TECHNICAL TERMS

The history of tiles is dominated by technical terminlogy and this can be confusing and baffling to the uninitiated. Below are posted some of the main terms with brief definitions of their meaning.

Aerography
The application of colours to a tile by spraying through stencils, or over natural materials such as leaves. Aerography is sometimes detectable by small speckles of colour adjacent to the main images.

Architectural ceramics
Broad term covering bricks, chimney pots, roof tiles, wall tiles, floor tiles, and terracotta and faience facings. However, the term is frequently used with reference only to decorative interior and exterior finishes, thus excluding bricks, roof tiles and the like.

Arista or Cuenca
From the Spanish, meaning ridge (Arista) or bowl (Cuenca). A mould is used to form a pattern as hollows in the surface of a tile, leaving a raised outline to keep different glaze colours separate.

Biscuit
Unglazed and incomplete ceramic articles, for instance a tile after initial firing but prior to decorating or glazing.

Carraraware
Trade name for a glazed stoneware, single fired to a matt (often white) finish, developed by Doulton’s of Lambeth during the late 1880s and used from 1888 in architectural work; it was produced until 1939.

China clay
A highly refractory clay, also called kaolin; an essential ingredient of porcelain. In Britain most china clay is mined in Cornwall and Dorset

Coade stone
A high quality terracotta made in Lambeth by Eleanor Coade from 1769; it closely imitated stone in its colour and texture.

Cuerda seca
From the Spanish, meaning dry cord; decorative technique using wax and powdered manganese lines to separate glaze colours. The sunken manganese lines remain after firing as black outlines.

Delftware
Derived from the Dutch town of Delft but applied to Dutch and English tiles decorated with opaque white tin glaze onto which a design is painted, frequently executed  in blue or purple. Tile designs invariably include corner motifs and a central subject; large tile panels were also made using this technique.

Dust-pressing
The compacting of dust clay, a fine powder clay with low moisture content, in a tile press. The process was invented by the engineer Richard Prosser in 1840 and commercially exploited by Herbert Minton who in the 1840s operated the patent under licence. It became the great revolutionary breakthrough for industrial tile production. A tile press has a square metal mould with an adjustable plate at the bottom set at a certain depth for a required thickness. The cavity of the mould is completely filled with slightly moist dust clay and the surface levelled. By turning the great flywheel, the screw, with a square metal plate at its lower end, is brought down and compresses the dust clay slowly, allowing the air to escape. It is then raised slightly only to be brought down again with great force compressing the clay and forming the tile. The tile is removed from the press and then fettled to remove the thin featheredge or burr along the edges of the tile. When dry, the tile is biscuit fired and is ready for decorating and glazing.

Earthenware
A term applied to ware having porous bodies fired around 1000 Celsius which may or may not be covered by a glaze.

Email ombrant
Email ombrant (or shadowed glaze) refers to relief tiles with tinted translucent glazes which pooled in the hollows of the design, producing graduated tones; the glaze was designed to finish with a flat surface. Sherwin and Cotton were the chief exponents of this technique. Sherwin and Cotton’s modeller George Cartlidge later set up his own company bringing with him his technique of producing almost photographic likenesses.

Encaustic tiles
An encaustic tile (sometimes also called inlaid tile) is one in which the design is reliant upon the contrast of coloured clays let into the body of the tile, rather than surface decoration. During the Middle Ages this type of tile was frequently used to pave the floors of cathedrals, churches, monastic buildings and royal palaces. Most medieval tiles consisted of a body of red clay with an inlaid design of white clay; a transparent glaze applied to the surface made the body look brown and the white inlaid clay honey‑coloured yellow. During the first half of the nineteenth century the manufacture of encaustic tiles was revived and were made either of plastic clay with slip infills or wholly of dust clay. In the 1840s, Minton & Co and Chamberlain were the first firms to manufacture replicas of medieval tiles successfully. Chamberlain covered their encaustic tiles with a transparent glaze, but Minton devised a method of heightening the effect of the inlaid design with a yellow enamel glaze, leaving the body of the tile unglazed. From 1842 onward Minton began to introduce colours such as blue and green to augment the basic red and yellow. Initially encaustic tiles were used mainly in churches but during the second half of the nineteenth century their use was extended to civic, public and domestic buildings.

Faience
Originally a name for tinglazed earthenware, taken from the Italian town Faenza, a major centre for glazed pottery production. In the context of architectural ceramics, faience refers to large blocks or slabs of ware glazed following an initial firing. Glazes may be translucent, opaque monochrome or polychrome, the ware may be relief modelled or flat, and the resulting blocks or slabs may be used as trim (applied cladding) or may be structural. Structural faience is normally heavily flanged or made in the form of hollow three dimensional blocks which are infilled during construction of the building. The glaze hides the colour of the clay, and it is the colour of the glaze that dominates the appearance of the faience. However, there is clearly a continuum of materials, beginning with terracotta (unglazed) and ending with highly glazed wares, not all of which are easily distinguishable.

Fireclay
A clay commonly found in coal mining areas that is resistant to high temperatures; it was used for various types of architectural ceramics.

Glaze
An impervious (usually vitreous) glassy material used to cover a ceramic article to prevent the clay body from absorbing liquids or to give it a more attractive appearance. Glaze is applied to a tile by dipping it into or spraying on to its surface a solution of powdered glaze suspended in water, then firing at a high temperature.

Impressed decoration
Linear decoration impressed into a tile through the use of a metal mould bearing the design in raised outline. This form of decoration is sometimes known as counter-relief or line-impressed. 

Lockback
Undercut indentations on the backs of dust-pressed tiles which helped to ‘lock’ the tiles more firmly to the cement to which they were fixed.  Some firms, including T. & R. Boote and H. & R. Johnson, took out patents for their particular type of lockback. 

Lustre
A rich iridescent sheen used as a decorative effect, achieved by depositing a thin film of metal derived from metallic salts mixed with the glaze on the surface of the tile, and firing in a reducing (low-oxygen) atmosphere. Ruby lustre, derived from copper sulphate, was the most common but other colours including gold, green and blue could be achieved through the use of different metal salts.

Maiolica and Majolica
The term maiolica is used for decorated tin‑glazed earthenware pottery and tiles made in Italy during the fifteenth and sixteenth centuries with intricate decorations and scenes painted in blue, yellow, green and orange. In the nineteenth century the term majolica became associated with relief tiles and pottery decorated with colourful opaque glazes.

Marmo
Trade term for a new type of faience with an almost matt glazed white finish developed by the Leeds Fireclay Co and launched as Burmantofts Marmo around 1908. The firm’s advertisements compared Marmo to white marble in colour surface and effect, and stated that an eggshell glaze was used in its manufacture.

Marsden’s patent    
Marsden’s patent (or Wedgwood’s patent impressed tile) was a process used in the 1880s. Tiles decorated in this manner show a finish similar to barbotine, only with sharp edges to the relief decoration; stencils were an essential part of the method of manufacture (although they are not mentioned in the patent). Most tiles decorated in this way have a stippled background applied before the relief decoration; some show the use of up to three stencils laid one after another.

On-glaze decoration
In this process the decorator works on a ready-glazed blank; this enabled individuals and firms to buy glazed blanks and decorate them. Nineteenth-century tiles with on-glaze decoration are easily identified because the hand-applied colours were fired at a lower temperature and have a duller finish than the glaze they were painted on, and occasionally stand a little proud of it; there was also a much enhanced palette of colours.

Pâte-sur-pâte
From the French, meaning paste-on-paste. The process involved building up thin layers of slip using a brush, until the intended decoration was achieved; often white slip was layered on a black ground. A mechanically formed and less subtle version of the process was also available.

Plastic clay
Soft and easily mouldable clay.

Relief tiles
Tiles bearing a raised design; the design is formed when plastic clay or dust clay is pressed into a mould.

Salt glaze
A glazing process actually carried out in the kiln. The ware is fired up to 1100 ‑1200°C and salt is then thrown on the fire, where it volatilises. The salt vapours settle on the wares in the kiln and react with the clay to form a sodium aluminosilicate glaze, which is acid and pollution resistant. Mainly used for sewer pipes and chimney pots but occasionally for tiles and architectural ceramics. Salt glaze can be brown or white depending on what type of clay is used. 

Sgraffito
The technique of cutting through a layer of slip revealing a contrasting clay colour below; fine lines were drawn with a stylus, bigger areas with a knife.  

Silk-screen printing
Carter’s of Poole carried out experiments with silk-screening before 1939, and were first to print on to the biscuit by this method, which became common from the 1950s. Silk-screening is essentially a method of pushing viscous colour through silk by means of a rubber leaf or squeegee.

Slip
Clay mixed with water to form a liquid of a smooth pouring consistency.

Stencil
Stencil decoration may be carried out using a fine grained sponge, spraying (when it is known as aerography) or a brush through a stencil; the finer lines were added by hand.

Stoneware
Ware made from very siliceous clay or a composition of clay and flint which can withstand extremely high firing temperatures. This results in a hard and glassy or vitrified body, which is impervious to water and thus suitable for external use.

Terracotta
Unglazed clay ware; from the Italian, meaning baked earth. Dried, shaped pieces are kiln-fired at 1150-1250°C, then allowed to cool gently before unloading. Terracotta normally ranges through buff to red in colour and is frequently used as a dressing on brick buildings.

Tin glaze
A glaze made by adding tin oxide to lead glaze; when fired it becomes an opaque white colour. Much used in the production of maiolica and delftware tiles

Transfer printing
The transfer of images to tiles from engraved copper plates, lithographic stone or wood block by means of gelatinous ‘bats’ or thin transfer paper. Transfer images can be applied on the glaze or under the glaze. John Sadler first used transfer printing on white tin-glazed tiles in Liverpool in 1756. During the 1830s Copeland produced under-glaze and on-glaze printed tiles. Printing flat colours (block-printing) from metal or stone using transfer paper was invented in 1848 by Collins and Reynolds, and developed for use on tiles by Minton. During the second half of the nineteenth century, transfer printing in its various forms became one of the main methods of decorating mass-produced tiles.  

Tube-lining
Piping thin trails of slip onto a tile from the nozzle of a handheld rubber bulb to form raised lines separating areas of coloured glazes. This method was especially suitable for one-off designs and small runs such as pictorial panels. A similar but mass-produced effect (imitation tube-lining) was achieved by using metal dies on which the outlines were depressed, thus producing raised outlines on the tile.

Underglaze decoration
Printed or painted decoration applied to biscuit-fired tiles prior to a transparent glaze. Because the decoration is completely covered by the clear glaze, it is extremely durable.