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Scumming: Unwanted deposits of soluble salts which sometimes appear on the surface of a raw dry pot, or more significantly, on bisc-fired ware. After bisc firing, areas where scum is apparent may be slightly fused and therefore resistant to glaze take-up. Scum is unlikely to resist glaze take-up on a raw pot.
One of a group of rock-forming minerals, the most abundant group in the Earth's crust. They are the chief constituents of igneous rock and are present in most metamorphic and sedimentary rocks. All feldspars contain silicon, aluminum, oxygen, linked together to form a framework; spaces within this structure are occupied by sodium, potassium, calcium, or occasionally barium, in various proportions.
Feldspars form white, gray, or pink crystals and rank 6 on the Mohs' scale of hardness.
The four extreme compositions of are orthoclase, KAlSi3O8; albite, NaAlSi3O8; anorthite, CaAl2Si2O8; and celsian, BaAl2Si2O8. These are grouped into plagioclase feldspars, which range from pure sodium feldspar (albite) through pure calcium feldspar (anorthite) with a negligible potassium content; and alkali feldspars (includingorthoclase and microcline), which have a high potassium content, less sodium, and little calcium.
The type known as moonstone has a pearl-like effect and is used in jewelry. Approximately 4,000 metric tons of feldspar are used in the ceramics industry annually.
Ceramic change: The point at which
chemically combined water is driven off
from clay molecules, so that the clay
becomes pot, and can no longer be
slaked down. This change is permanent
and irreversible, and is widely said to
take place at 573degrees C..although it
is, in fact, an ongoing process which may
begin as low as 350C., and may still be
taking place a temperatures as high as
Flocculent/flocculant: Acids (or salts
which in solution behave as acids) which
alter the electrostatic charges of fine
particles in a suspension, reducing their
mutual repulsion, and thus increase their
forces of attraction. This causes
particles to floc together, increasing
friction and viscosity, thus apparently
thickening the suspension without
removing any water from it.
Cristobalite inversion: A sudden change
in the physical structure of silica in its
cristobalite phase. This change causes
sudden expansion on heating (with
corresponding contraction on cooling
from alpha to beta form and takes pace
a temperatures between 220C and
280C(227C is widely taken as a guide).
Resist: Any material which is used to create a barrier between the surface of a pot and an applied treatment, such as a slip or glaze coat. Resists may be either wax (hot paraffin wax or cold wax emulsion), latex (e.g. carpet glue, copydex etc.), paper (florists paper is good, but newspaper will do for making templates or stencils), or found materials
such as lace or leaves. Salt/vapour glazing: The glazing of pots in the kiln by introducing damp salt (or other soda compound) into the kiln at high temperatures. The salt decomposes and volatilises, and the soda thus released into the firing chamber the kiln and furniture too), thus forming a glaze.
Vitrification: The process by which silica is converted to glass by the action of heat and fluxes. A vitrified body is one in which some of the silica in the body has been converted from its crystalline to its glassy phase. If all of the silica in a body has been vitrified, it may slump in firing, and the finished pot will be very brittle.
If you are just starting out with ceramics you may find the technical jargon intimidating. Look through our Ceramic Glossary and Ceramic Terms to learn about the craft, tools and techniques of ceramics. Learn from our ceramics guru about the business, art and production of high quality ceramics.
Raku: A Japanese word which loosely translates as 'enjoyment', and which strictly only refers to pottery by the potter who holds the Raku title. In the
West, however, the work has become associated with a particular technique which generally involves placing pots into the already hot kiln, and often then carbonising the pots by removing them directly from the hot kiln to a bin of combustible material such as leaves or sawdust.
Group of clay minerals, such as kaolinite, Al2Si2O5(OH)4, derived from the alteration of aluminum silicate minerals, such as feldspars and mica. It is used in medicine to treat digestive upsets, and in poultices.
Kaolinite is economically important in the ceramic and paper industries. It is mined in the UK, the US, France, and the Czech Republic.
Pottery, clay that is chemically altered and permanently hardened by firing in a kiln. The nature and type of pottery, or ceramics (Greek keramos,”potter's clay”), is determined by the composition of the clay and the way it is prepared; the temperature at which it is fired; and the glazes used.
An anvil is a pebble or piece of wood used to beat pot walls. The anvil is used inside the pot, a beater or paddle is used outside. The process compresses the clay, thins the pot wall and enlarges the pot. To assist the clay to stretch without cracking, the anvil is often 'pecked' that is, it has its surface covered with small chipped holes.
Burnishing for a Blackfire
The technique of burnishing pottery can be traced back to ancient times.
Burnishing involves no more than rubbing the clay surface with a smooth tool to produce a mirror-smooth surface. In reality it has a compressing effect on the clay particles. It can be done when the clay surface is leather hard and up untilit is almost completely dry. Most clays are suitable for burnishing although the finer the clay the smoother the burnished surface.
Suitable tools for burnishing include: Smooth rounded beach pebbles, The convex side of metal spoons and smooth knife handles. After the pot is smooth, draw your design with lead pencil then scratch around design with a knife. Designs can either be geometric or organic. Once the pot has been blackfired it can be left without further treatment or polished with oils to enhance the shine.
Pinholing: a glaze fault which causes
pitting of the glaze surface. It is caused
by bubbles rising to the surface during
drying or firing, and being prevented
from healing over because of the
viscosity of the glaze melt, which may
be caused either by its composition, or
Frit/fritt: Materials which have been
prepared for use by melting (usually
together with at least one other
material-often a pure silica-to form a
fused compound) and grinding. This may
be done for a number of reasons,
including: reducing solubility, safely
containing poisons, reducing volatility
soluble alkalis added to a glaze slop
which bring about ion exchange, thus
causing clay particles in the suspension
to repel eachother. This repulsion
reduces friction between the particles,
thus increasing the fluidity of the
suspension without the addition of more
Bisc: A general term for used to
describe a prelominary firing of unglazed
Biscuit firing: A preliminary firing of
unglazed ware beyond the point of
ceramic change, but below the point of
fusion, so that the ware remains
porous. Biscuit may refer to finished
unglazed wares, despite the fact that
these may be fired to vitrification.
Bisque: Generally refers to unglazed
ware fired to maturation (and in some
cases, to vitrification)before the
application of a glaze whose maturing
temperature is lower than that of the
Quartz inversion: A sudden change in the physical structure of silica in its quartz phase. This change causes sudden expansion on heating (with corresponding
contraction on cooling) from its alpha to its beta form and takes place at temperatures between 550C and 575C (573C is the usual given temperature).
Souring: A process of organic breakdown which releases acids into the water of plasticity in a clay body. These acids bring about the flocculation of fine particles, thus improving the body's placicity and strength.
Stoneware: A high-firing clay body. Stonewares are fired to the point of vitrification, and are therefore much harder, stronger and more brittle than lower fired earthenwares
Transparent vitreous coating for pottery and porcelain, which gives the object a shiny, protective finish and helps to keep it from leaking and chipping. Glaze is applied by dipping a formed ceramic body into it or by painting onto the surface. It is fixed by firing in a kiln.
Different mineral glazes will combine chemically with different bodies (according to the minerals present in the clay). Glazes may be alkaline, lead, leadless, tin, salt, or feldspathic. Glazed pottery is first known from the mid-to late-Neolithic in Egypt, where glass was first made.
Raw glaze: Although this term is frequently used by studio potters to refer to the practice of glazing pots when they are raw, it strictly (particularly in industry) denotes a glaze which is, itself, composed of raw, unfritted materials.Reduction firing: The firing of ware in a kiln where there is an insufficient supply of oxygen in the in the chamber, so that combustion is incomplete, causing the
flames to draw on the combined oxygen in the fabric of the ware, which has the effect of reducing oxides to their respective metal forms.
|Sheri Ann Richerson|