Pottery, for Artists Craftsmen & Teachers Part 13

=Gold.=--Used in solution for delicate purples and l.u.s.tres.

=Gypsum.=--When calcined gypsum becomes plaster of Paris, these two materials, together with the allied marble, limestone, and alabaster, are widely used in pastes (such as Parian), slips, engobes, and variously to impart fusibility or colour properties to glazes.

=Iron, Oxides of.=--Have a wide range of colour, from yellow to purple. They are used to stain glazes and colour bodies. They impart fusibility to clays and are carefully excluded from fine white bodies.

=Kaolin.=--A fine, white, very pure, and infusible China clay, almost pure alumina and silica. Chiefly used in the manufacture of porcelain and fine earthenware.

=Lead (Oxides and Carbonates of). White Lead, Red Lead, Litharge.=--Are very widely used as a safe and cheap flux.

Poisonous. It cannot be used in those glazes that have to stand a high fire.

=Lime.= (_See_ =Calcium=.)

=Lynn Sand.= (_See_ =Quartz Sand=.)

=Magnesia.=--A white metallic element present in small quant.i.ties in most clays.

=Manganese.=--The black and brown oxides of this hard metal are much used to stain slips and bodies, and to colour glazes brown or purple.

=Marls.=--Amorphous deposits of lime, sand, and clay, very coa.r.s.e in texture. Used in making saggars, drain pipes, and similar appliances.

=Nickel.=--A hard metallic element, the oxides of which are found useful in preparing blacks, greys, and greens.

=Nitre or Pota.s.sium Nitrate, or Saltpetre.=--A vitreous and aqueous compound, used in some glazes.

=Plaster of Paris.= (_See_ =Gypsum=.)

=Potash.=--Pota.s.sium carbonate or the leached ashes of plants.

Used from earliest times as a powerful alkaline flux.

=Potash, b.i.+.c.hromate of.=--Used for pinks and crystalline effects.

Poisonous.

=Quartz or Quartz Sand.=--Like Lynn or silver sand. This mineral is pure silica and free from lime, although the sands may contain some small percentage of iron. Used much like flint for bedding or with alkaline fluxes for the finest glazes.

=Rutile. Oxide of t.i.tanium.=--Used variously to impart a yellow tinge to porcelain, and colour and irregularity to some glazes.

=Salt.=--Sodium chloride. Sometimes used in glazes, but best known in connection with salt glazing. It vaporizes at about 1200 C., forming a silicate or hard, thin skin of glaze over the clay.

=Silica.=--A hard, colourless crystalline element; found pure, as in quartz, or in combination with alumina and alkalies, as in all clays. Present in all glazes.

=Soda. Sodium Carbonate.=--Product of the decomposition of salts with acids. It is a strong alkaline flux and much used in glaze and gla.s.s-making.

=Silver Sand.= (_See_ =Quartz=.)

=Tin, Oxide of.=--Used from the earliest times to impart opacity to glazes.

=Tincal.= (_See_ =Borax=.)

=t.i.tanium.= (_See_ =Rutile=.)

=Whitening.= (_See_ =Lime=.)

=Zinc, Oxide of.=--A white metallic oxide; used to brighten and stabilize glazes and colours.

POTTER'S TERMS

=Bags.=--Chimneys or walls of fire bricks built to protect the ware from flame.

=Baitings.=--The feed of fuel during firing.

=Bat.=--Any flat slab of plaster, biscuit, or fire clay.

=Biscuit.=--The fired but unglazed clay.

=Blowing.=--The shattering of the clay shape when biscuiting.

Usually due to hurried firing or the sudden access of heat, and the consequent generation of steam.

=Blunger.=--A machine for mixing clay.

=Bungs.=--Piles of filled saggars.

=Chuck or Chum.=--The cone or cap used to support shapes during turning on the lathe.

=Clamming.=--The wet marl, sand, or siftings applied to cracks in the hatches or doors of kilns to retain the heat during firing.

=Craze.=--The minute cracks that appear in a badly fitting glaze.

When arrived at by design, as in some Chinese work, it is termed a crackle, but there is then no fissure.

=Drawing.=--Unpacking the kiln after firing.

=Engobe.=--A dip or outer covering of slip; usually applied to inferior bodies to improve their appearance.

=Fat.=--Clays that are sticky or greasy are sometimes termed fat by potters.

=Fettle.=--To touch up, and remove traces of seams, cast lines, etc.

=Fluxes.=--Those materials which by their addition to paste or glaze render them fusible, although they may not always be fusible themselves.

=Glost.=--The glazed ware, usually applied to the glaze in firing, as glost-oven.

=Green.=--The clay shapes before biscuiting.

=Jigger.=--The wheel on which shapes are moulded with the aid of a jolley or profile.

=Joggle.=--The natch or key in a mould to insure correct adjustment and prevent slipping.

=Lawn.=--The fine mesh gauze through which glazes are strained.