Pottery, for Artists Craftsmen & Teachers Part 12

The clay should be dried, then broken up with a hammer, and mixed with water, and the resultant "slurry" pa.s.sed through a sieve (No. 80). The slip is allowed to settle and the water siphoned off. The thick slip is then dried on the plaster bats until stiff enough to work up between the hands. From this clay a tile, a plate, and a vase should be made and fired. If the pieces stand a fire of about 1100 (cone .03) without buckling, splitting, or crumbling, the clay should do quite well for school work. Possibly when screened fine enough for working, the clay may be too rich or _long_ and will split at a moderate fire. Then the screenings might be pounded in the mortar, pa.s.sed through the sieve, and added to the slip. Again, ground pitchers, fine grog, kaolin, or calcined flint could be tried as stiffening agents. In the unlikely event of the clay being too refractory or _short_, a portion of rich, fusible, or _fat_ clay might be added, or the addition of powdered spar tested. (See chapter on Clays.) The colour of the body will hardly matter for schools; indeed a brown, red, or cane-coloured clay will give better results than a staring white paste, when working out simple school problems.

Where necessary, tin glaze could be used for a white ground, or an engobe; that is, a dip of white clay slip over the coloured body. For glazing, a leadless glaze is strongly to be advised. Lead is often indispensable to the craftsman, and with care need not become a danger; but in schools a lead glaze is positively harmful.

A glaze with a borax base, if ground dry and mixed with water and re-ground before sieving, will give little trouble if used immediately. It will answer for all grade work and may be used for spraying, dipping, pouring, or painting, with absolute safety.

The ground pitchers and grog may be obtained by pounding up broken biscuit and pieces of fire tile, respectively. This, and the glaze grinding, is, of course, laborious work, and suggests correlation with the Physical Education Department. The drip pan and the round tins make excellent moulds for casting drying bats and working bats.

For casting purposes plates and shallow bowls may be moulded in one piece as described, p. 26. If no lathe be handy, glazed vases may be used as subst.i.tutes, the "waste" being added in plasticine to the neck and base.

For tile-making, strips nailed on a stout board will serve in place of tile boxes. The clay is rolled out on cheesecloth with a rolling pin.

Various other expedients for drying cupboards, damp-box, etc., will suggest themselves as the course develops.

The above equipment need not be very costly. With it the students should be capable of producing all kinds of tiles, built, pressed, and cast shapes, decorated in relief, with inlays or in colours or glaze.

SIMPLE RAW GLAZES. COLOURLESS

=================================================================== NO. MATERIALS PARTS SIEVE NO. CONE METHOD OF USING ------+---------------+-----+----------+----+---------------------- I Lead oxide, red 50 100. Mesh .03 Applied evenly Glossy China stone 30 with a brush to Flint 10 the _green_ shapes. Fired very slowly. Earthenware body.

II Borax 70 80. Mesh 2 Green shapes Glossy China clay 10 dipped thick Felspar 75 and slowly fired.

Flint 20 Stoneware body.

Whiting 25 III Borax 360 100. Mesh .03 Ground dry for Glossy Silver sand 160 1/2 hour Wet for China clay 120 1-1/2 Used when Whiting 20 fresh on biscuit Flint 10 (earthenware body) for under-glaze painting.

IV Lead carbonate 130 80. Mesh .04 Used with metallic Glossy Calcined kaolin 150 oxides for Flint 50 simple colours on Felspar 50 earthenware Whiting 10 body; both green Zinc oxide 10 and biscuit.

V Lead carbonate 375 120. Mesh .04 Used thick on Matt Kaolin 210 hard white Felspar 175 earthenware (CC) Flint 120 body.

Whiting 105 Zinc 25 VI Lead carbonate 120 100. Mesh .02 Used thick on Matt China clay 50 stoneware body.

Felspar 80 Coloured with 3 Flint 15 to 7 per cent of Whiting 45 glaze stains or U. G. colours.

The proportion of lead and whiting may be varied as found expedient.

VII Borax 70 Enamel Lead carbonate 300 China clay 50 80. Mesh .07- Used with various Felspar 120 .05 combinations Lynn sand 50 oxide, and iron Tin 40 oxide and copper carbonate, giving wide range of blues and greens.

On stoneware body.

All the above colourless glaze ma.s.ses may be coloured with combinations of the various metallic oxides, or from 3 to 7 or even 10 per cent of glaze stains or under-glaze colours.

APPENDIX II

GLOSSARY

=Alumina, or Oxide of Aluminium=, is one of the most abundant of earths. Combined with silica it is the chief const.i.tuent of kaolins and China clays. It imparts refractory qualities to clays and is an indispensable ingredient of pure glazes. Pure alumina or calcined Aluminium is a chemical product.

=Ammonia.=--A volatile gaseous matter, found in some clays.

Alkaline in action.

=Antimony.=--A silver-white metallic element, used with other oxides as a colourant or to give opacity in glazes.

=a.r.s.enic.=--A non-metallic volatile element, used in glaze making.

=Barytes.=--A heavy spar used with clays to introduce density and vitrescence.

=Bauxite.=--A very aluminous earth, used in preparation of pure alumina and to render clays refractory.

=Boracic Acid.=--The natural and, usually, impure product (boric acid being free from chemicals).

=Borax.=--The combined chemical product of soda and boracic acid.

Used as a strong flux in glazes.

=Calcined Bones.=--The residuum of burned bones, used to stiffen artificial porcelain.

=Calcined Kaolin.=--Kaolin after it has been subjected to heat to drive off the water combined with it.

=Calcium Carbonate (Whiting).=--Found as a white rock, and ground to pure powder. Used with clays for soft bodies. Gives durability to glazes.

=Calcium Oxide (Lime).=--A widely distributed earthy matter.

Imparts fusibility to clays, in nearly all of which it is present in varying proportions.

=Chrome, Oxide of.=--Used in making greens, browns, and blacks.

Stands a high fire.

CLAYS:

=Ball Clay.=--Blue and black. Very plastic clays. Used with non-plastic materials, such as flint, stone, felspar, or whiting, to form fine earthenwares.

=Cane and Red Clays.=--Clays coloured by the presence of ferric oxide, and used extensively for bricks, terra-cotta tiles, and common pottery.

=China Clay.=--A yellowish-white, non-vitreous clay, product of the decomposition of granitic or felspathic rocks. Cornish China clay is exceptionally white, pure, and plastic. It is widely used with China, or Cornwall stone and calcined bones, to make bone porcelain. Felspar is added to render it vitreous. Mixed with ball clays, pipe clays, flint, and stone, it makes the various cla.s.ses of earthen and stone wares.

=Pipe Clay.=--A very white, smooth clay. Less plastic than ball clays. Much used for making slips, engobes, and enamels.

=Saggar Clays or Fire Clays.=--Coa.r.s.e refractory clays strengthened by the addition of grog, used for saggars, fire tiles, and bricks.

=Cobalt Oxide.=--The oxide of the steel-grey hard metal. Extremely valuable in pottery, making all shades of blue for under-glaze printing or staining. With iron or copper gives blue-greens.

=Copper, Oxides of, and Carbonate.=--Red, green, and black oxides of copper have been of the utmost value to potters. They are used to produce green, blue, turquoise, red, and crimson. Its extraordinary changes in reducing or oxidizing fires are of the greatest interest to the experimenter.

=Cornish or China Stone.=--A rock composed of felspar and quartz.

Its vitrification (about 1400 C.) imparts hardness and density to China clays. It is a valuable const.i.tuent of glazes. First known as "moorstone" or "growan."

=Earthy Colourants.=--Rarely used in modern commercial pottery, except for salt-glazed jars, crocks, and peasant pottery.

=Felspar.=--A fusible rock found almost pure or in combination with potash and soda, the greater the percentage of alkalies the more fusible being the spar. It is used to replace more refractory materials in clay and to stiffen glazes.

=Flint.=--A pure silica with slight traces of calcium. Found in pebble form on seash.o.r.es. Calcined and ground to a white powder, it is widely used to impart whiteness and strength to clays.

Invaluable for bedding and packing in kilns. Used with the fluxes,--lead, borax, potash, and soda,--to make glazes and gla.s.s.

=Fluorspar.=--A combination of fluorine and calcium, more fusible than felspar, and of a white colour, felspar being pink.

=Galena.=--Lead sulphide, a highly poisonous material used on "peasant" pottery, giving a soft, yellowish, transparent glaze.