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 Originally made from quarried stone, they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 ¼" to ½"
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.
 Encaustic floor tiles were decorated with traditional as well as original designs. Over time, the decorations can be worn thin by heavy traffic. Photo: NPS files. |
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 ¼" 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 coarser 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.
"Preserving Historic Ceramic Tile Floors" an Historic Preservation Brief
