Pottery, for Artists, Craftsmen & Teachers

Enamel kilns are used for the final processes where the glazed pot is painted with over-glaze or enamel colours. They are of no great size and are made of fire-clay slabs or even iron plates when the enamel is soft. The flames play all round the muffle or fire-clay box during the firing, but no flame or fume is allowed access to the inside. (Fig. 68.)

In packing a kiln with biscuit or glaze much care, thought, and expedition have to be exercised. An experienced setter is essential in a factory if the pots are to have every chance in the fire, for all kilns vary and have their hot and cool corners. First, the floor is well bedded with quartz sand or flint that will not vitrify but will provide a good setting for the bottom saggars. These saggars are fire-clay boxes, round or oval, rarely square, and without lids, in which the pots are placed. Piled one above another they form the bungs, the bottom of one supplying the top of that beneath. These bungs are built at intervals that permit the flames to penetrate between them and give a good even fire all over the chamber.

In actual practice some parts of a kiln are hotter than others, and it is here that a good setter shows his capabilities, setting the thin wares in the softest places and putting the heavier biscuit or hard glaze in the hottest corners. With biscuit the setting is not difficult except where delicate or friable ware may need very careful bedding and propping. With clean saggars the biscuit may touch the sides, and a competent man will pile an enormous amount of biscuit into a kiln without risk. In the glost kiln the ware requires gentle handling and must touch nothing but the spur or support. The saggars are usually given a saggar wash of lead and stone to prevent them absorbing glaze from the wares, for a glazed pot placed too near a raw saggar is very likely to come out with a thin or dry patch.

When the pot is firmly placed on its stilt, a roll of pugging is placed round the rim of the saggar; this provides a firm bed for the next above it and also stops the entry of dust and flame. This pugging is made from clay or marl mixed with ground sherds, sieved gravel, or some non-vitreous dust to prevent it sticking to the saggars. The plugging is made malleable with a little water and rolled out by hand or pressed through a die. It will readily be seen that a carelessly built bung of any height may slip in the stress of firing, and its fall would most likely involve others, whilst any slight movement may be sufficient to cause a vase to topple off its stilt. In a down-draught kiln the bungs over any vent must be raised on fire tiles to permit the escape of the flames. When all the bungs are filled up and piled in position, the trials and cones placed, the hatch is bricked up. Spy holes are left where necessary, and the whole well clammed to prevent the loss of heat during firing. When fired, the hatch may be very gradually loosened to accelerate the cooling.

The fireman’s job is one of the most arduous and important of all the prolonged processes of potting. Coming as it does at the end, it may make or mar kilns of great value, and it requires more than ordinary judgement and nerve. The chief troubles arise from bad or irregular draught or impure fuel. They express themselves in the form of clinkered holes, excessive smoke, and irregularly fired or sulphured ware. The term sulphured is applied to pots that come out gloomy or dulled and is said to be equally due to a reducing fire or an oxidizing one. In the former an insufficiency of air causes excess of carbon in the kiln and the absorption of oxygen from the glaze. In the latter an excess of air (oxygen) allows sulphur vapour, if present, to attach itself to the glaze.

The fire bars, fire holes, and flues must be kept clear and the fuel carefully selected if these things are to be avoided. The aim, as previously stated, is a gradually increasing fire, sharper towards the finish. For this purpose some potters finish off with wood which gives a long flame free from sulphur and clears the glaze. Even when sulphured, a clear flame at the finish will remove many ill effects. The baiting should be fairly heavy at the start, lighter and more frequent towards the finish, when smoke in any quantity should be avoided.

For temperature gauges, the cones previously described are used and should be placed in an average place, screened as much as possible from draught or flame. Small sight pots and glaze rings placed near the spies can be hooked out and examined and are valuable aids, but their exposed position and the corrosive action of the flames must be taken into account. Pyrometers are also used, but with all these aids the experienced fireman pays due regard to sight and colour.

For a craftsman, the kilns that have been described are unsuitable on account of their size, if they are not altogether beyond his means. But a kiln of some sort is indispensable to a potter. The craftsman is nothing if not inventive. Small trials can be fired in crucibles in an open fire or on a gas ring. I have heard of some preliminary success with a gas cooker, discounted later by a frontal attack from the cook. Considerable knowledge can be gained from small, easily constructed, experimental kilns.

The small trial kiln shown (Fig. 70) was constructed with a stout old crock, over which was built a core of bricks plastered with marl. The fire was started at each corner at the bottom, and when well alight, fed from the top with coke. A small spy at one corner closed with a piece of biscuit gave some idea of the progress of the firing. This, of course, had to be practically rebuilt at each firing, but as a makeshift was quite satisfactory.

The gas and oil kilns described in Chapter IX are excellent as far as they go, and indeed the only practicable kind for indoor schools. But their restricted size soon becomes irksome to a craftsman, whilst the expense of firing makes only the finest work remunerative. For over-glaze work they are excellent, but for some reason glazes fired in them seem to lack some of the richness and maturity the same glazes exhibit when fired in the slower and more soaking fire of a brick kiln. The dug-out kiln here depicted (Fig. 71) would be quite suitable for summer schools or for a craftsman making soft peasant pottery. The section and sketch will indicate its construction. The materials are hard bricks and stout old boiler plates, or sheet iron. To pack or unpack, the middle section of the roof would have to be removed each time, and all glazed pots would need protection from scalings and gravel from above. The roof will sag at any big heat, and if of thin iron, will need propping. The firing would be done with soft coal or wood; a very slow start, with a brisk draught and a long flame at the finish.

Of course the front will be hotter than the back, but if saggars can be obtained, the glaze may be placed in them to the front with the biscuit protected behind. Clay shapes fired up at the front may bend towards the fire, and any broken crocks should be used to screen them.

The making of rough saggars is not difficult if a supply of fire clay is to hand, or clay and grog will serve at a pinch. The clay is rolled out and the saggar stuck up, much as described on page 32. Every joint must be carefully welded and the whole thoroughly dried. Then they are fired up in the kiln, very gently at first, and carried up to a temperature considerably above that which they will be subjected to when in use.

For small trial crucibles ordinary clay mixed with pitchers and powdered coke proves satisfactory; the coke when fired out renders the body porous and the heat penetrates more swiftly.

To construct the muffle kiln shown at Fig. 72 the aid of a competent bricklayer would probably be required. In this kiln glaze and biscuit would fire up without saggars, but will take rather longer. The plan and elevation of this particular kiln are given with all reserve. The design would probably require considerable adjustment and modification before complete satisfaction was obtained.

A down-draught kiln, although more difficult to construct than an up-draught, is more economical in the end and does its work more evenly. For the benefit of those that may like to build a small and moderately cheap kiln drawings are here given. This kiln was built by me when I had very little practical experience of kilns other than gas, but it served its purpose well. It was not banded, but this is essential if the kiln is to stand hard and frequent fire. Firebricks were used for the fireholes, flues, floor, core, and dome; strong stock bricks for the rest. The glazed ware, which was fired harder than the biscuit, was saggared in bungs as usual up to about five feet; the biscuit piled on top and protected by old saggars and cracked pots from the roughest fire. There were no bags in this kiln, but the saggars used were very strong and had stood a much greater heat than that to which they were subjected in this kiln, so that they showed very little wear or tear after twenty firings. The stack of sixteen feet gave a good sharp draught, increased if necessary by the addition of an iron chimney and regulated by an iron damper. A wind screen or hovel is advisable for rough nights, and some sort of roof is necessary to protect the crown from the weather. It is important to have the arch of the hatch very strongly built, as it has to stand a lot of strain, and an iron support too near the fire soon corrodes and needs replacing. Another essential is that the site be as dry as possible and the foundation made solid with concrete; otherwise even a small kiln is liable to settle and crack. With a little extra expense a kiln of this kind could be banded round the impost and fireholes, thus considerably prolonging its life.

In firing this kiln about half a ton of coke and three quarters of a ton of good hard coal, giving a long flame, was used. Coke for the slow fire was first started on the bottom and maintained for sixteen or eighteen hours, lifted up on to the bars for another six or eight hours, when the saggars would begin to show signs of colour. The coal fire was then started about the 24th or 25th hour and continued another 18 hours, more or less, according to the varying conditions, making in all some 40 or 45 hours. This gave a very evenly graduated heat from cone 1 at the base to cone .03 at the top. Bags were tried experimentally, but whilst giving a more uniform heat, took much longer to fire up. At the finish of the firing the fireholes were bricked up, the damper closed when the fires died down. In about 24 hours the vent and the hatch were eased a little at the top, and in 48 hours it was pulled down and the drawing commenced. Packed with the hard glazes at the bottom and the soft at the top this kiln answered excellently for the purposes for which it was required.

CHAPTER XVI

The study of the fictile art of the potter, even from the theoretical side alone, cannot fail to quicken and broaden education. The antiquity of the craft, stimulating research amongst the records of ancient civilizations, brings to light customs and habits bearing very closely upon the earliest struggles of man to emancipate himself from mere brute surroundings. The primitive decorations rudely scratched on clay vessels antedate and forecast the hieroglyphic and sign languages of all nations.

It would be but hyperbole to claim that without clay the Mosaic tablets would have remained unwritten, but indubitably the clay cylinders of Assyria gave a strong impulse to the development of ciphering and writing and the spread of learning,—an impetus not to be derived from the obstinate granite medium so generally employed by the Egyptians.

It is this amenable ductile quality, so easily receptive of the most emotional touch, that has made and still makes clay such an admirable medium of expression for the young,—whether young in the history of the world or young in actual years. And this malleability is accompanied by a tenacity that permits slow building up, remodelling, and high finish, suitable to work of the most painstaking character. To this is added the fixed, unalterable quality imparted by fire, so that pottery more than any other craft preserves an imperishable record of the ages.

This positive chronicle is valuable alike to the savant or the student. Indeed the most trivial child’s toys of the Hellenes, the quaint water pots of the Peruvian peasant, or the unassuming tea bowl of the esoteric followers of Riku may chance to convey to the sincere student a clearer idea of the habits and thoughts of their producers than many a pedantic treatise or translation.

Coming down to points in close contact with the curricula of schools, we all subscribe to the dictum of Ruskin that “Everyone, from the King’s son downwards, should learn to do something finely and thoroughly with his hands.” What then more suitable than sympathetic clay wherein to fashion the first fancies of the child mind. It is a medium at once attractive and easy to mould, giving a tangibility and reality to forms and things that can never be obtained by drawing or painting. Then the limitless uses to which clay is put, and, with the development of hygiene, increasingly will be put, have the closest bearing upon the everyday life of the child. They are intimately connected with other studies that cannot fail to be rendered more attractive by working in clay.

But clay work is a branch now so universal that it seems unnecessary to dwell upon its advantages to the kindergartener.

The valuable remedial effects of clay work upon the defective are perhaps less widely known. The manipulation induces a most beneficial concentration and provides a fine discipline without a trace of inimical restraint. Turning to higher grades, the use of clays should foster an interest in the formation, composition, and disintegration of rocks, and in the properties of the products so engendered; in short, a liking for geology.

With the making of simple glazes and colours will awaken an intelligent curiosity concerning the nature of minerals and metals, their actions and reactions in the fire; a lively sympathy only awaiting a touch to turn it into a love for chemistry and physics. Then as power and ambition and craftsmanship develop, there must needs be a study of the history of ornament. This impinges too closely upon history and geography to fail to increase the student’s attraction towards these more remote but allied fields.

Finally, is it not in the realm of æsthetics that there looms the ultimate reward? The proper pursuit of pottery must eventually lead us “towards that idealization of daily life ... and the road that connects the love of the beautiful with the love of the good is short and smooth” (President Eliot). In the hurried curricula of to-day art plays a rather sorry part. Little time indeed is left for contemplation, for the realization of all that beauty and harmony in our surroundings may mean to us in our everyday work.

The making of a bowl, with the concentration required to shape it in a manner at once beautiful and serviceable, must quicken the perception of beauty and sharpen the quality of judgement, not only for things fictile, but in far wider fields. Thus the things of everyday contact—the tableware, the chairs, the doors, the windows, pictures, ornaments, hangings, and fittings—will all come in for intelligent scrutiny and criticism. This in turn will be carried on and over into matters civic. This must result in a careful estimation, selection, and appreciation of our surroundings, bringing them into harmony with our cultivated thoughts and so enabling us to get through the day’s work with the least amount of useless friction and with the greatest possible measure of enjoyment, well-being, and well-doing.

APPENDIX I

The divinity that presides over potting is an expensive as well as an exacting mistress. The equipment of even a small pottery is, unfortunately, a matter of considerable expense. Try it from whatever angle we may there is the cost of the kiln to be faced, besides a host of other small but cumulative expenses. The first essential is, of course, a roomy workshop with if possible a top as well as a side light. If the craftsman means business, he should remember that the initial cost of a kiln is often in inverse ratio to its upkeep. If he would aspire to big things, full-bellied pots, plaques, reliefs, and figures,—and every craftsman would,—a brick kiln will be wanted. One holding a dozen saggars could be built, but where some experience has been obtained with materials and processes, a larger one would be more economical. With oil or gas kilns of the ordinary school size the cost of firing and the extra time is proportionately too great to permit of any but comparatively high-priced pots being turned out. This may serve in some cases, but usually it is not practical potting where a livelihood has to be obtained. Where only the painting is to be fired on, an oil or gas kiln is exactly what is wanted.

In this appendix is given a plan of a workshop that has all the equipment necessary for the whole-hearted pursuit of the craft. In such an one a good craftsman, capable of modelling and painting decently a figure or a panel, a good thrower, and a handy boy could work wonders. They would be capable of turning out a surprisingly wide range of “pots”: jugs, mugs, pots, bottles, bowls, buttons, dishes, plaques, panels, vases, tiles, and statuettes,—useful and beautiful things. Anything in fact worth doing can be done except fine tableware or those articles that by their nature demand more mechanical accuracy than is possible, or even desirable, for a craftsman to exhibit. Where much plaster turning for moulds was attempted, a lathe would be required; ordinarily the hiring of one should be practicable and expedient. Where only built or cast shapes were attempted, the wheel and its long years of drill might be dispensed with, and it is possible, with strong individual work of high finish and fine quality and the consequently restricted output, that an oil or gas kiln would give economically practicable results. Between the kiln for firing decoration simply painted on the ready-made shape to the full equipment here described will be found several modifications, but to try the craft without a kiln of some sort is an imbecile proceeding.

Small brick kiln supplied with saggars.

Small muffle kiln—oil or gas—for over-glaze and lustres.

Small enameller’s kiln for firing quick trials.

Kick wheel, and tools for throwing.

Clay bin, zinc-lined.

Damp-box.

Drying cupboard.

Plaster bin.

Pot boards and brackets.

Table, strong and heavy.

Clay: white, red, buff.

Plaster.

Glaze materials.

Oxides, lustres.

Under- and over-glaze colours.

Modelling tools, callipers.

Painter’s outfit.

Brushes, straight-edges.

Shellac, beeswax, French chalk.

Gum arabic and tragacanth.

Glaze tubs, teak.

Sieves, glaze and slip, Nos. 80, 100, 120.

Buckets.

Bowls, enamelled and earthenware.

Small porcelain ball mill, hand or power.

Spray and pump (respirator).

Small outfit for carpentry.

Files, sheet iron, and zinc, wire and cutters, cords, sandpaper.

Benches and shelves ad lib., odd cupboards, chairs, etc.,

Shovels and slicer for firing, tongs for trials.

Two large tubs and rubber tubing.

Sand and flint.

Spurs, props, fire tiles.

Tile boxes.

Disc (emery) for grinding.

Small pestle and mortar.

Jugs and funnel.

Potter’s knife, sponges.

Whirler.

Turning tools and lathe.

Temperature indicators.

Oil can, oil, waste.

Callipers and compass, rulers.

Most of this equipment has been previously described and needs no further comment.

The pot boards and brackets are simple but indispensable devices. The boards are about six feet long, iron shod or cross battened to prevent warping, and six or nine inches in width. The brackets of any serviceable kind are fixed to the wall at convenient distances. When throwing, turning, or glazing, the pots are stood on one of these boards to dry, and each board as filled is slipped onto the brackets. Thus the pots may be carried about to the kiln, drying cupboard, or glaze tubs without loss of time or frequent handling.

The table must be stout enough to withstand the heavy work of wedging and should have a top of hard wood. Teak or hard-wood glaze tubs have the advantage of not breaking either themselves or pots accidentally knocked against them. Further, some glazes stick badly to porcelain or enamel tubs.

In the small pottery plotted here, the equipment and arrangement were as follows:

An anthracite stove with the pipe running into the large room warmed the workshop in winter, but no wet or half-dry pots were left where the frost could get at them.

The glaze materials, oxides, colours, painting paraphernalia, finished pots, trials, and trial kiln were in the small room. The wheel had a good top and side light.

The drying cupboard, plaster bin, and moulds were at the end nearest the stove; the clay bin, damp-box, and sink farthest away.

All the walls were copiously supplied with brackets and shelves and handy benches.

Outside, in a well-built lean-to, was the muffle kiln for onglaze and lustre decoration.

This was well bracketed and shelved for the biscuit, and here was done the glazing, handy for packing in the brick kiln just outside. This was protected from the weather and other lean-to’s held the saggars, coke, and coal.

The teacher with ample funds and a free hand will find the previous chapter all-sufficient, but in many cases the purchase of a kiln will nearly exhaust the allowance and the rest of the equipment becomes sketchy.

The indispensable appliances are as follows:

A kiln, with fire tiles or shelves, props, spurs, and stilts, etc., for packing. A good clay bin and sieve for slip (No. 80) with a tub and two pails.

Scales and weights, pestle and mortar and glaze lawn (No. 100), shot for weights.

Plaster, for drying bats and working discs.

Large drip pan and three round pans.

Several jugs and bowls.

Spoons (wooden), knives, and big brushes.

Oil, gum, boards, strips, rolling pin.

Hammer, saw, iron straight-edge, sponges.

Glass slab and muller, palette knife and brushes for painting.

An atomizer or spray pump.

Glaze materials:

Kaolin, China stone, flint, silver sand, whiting, felspar, borax. A supply of ground pitchers and grog, cones.

Metallic oxides:

Tin, white, oxide of, iron, copper, manganese, cobalt, etc.

Under-glaze colours to taste.

Glass jars with lids to contain materials. Gummed labels, India ink.

For a school in the country or where ground is available, a kiln like the one shown at p. 164 should be practicable. It costs very little to build or to fire. Next comes the question of the clay. This is one of the most abundant of nature’s materials, and almost any river bank or creek will supply clay of some kind. Any sort of clay near to hand should be thoroughly tested before going to other or distant sources.

The clay should be dried, then broken up with a hammer, and mixed with water, and the resultant “slurry” passed 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, passed 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 substitutes, 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.

All the above colourless glaze masses 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 constituent 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.

Arsenic.—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 classes 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.—Coarse 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 constituent 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 seashores. 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 glass.

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.

Gold.—Used in solution for delicate purples and lustres.

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 quantities 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 coarse 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 Potassium Nitrate, or Saltpetre.—A vitreous and aqueous compound, used in some glazes.

Plaster of Paris. (See Gypsum.)

Potash.—Potassium carbonate or the leached ashes of plants. Used from earliest times as a powerful alkaline flux.

Potash, Bichromate 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 Titanium.—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 glass-making.

Silver Sand. (See Quartz.)

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

Tincal. (See Borax.)

Titanium. (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.

Long.—A clay is termed long if very ductile and tenacious.

Muffle.—Usually the fire-clay box or interior of a small kiln, but applied to any kiln to the inside of which the flames have no access.

Natch. (See Joggle.)

Oxidizing.—The ordinary method of firing gives an atmosphere in which there is always sufficient oxygen to consume all the carbon or combustible gases. If oxygen is present in excess, it causes reactions known as oxidizing.

Pitchers.—Finely ground biscuit. Added to some clays to increase refractories or porosity. Moulds made in such clays and fired are termed pitcher moulds.

Potsherds.—Any broken biscuit or pot, sometimes used for pitchers.

Potting.—A colloquialism used to designate the ceramic industry.

Pugging.—The roll of infusible clay placed between each saggar when building bungs.

Reducing.—The reaction that accompanies the introduction of smoke or gas containing carbon in a very finely divided state into a kiln during the process of firing glaze. Reduction is now widely employed in obtaining fine lustre effects.

Refractory.—Hard, infusible.

Rich.—Used of clays that are long and fusible, such as red clays.

Riffle.—A grooved and toothed plaster tool of steel.

Saggars.—Or seggers. The fire-clay receptacles in which the glazed ware is set during the firing.

Setters.—Supports used when packing friable biscuit.

Short.—A word used to denote a clay that crumbles or is difficult to pull up on the wheel.

Sieve.—Sometimes called a lawn, more correctly a screen for clay or slip.

Slip.—The sieved clay or paste in creamy liquid condition as used for slip decoration, engobes, or casting.

Slub or Slurry.—Clay mixed with water but not sieved, as with slip.

Spy.—The small hole, kept plugged, through which tests and cones are observed.

Stunt.—Or dunt. To crack or split on cooling.

Turning.—The shaving down of the clay shape on a lathe, to impart lightness and finish.

U. G.—Under-glaze (applied to colours).

Vent.—A hole to aid the even distribution of fire in a kiln or to accelerate the cooling off.

Waster.—Commercially, a spoiled pot; defective ones are termed “seconds.”

Wedging.—The beating or slamming operation usually employed to expel air or correct inequalities just before clay is used by the thrower.

Whirler.—A circular support pivoting on its centre, used in casting or banding; similar to a banding wheel, but usually heavier.

MATERIALS, TERMS, ETC.