Aspects of Analysis –Ceramics
Pottery, ceramic, ceramic ware, pottery ware, ware, china, pot, vessel, clay form, fired clay,
Key Interests of Modern Analysis
. Identify and record materials
. Date
. Infer or determine site use
. Identify site archaeology, and recommend and justify continuing finds or site work
Key Aspects of Modern Analysis
. Ceramic petrology: study of burnt and fired clay and archaeological artefacts made from
this material, component materials and their association –aid to determine source and
production techniques
. Chemical analysis: study of the presence/absence and quantitative data on chemical
content, a diagnostic feature and can be measured using Energy Dispersive X-Ray analysis
of a polished thin-section, X-Ray fluorescence (XRF) detects elements heavier than
fluorine, irradiated with x-rays, measures remissions that characterize composition. Nondestructive. Good for glazes and colours.
. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) or Mass
Spectroscopy (ICP-MS): vapourisation of pottery dissolved in acid, producing excited atoms
measurable by mass or wavelength.
. Neutron Activation Analysis: powdered samples, irradiated in reactor, measures isotopes.
. typology
Common Terms
. ceramic: made from rock, mineral fragments (temper or inclusions) and clay minerals –it is
an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling.
Ceramic materials may have a crystalline or partly crystalline structure, or may be
amorphous (e.g., a glass). Because most common ceramics are crystalline, the definition of
ceramic is often restricted to inorganic crystalline materials, as opposed to the noncrystalline glasses.
. Ceramic: object subject to high-temperature firing that bonds a glaze
. Ceramic: the word ceramic is traced to the Greek term keramos, meaning pottery or potter.
Ceramics are defined as products made from inorganic, non-metallic materials with a
crystalline structure, usually processed at a high temperature at some time during their
manufacture.
. Clay: basic material for all ceramic ware is clay, a soft earth that is plastic, or may be
molded with the hands, consisting of hydrated silicates of aluminum. It is the result of the
wearing down and decomposition, in part, of feldspathic rocks containing aluminous
minerals, such as granite.
. inclusions are distinguished from clay matrix (as material less than .1mm)
. voids, usually a disintegrated, burned or dissolved inclusion
. lenticular voids: by product of mixing process and the folding in of air,
. laminae: void, when a relict of original stratification
. post-production:
. Structural, including bricks, pipes, floor and roof tiles
. Refractories, such as kiln linings, gas fire radiants, moulds, steel and glass making
crucibles
.
. Whitewares, including tableware, wall tiles, pottery products, and sanitary ware
. Technical, -modern, sometimes non-clay, space shuttles etc.
source: clay, outwash sands, blown outwash sands (loess), white clay (Kaolin) for
china/porcelain
Direct Analysis
. utilization: optical examination for microwear features –striations, polishes, microflaking,
micotopography,
Indirect Analysis and Traditional Explanation
. as other objects, as discussed in other seminars
Archaeological and ceramic pottery parts
Sherd/shard/potsherd: a piece of fired ceramic
Ostracon –sherd with written or inscribed writing
rim/lip
neck
body
base
foot
spouts
handles
decoration
castellation
concretions
chemical weathering (leaching)
mechanical weathering (abrasion)
Ceramic Types
Agate – earthenware made to resemble the semi-precious stone agate. It is created by building up
irregular layers of white, green, blue, and brown clay.
Basalt – a type of hard, unglazed, black stoneware that is uniformly and densely grained.
Bisque Ware – the first firing of clay. From the French for “twice fired,” a reference to the
preliminary firing involved in the production of the glassy frit used in French soft-paste
porcelain. Biscuit firing hardens the clay in preparation for glazing. Some sculptural
biscuit pieces are deliberately left unglazed and undecorated.
Blue-and-White Ware –porcelain and other white ware that has a decorative blue pigment beneath
the glaze. The pigment is derived from cobalt.
Bone China –combines bone ash with the hard-paste porcelain ingredients kaolin and china stone,
in a formula of 50% bone ash, 25% kaolin, and 25% china stone. This formula is still used
today.
Celadon –stoneware with a feldspathic glaze that gives it a bluish-green colour.
China – a term commonly used to describe ceramic wares, named after the country of origin.
Clobbering – porcelain that is originally white or blue and white that is touched up with various
other colours
Combed Ware – clay objects decorated with dots or lines made by pressing a comb into damp clay
or by dragging a comb across surface of the clay.
Dipped Wares – ring-shaped or banded decoration. Called dipped because they are dipped in a
container holding coloured slip.
Earthenware – an opaque, porous and coarse ceramic ware that is fired at a relatively low
temperature (700º–1200ºC). –sometimes twice fired
Porcelain –a white, translucent, vitrified clay body fired at a very high temperature (1250º–1450ºC).
Slipware – a decorative technique using slip, a liquid mixture of fine clay and water. The slip can be
coloured with oxides or coloured clays and applied to the vessel by dipping, painting, or
trailing, like icing on a cake.
Terra Cotta – a lightly fired, unglazed earthenware usually reddish in colour. It has frequently been
used by sculptors and modellers to produce models or studies for more finished pieces in
other materials.
Redware – a type of earthenware made from clay that contains a large proportion of ferrous oxide,
giving it a red color.
Hard-paste Porcelain – true oriental hard-paste porcelain is made from white china clay (kaolin)
and china stone (petuntse or feldspar, a silicate of potassium and aluminum). These two
ingredients are fired together at a high temperature (1250º–1350ºC) to produce a glassy
matrix.
Soft-paste Porcelain – sometimes referred to as artificial porcelain, soft-paste porcelain is made
without kaolin. France was the first country to successfully manufacture this type of
porcelain in the 17th century.
Porcelaineous Stoneware –a high-fired white ware that is similar to, but not as glassy as, true
porcelain.
Stoneware – a dense, fine-grained, non-translucent, vitrified clay body that is impervious to liquids
and fired at a high temperature (1200º–1350ºC). The clay contains significant amounts of
aluminum silicates.
Clay
Fireclays – these clays contain a slightly higher percentage of fluxes than pure clay. They usually
fire to a grey colour and mature at between 1371º–1482ºC, a higher temperature range
than that typically used in firing pottery and glazes. Fireclay is often used to make muffle,
firebrick, and kiln linings.
Greenware – this refers to hard clay objects that have not yet been fired. In this state, moisture can
be added to turn the clay back to a malleable form. Despite its name, it may be grey,
turquoise, yellow, or blue.
Kaolin – this very fine, pure, white-firing natural clay is used to make porcelain paste. Kaolin forms
when the mineral feldspar decays, and is sometimes called “China clay” because of its
prevalence in China.
Paper Clay – any clay that has had paper added to it. This makes the clay piece stronger and more
pliable, and can be added to, cut apart, reassembled, and mended even after it has dried.
Paste – clays and other materials that compose the clay body, exclusive of the glaze. Paste can be
described as coarse-, medium-, or fine-grained and in terms of its porosity.
Porosity –the state of being porous. This is an important factor in the creation of ceramics. Clay
can be too porous or not porous enough, conditions which may result in cracking. Clay
can be combined with flint, grog, or another type of clay to adjust its porosity.
Plastic
leather-hard
dry
bone dry
Decoration
Bas Relief – a type of relief in which figures or shapes protrude only slightly from the background.
Inlay ( “Mishima”) –a decoration technique that involves pressing coloured or contrasting clay into
incisions made on a clay object. The excess clay is scraped off when it has partially dried.
Insizing – decorating by cutting designs into the clay with a sharp tool or wire.
Istoriato – an Italian term meaning story. Refers to imagery on ceramic surfaces that tells a story.
Lustre –applying an iridescent decoration to ceramics by depositing a metallic film onto the glaze.
Oxides of silver, copper, or gold are dissolved in acid, combined with an oily medium and
painted to the ware. The object is then fired in a reduction (oxygen-starved) kiln, causing
a metallic film to adhere to the ceramic surface that becomes iridescent when burnished.
Relief – the protrusion of figures or shapes from a background.
Sgraffito –scratching through a coloured slip to expose another colour underneath.
Trailing –lines or dots of coloured slip are dribbled on the surface of a clay body.
Wax Resist – a decoration technique in which warm wax is applied to pottery or to the pottery’s
glaze so that subsequent layers of glaze do not adhere to the waxed areas.
Impression
Production
General terms: raw clay, crush, powder, mix, sieve, dice, temper, kneading, lubricate, glaze,
thrown, body, mix, clay, kiln, pit firing, firing, basic types: pinch pots, coil pots, slab pots,
thrown pots, hand formed sculptures/other
Biscuit – the first firing of clay. From the French “twice fired,” a reference to the preliminary firing
involved in the production of the glassy frit used in French soft-paste porcelain. Biscuit
firing hardens the clay in preparation for glazing. Some sculptural biscuit pieces are
deliberately left unglazed and undecorated.
Bisque - another term for “biscuit,” meaning earthenware or porcelain that has been fired once but
not glazed.
Body/matrix –the name used to describe the physical composition of any type of ceramic ware, as
opposed to its decoration or glaze.
Burnishing –the process of rubbing the surface of a leather-hard or dry clay object with a hard
object, such as a stone or a piece of glass or metal. This creates a polished look and
reduces the piece’s porosity, making the clay more resistant to water absorption.
Coiling – creating the walls of ceramic forms by melding together rolls of clay.
Kick Wheel – a potter’s wheel is powered by the artist’s foot, which kicks or pushes a stone or
concrete base.
Kiln –
a furnace or oven used to fire pottery and fuse glass. Kilns can be powered by natural
gas, propane, wood, coal, or fuel oil.
Low-Fire Kiln –a kiln that heats to a temperature high enough to fuse the clay into a solid mass, but
too low to make the clay totally non-absorbent.
Fire –
to bake clay in a hot kiln or oven. To harden clay, the temperature must be high enough
to fuse the clay particles. High firing (1200°–1400°C) is used for porcelain, moderate firing
(1200°–1280°C) is used for stoneware, and low firing (800°–1100°C) is used for
earthenware. biscuit (first firing, not glazed, porous), bisque (USA-first firing), glost firing
(second)
Maturing Point – the temperature and time during firing at which the clay or glaze attains its ideal
state of solidity and density.
Muffle – a kiln or a compartment in a kiln in which pottery can be fired without exposure to direct
flame.
Oxidation –a kiln firing in which the pottery is exposed to a full supply of oxygen. Electric kilns
perform these firings, and oxidation occurs between 704º and 1149ºC. All pottery goes
through an oxidation stage during firing, although it is called oxidation firing only when the
complete process is fulfilled. Otherwise, it is known as reduction firing.
Oxides – pigments derived from metallic oxides and used to decorate ceramic bodies and glazes.
Pure metallic oxides include cobalt for blue, copper for green, iron for brown, manganese
for purple, antimony for yellow, and various other compounds to produce red and black.
Metal-oxide pigments are called underglaze colours when painted over the glazed
surface and refired. In-glaze pigments are painted onto the unfired tin-opacified leadglazed ware, which fuse together during firing. Overglaze enamel colours provide the
greatest range of colours because of their low-firing requirements.
Pinch Pot –a clay form created by manually pinching and manipulating a mass of clay into the
desired shape.
Reduction – a kiln firing in which there is an intentional absence of oxygen. This type of firing
produces carbon monoxide, which functions to extract oxygen from the clay and glaze,
resulting in colour changes to the piece.
Pottery – refers to any type of ceramic ware or the workshop where ceramic ware is produced.
Vitrification –the conversion of clay into a hard, glass-like substance by firing it at a high
temperature.
Wheel Throwing (Throwing) – using a potter’s wheel to produce pottery. A mass of clay is placed in
the exact centre of the wheel head, and an opening is formed in the middle of the clay.
Potter’s Wheel (turntable)– a flat, rotating disk used for throwing clay.
Glazes, Slips and related
Cobalt – the blue pigment used in the decoration of blue-and-white ware.
Crackle –the intentional creation of a fine network of cracks in the glaze on a clay surface,
sometimes darkened for accentuation. Crackle occurs during the cooling process when
the clay and the glaze contract at different rates.
Crawling – a part of the underlying clay surface that is exposed when the glaze separates from the
body.
Crazing – the unintentional cracking of glaze on a clay surface. This occurs during the cooling
process when the clay and the glaze contract at different rates.
Enamel – overglaze enamels are low-fired metallic oxides used to decorate ceramic wares. The
frit, suspended in an oily medium, is painted on the glazed surface, and the suspension
material is burnt away.
Engobe – liquid clay slips that are applied to the surface of wet clay, leather-hard clay, greenware,
or bisque ware. Its purpose is varied: it can be added to change the colour or texture of a
piece or to prepare it for additional decoration.
Feldspathic Glaze – used on hard-paste porcelains, this glaze is made of the same kaolin and
petuntse as the body but in different proportions. It fuses into a type of natural glass when
heated to a very high temperature
Glaze –a glassy coating fused with a ceramic body by firing, creating a watertight surface. The
glaze consists of silica, stabilizers (alumina), and various fluxes, which are added to lower
the required melting temperature.
Lead Glaze –a clear glaze in which lead oxide serves as the principal flux. Lead glazes can be
pigmented with a variety of metallic oxides.
Opaque Glaze – a non-transparent glaze that conceals the clay beneath it.
Overglaze – the application of low-fire coloured glazes to clay pieces that have already been
glazed and have usually already been fired.
Salt Glaze (Soda Glaze) – a glaze formed on stoneware by throwing common salt into the kiln at
peak temperatures. The salt vaporizes into sodium and chlorine. The sodium combines
with the silicates in the clay stoneware to form a thin, colourless glaze that resembles the
texture of orange peel.
Tin Glaze – a decorative technique invented in 9th-century Mesopotamia for use on low-fired
earthenware. Already-fired biscuit vessels are covered in a clear lead glaze with tin added
as an opacifier to create a smooth white surface, imitating porcelain after a second firing.
Pouncing – a fine powder, such as powdered charcoal, that is sprinkled over a stencil to create a
design on the underlying clay piece.
Scratch Blue – a process whereby a design is incised into pre-fired stoneware clay and a blue
pigment is rubbed into the incised design.
Underglaze Decoration – colour that is applied to greenware or bisqueware before it is glazed or
fired.
Slip - is an aqueous suspension of a clay body, which is a mixture of clays and other minerals
such as quartz, feldspar and mica
Temper, Flux, Inclusions and related
Temper - anything added to improve plasticity/workability of clay and strength of ceramic
Flux – a base oxide that lowers the melting point of vitrification, the fusion of particles that occurs
in stoneware, porcelain, and overglaze enamels. Fluxes include alkaline materials such
as sea salt, feldspar, soda, potash, bones, ferns, seaweed, and lead
Feldspar – an abundant group of rock-forming, hard crystalline minerals containing potassium,
sodium, calcium, silicates of aluminum, and sometimes barium. It is used widely as a flux
to produce stoneware, porcelain, and glazes.
Frit –
a glassy frit is composed of silica (found in sand or quartz), a flux (potash or soda), and
metallic-oxide pigments. The mixture must be calcined (melted) and ground to a powder
before being added to soft-paste porcelain or overglaze enamels.
Grog – usually made of fireclay or a comparable type of clay that has already undergone the
firing process, grog is added to clay to diminish shrinking and cracking as the clay dries,
to increase durability, workability and strength, to protect against thermal shock, and to
add texture. Grog is often added to wares such as raku and flameware because of their
vulnerability to thermal shock.
Sources:
Alan Vince Archaeological Consultancy
Gardner Museum
Virtual Archaeology Online
Pottery Experts UK