Preservation Briefs: 23 Preserving Historic Ornamental Plaster

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Preservation Briefs 23, National Park Service, Pad
Lath & Plaster
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The link immediately below connects to the latest version of Preservation Brief 23:

Anne Grimmer

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From the time America struggled for a new identity as a
constitutional republic - and well into the 20th century - its
architecture and its decorative detailing remained firmly rooted in
the European classicism of Palladio, Wren, and Mansart.

Together with skilled masons and carpenters, ornamental plasterers
saw their inherited trade flourish from the mid-18th century until
the Depression years of the 1930s.  During this two hundred year
period, as the Georgian and Federal styles yielded to the revivals
- Greek, Rococo, Gothic, Renaissance, and Spanish - decorative
plaster reflected each style, resulting in the wide variety of
ornamentation that survives.  The traditional methods of producing
and installing interior decorative plaster were brought from Europe
to this country intact and its practice remains virtually unchanged
to this day.

Like flat walls and ceilings, historic ornamental plaster is made
of gypsum and lime which are stable and durable materials.  An
extremely versatile material, plaster can be modeled, cast,
incised, colored, stamped, or stenciled.  However, as an integral
part of the building system it is subject to the typical problems
of water intrusion, structural movement, vibration and insensitive
alterations, both incrementally and from adaptive use projects.
This Preservation Brief has been prepared to assist property
owners, architects, contractors, and Federal agency managers in
identifying the causes of ornamental plaster failure, specifying
repair and replacement techniques and engaging qualified
professionals to do the work.  The scope of this Brief is limited
to the repair and restoration of existing ornamental plaster;
certain forms of decorative plaster such as scagliola, composition
ornament, and artificial Caen Stone are not addressed, nor is the
design and installation of ornamental plasterwork in new
construction.  Finally, guidance on using substitute materials to
match the historic appearance of ornamental plasterwork - a
legitimate option within the Secretary of Interior's Standards for
Historic Preservation Projects - is not discussed here, but will be
the subject of another Brief on interiors.



As builders and architects were hired by an increasingly affluent
clientele, ornamental plaster shops developed from the single
artisan operations of the 18th century into the complex
establishments of the early 20th century.  American plaster studios
employed immigrant and, later, native craftsmen.  Plasterers'
guilds were in existence in Philadelphia in the 1790s.  In 1864, a
plasterers' union was organized in the United States with members
from the British Isles whose work there had been limited to palaces
and churches.  English and European craftsmen came to America where
the demand for their skills had increased by the decade, offering
them the unparalleled opportunity to open their own shops.  Over
the years, plaster elements became so popular in decorating
interior spaces that a major industry was established.  By the
1880s, catalogs were available from which property owners could
select ornamentation for their splendid new buildings.


Historically, ornamental plasterwork has been produced in two ways:
it would be run in place (or on a bench) at the site; or cast in
molds in a workshop.  Plain plaster molding without surface
ornamentation was usually created directly on the wall, or run on
a flat surface such as a plasterer's workbench and attached to the
wall after it set.  Ornament such as coffering for ceilings,
centers for light fixtures (medallions), brackets, dentils, or
columns were cast in hide glue (gelatin) or plaster molds in an
off-site shop, often in more than one piece, then assembled and
installed in the building.


Three decorative plaster forms in particular - the cornice, the
ceiling medallion, and the coffered ceiling - historically
comprised much of the ornamental plasterers' business.  These forms
appear individually or in combination from the 18th to 20th
century, irrespective of stylistic changes.

For example, an elaborate parlor cornice consisted of plain
moldings made of gypsum and lime run atop temporary lattice strips
around the room.  Tooling for plain-run moldings called for a sheet
metal template of the molding profile mounted on a wooden "horse".
Mitering was accomplished using a plaster and lime putty gauge
(mix) tooled with miter rods at the joints.  Decorative
"enrichments" such as leaves, egg and dart mouldings, and bead and
reel units were cast in the shop and applied to the plain runs
using plaster as an adhesive. Painting, glazing, and even gilding
followed.  Large houses often had plain-run cornices on the upper
floors which were not used for entertaining; modest houses also
boasted cornice work without cast enrichment.

Among the most dramatic of ornamental plaster forms is the parlor
ceiling medallion.  Vernacular houses often used plain-run
concentric circles from which lighting fixtures descended, usually
hung from a wrought iron hook embedded in the central ceiling
joist.  More elaborate medallions were composed of shop-cast
pieces, such as acanthus foliage often alternating with anthemia or
other decorative designs.  Medallions usually related stylistically
to the cornice ornament found in the room and could be created with
or without a plain-run surround.  Of particular importance to the
art of ornamental plaster was the mid-19th century double parlor
plan.  Architects often specified matching medallions of robust
proportions and ornamentation.  Later, in 20th century American
Colonial Revival architecture, architects called for Federal style
ceiling medallions.  Some of the more successful were graceful
one-piece units, utilizing classical motifs such as garlands and
swags, and in their simplicity, reminiscent of Adamesque designs of
the 1760s.

Yet another significant decorative form is the coffered ceiling.
Coffering units were cast in the shop or on site, then installed
with hanging wires to form the ceiling.  Ceiling design varied from
period to period as to depth, panel shape, and ornamental
complexity.  Not always flat, coffering is seen inside domes,
within barrel vaults and groin ceilings, along overhead ribs and
soffits.  Rosettes are usually centered in the panels and often
enrich the intersections of elaborate stiles bordering the panels.
Flat ceiling coffers are generally identical in reflected plan; on
domed or barrel ceilings, coffers differ from course to course so
as to appear identical from various sight lines.  The finish
treatment of a coffered ceiling frequently exhibits the height of
the painter's craft.  Foremost examples of ceiling coffering
include the United States Capitol, and Washington D. C.'s Union
Station.  As a popular decorative form with inherent acoustical
benefits, the coffered ceiling is seen across the United States in
many large public spaces such as theaters, courthouses, railroad
stations, and hotels.

Unfortunately, these supposedly enduring decorative forms created
by ornamental plaster tradesmen are subjected to the ravages of
both nature and man and, consequently, seldom remain as originally
designed.  Minor changes of taste are perhaps the least injurious
to plasterwork.  Considerably greater damage and deterioration are
caused by radical changes in building use and poor maintenance
practices.  Fortunately, in most cases, the form, detailing, and
finish of historic ornamental plaster can be recaptured through
careful repair and restoration.



For flat plaster walls and ceilings, as well as decorative forms,
the system to attach interior plaster to walls and ceilings
primarily consisted of 1/4" x 1-1/4" wooden lathing strips nailed
3/8" apart against studs and joists.  First a scratch coat
consisting of sand, lime, and cattle hair was troweled on the lath
and pressed through the slots so as to slump over and form "keys".
Next, a brown coat was applied to establish flat and plumb
surfaces.  The earliest plasterwork consisted of two coats of lime
and sand plaster; later in the 19th century, a third or finish coat
was applied that consisted of both lime and gypsum.  Decorative
units were generally attached to the substrate using plaster as an


Failure of the substrate is more typical than failure of the
plaster ornament itself.  Among the reasons for deterioration,
structural movement and water intrusion are the most deleterious.
Buildings move and settle, causing deflection and delamination
which result in stress cracking.  These cracks often begin at the
corners of windows and doors and extend upward at acute angles.
Roof or plumbing leaks make finishes discolor and peel and cause
efflorescence, especially on plain-run or enriched cornices.
Unheated buildings with water intrusion are subject to freeze-thaw
cycles which ultimately result in base coat and ornamental plaster

In addition, keying and adhesive properties may be further
jeopardized by weak original mixes that were improperly applied.
Substrate failure typically results from faulty lathing or rusty
lath nails, causing ceilings to fall.  In the 20th century,
vibration from heavy vehicular traffic, nearby blasting, and even
repeated sonic booms may contribute to damaging ornamental plaster.
Inadequate support in an original design may also be to blame when
particularly heavy units have simply broken off over time.
Finally, new mechanical systems, suspended ceilings and partition
walls insensitively installed in adaptive use projects, show little
regard for the inspired decorations of earlier periods.


Plaster failure is a matter of degree.  For example, top coat
failure can be repaired by applying a new finish coat over a sound
early substrate. Also, if cracking or loss of all three coats has
occurred and is not combined with major structural failure, it can
be repaired much like flat wall plaster. For ornamental plaster,
however, repair beyond patching is often equivalent to targeted
replacement of entire lengths or portions of run-in-place and cast
ornamentation. Pieces that are deteriorated or damaged beyond plain
patching must be removed and replaced with new pieces that exactly
match the existing historic plaster.  For this reason, partial
restoration is often a more accurate term than repair.  But
whichever term is used, it is not recommended that repair of
ornamental plaster be undertaken at any level by property owners;
it is a craft requiring years of training and experience.  A
qualified professional should always be called in to make an
inventory of ornamental plaster enrichments and to identify those
details which are repairable on site and which should be removed
for repair or remanufacture in the shop.


Once the cause and extent of damage have been determined,
treatments such as shoring, stabilization, and limited demolition
can begin, preparatory to repairing or restoring historic
ornamental plaster.

First, roof or plumbing leaks must be repaired to eliminate the
problem of water intrusion.  General structural repairs should be
undertaken to arrest building movement, which weakens the base coat
plasters to which the ornamental enrichments are attached.
Ornamental plaster deflection should be corrected by shoring from
below followed by re-anchoring.

Testing for poor adhesion of base coat to lath or ornament to base
coat, should be conducted to reduce further loss of enrichment.
Adaptive use intrusions should be carefully removed to protect the
existing decorative plasterwork.

Code-required fire suppression systems should be evaluated at this
time.  Modern building codes may require heat/smoke/flame detectors
and automatic sprinkler systems of various types and applications.
Fire suppression systems as well as all mechanical systems (HVAC,
plumbing and electrical systems) should be designed so that they
accomplish their purpose with minimal impact on the decorative
plaster.  Plumbing for an automatic sprinkler system, for example,
can be run above new and existing coffering so that the sprinkler
heads barely protrude from the rosette centers in the coffered
design.  Access should be provided for future system maintenance or


Before discussing how decorative forms such as cornices,
medallions, and ceiling coffers are repaired on site and in the
shop by ornamental plasterers, the "shop tour" explains traditional
casting processes used in conjunction with updated materials.  A
shop tour can be exciting, but confusing to the layman without some
explanation of modeling, molding, and casting activities.  For a
prospective client, a visit to the plaster studio or site can be of
value in choosing a qualified plastering contractor.


Generally, a highly functional shop should look well
organized--that is, not in disarray with remnants of past projects
lying about to impede current production.  Old molds may be in
abundance, but hanging from the wall or otherwise "on file".
Machinery (saws and drill presses) and hand tools should appear
well maintained.  In short, one might evaluate such a studio as one
does an auto mechanic's shop: does it inspire confidence?  This is
the time to look around and ask questions.  What is the shop's past
project work experience?  Is the firm mostly involved in new
construction work or total reconstruction?  More important than the
way the shop looks, is the personnel sufficiently experienced in
making repairs to historic decorative plaster?  What about training
and apprenticeships?  How did the staff learn the trade?  The more
that is known about the total operation the better.


Familiarity with contemporary molding rubbers is desirable.  There
are several formulations currently on the market.  In the past,
flexible molds were made with hide glue melted in a double boiler
and poured over plaster originals which had been prepared with an
appropriate parting agent.  Of the newer rubbers, latex (painted on
the model coat by coat) is time consuming and has little
dimensional accuracy; polysulfide distorts under pressure; and
silicone is needlessly expensive.  Urethane rubber, with a 30
durometer hardness, is the current choice.  Urethanes are
manufactured as pourable liquids and as thixotropic pastes so that
they can be used on vertical or overhead surfaces.  The paste is
especially useful for on-site impressions of existing ornament; the
liquid is best used in the shop much as hide glue or gelatin was
historically.  Urethane rubber has the ability to reproduce detail
as fine as a fingerprint and does not degrade during most
ornamental plaster projects.  No flexible molding material lasts
forever, so spare casts should be maintained for future remolding.


Molding plaster will also be in evidence; it is the product most
similar to that used historically.  This plaster is finely ground
to accept the detail of the rubber molds, not so hard as to
prohibit tooling, and combines readily with finish lime.  High
strength plaster is available in varying densities, some with added
components for specific purposes.  Most shops maintain these
varieties, but use molding plaster for typical work.


The contractor's familiarity with sheet metal is critical.
Accurate template blades are required to reproduce both straight
and curved sections of moldings.  The blades must be carefully cut,
filed, and sanded in order to form exact reproductive units.  A
tour of a sizable shop will include observation of running
techniques and the results of this activity should be much in
evidence.  Regardless of size, these runs should be smooth and true
when made by qualified craftsmen.


Models, whether of capitals, cornices, medallions or cartouches,
are made as whole units or in parts depending on project demands.
Completeness, accurate dimensions, and attention to historic styles
are essential ingredients of successful models.  Each part of a
model has a name, i.e., dentil, guilloche, rinceau or bolection
molding, modillion, egg and dart, and the designers and restorers
of these ornaments should know their names.  Failure to identify
these parts correctly should be of concern to a prospective client.


Molds are "negative forms" produced from completed models.  Simple
flood molds require a separator or barrier coat over the original
and a surrounding fence to prevent the liquid rubber from leaking
out.  Larger or more complicated molds are made in pieces or with
a layer of rubber supported by a plaster shell or mother mold
attached to a wooden or metal frame.  Following completion of a
successful mold, the original model is discarded because it is now
possible for it to be accurately reproduced.


Casting operations should appear clean and efficient.  A skillful
caster's output can be voluminous and often looks effortless as it
is being produced.  Raw materials are close at hand, molds are
rarely without curing plaster in them, production is stored so as
not to warp while it is still wet and each cycle, from mixing to
pouring, setting, and demolding is accomplished so as not to waste
time or break plaster casts.  A good caster generally obviates the
need for a finishing department.

Two other aspects should be noted.  Shipping facilities are
critical to move the product to the restoration site safely.
Drawing and design space should be separate from the production
floor.  In summary, the modern ornamental plaster shop inevitably
looks quite different from that pictured earlier in this
Preservation Brief, but, with the exception of contemporary tools
and materials, the operations are the same.  The following sections
discuss how repairs are made by today's plaster tradespersons.



A plain run or ornamented plaster cornice, which has undergone
damage or severe deterioration can often be repaired.  Footage
which is beyond repair should be identified and carefully
demolished to expose the underlying structure beneath to which the
molding was secured.  To replace the missing lengths, the first
step is to obtain a cross-section, or profile, through the cornice
from finish ceiling to finish wall lines.  This is best
accomplished using one of these methods:

1.   A section through the cornice may be determined by sawing
    through the molding, inserting a sheet metal blank in the slot
    and tracing the profile directly on the template.  This is
    considerably more accurate than the profile gauge, but will
    require repointing the saw kerf; alternatively, the cut may be
    made on one of the deteriorated pieces, provided it was
    removed as an intact unit.

2.   The section may be obtained by making a thixotropic rubber
    impression of the molding, casting the result in fresh plaster
    and sawing through the cast to transfer the cross-section to
    a sheet metal template.

With the section determined, it is drawn onto 22-gauge galvanized
sheet metal, cut with tin snips and carefully filed to the line.
The template is checked periodically against the original profile
to assure a perfect match.  With the template blade finally
complete, it is nailed to stock and slipper, ready for running the
replacement footage.

Short lengths of new cornice are best run on a bench using gypsum
and lime; the reproduction molding should be somewhat longer than
the required length.  The new footage is cut and fit in place to
match the existing cornice, then securely countersunk screwed to
studs, joists and/or blocking.  The resulting joints are pointed
with flat mitering rods, flush with adjacent members.

Longer lengths of cornice may be run in place, much as they were
historically.  Care should be taken that the position of the
running mold engages with the existing work at either end of the
run.  Yet another method is to bench run the cornice to five or six
feet, make a rubber mold of the model, and pre-cast the replacement
parts either at the site or in the shop.

If the damaged cornice is ornamented, samples of the enrichment
should be removed, making sure that whole original units are
obtained.  This is a difficult process, since these units were
stuck into plain-run recesses called "sinkages" using plaster as an
adhesive.  In order to insert a flat chisel behind the ornament to
break the bond, some units may have to be sacrificed.  Sacrifice
should be minimal.  The excised enrichment should then be removed
to the shop for rubber molding and casting either with or without
the paint buildup, depending on the demands of the project.
Whereas molding with several layers of paint make it hard to
discern new casts from originals, paint-stripped molding reveals
the remarkable talents of the period model makers.  As noted,
contemporary rubber materials have "fingerprint detail" capability.
Modern casts are then applied to the new or original runs, again
using plaster as an adhesive.


Ceiling medallions are often in greater jeopardy than cornices
because the joist-lath basecoat support system is susceptible to
deflection and the force of gravity.  The problems of ceiling
failure are more frequent in the centers of parlors because
circular-run and shop-cast ornament is often quite heavy and was
not historically attached with any additional mechanical fasteners
such as bolts and screws.

If the lath or keys have failed, the plaster ceiling ornament may
be saved, in whole or in part, by removing floor boards above, then
drilling and injecting each lath with an elastic acrylic or epoxy
material to reattach plaster to lath, and lath to the joists.  This
is a recently developed procedure which should only be undertaken
by experienced professionals.  The consolidation and reattachment
process has been used successfully in period structures with
dramatic results when important plaster and painted surfaces would
otherwise have been lost.

Historic lighting fixtures often hung from elaborate ceiling
medallions.  When these fixtures were later converted to gas and
electrical service, the central ornamental plaster canopies were
sometimes damaged by insensitive tradesmen.  More recent adaptive
use projects may have caused additional damage.

Damaged ceiling medallions can be repaired by carefully removing
representative plaster ornamentation, molding and recasting in the
shop and replacing the new enrichments so that they align perfectly
with the original pattern.  Polyvinyl acetate bonding agents are
applied to the background and ornament so that the adhesive plaster
grips tightly.  Alternatively, a severely damaged medallion can be
replaced using the fragments as physical documentation to cast a
visually accurate replacement.

Sections of plain-run circular molding may also be repaired by
determining a section through the run and the radius from molding
to pivot point.  As with cornices, the run should be made on a
bench to a length greater than required, then cut and fit in place.
Circular run sections are installed using plaster adhesives on
bonded surfaces or modern construction adhesives after referring to
manufacturers' instructions as to whether the adhesive is
recommended for use on wet or dry materials.  Coarse-threaded,
galvanized screws are often countersunk to aid the bond; if
possible, the screws should be inserted at points that will
ultimately be covered with cast enrichments.

Ceiling medallions frequently appear in matching double parlors.
It is not unusual for one ceiling to fail while its mate remains
undamaged.  The flat plastered ceiling over the location of the
missing medallion often has a "ghost," confirming that a ceiling
medallion once ornamented the parlor.  The missing medallion may be
remanufactured by securing a section, dimensions, and samples of
cast enrichments from the surviving ornament and accurately
following the original procedure.  The ceiling on which the new
work is to be set should be examined for its soundness and, if
necessary, relathed (with self-furring metal lath) and plastered.
The pivot point for a circular run is screwed into a wooden block,
force-fit into the center electrical box, and removed after the run
is completed.

After 1850, particularly in the South, ceiling medallions were
often designed with cast ornament only; no plain run surround was
used.  Repair of such medallions proceeds as described above but
without bordering molding.

An important point needs to be made about adding ceiling medallions
(or any other kind of ornamental plaster element) when there is a
lack of historical evidence.  If there is no ghost mark or other
documentation, indicating a medallion once existed, then the room
should remain unornamented as it was historically.  Adding
conjectural ornamentation of any type or material (i.e., shop-cast
or glass fiber reinforced plaster or polystyrene foam substitutes)
can create a false sense of historical development contrary to the
preservation principles stated in The Secretary of the Interior's
Standards for Historic Preservation Projects.  However, if there is
clear indication that a ceiling medallion once existed, but there
is inadequate documentation for its replacement, a medallion
compatible with the room's historic character may be considered.
Professional advice should be sought.


Like cornices and medallions, coffered ceilings suffer from poor
maintenance practices and structural problems; however, these
individually cast ceiling units are particularly vulnerable when a
building is being rehabilitated and great care is not taken in
executing the work.  In the most serious of cases, portions of a
roof can collapse, dropping heavy debris through the hanging
coffering panels, and demolishing large portions of the

But even this level of damage can usually be remedied by
restoration professionals.  Immediate action calls for shoring the
areas adjacent to the damage, and inspecting the hanging apparatus
for unforeseen detachment and deflection.  New channel iron is used
to stabilize the existing coffers and ties reinforced, as
necessary.  An intact coffering unit is then identified and
carefully removed to a casting shop for molding and casting.  When
rehung, the units are painted to match the historic coffering.

Coffered ceilings appear with plain run or enriched cornices.  In
most cases it is recommended that the cornice be repaired first in
order to achieve straight and level moldings.  Then the damaged
coffers should be replaced with the matching new coffers and the
joints between pointed.  Access from above is critical.


When ornamental plaster damage or deterioration has been
identified, the historic property owner, architect, or developer
should secure the services of a reputable restoration contractor
before proceeding further.  It is clear as more and more projects
are undertaken, that there is a wide disparity of skills within the
trade today.  This is partly due to the introduction of gypsum
board as a substitute for traditional plastering.  As gypsum board
became popular after World War II, plasterers saw the demand for
their skills decline.  Plastering techniques were forgotten because
they were often not passed down within shops and families.
However, ornamental plaster studios have seen a resurgence in
demand for their services in the last decade, particularly as more
historic buildings are rehabilitated.

Locating an experienced contractor who is suitable for your
particular project is the goal.  First, many professional
preservation organizations can provide references for suitable
restoration contractors.  Local plasterers' unions should also be
able to identify contractors with experience in ornamental plaster
restoration projects.  Architects with preservation and restoration
project experience may recommend contractors they feel have done a
good job for them in the past.  Museums with period rooms have
engaged craftsmen to assemble the backgrounds for display of
antique furniture and decorative arts.  Finally, historical
societies, either national, state, or municipally organized, may
have funded projects which repaired and restored ornamental

Once several contractors have been identified, their specific
abilities need to be evaluated.  Prospective contractors should be
invited to visit the job site to see and define the scope of work;
written proposals, including prices from all bidders, are essential
for comparison.  References should be provided and investigated.
An outside consultant may be engaged or an informal adviser
designated to aid in evaluating the experience and proposals of the
bidders.  To get a total picture, a completed project should
ideally be visited by the prospective client with the contractor
present to answer questions which often arise.

Finally, although this may not always be achievable, the bidder's
studio may be visited, preferably on a normal working day (see A
20th Century Shop Tour, above).  Alternatively, the bidder may be
visited while working on site.  Some ornamental plasterers simply
do not have shops.  They prefer to cast on site, adhering the casts
while the plaster is wet, and coordinating the job closely with the
architect, who inspects each unit as it is cast and before it is


Decorative plasterwork is usually a component of the historic
character of interiors and, consequently, The Secretary of the
Interior's Standards for Historic Preservation Projects call for
its protection, maintenance, and repair.  Where decorative
plasterwork has deteriorated beyond repair, it should be replaced
to match the old.  Based on physical documentation, both repair and
replacement can be accomplished using traditional molding plaster
and casting procedures, together with the best of the modern
molding materials available.  Once a "lost art" after the
Depression years, the skills of today's ornamental plasterers are
increasingly in demand as part of historic preservation project
teams.  The ingenious and inspired decorative work created by our
earlier architects and artisans can now be assured an extended

                             END OF SECTION

Last Reviewed 2012-09-06