, cast stone generally may not be tooled in place to reduce lippage of 

uneven surfaces at joints. Tooling often exposes coarse aggregate from below the surface. 

Surface Refinishing 

While re-tooling deteriorated natural stone may sometimes 

be appropriate, restoring the original appearance of cast 

stone where surface erosion has occurred is difficult or 

impossible. Tooling or grinding of the cast stone surface 

may expose coarse aggregate and will not, in any case, 

restore original patterned pigmentation that has weathered 

away (Fig. 17). Silicate paints or masonry stains may be 

applied in patterns to replicate the original appearance, but 

may not be durable or completely successful aesthetically. 

Where matrix has eroded, it is advisable to accept the 

weathered appearance of the cast stone, unless extensive 

replacement is mandated by other factors. 

Because cast stone depends on exposed aggregate to 

achieve its aesthetic effect, the use of an applied cementitious surface coating dramatically alters the visual effect of 

the material and is inappropriate as a cast stone repair technique. A cementitious surface coating can also trap moisture in the cast units and cause surface spalling. 

Figure 18. Long or thin cast 

stone elements require reinforcement. When possible, 

reinforcement of new cast 

stone should be made of stainless steel. 

11 
Figure 19. Molds for the replication of cast stone may be made on site using 

rubber materials and reinforcement. The molded profile of this window pediment will be used to run a plaster mockup, from which a production mold will 

be made. Photo: Michael Devonshire. 

Replacement of Historic Cast Stone 

Installations 

Individual cast stone units, which are subject to repeated 

wetting (such as copings, railings and balusters) and exhibit severe failure due to spalling or reinforcement deterioration, may require replacement with new cast stone (Fig. 18). 

Fortunately, a number of companies custom manufacture 

precast concrete units and can replicate deteriorated units 

in existing buildings (Fig. 19). The variables involved in 

manufacture are considerable, and it is wise to use a firm 

with experience in ornamental and custom work rather 

than a precast concrete firm which manufactures stock 

structural items, concrete pipe, or the like. Several trade 

organizations, including the Cast Stone Institute, the 

National Precast Concrete Association, and the 

Architectural Precast Association, have developed recommendations and / or guide specifications for the manufacture of cast stone and precast concrete. These specifications 

set standards for characteristics such as compressive 

strength and water absorptivity, and discuss additives such 

as air entraining agents and water reducing agents, which 

influence the longevity of new cast stone. Trade references 

and guide specifications should be consulted before contracting for replacement of historic cast stone. 

Fabrication Defects in New Cast Stone 

While the cement matrix coloration and aggregate considerations previously described require the most careful attention, project staff should also look for defects which are 

common to cast stone fabrication: 

Air bubbles. Small pits on the surface of the stone may form 

if the unit is not given adequate vibration to release 

trapped air during pouring. Bubbles can also be a problem 

12 
Figure 21. Surface cracking 

may reduce the durability of cast 

stone units. Cracking is often 

problematic on reinforced elements with thin "necks," such 

as balusters, unless curing is 

carefully controlled. 

when end casting long items such as columns or railings, 

where it is difficult to vibrate bubbles away from the finish 

surface of the unit (Fig. 20). 

Surface cracking or checking. Overly wet mixes and insufficient moisture during curing can result in surface cracking 

of large castings, such as columns. Such cracking dramatically reduces the durability of new cast stone. Small reinforced elements, such as balusters, also frequently crack at 

thin "necks" in the castings (Fig. 21). 

Aggregate segregation. Cast stone formulations generally 

include a range of coarse aggregates (crushed stone) and 

fine aggregates (sand) (Fig. 22). When units are vibrated to 

assure compaction of the mix and liberate trapped air bubbles, coarse aggregates may begin to settle and separate 

from the paste of cement and sand. Aggregate segregation 

results in a visible concentration of coarse aggregate at one 

end of the casting. Segregation is more problematic when 

end casting long pieces, such as columns. 

Surface rippling or irregularity. Production 

molds for fabrication are often made of rubber mold facings encased in larger "mother 

molds" of plaster and wood (Fig. 23). 

Vibration can loosen the rubber facing from 

the outer mold and result in rippling or 

irregularities on the surface of the finished 

casting (Fig. 24). Even when rippling is not 

noticeable, irregularity caused by mold 

movement can make it difficult to line up 

surfaces of adjacent units when assembling 

cast stone installations. 

Figure 22. Most cast stone formulations use a mixture of coarse and fine aggregates. Coarse aggregates 

are not usually as visible from the surface, but may settle in end cast pieces. 

Mold lines. Freestanding elements, such as 

columns, must be cast in two-part molds, 

which are separated to release the completed cast piece. If the mold parts do not join 

tightly, some leakage of cement paste will 

occur at the mold joint, resulting in a projecting line on the surface of the casting. 

This is generally tooled off before the casting completely cures. A mold line will be 

visible on the completed piece if the projecting material is not completely removed, or 

if the tooling at the mold line does not 

match the adjacent surface of the casting. 

Tooling at mold lines may also expose contrasting coarse aggregate beneath the surface of the casting. 

13 
Other Considerations for Replacement of Cast Stone 

Several other considerations are worth noting when it is 

necessary to replace historic cast stone elements with 

matching new cast stone. 

R~inforcement. The alkalinity of new concrete generally provIdes adequate protection to steel reinforcement. In 

exposed areas where deterioration due to rusting of reinforcement has previously been a problem, however, the use 

of stainless steel reinforcement is recommended. 

Surface finishing. Post-fabrication surface tooling of new 

cast stone is not currently common. Sandblasting is typically used to remove the surface film of cement and expose 

the aggregate. For replacement units replicating historic 

cast stone pieces in highly visible locations, it is sometimes 

P?ssible .to make a mold of a sound or repaired existing 

pIece to mcorporate the original tooling in the casting process. If the historic unit is too deteriorated to use as a pattern, a plaster model may be made to replicate the damaged piece. This is tooled to replicate the desired surface 

treatment or appearance, and a production mold is then 

made from the plaster model. 

Moist curing. Surface crystallization of soluble salts (efflorescence) during curing may lighten the surface of some 

precast units, especially those simulating darker stone. 

Some manufacturers use a series of wetl dry curing cycles 

or washing with acetic acid to remove soluble salts that 

might otherwise discolor finished surfaces. For most wet 

cast products, simple moist curing under a plastic cover is 

sufficient. 

14 

Figure 23. Production molds (left) made 

of durable rubbers backed with wood and 

plaster supports are used to fabricate new 

cast stone. Production molds may be 

made from original elements which have 

been repaired with epoxies and plaster, or 

from new plaster mockups. (below) 

Appropriateness of Glass Fiber Reinforced 

Concrete as a Replacement Material 

Light-Weight Alternative 

Glass fiber reinforced concrete (GFRC) is more and more 

frequently encountered in building rehabilitation and is 

used to replicate deteriorated stone and cast stone, and 

even architectural terra cotta (Fig. 25). This is a relatively 

new material that uses short chopped strands of glass fiber 

to reinforce a matrix of sand and cement. GFRC has 

become a popular low-cost alternative to traditional precast 

concrete or stone masonry for some applications. 

Fabricators use a spray gun to spray the mortar-like mix 

into a mold of the shape desired. The resulting concrete 

unit, typically only 3,4" thick, is quite rigid, but requires a 

metal frame or armature to secure it to the building substrate. The metal frame is joined to the GFRC unit with 

small "bonding pads" of GFRC. 

GFRC has a dramatic advantage over traditional precast 

concrete where the weight of the installation is a concern, 

such as with cornices or window hoods. Many cast stone 

mixes can successfully be replicated with GFRC. Where it 

is used to simulate natural stone, GFRC, like cast stone, is 

most appropriate for simulation of fine-grained sandstones 

or limestones. 
Not for Use in Load Bearing Applications 

Because the GFRC system is in effect a "skin," GFRC cannot 

be used for load bearing applications without provision of 

additional support. This makes it unsuitable for some 

tasks, such as replacement of individual ashlar units. It is 

also not appropriate for small freestanding elements, such 

as balusters, or for most columns, unless they are engaged 

to surrounding masonry or can be vertically seamed, which 

may significantly alter the historic appearance. GFRC units 

must also allow for expansion and contraction, and are generally separated by sealant joints, not by mortar. A sealant 

joint may be unacceptable for some historic applications; 

however, substitution of GFRC for cast stone may be appropriate when an entire assembly, such as a cornice, roof 

dormer, or window hood, requires replacement. Great care 

must be taken when detailing a GFRC replacement for 

existing cast stone. 

Deterioration of GFRC 

Because it is a relatively new material, the long 

term durability of GFRC is still untested. When 

GFRC was first introduced, some installations 

experienced deterioration caused by alkaline sensitivity of the glass fiber reinforcement. Alkali 

resistant glass is now used for GFRC manufacture. Even when the GFRC skin is well manufactured, however, the steel armature and bonding 

pad system used to mount the material is vulnerable to damage from leakage at sealant joints or 

small cracks in wash surfaces. The use of galvanized or stainless steel armatures, and stainless 

steel fasteners and bonding pad anchors is advisable. 

Figure 25. Glass Fiber Reinforced Concrete (GFRC) is sometimes used to replicate deteriorated elements of cast or fine 

grained natural stone. Because the GFRC element is a rigid, but 

relatively thin shell, it must be supported and attached with an 

interior framework of steel. The attachment hardware inside this 

GFRC cartouche (left) will not be visible when the unit is 

installed (armature visible at right). Photos: Towne House 

Restorations, Inc. 

Summary 

Figure 24. Separation of an interior rubber mold 

from a plaster mother mold during casting may 

result in rippling or irregularity of the completed 

casting. 

Cast stone--a mixture of water, sand, coarse aggregate, and 

cementing agents--has proven over time to be an attractive 

and durable building material, when properly manufactured. It gained popularity in the 1860s and, by the early 

decades of the 20th century, became widely accepted as an 

economical substitute for natural stone. Unfortunately, 

much historic cast stone is unnecessarily replaced when it 

could easily be repaired and preserved in situ, or left 

untreated. Appropriate repair of damaged units can 

extend the life of any cast stone installation. Because of the 

necessity of matching both matrix color and aggregate size 

and ratio, conservation projects which involve repair or 

replication of cast stone should allow adequate lead time 

for the assembly of materials and the preparation of test 

samples. Understanding which conditions require repair, 

which warrant replacement, and which should be accepted 

as normal weathering is key to selecting the most appropriate approach to the protection and care of historic cast 

stone. 

15 
Selected Reading 

Childe, H.L. Manufacture and Uses of Concrete Products and Cast Stone, 

London: Concrete Publications Limited, 1930. 

Jester, Thomas c., ed., Twentieth Century Building Materials, New York, 

NY: McGraw-Hill, 1995. 

Precast/Prestressed Concrete Institute, Architectural Precast Concrete, 

2nd ed., Chicago, Illinois: Precast/Prestressed Concrete Institute, 1989. 

Whipple, Harvey, Concrete Stone Manufacture, Detroit: ConcreteCement Age Publishing Company, 1918. 

Acknowledgements 

Helpful Organizations 

Cast Stone Institute 

10 West Kimball Street 

Winder, GA 30680-2535 

National Precast Concrete Association 

10333 North Meridian Street, Suite 272 

Indianapolis, IN 46290 

Architectural Precast Association 

P.O. Box 08669 

Fort Myers, FL 33908-0669 

Kay D. Weeks, Heritage Preservation Services, National Park Service (NPS), served as project director and general editor. The author wishes to thank 

Alan Barr of Towne House Restorations, Inc., Ron Moore of Western Building Restoration, architect Theo Prudon, and conservator David Wessell of 

the Architectural Resources Group for their assistance in the preparation and review of this brief. Chuck Fisher and Anne Grimmer, Technical 

Preservation Services, NPS, offered valuable comments during its development. MJM Studios provided access for photography of cast stone fabrication. Michael F. Lynch, Vice President, SPNEA, and Michael Devonshire, Principal, Jan Hird Pokorny Associates, generously lent images from the personal collections for the Brief. 

This publication has been prepared pursuant to the National Historic Preservation Act, as amended, which directs the Secretary of the Interior to 

develop and make available information concerning historic properties. Comments about this publication should be directed to: Chuck Fisher, 

Technical Publications Program Manager, Technical Preservation Services, National Park Service, 1849 C Street, NW, NC-200, Washington, DC 20240. 

<www2.cr.nps.gov /tps> This publication is not copyrighted and can be reproduced without penalty. Normal procedures for credit to the authors and 

the National Park Service are appreciated. Unless otherwise noted, all photographs in the brief are by Richard Pieper. The photographs used in this 

publication may not be used to illustrate other publications without permission of the owners. 

ISSN: 0885-7016 September 2001 

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