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Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 40 Preserving Historic Ceramic Tile Floors

Procedure code:

0931002S

Source:

National Park Service, Preservation Assistance Division

Division:

Finishes

Section:

Ceramic Tile

Last Modified:

09/11/2012

Details:

Preservation Briefs: 40 Preserving Historic Ceramic Tile Floors



PRESERVATION BRIEF: 40

PRESERVING HISTORIC CERAMIC TILE FLOORS

 

Link below connects to the NPS website and latest version of PB #40:

 

http://www.nps.gov/history/hps/tps/briefs/brief40.htm


Anne E. Grimmer and Kimberly A. Konrad

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documents
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01121-8
              $1.50

Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


THE SECRETARY OF THE INTERIOR'S "STANDARDS FOR HISTORIC
PRESERVATION PROJECTS" EMBODY TWO IMPORTANT GOALS: 1) THE
PRESERVATION OF HISTORIC MATERIALS AND, 2) THE PRESERVATION OF A
BUILDING'S DISTINGUISHING CHARACTER.  EVERY OLD BUILDING IS UNIQUE,
WITH ITS OWN IDENTITY AND ITS OWN DISTINCTIVE CHARACTER.  CHARACTER
REFERS TO ALL THOSE VISUAL ASPECTS AND PHYSICAL FEATURES THAT
COMPRISE THE APPEARANCE OF EVERY HISTORIC BUILDING.


***INTRODUCTION***

With a tradition that dates to ancient civilizations, ceramic (any
product manufactured from a nonmetallic mineral such as clay, by
firing at high temperatures) tile flooring can be found in a
variety of settings in diverse cultures and structures, including
residential buildings ranging from large apartment buildings such
as government offices and schools, and religious buildings such as
cathedrals and mosques. Historically, its widespread use may be
attributed to the fact that a readily available natural material -
clay - could be converted by a relatively simple manufacturing
process - baking or firing - into a very durable, long-lasting and
attractive floor tile that is easy to maintain. Ceramic floor tiles
exhibit a versatility of colored glazes and decoration, and they
range from the plainest terra cotta tiles to highly decorated
individual ceramic tiles and elaborately patterned tile floors.
Their modularity, as standardized units, make them easy to fit into
different sized spaces which also explains much of the popularity
of ceramic tile floor tiles throughout history.

This Brief begins with an overview of ceramic tiles as a
traditional flooring material. It includes an explanation of the
various kinds of historic floor tiles used in the United States and
how they were made.  General guidance is given on preservation
treatments, focusing on maintenance, and when necessary, selective
replacement of damaged floor tiles.  The Brief is intended to
provide owners and managers of historic properties with an
understanding of the significance and historical background of
ceramic floor tiles, and a basic awareness of maintenance
techniques and various deterioration problems to which tile floors
are especially prone.  In the case of significant historic ceramic
tile floors, a professional conservator of ceramics should be
consulted to advise in matters of repair, restoration or
conservation. Historically, ceramic tiles were used on walls as
wainscoting, on fireplace hearths and fireplace surrounds, and even
on furniture, as well as for flooring. However, because floor tiles
are subject to greater damage and deterioration, they are the
primary emphasis of this Brief.  Highlights include: a short
history of ceramic floor tiles; a description of ceramic tile
types; a summary of traditional installation methods; maintenance
techniques; and guidance on repair and replacement.


***THE TILE-MAKING PROCESS***

Clay is an earthen material, moldable or plastic when wet, non-
plastic when dry, and permanently hard when baked or fired. It is
widely distributed geographically, and often found mixed with sand
in soils of a loam type - a mixture of clay, silt and sand.
Relatively pure clay is not usually a surface deposit, although, in
some cases it may be exposed by erosion. Clay types vary throughout
the world, and even within a region. Each type of clay possesses a
unique combination of special properties such as plasticity,
hardness and lightness, as well as color and texture, which makes
some clays better suited for one kind of ceramic than another. The
correct clay mixture needed for a particular purpose can be created
by blending clays and adding other materials, but using the wrong
type of clay can result in expensive production problems such as
crazing (the formation of tiny cracks in a tile glaze) or warping
of the tile itself. Traditionally, chalky clays have been preferred
for many kinds of ceramic tiles, in part because they produce, when
fired, a white body which is desirable for decorating. Other
materials can be added, including grog (or ground-up fired clay)
that helps aerate the clay and prevents warping, speeds firing and
reduces shrinking, or calcined flint, to harden it.

There are several methods used for making ceramic tiles: extrusion;
compaction or dust-pressing; cutting from a sheet of clay; or
molded in a wooden or metal frame.  Quarry tiles are extruded, but
most ceramic floor tiles, including traditional encaustic,
geometric and ceramic 'mosaic' tiles are made from refined and
blended ceramic powders using the compaction method, known as dust-
pressing.  Encaustic tiles, which were made by dust-pressing, are
unique in that their designs are literally 'inlaid' into the tile
body, rather than surface-applied. Once formed, tiles are dried
slowly and evenly to avoid warpage, then fired in a special kiln
that controls high, even heat at temperatures up to 1200 degrees
Celsius (or approximately 2500 degrees Fahrenheit) for 30-40 hours.
Higher temperatures produce denser tiles with harder glazes. Most
ceramic tiles require only one firing to achieve low porosity and
become vitrified or glasslike, but some, especially highly
decorated tiles, are fired more than once.  Non-vitreous and semi-
vitreous tiles are fired at lower temperatures and are much more
porous.


***HISTORICAL BACKGROUND***

Historically, the use of ceramic floor tiles goes back to the
fourth millennium B.C. in the Near and Far East. The Romans
introduced tile-making in Western Europe as they occupied
territories. However, that art was eventually forgotten in Europe
for centuries until the 12th century when Cistercian monks developed
a method of making encaustic floor tiles with inlaid patterns for
cathedral and church floors.  But this skill was again lost in the
16th century following the Reformation.  Except for finely decorated
wall tiles made in Turkey and the Middle East, and Delft tiles made
in Holland in the 17th century, ceramic floor tiles were not made
again in Europe until almost the middle of the 19th century.

The modern tile industry was advanced by Herbert Minton in 1843
when he revived the lost art of encaustic tile-making in England.
The industry was further revolutionized in the 1840s by the 'dust-
pressing' method which consisted of compressing nearly dry clay
between two metal dies.  Dust-pressing replaced tile-making by hand
with wet clay, and facilitated mechanization of the tile-making
industry. Throughout the rest of the 19th century, dust-pressing
enabled faster and cheaper production of better quality floor tiles
in a greater range of colors and designs. In the 1850s encaustic
tiles were selected for such important structures at the new Palace
at Westminster in London, and Queen Victoria's Royal Residence on
the Isle of Wight. By the latter part of the 19th century, despite
the fact that encaustic tiles were still quite expensive, they had
become a common flooring material in many kinds of buildings.

Development of the Tile Industry in America:  

Although plain, undecorated ceramic tiles were traditionally a
common flooring material in many parts of the Americas, especially
in Latin and South America, ceramic floor and roof tiles were
probably not made in the North American colonies until the late 16th
or early 17th century. It was, however, in the Victorian era that
ceramic tile flooring first became so prevalent in the United
States.  The production of decorative tiles in America began about
1870 and flourished until about 1930.

Like so many architectural fashions of the day, the popularity of
ceramic tile floors in America was greatly influenced by the noted
architect and critic, Andrew Jackson Downing. In his book The
Architecture of Country Houses, published in 1850, Downing
recommended encaustic floor tiles for residential use because of
their practicality, especially in vestibules and entrance halls.

The 1876 Philadelphia Centennial Exposition, with its European and
even a few American exhibits of decorative floor tile, was a major
factor in popularizing ceramic tile floors in the U.S.  Initially,
most ceramic tiles - other than purely utilitarian floor tiles -
were imported from England, and their relatively high cost meant
that only wealthy Americans could afford them.  However, when
English tile companies realized the potential for profitable
export, they soon established agents in major U.S. cities to handle
their American business.  The English near monopoly actually
stimulated the growth of the U.S. tile industry in the 1870s
resulting in sharply decreased English imports by 1890.

The location of potteries and ceramic tile factories is dependent
upon the ready availability of suitable ball clay (clay that balled
or held together), kaolin (a white clay used as a filler or
extender), and feldspar (a crystalline mineral), and an accessible
market.  Since the cost of shipping the manufactured products
tended to restrict profitable sales to limited areas, this usually
determined whether a factory would succeed.  Although the United
States Pottery in Bennington, Vermont, is known to have made
encaustic tiles as early as 1853, the Pittsburgh Encaustic Tile
Company (later the Star Encaustic Tiling Company), was the first
successful American tile company, and is generally considered the
first to manufacture ceramic tile in the U.S. on a commercial basis
beginning in 1876.

At least 25 ceramic tile companies were founded in the United
States between 1876 and 1894. In the East, several notable tile
firms that were established in this period flourished in the Boston
area, such as the Chelsea Ceramic Art Works, the Low Art Tile
Works, and the Grueby Faience Company. Other East Coast companies
organized in the late 19th and early 20th century included the
International Tile & Trim Company, Providential Tile Company,
Mueller Mosaic Tile Company, and the Maywood Tile Company, all in
New Jersey; and the Moravian Pottery and Tile Works in Doylestown,
Pennsylvania.

Many factories were also established in the Midwest - in Indiana,
Michigan and, especially in Ohio.  In the last quarter of the 19th
century, the town of Zanesville Ohio, was the largest center for
pottery and tile-making in the world.  Some of the factories in
Zanesville included: Ohio Encaustic Tile Company; Mosaic Tile
Company; Zanesville Majolica Company; and J. B. Owens Pottery,
later to become the Empire Floor and Wall Tile Company.  The
American Encaustic Tiling Company, established in 1876, was one of
the first, and most successful manufacturers in Zanesville.  In the
early 1930s it was the largest tile company in the world, producing
large quantities of floor tile, plain and ornamental wall tile, and
art tile until it closed about 1935, as a result of the Depression.
The United States Encaustic Tile Company, Indianapolis, Indiana;
Rookwood Pottery, Cincinnati, Ohio; Cambridge Art Tile Works,
Covington, Kentucky; and Pewabic Pottery, Detroit, Michigan, were
some of the other well-known potteries in the Midwest.

Around the turn of the century, the industry began to expand as
tilemakers moved West and established potteries there.  Joseph
Kirkham started the ceramic tile industry on the West Coast in 1900
when he set up the Pacific Art Tile Company in Tropico, California,
after his company in Ohio was destroyed by fire.  In 1904 the
company became the Western Art Tile Company, surviving for five
years until it went out of business in 1909.  During the early 20th Century,
other companies were founded in Southern California, in and around Los
Angeles.  Batchelder & Brown, in particular, of Pasadena (later
Batchelder-Wilson in Los Angeles), was well-known for its Arts and
Crafts-style tiles in the teens and 1920s.  By the early 1940s California
had become one of the leading producers of tile, especially faience, in
the U.S.

Ceramic engineers, potters and artists not only moved frequently
from one pottery to another, but often struck out on their own and
established new factories when dissatisfied with a former employer.
Also, it was not uncommon for one company to reuse a defunct
factory or purchase another pottery business, change the name and
increase the product line.  As a result, many of the companies in
existence today are descendants of the early pioneering firms.

Changes in the Tile Industry:

The majority of ceramic floor tile made in the U.S. before 1890 was
encaustic, but various factories gradually began to develop and
produce other kinds of tiles.  The Trent Tile Company, among
others, started to manufacture both white and colored ceramic
mosaic tiles by the middle 1890s.  White vitreous wall tile became
available, as well as more decorative tiles with colored glazes,
such as the variegated faience glazes intended to give a more hand-
crafted appearance that were originated by the Grueby Faience and
Tile Company in 1894, and soon adopted by other potteries.

In the 19th and early 20th century, many ceramic tile firms had their
own engraving departments, while some used commercial designs
supplied by professional printers.  Well-known designers were often
commissioned to work on specific product lines for a particular
firm.  These designers worked for one firm after another which
resulted in similar designs being produced by different companies.
(Historic ceramic floor tiles were usually identified by a
manufacturer's or designer's mark on the back, if they were marked
at all.)  By the latter part of the 19th century ready-mixed glazes
and colors were also available.  This was a great advantage for
potters who, prior to this, had to mix their own colors and glazes.

During the 20th century, the floor tile industry continued to evolve
as much as it had in the previous century.  Modern methods of
production employed sophisticated machinery, new materials and
decorating techniques. In the years following World War II, there
were many advances in the industry. Commercially manufactured dust-
pressed tiles, which had previously required more than 70 hours
just in the kiln, could be made in less than two hours from the raw
material stage to finished tiles, boxed and ready to ship.  Dried,
unglazed tiles were sprayed with colored glaze evenly and
automatically as conveyors carried the tiles into the tunnel kilns,
and the extrusion process ensured that the tiles were cut to a
uniform thickness and size.  The changes and developments in the
production of floor tile brought forth a wide range of shapes and
sizes, along with new colors, glazes and decorating techniques.

After the turn of the century, fewer encaustic floor tiles were
used, particularly in residential architecture.  The introduction
of ceramic mosaic floor tiles was a factor in their decline.  The
development of rubber interlocking floor tiles in 1894, along with
other, more resilient, flooring materials, was instrumental in the
decreased popularity not only of encaustic tiles, but also other
ceramic tile flooring.  These new materials were not only cheaper,
they were not as fragile; they were also lighter and thinner, and
easier to install.

Ceramic mosaic tiles remained in common use through the 1930s in
part because an innovative development had made laying such small
tiles easier.  The tiles were pre-mounted in decorative patterns on
12" x 12" sheets of paper and sold ready to lay in cement.  This
greatly simplified the tile setter's work, and no doubt was a
significant factor in the increased popularity of ceramic mosaic
tiles.  Sophisticated mosaic floor designs became common in
entrance foyers of public and private buildings.  Small, white
unglazed tiles in round, square, octagonal or hexagonal shapes were
promoted for their sanitary qualities, particularly for bathroom
floors, while larger, rectangular, white, glazed tiles were used
for bathroom walls or wainscoting.  Colored tiles were also
popular, especially for bathrooms, and even kitchens.  Quarry tile,
which was larger and thicker than other ceramic floor tile of this
period, was often used in public buildings, as well as for entrance
halls, small studies, libraries, dining rooms and even living rooms
in private homes.  But, by the 1930s the fashion for art tile had
diminished to the point where floor tiles were, for the most part,
generally regarded as primarily utilitarian, as opposed to
important decorative elements.


***CERAMIC FLOOR TILE TYPES***

The thickness of historic ceramic floor tiles varied considerably
according to their intended use and when they were made.  Floor
tiles were thicker and harder than wall or ceiling tiles.  Stove
tiles, meant to retain the heat of the stove, were sometimes as
much as several inches thick.  Medieval floor tiles were usually
one inch thick; encaustic tiles of the Victorian era tended to be
slightly thinner.  Modern 20th century tiles, with the exception of
some art pottery tiles, are the thinnest, as a result of modern
manufacturing methods.  The backs of most, but not all, ceramic
floor tiles are covered with raised (or sometimes recessed) ridges,
circles or squares which help to increase the bonding capability of
the tile.

UNGLAZED AND GLAZED TILES

Ceramic floor tiles can generally be divided into two types:
unglazed and glazed.  Unglazed tiles include: quarry tiles;
encaustic and geometric tiles; and ceramic mosaic tiles, which can
be either glazed or unglazed.  Most other ceramic floor tiles are
glazed.

UNGLAZED TILES

Quarry tiles are the most basic type of historic ceramic floor
tile.  Originally made from quarried stone, they are machine-made
using the extrusion process. Quarry tiles are unglazed, semi-
vitreous or vitreous, and essentially are square or rectangular
slabs of clay baked in a kiln. The colors of quarry tiles are
natural earthen shades of gray, red and brown determined by the
clay and, to some extent, the temperature and duration of firing.
Quarry tiles, which range from 1/4" to 1/2" in thickness, are
available in square and rectangular shapes in sizes that include
3", 4-1/4", 6" (one of the most common sizes), 9" and 12" squares;
6" x 12", 6" x 9", 4-1/4" x 9", 3" x 6", and 3" x 9" rectangles;
and 4" x 8" hexagon shapes. Pavers or paver tiles are a simpler,
and tend to be somewhat cruder, version of quarry tiles. Like
quarry tiles, they are usually unglazed, but slightly thicker.
Machine-made pavers are either semi-vitreous or vitreous, and
generally formed by dust-pressing, although sometimes are extruded.
Hand-made pavers which are common in Mexico and southern Europe are
non-vitreous.

Encaustic tiles are a type of traditional unglazed - yet decorative
- floor tile, manufactured by the dust-pressed method. Whereas most
ceramic tiles are surface-decorated or decorated with impressed or
embossed designs created by a mold, encaustic tiles are unique in
that their decorative designs are not on the surface, but are
inlaid patterns created as part of the manufacturing process.
First, a thin, approximately 1/4" layer of fine, almost powder-dry,
clay was pressed into a mold with a relief design at the bottom
which formed a depression in the face of the tile. A second,
thicker layer of courser clay was laid over the first layer, then
covered with another layer of fine clay. This 'sandwich' helped
prevent warping and ensured that the body of the tile was strong
and had a fine, smooth surface. The layers of clay 'dust' were
compacted by presses, after which the mold was inverted and the die
removed, thus producing a tile with an indented or intaglio pattern
on top. After the tile dried, colored slip (liquid white clay
colored with dyes) was poured to fill in the intaglio pattern. Each
color had to dry before another color of slip was added. The
recessed area was overfilled to allow for shrinkage, and after
drying for several days, and before firing, the excess slip was
scraped off the surface by a rotating cutter that created a flat,
although not completely smooth, face. Problems might arise during
the firing. Due to the dissimilar rates of contraction of the
different clays, the inlaid clay could shrink too much and fall out
of the tile recesses; or, the tile could be stained by the
different pigments used for the design if impure or unstable.

By the 1840s, encaustic tiles were made entirely with almost-dry
clay using the dust-pressed method. This served to eliminate the
possibility of staining the body of the tile with other colors and
permitted the use of more colors on a single tile. Thus, an
encaustic tile can sometimes be dated according to the complexity
and the number of colors in its pattern. Red tiles with white
figurative patterns were generally the earliest, followed by brown
and buff colored tiles. In the 1860s, blue tiles with yellow or
buff patterns were popular, succeeded by more subtle color schemes
featuring a 'chocolate' red with a soft grey. By 1860, up to six
colors were used in a single tile to form a pattern. Toward the end
of the century, white encaustic tiles with a black or gold design
were common, as well as tiles with complicated color patterns of
white, black, gold, pink, green and blue. Encaustic tiles were
decorated with traditional as well as original designs. Some,
particularly intricate, designs were painted on the surface of the
tile with opaque colored glazes, instead of being inlaid. Most
major tile manufacturers sold many of the same pre-formed encaustic
floor tile patterns through catalogues. Encaustic tiles were
produced in a variety of sizes, mostly square or octagonal in
shape, and almost any design could be custom-made for a special
purpose or to fit a particular space. Historic, 19th century
encaustic tiles were generally slightly less than 1" thick, about
15/16." Cheaper tiles of lesser quality were also made of clay or
cement. These designs resembled those commonly found on encaustic
tiles but applied as a transfer printed pattern, or using a multi-
color lithographic or silkscreen process. These are still
manufactured and popular in many parts of the world.

Smaller, single-colored versions of encaustic tiles that, when
assembled together form a geometric pattern, are called geometric
tiles in England. However, in the United States they are generally
not differentiated from encaustic tiles. Based on the geometric
segments of a six-inch square, they were typically rectangular,
square, triangular or hexagonal in shape, and about the same
thickness as patterned encaustic tiles. Geometric tiles were
especially well suited for decorative borders, and a wide variety
of floor designs could be created with their many shapes, sizes and
colors - either alone or combined with patterned encaustic tiles.
The cost of producing geometric tiles was much less than of
encaustic tiles because each tile involved only one type of clay
and one color. By the end of the 19th century, over 60 different
shapes and sizes of geometric tiles were available in up to ten
colors, including buff, beige or tan, salmon, light grey, dark
grey, red, chocolate, blue, white and black.

Ceramic mosaic tiles are essentially smaller versions of geometric
tiles (usually no larger than 2-1/4", and no thicker than 1/4")
ranging in size from 1/2" to 2-3/16", in square, rectangular or
oblong, hexagonal, pentagonal and trapezoidal shapes. Both vitreous
and semi-vitreous mosaic tiles were available, unglazed in solid or
variegated colors with a matte finish, or glazed in unlimited
colors. Single, one-piece tiles were also fabricated to give the
appearance of multiple mosaic pieces. This was achieved with a
mold, which gave the appearance of recessed mortar joints
separating individual 'mosaics.'

GLAZED TILES

With the exception of quarry tiles, encaustic tiles, and some
mosaic tiles, most ceramic floor tiles are decorated with a glaze.
While unglazed tiles derive their color solely from the clay, or
from oxides, dyes or pigments added to the clay, the color of
glazed tiles is provided by the glaze, either shiny or matte. Some
potteries specialized in certain kinds of glazes and were famous
for them. The earliest and most common method of clay tile
decoration made use of tin-glazes which were essentially
transparent lead glazes. Tiles were either dipped into the glaze or
the glaze was brushed on the tile surface. Glazes were generally
made with white lead, flint, or china clays ground up and mixed
with finely ground metallic oxides that provided the color. Colored
glazes were commonly know as 'enamels.' Colors included blue
derived from cobalt, green from copper, purple from manganese,
yellow from antimony and lead, and reds and browns from iron. An
opaque glaze was created by adding tin oxide.


***LAYING CERAMIC TILE FLOORS***

19th century Techniques:

Aside from the use of improved tools and modern materials,
installation methods have changed little since the mid-19th century.
M. Digby Wyatt, an architect for one of the major 19th century
encaustic tile manufacturers in Britain, Maw & Co., described this
procedure for laying encaustic and geometric tiles in 1857:

First, either an even layer of bricks, a 2-1/2" bed of concrete of
quicklime and gravel, or a mixture of Portland cement and clean
sharp sand was laid to prepare a solid foundation for the tiles. If
the tiles were to be laid over an existing wooden floor, the floor
boards had to be pulled up, sawn into short lengths and fitted
between the joists. Concrete filled in the spaces and made the base
flush with the upper face of the joists, and created a level
surface finished within 1" of the finished floor line. A layer of
cement mortar was then laid on top. This allowed the tiles to fit
in the same amount of space as the floorboards they replaced. (The
traditional practice of sawing the original floor boards and
fitting them between the joists, still used today to maintain a low
finished floor profile, has resulted in numerous cracked tiles and
other failures. Instead, a better approach is to leave the existing
floor boards, if they are in good shape, and install a cementitious
backer board (CBU) available in thicknesses ranging from 1/4" to
5/8" as the setting bed for the tiles.)

Before laying the tiles, skirting boards or shoe moldings were to
be removed, and replaced after the tiles were laid. This eliminated
having to cut the outer tiles to fit exactly, and resulted in a
neater appearance.

Next, the floor design was marked off with mason's string or chalk
lines which divided the space into equal quadrants. The first
section to be laid out was defined by two parallel strips of wood,
or guide pieces, about 4" wide. A level thickness of cement was
spread between these strips. The tiles, thoroughly soaked in water,
were laid in cement and leveled with a straight-edge. The
foundation had to be kept wet while the tiles were being laid.
Small strips of wood temporarily placed at right angles to the
guide pieces helped keep elaborate patterns straight.

When the bed was hard, the joints were filled with pure cement
mortar - sometimes colored with lamp black, red ochre or other
natural pigments - mixed to the consistency of cream. Excess mortar
was wiped off the tiles with a piece of flannel or sponge.

A newly-laid tile floor could not be walked on for 4-6 days until
the cement hardened properly. Occasional washing would remove the
saline scum that often appeared on the surface right after the
tiles were laid.

20th Century Techniques:

Almost 50 years later, in 1904, the Tile Manufacturers of the
United States of America published Suggestions for Setting Tile
with the intent of bringing tile-laying up to a uniform standard.
This guidance was very similar to that given by Wyatt. But, there
were some differences, such as using hollow clay tile as a
foundation material and heavy tar paper when laying tile over a
wooden floor to protect the floor boards from the moisture of the
mortar mix. Emphasis was placed on using the best quality cement,
sand, and purest water to obtain a durable tile floor. Soaking the
tiles before setting was no longer necessary, but using stiffer
mortar was suggested to prevent if from rising up between the
tiles.

Tile-laying methods changed somewhat more later in the 20th century,
mostly due to the availability of new materials and techniques. By
the 1920s small ceramic mosaic tiles were manufactured as 12"
square sheets held together by a face-mounted paper 'skin.' This
made it possible to lay the 12" square of tiles as a unit rather
than each of the small tiles individually. Mounting the tiles
directly in the cement resulted in a very strong bond. But the
face-mounted paper obscured the tiles from view making it difficult
for the tile-setter to see if the tiles were being laid straight.
The fact that the paper was not removed until after the tiles were
firmly set in the cement bond coat further complicated realignment
of crooked tiles. This paper 'skin' was eventually replaced with a
fabric mesh backing. This permitted the tiles to be aligned as soon
as the moisture from the bond coat loosened the mesh from the back
of the tile; it also allowed a single tile to be cut away from the
mesh and repositioned immediately. Although the fabric mesh made
tile setting faster, sometimes it also resulted in a weaker bond by
reducing the contact area between the backs of the tiles and the
bond coat.

Following World War II, different methods of preparing a foundation
for a ceramic tile floor were developed to be more compatible with
new materials, such as reinforced concrete, expanded wire mesh,
polyethylene and waterproof plywood, new adhesives and grouts also
facilitated tile installation, and an increased variety of epoxy
and cement mortars allowed for different setting bed thicknesses.
But today, after half a century of practical application, some of
these 'new' materials, such as plywood, particle board, oriented
strand boards and other wood panels, are no longer recommended for
use with ceramic tile.

Mortar beds are lighter, more flexible, and much thinner than they
were previously, having shrunk from several inches to as thin as
3/32". A greater variety of materials are used for setting ceramic
floor tiles, including bonding agents and waterproof membranes.
Basic installation methods have not changed significantly, but they
vary according to the type of subfloor on which the tile is to be
laid. While the same concerns for level underlayment and strong
adhesion exist, advancement has occurred mostly in the increased
speed and ease of laying the tiles.


***HISTORIC CERAMIC FLOOR TILE: PRESERVATION AND MAINTENANCE***

Before undertaking any work more complicated than regular
maintenance or a very simple repair on a significant historic
damage has occurred, it is recommended that a professional

 


ceramic tile floor, preserving ceramic tile, repairing ceramic tile, Preservation Brief 40