Each item is thus disposed of until the list is complete. These figures are, however, given in equivalents and each must be multiplied by the equivalent weight of the substance used.
These amounts are weighed out in grams, put upon the mill with half a pint of water, and ground for about an hour. When taken off, the jar and porcelain balls are washed with plenty of water and the washings saved. The glaze, thus diluted, is strained through a lawn of 120 mesh and laid aside to settle. The clear water is then siphoned or poured off and the glaze is ready for use.
For glazing the glaze should be as thick as cream. A finger dipped into it should show a white coating which cannot be shaken off. The pottery to be glazed should be first soaked in clean water until all absorption has ceased. It is then wiped dry and plunged into the glaze bath, or, if the piece be large, the glaze may be poured over it. The piece is gently shaken to distribute the glaze evenly and it is then set aside to dry. Before glazing a piece everything should be prepared. A stilt or support upon which to set the wet glazed pottery, and a bowl of water in which to wash the fingers so as to save all the glaze. It will be found best to glaze the inside of the piece first. It should then be well shaken to remove as much glazeas possible before beginning the outside. A thick glaze inside is almost sure to run down to the bottom where it will form a pool and perhaps burst the piece.
Before firing, the bottom of the pottery should be carefully trimmed. Any excess of glaze is removed and the point of contact with the table is sponged clean. Then, when the piece is set in the kiln the bottom will not be inclined to stick.
The texture of the matt glaze is always pleasing and the artist is not content unless at least some of his work can be finished in this way.
Matt glazes are not underfired glazes nor are they deadened by acid or sand blast. They are produced in two ways. First, by an excess of alumina which is believed to cause the formation of certain compounds in the glaze, and, second, by an excess of silica which produces a devitrified surface. It was mentioned in the last chapter that a glaze free from alumina will devitrify or become dull. This is undesirable when a glaze is intended to be brilliant but it may be controlled and turned to advantage in theproduction of a certain type of matt. The successful preparation of this silica matt is extremely difficult. In fact, in the studio kiln it is almost impossible. These small kilns are apt to cool with great rapidity whereas, in order to produce the silica matt the kiln must be cooled very slowly, hours and even days of cooling being sometimes necessary.
The alumina matt is more simple and its texture is quite satisfactory, being, in the opinion of some, the more pleasing of the two.
It was mentioned in the last chapter that the best bright glazes for low temperature work are bisilicates, having an oxygen ratio of 1:2. The alumina matt has an oxygen ratio of about 3:4. This is secured in the following manner. The RO content may consist of any of the bases used in bright glazes, the proportion of each being adjusted in accordance with the desired point of fusion. The alumina content is rather higher than in a bright glaze and should not fall much below .3 equivalent, .35 equivalent is even better. The silica is adjusted in accordance with the following equation:
SiO2= 3(3Al2O3+ 1) / 4
Now if the alumina content be placed at .35 equivalentthis would work out:
SiO2= 3(1.05 + 1) / 4
Or:
SiO2= 6.15 / 4 = 1.5375
But as such a complete fraction is not necessary it may be stated as 1.54 equivalent. The formula would therefore be:
RO, Al2O3.35, SiO21.54
The RO content should not be too fusible. Lead oxide is desirable up to about .5 equivalent and it is an advantage to use feldspar so that K2O may be introduced. Calcium oxide is also good but zinc oxide must be used sparingly as it is apt to suffer if overfired. The high content of alumina necessitates a good deal of clay and as this, if used raw, would make the glaze too plastic and cause it to crack, it is best to calcine a part of it, thus removing the combined water and changing the equivalent weight from 258 to 222. The calculation will then proceed as in the case of a bright glaze.
The mix, therefore, is:
This will give a silky matt glaze, nearly white, maturing at about cone 1. If a lower fusing point is desired the white lead may be increased at the expense of the whiting or if the glaze prove too fusible the reverse will correct it. The flint may be omitted without damage.
The grinding of a matt glaze is of great importance. It is better to have it too coarse than too fine. Grinding for one hour on the ball mill should be ample and if the glaze be then strained through 120 mesh lawn all coarse particles will be arrested. A glaze that is too fine will crack and peel off or will curl up in the kiln.
More than half the success of matt glazes lies in the using. It is necessary that the coating of glaze be very thick or the true texture will not be developed. When the glaze is taken from the mill plenty of water may be used in order to wash the apparatus clean and to save all the glaze. This is set aside in a deep bowl to settle. After some hours the clear water is carefully drawn off with a siphon.
Half an ounce of gum tragacanth is put to soak in a quart of clean water. After twelve hours the gum will have swollen to a jelly-like mass. This is now worked vigorously with a Dover egg-beater or in a Christy mixer and again set aside. After another twelve hours the operation is repeated and the solution is a clear syrup of the consistency of thin molasses. A drop or two of carbolic acid or other germicide should be added to prevent decomposition. This mucilage should be prepared in advance. To the glaze batch from which the water has been removed a tablespoonful of the mucilage is added. If more of the glaze than the single batch has been weighed out then more mucilage will be necessary. The mixture is to be stirred very thoroughly and it will be found to thicken under the hand. It must be very much thicker than the bright glaze. In fact, the thicker it is the better, only that it must flow sufficiently so that the pottery may be covered with a smooth coating, avoiding lumps. Matt glazes do not correcttheir own faults in the kiln as bright glazes do. Every finger mark will show and, consequently, the glazing must be done with the greatest care. The process is the same as that described for bright glazes, except that as much glaze as possible is left on the ware. No more shaking should be done than will suffice to secure a smooth coating. It is well to place the pieces upside down to dry.
For the inside of the pieces a matt glaze may be used or a thin coat of clear glaze at the pleasure of the worker. If the latter, care must be taken that none of the inside glaze is allowed to run over the edge.
In firing, the pottery is sometimes placed on a stilt but this is not absolutely necessary. For a support a flat piece of burned clay may be used and this should be covered with an infusible wash to prevent any possibility of sticking. Equal parts of kaolin and flint make a good wash. The wash is worked up with water into a slip and applied with an ordinary brush.
Fritted glazes, like raw glazes, are clear and brilliant and for most purposes the latter will suffice. Since, however, the aim of this work is togive as complete information as may be the fritted glaze will not be omitted.
A fritt is a melt or compounded glass and the purpose of it is to permit the use of certain ingredients which are not available in the raw state. As glazes are ground in water it is essential that the substances used be insoluble. This condition would prohibit advantage being taken of borax, boric acid, and soda ash, if it were not for the possibility of rendering these insoluble by the operation of fritting.
The following is an example of a fritted glaze:
This will be produced in accordance with the usual calculation by the mix:
The borax contains the required amount of both soda and boric acid and the potash is supplied by thefeldspar. Borax, being soluble, must be melted with certain other ingredients into an insoluble glass, thus:
These ingredients are weighed out in double quantity to guard against loss in melting and are fused either in the kiln or in a special furnace. A good fritting furnace is the No. 15, made by the Buffalo Dental Manufacturing Company. The charge is put into a plumbago crucible and when melted is poured out into water. This breaks up the fritt and renders it easy to grind. A similar crucible may be used in the kiln but as the fritt becomes very hard when cold and a crucible must be broken each time, the furnace method is better. If the fritt as given prove too sluggish to pour freely, the feldspar may be omitted, being added, of course, to the glaze mix. The melted weight of the fritt must now be calculated.
Borax contains in each equivalent 180 parts water. Whiting contains in each equivalent 44 parts carbonic acid. Both water and carbonic acid pass off in the melting, thus the 76 parts of borax will be reduced in weight to 40 parts, and the 25 parts of whitingwill be reduced to 14 parts. Spar and flint undergo no loss. The fritt after melting will therefore be:
And the final mix for the glaze will be:
This is ground on the mill as already directed and is ready for use.
Fritted glazes are better than raw glazes for certain classes of ware. They are usually whiter and less easily scratched. They are, moreover, better for use with underglaze colors and are, as a rule, more easily melted. It is never necessary to make a fritt for the preparation of matt glazes.
While the purpose of this work is not so much to put ready-made materials into the hands of the craftsman as to enable him to work out his own plans, it is recognized that there are some workerswho lack the training and even the patience to do this. For these, the following recipes are given, but with the proviso that no recipe can be regarded as perfect for all conditions. Just as an untrained cook can spoil a dinner even when surrounded by cookery books, so the best of recipes will fail when unskillfully treated. One must be prepared to recognize the faults which are sure to develop and to correct them in an intelligent manner. The previous chapters should therefore be carefully studied, not alone for the information but because "the joy of the working" depends greatly upon the knowledge one has of the operations involved and a modest confidence in one's own powers.
1. Bright raw glaze.
2. Bright raw glaze.
3. Bright fritted glaze.
4. Matt glaze.
5. Matt glaze.
For colored glazes add to any of the above:
The coloring oxides should be weighed out and ground with the glaze. Any of the colors may be mixed together in order to modify the hue obtainedor the amount of each coloring oxide may be varied to give a stronger or weaker value.
Opaque tin enamel.
While it may chance that body and glaze and fire are so adjusted that faults do not develop, this state of things is rare. Besides, it is always possible that an occasional trouble may arise, hence it will be well to recount a few of the commonest defects with the method of cure. A cure is not necessarily specific. There may be a complication of causes but the remedy indicates the line along which relief will be found.
1. Crazing. Fine cracks appear in the glaze but do not penetrate the body. There are many causes. The body may be underfired or overfired. In the former case the crazing does not always appear at once and it grows worse upon standing. In the latter case the glaze is found to be crazed when taken from the kiln and it does not extend even after long standing. The glaze may be underfired. In this case the lines of the crack are broken and irregular, one often changing its direction without meeting another crack. In all these cases the remedy is obvious.
Crazing also occurs when both body and glaze are correctly fired but there is an inherent disagreement in expansion. In such a case a little flint added either to the body or to the glaze will tend to cure the trouble but it must be remembered that the addition of flint to the glaze is apt to render it less fusible and therefore while one craze may be cured another may be caused. The addition of flint to the body is the simplest remedy.
2. Shivering or peeling. This is the reverse of crazing and is caused by the glaze being too large for the body. It almost always appears immediately the ware is cooled. The symptoms are that edges or convex surfaces are pushed off and even the ware itself is shattered. The remedy is to decrease the flint in either body or glaze.
3. Blistering. Glazes, both bright and matt, are apt to develop blisters at times. These may be yet unbroken when the kiln is opened or they may have melted down to a small crater, a ring with a depression in the center. The cause of this fault is usually to be found in the body. All clays contain sulphur and when a clay is aged this develops an acid which rises to the surface of the ware when dried and causes a scum. The glaze attacks this sulphate scum and a gas is generated which boils out and causes the blisters. If old clay blisters and new clay does not it may be regarded as certain that this is the cause. A little barium carbonate added to the clay will help to effect a cure. About one per cent. is usually enough. Clay so treated, however, must not be used in plaster molds as the barium attacks the plaster. If the cause be not found in the clay it may exist in the glaze itself. Some glaze ingredients contain impurities in the form of sulphates and these will cause blisters.
4. The glaze flows, leaving bare places. It is too fluid, add a little clay and flint.
5. A matt glaze burns to a bright surface. Matt glazes must be used in a very thick coat. If too thin they will inevitably brighten. The fire may be too high. The fire may be "reducing," that is, with insufficient air.
6. The glaze crawls or rolls up in lumps. Notice whether the glaze is cracked before burning. If soit will surely crawl. Too fine grinding is usually the cause of this trouble. Too much clay in the glaze may cause it, or a too porous body. A body which is underfired will almost certainly cause the glaze to crawl.
7. Pinholes appear in the glaze when cool. Too rapid cooling is the cause.
The glory of the Persian and Egyptian blue is too alluring for potters to withstand. Though the pursuit of this glory leads one into all kinds of disasters and failures, the avenues of research that it opens add unending fascination to the study. Even one beautiful glowing pot out of twenty or more efforts is a stimulating achievement though it should not be thought that this is the usual proportion.
It is a continual source of astonishment that with a slight variation of glaze formula a positive green will swim into a vibrating blue. The addition or substitution of one substance or another in the glaze mix may be the key to an unexpected transformation and may give the potter a new palette of color.
The clay body has a very positive effect on alkaline glaze both in its composition and its color. This isespecially true under a transparent glaze where the effect is considerable since the color of the glaze would be modified by the red or buff clay showing through.
If, therefore, the object of the potter is to obtain a brilliant "Persian" blue, a white clay body must be composed or a whiteengobeapplied over the buff or red clay to hide the color.
The Persians and Egyptians used a coarse, sandy body high in silica and covered the roughness of the clay with a fine whiteengobeon which they painted their decorations in various colors. The whole was finally covered with the transparent alkaline glaze.
While the effect of colored clay under opaque glaze is less pronounced, it still makes sufficient difference to be considered.
The wordengobeis French and refers to a thin coating of clay, also called a slip, laid over a colored body to change the color or over a coarse body to give a finer texture.
Theengobeis usually composed of china clay, flint, and feldspar much as a white earthenware body is constituted but with a larger content of flint. Ball clay may also be used but the color is not so white.
The mixture of porcelain given on page forty will make anengobesuitable for many clay bodies. If it should crack on drying more flint should be added.
Anengobemust, of course, be put upon the unburned or green clay ware and this should be leatherhard, not dry. The body with theengobemay be burned before glazing or the glaze may be put upon the unburned ware and the whole subjected to one fire only.
The ingredients in alkaline glazes are soda-ash, whiting, feldspar, flint and oxide of tin. The following is an example of a fritted glaze:
The entire batch is fritted and ground in a ball mill with the usual amount of water for fritt grinding, adding a tablespoonful of gum tragacanth mucilage to the batch after it is sieved. The glaze should be the consistency of heavy cream when used.
It is also possible to use an alkaline glaze in the raw or unfritted state. This necessitates grinding by hand in a mortar, but great care must be taken to mix the dry ingredients thoroughly before adding water and to stir the glaze constantly while pouring in the water, otherwise the soda-ash will cake and harden and be very difficult to break up. A batch of glaze can be ground by hand in fifteen or twenty minutes if done vigorously. It is then put through a 120-mesh sieve. The consistency is of importance. If too muchwater has been added and the glaze has become thin, it cannot be used successfully and should be discarded. Unfritted alkaline glaze does not keep well when moist but the ingredients can be ground dry and kept ready to be moistened as needed.
The following is an example of an unfritted alkaline glaze:
For color add the following oxides to a batch.
1. Egyptian blue, opaque—from 5 to 8 grams of black oxide of copper—16 grams of oxide of tin.
2. Persian blue, opaque—from 8 to 10 grams of black oxide of copper—16 grams of oxide of tin.
3. Sapphire blue—1 gram black oxide of cobalt.
4. Aubergine—9 grams black oxide of manganese.
The clear glaze without any coloring oxide can be used over any of the colored glazes. This is sometimes necessary when the colored glaze contains such a large proportion of coloring oxide as to show black on the surface.
The application of alkaline glaze is very important. Any of the three methods of pouring, dipping, and brushing can be employed. Brushing seems to givethe best results but the glaze must be put on thick, in two or three coats, to give quality.
The firing is interesting and important because of the varied effects it develops from the same formula. The range of temperature is great, varying from cone .05 to 1, developing the alkaline glaze according to the result desired. If the biscuit is soft fired the color will be more intense; if hard fired, the color will be much lighter in value with a high sheen on the surface. An unfritted alkaline glaze burned to .05 develops a soft matt finish.
Where the color of a transparent Persian blue comes out olive green, too little glaze has been used on the piece or the buff of the clay has modified the color. Bubbles mean undeveloped glaze or sulphur in the clay or fuel. Black scum shows an excess of copper in the batch, or reduction in the fire. Sand paper surface proves too low firing or too thin a glaze.
If one desires to reproduce the underglaze Persian decoration the black outlines may be drawn with a black underglaze color mixed with clay. A little mucilage must be added to secure smooth working. The turquoise blue is copper oxide, the dark blue cobalt, and the purple manganese. The oxides must be diluted with white clay and used rather thin. The Rhodian red is a finely ground red burning clay mixed with a little flint. This red must be laid on quite thickly. It will probably be found necessary to fire the painteddecoration to about cone .03 before glazing. The glaze may be either quite clear or slightly tinted. Another effect may be produced by using the black outline alone under a peacock blue or turquoise glaze.
A great many modifications and additions to this subject will suggest themselves to the potter as he works, and a continual study of the masterpieces of the Persians in the museums will prove the greatest inspiration.
The necessity for some kind of decoration upon the clay will always be a point of difference amongst artists. Some prefer the simple form with a glaze treatment only, others consider that the surface should be broken up by design. The question will not be debated here. The aim of this hand-book is instruction and the individuality of the worker is to be encouraged. Directions for executing the different treatments do not imply that these elaborations are advocated. That must be left to the inspiration of the worker.
Decorations may be applied upon the soft clay by incising, inlaying and embossing; upon the dry clay or upon the burned pottery in color under the glaze or with no glaze at all; in the glaze by the use of colors or colored glazes; or over the glaze with colors and enamels. Each of these methods possesses special features. Each has its own possibilities and limitations and these should be mastered by the craftsman.
As in the production of form a well-planned design should be prepared. The first sketch should be madeon paper or on a slab of clay but the fitting and final arrangement are best made on the piece itself.
Incising consists in the excavation of a shallow trench or trough on the surface of the clay. The vase or jar having been finished should be kept in a damp place so that the clay does not dry out completely. The design may be made in India ink with a brush. A steel tool with a narrow chisel end is used for cutting and care must be taken that the clay is in such a condition of moisture as will admit of a clean trench being dug without any rough or broken edges. The bottom of the trench need not be very smooth but the edges should be sharp and the lines well defined. At the same time a mechanical hardness of finish is to be avoided. The plastic nature of the clay should be kept in mind and every surface, though decided in character, should be soft and expressive. This result can be secured by working over the cutting with a moist camel-hair brush. The work must not be mopped so as to leave a woolly effect, but a little sympathetic penciling will remove the hard lines of the tool.
There are two possible developments of incised work. The details of the design may be excavated or the background may be cut out leaving the drawing in relief.
In modeling embossments the piece should be a little softer than for incising. It is important that inany clay work attached to a clay body the same amount of moisture should be present in both parts. This is not entirely possible in modeling upon forms which have already been shaped, for if the form be as soft as modeling clay it will not bear to be handled, while if the clay were as hard as the form it could not be worked. A compromise is therefore necessary. The vase must be kept as soft as possible consistent with holding its shape and the clay must be as stiff as the working will allow.
As little water as possible should be used and the modeling should not be brought to its full height at once. If the clay be laid on little by little there is much less chance of cracking. Low relief is sometimes produced by painting in slip but here even more care is necessary. The slip should be laid on with a brush in thin coats, each coat being allowed to stiffen before another is applied and the whole work being kept moist.
An atomizer with clean water is useful in this regard. The work, being kept on a whirler or turntable, is sprayed now and then with water and thus prevented from becoming too hard.
When the slip work has been raised to the desired height the surface is tooled over so as to remove the brush marks. This is the method which has been brought to such perfection by the French artists and by them namedpâte-sur-pâte.
Modeled work is generally carried out in the same clay as that of which the form is made and depends upon high relief for its effect. Slip painting is usually done in a different color and if a light-colored slip be used upon a dark clay, the latter is partially seen through the coating in the thinnest places. This fact is made use of to accentuate the shadow effects.
In using one clay over another great care must be taken to insure that the fire shrinkage is the same. The white body already given, or indeed, any light colored clay, may be tinted by the addition of under-glaze colors. The dry color, if sifted very fine, may be added to the plastic clay by thorough kneading and wedging but it is better to work up the clay into a slip and to stir in the color. The tinted slip is then lawned two or three times and dried out on plaster or used in the slip state as the case may be.
A trial should be made before any important work is undertaken, both to see that the color is right and to discover any discrepancy in shrinkage. If a clay shrinks too much, a little ground flint may be added. If it shrinks too little, a little ball clay will correct it. The tint produced by the color is apt to darken in the kiln but the general hue will be similar to that of the color used.
For some classes of work a native red clay gives admirable results. It may be lightened by the use of kaolin and flint and darkened by adding burnt umber.These colors are more satisfactory than greens and blues in clay because the brown and red tones are natural, the others are artificial.
If a good buff-burning clay be available, it forms the best possible foundation for color work. Burnt umber will darken it and red clay may be mixed with it, always having regard to the matter of shrinkage already mentioned.
Very pleasing effects may be produced by inlaying one clay with another. The pattern or design is first cut out as described under incising and then the second clay is pressed, morsel by morsel, into the excavation. The surface is cleaned off level with the body of the piece and the whole may be either polished or glazed.
A plastic clay can be polished when leather hard and the finish will remain after firing. Any tool of steel, boxwood or ivory will do the work but a good supply of patience is needed so that the whole surface may be uniformly treated.
For color decoration upon the pottery, ordinary underglaze colors are used, either upon the unburned clay or upon the burned ware commonly called biscuit. For use upon the clay, the colors should be mixed in water, using a little molasses, sugar, glycerine or gum arabic to make the color flow easily from the brush. Before burning, a little glaze should be sprayed over the work with an atomizer. Any ordinary fusibleglaze will do. It is diluted with a good deal of water as only the very thinnest coat is necessary. The spray should not be held long in one place or the water will flow and smear the color. If the piece be turned slowly around the clay will absorb the water as it is applied. If this spraying be not done the colors will be apt to rub off after burning. Under-glaze colors are not fusible and hence they come from the fire as dry powders.
The work on the biscuit is much the same except that turpentine and fat oil constitute a better working medium. When dry the spray should be applied as before.
Kilns and burning form the pivot upon which the art of the potter turns. M. Doat has said, "A potter can no more express himself without his kiln than can a violinist without his violin," and yet there are some who try to make out by sending their work to some nearby pottery to be burned. Let it be at once understood that he who finds it impossible to procure and manage a kiln had best take to some other craft.
Kilns are of two types, open and muffle. In the open kiln the flames pass through the firing chamber and the ware may be exposed to their action, as in stoneware and brick; or it may be enclosed in the fire-clay cases, called saggers, as in the many forms of pottery, dishes or faience. The muffle kiln is a closed chamber which is surrounded by flames but which is not entered by them. These kilns are used in the manufacture of terra cotta and heavy enamel wares, and the portable kilns made for studio use are of this type.
There are certain advantages to be gained in the use of either type of kiln but inasmuch as the openkiln involves the use of saggers and as, moreover, it must be properly constructed of fire-brick by a skilled mason, it will be best to consider only the portable studio kiln.[M]
It must not be expected that any kiln will give perfect satisfaction. Neither built kiln nor portable kiln will do this, but either may be relied upon to do excellent work in the hands of those who will take trouble. A kiln of the proper size having been purchased, it must be carefully installed. A good chimney is an absolute necessity and if one can be built on purpose it will be best. It should be at least twenty-five feet high with the bottom lined with fire-brick to a height of six or eight feet. The portable kiln is set on iron legs which raise it about one foot from the floor. This is not enough for easy work and a platform of brick or stone, ten inches high, should be prepared. This will greatly simplify the observation and management of the burners which are beneath the kiln, and if it should make the inside of the muffle hard to reach, it is easier to stand on a box to attend to the kiln than it is to go on one's knees to the burner.
The kiln room should have a cement floor and should be both well drained and well ventilated. At the window there should be a stout bench where thework of preparation may be done and at a convenient spot there should be shelves for stilts, cones, wash, stopping and all the minor accessories of burning. If there is room for a barrel of oil it will be a convenience, and if the room be fire-proof the insurance company will not object.
The kiln having arrived it is mounted on the platform and the asbestos-lined pipe is securely connected with the chimney. The inside of the muffle is examined with care to see that no part has been jarred in transit. The reservoir cans are filled with oil and a slow fire is started. This should be allowed to burn very gently for an hour or two in order to thoroughly dry out and season the kiln. It is a good plan to make up a wash of equal parts of kaolin and flint and to brush this all over the inside of the kiln. It should not be put on so thick as to shell off from the walls but at the bottom a good coating may be laid. This protects the walls of the kiln from the attacks of glaze and will make them last longer.
In order to fill the kiln economically a number of props and bats must be provided. Some of these are sent out with the kiln but one is always needing odd sizes and extra pieces. The props are simply legs of burned clay; they are of any height desired and should be thick enough to stand alone. The bats are slabs of burned clay and they rest on the props to form shelves. The bats must be thick enough to bearthe weight of any pieces which they may be called upon to support, but they need not be very large as two or more may be used to bridge the width and length of the kiln. Bats and props are best made of sagger clay to which has been added about one-third of crushed fire-brick. Broken bats serve well for this after the first supply has been secured. This crushed burned clay, called grog, has a very important influence upon wares which have to be heated again and again. The size used should be about what will pass through a 16-mesh sieve, and if the dust be sifted out through a 48-mesh sieve, the resulting ware will be stronger. That is, only the grog which passes a 16 sieve but lies upon a 48 sieve should be used.
The relative proportions of clay and grog in the mix will depend somewhat upon the nature of the clay. Three parts of clay to two of grog by measure will be about right.
The first charging of the kiln should be with pieces of no great importance. The temperature in different parts must be carefully ascertained. In order to do this a number of pyrometric cones[N]are prepared in groups of three.
Let us suppose that the work is intended to be carried out at a temperature of Cone No. 01. The numbers run both ways from this. The higher orless fusible cones are, 1, 2, 3, 4, etc., up to 36, and the more fusible numbers are 02, 03, etc., down to 022. If the firing is to be to Cone 01, numbers 02, 01 and 1 are selected and set upright in a small strip of soft clay. Eight or ten of these groups of three cones are to be prepared for the first firing, so as to test the kiln, one group is placed in each corner, at the bottom, and another in each corner on a shelf, which is arranged opposite the spy-hole in the door. In the middle of this, where it can be well seen through the hole, one of the groups of cones is placed. They must be set so that all three cones are visible as the kiln is being fired.
The kiln is now filled up on both levels with pieces of pottery. To burn an empty kiln is not a reliable test. On the first occasion the fire should be started in the morning because no one can tell just how long the burn will take. When this time is ascertained it is best to start the fire so that the kiln will be finished by early evening. The cooling then takes place at night and there is no temptation to open the door too soon.
The fire is started slowly and the flow of oil is gradually increased as the muffle begins to glow. The work here needs practice, nerve and judgment. A good deal of smoke will be seen at the chimney at first but this should disappear as the kiln grows red. If the fire be urged too strongly at the beginning fuelwill be consumed to no purpose, the only result being the choking of the flues with carbon. As the red becomes visible through the spy-hole, more oil may be supplied, but notice must be taken that the smoke at the chimney does not increase. The ideal firing is where there is no smoke but this cannot be reached until the kiln is hot enough to cause the smoke to burn.
Persons who have burned kilns for overglaze work will find the method of burning pottery very different. Instead of a fire brought as rapidly as possible to the finishing point, there must now be a slow soaking burn in which the heat shall have time to saturate the ware.
The cones in front of the spy-hole must be observed from time to time and presently as the kiln reaches a bright cherry red, number 02 will begin to bend at the tip and will gradually arch over until the point touches the shelf upon which the cones stand. By this time number 01 will have begun to bend and when the point of this has touched the shelf, the firing is over and the oil is shut off.
It requires some resolution to leave a kiln until morning but it is conducive to early rising anyway. The kiln need not be quite cold but it will help the kiln itself to wear better and the pottery will be better if nothing is done until everything can be handled without gloves.
The cones are now taken out and a diagram is made of each level with the bend of each cone accurately drawn. This diagram should be mounted and hung on the wall for reference. It is not well to trust to memory. It will probably be found, in the type of kiln we are discussing, that the cones on the bottom have bent further than those on the shelf. That is, the bottom is somewhat the hotter.
The variation in the kiln is not necessarily a disadvantage. It may be utilized in burning wares of different kinds. For example, if the bottom prove much the hotter, the biscuit ware may be placed below and the glazed pieces on the shelf. In such case the shelf itself should be washed with a good coating of clay and flint in order to protect it from casual drops of glaze.
If a number of small pieces are being made, more than one shelf should be set up. The legs may be just a little taller than the tallest of the small pieces, but the art of placing or filling a kiln economically consists in making selection of pieces which fit well together both as regards height and shape. Thus, pieces which are large at the base may be dovetailed in with others of which the base is smaller than the upper part. In the case of clay ware the pieces may be set close together or even piled one upon another. There is no danger of sticking unless the ware isburned to complete vitrification. The glazed pieces must not, of course, touch each other.
It will be seen, from these instructions, that there should be a good assortment of wares from which to select. Economical firing cannot be managed if a burn be attempted whenever a piece is ready, and patience must be exercised so as to fill the kiln to advantage.
It is important that anyone attempting to burn a kiln should have some understanding of the phenomena of combustion. Many things occur in the firing which, without such an understanding, are not easily explained but which become perfectly clear when considered in the light of simple chemical science.
Combustion means oxidation or a combination between the elements of the fuel, principally carbon and hydrogen, and the oxygen of the air. This combination is a chemical action and as it proceeds heat is liberated. With a given amount of a specific fuel and a given amount of air there is always the same amount of heat, but the rate at which this heat is given off varies with the time occupied in the operation. Heat may be generated slowly which means a low temperature, or the same volume of heat may be generated rapidly, occupying a much shorter time and developing a higher temperature. From these statements it will be seen that there is a difference between heat and temperature; heat means volume, temperaturemeans intensity. Thus the temperature derivable from a given amount of fuel depends upon the rapidity with which it is burned.
Combustion may be either complete or incomplete. In the former case enough air is supplied to oxidize all the fuel with, usually, some excess. The contents of the kiln are then bathed in the heated oxygen and the condition of the burning is called oxidizing. When the combustion is incomplete, on the other hand, there is a deficiency of oxygen. The kiln is charged with hot carbonaceous gases and smoke, and these, being hungry for oxygen, will abstract it from any substance which may be present. This condition is called reducing because the compounds which exist in clay or glaze are deprived of oxygen and thus reduced to a lower state of oxidation.
In burning a kiln one should be able to produce either of these conditions at will because there are certain wares which require one or the other in order to secure the best results. To put the matter in a nutshell, oxidizing conditions are induced by a strong draft and open flues, reducing conditions are obtained by closing the air inlets and using a liberal amount of fuel.
The subject of "Grand Feu Ceramics" has been so ably developed by M. Taxile Doat in his admirable treatise[P]that it will be unnecessary to go deeply into the matter, but in order that the reader may be aware of what is involved, some description of the technique will be given.
Hard-fired wares are divided into two classes, porcelain and stoneware. The latter is called by the French, "Grès," an abbreviation of the name "Grès de Flandres," the stoneware made in the low countries in the sixteenth century. Both these wares are, technically, once fired, that is, the body and glaze come to maturity at one and the same burning. The biscuit ware is often given a low burn at first in order to facilitate handling, but this leaves the body very porous and is in no sense a maturing fire. The glaze is laid upon this porous ware, or upon the unburned clay if preferred, and then comes the high fire or "Grand Feu" of the French.
A mix for a porcelain body has already been givenbut if the ceramist means seriously to attack the porcelain problem he will have to do some experimental work for himself. The Georgia kaolin mentioned in the recipe on page 40 is a good, plastic clay but it is slightly off color. It may be necessary to improve the color by the use, in part, of another kaolin such as the Harris clay from North Carolina.[Q]
Furthermore, in the preparation of a fine porcelain it is necessary to grind the whole mix upon a mill. The mill used for glaze grinding will answer every purpose and care must be taken that the grinding, while carried far enough, be not too long continued. A certain amount of fine grit in the body mass is necessary but only by constant practice can the right point be reached. In making these experiments each step should be faithfully noted in a handy book. The amount of water to a given weight of clay and the duration of the grinding should be accurately observed and written down. It is most unwise to trust to memory.
The process of casting may be used for porcelain as already described, but the very best of workmanship is necessary. The hard fire to which the porcelain is subjected reveals every error which has occurred in the making. The same thing applies to wheel work. Not only is great skill required in orderto shape the tender porcelain clay on the wheel but the very essence of the porcelain is its lightness, to produce which by craftsmanship a long and arduous course of training must be endured.
Stoneware is free from many of these difficulties and, consequently one who attempts the conquest of high-temperature wares is advised to begin with this. Stoneware clay need not be a mixture. There are many clays which can be used for the manufacture of grès with no more preparation than that laid down for common clays.[R]It sometimes happens that a clay will need the addition of a small quantity of flint or spar but this does not amount to a difficulty.
Stoneware does not present the same manufacturing difficulties as are found in porcelain. The clay is quite plastic and can be easily shaped on the wheel; casting is scarcely a suitable process for this ware. The essence of stoneware is strength and virility, just as that of porcelain is lightness and grace. Each ware has forms suited to itself and it is a mistake to depart from these essential characteristics.
After shaping and drying the technical manipulation of both wares proceeds along the usual lines. The first fire is at a very low temperature. The melting point of silver (cone 010) is enough in nearly every case. This leaves the ware in a soft and porouscondition but hard enough to resist the action of water. The process of glazing has already been described but the composition of the proper glazes differs from that of low temperature glazes.
Porcelain is always burned in a reducing fire; stoneware may be burned either reducing or oxidizing. The temperature at which the glaze is burned is very high, it must be, in fact, the maturing point of the body itself.
The simplest form of porcelain glaze is that represented by the formula—
Which is carried out in the following mixture:
The glaze is ground for use.
The same glaze will also serve for stoneware but it will burn to a brilliant surface whereas stoneware is better when finished with a matt texture.
The following is a stoneware matt glaze:
Of which formula the mixture is—
The porcelain glaze is at its best when uncolored. The matt glaze will be more interesting when used as a colored coating.
The following are a few suggestions for colored matt stoneware glazes. To the glaze batch, 329 parts, add:
Rutile has not before been mentioned. It is a crude oxide of titanium and is exceedingly useful in high temperature work for producing odd, mossy and crystalline effects.
These mixtures make no pretense to be complete, they are given as suggestions only because if the artist-potter is to be successful he must be prepared to compound glazes which are the expression of his own individuality.
For burning high-temperature wares the kilns already described may be used but upon purchasing it should be stipulated that the kiln is to stand burning up to cone 11 or 12. Successful porcelain can be made at cone 10 but better results are secured at cone 12, though, of course, the wear upon the kiln is proportionately greater. Stoneware requires a burn of about cone 9, higher or lower according to the clay used but fine results must not be expected below cone 7 nor is it necessary to go higher than cone 10.