The Potter's Craft: A Practical Guide for the Studio and Workshop

CHAPTER V
The Preparation of the Clay

A clay having been selected in accordance with the tests described, it becomes necessary to prepare it for use. A fairly large supply should be obtained and stored in a dry place. Most natural clays need some kind of cleansing for there are almost always foreign substances present. This cleansing is accomplished by reducing the clay to the fluid known as slip. The necessary appliances for making slip are as follows:

• A large sieve of quarter-inch mesh.
• A small wire sieve of about 14 meshes to the inch.
• A large barrel.
• Two galvanized pails.

The clay is, after drying, powdered and sifted through the large sieve. One of the pails is half filled with clean water and the clay, handful by handful, is sprinkled into it. The clay rapidly absorbs the water and sinks to the bottom. The addition of clay is continued until a small mound rises through the water, when the whole is left to soak for an hour. The bared arm is then plunged into the pail and the mass stirred vigorously. A stick or paddle will serve, of course, but the potter learns a great deal by the feel of the clay and therefore the hand is best. It is said that he is a poor sailor who will not dip his hands in the tar bucket and in like manner, he is a poor potter who fears the slip tub. This stirring will tell a good deal about the probable working of the clay. It may be stony or sandy or greasy. The large stones and roots will have been removed by the sieve but now, after thorough mixing, the slip is poured through the small sieve into the barrel. Both pails may be kept going at once, one being filled while the other is soaking and so on until the barrel is full or, at least, a good quantity of slip has been prepared.

If the clay prove very sandy it should be washed. The mixture in the pail having been well stirred is allowed to stand for a definite time, say one minute. The slip is then poured into the second pail and it will be found that a quantity of sand has settled. This is thrown away and the slip in the second pail is examined. If enough sand has been removed, the slip may be poured into the barrel, using the fine sieve as already described. If still sandy the process should be repeated, the settling being for two minutes. Experience is the best guide in this operation but all the sand should not be removed.

When the barrel is full of slip it is allowed to stand over night when some inches of clear water will be found at the top. This is removed with a siphon which may be made of a piece of lead or rubber pipe. The removal of the water results in the thickening of the slip and the contents of the barrel should be thoroughly stirred with a long wooden paddle to insure a uniform consistency. If the slip is found to be still thin another settling and removal of the water will thicken it.

The slip thus prepared will keep indefinitely, provided that it is not allowed to become dry by evaporation. It improves greatly with age. This is the material which is used for casting as will be described later but for plastic work it must be still further thickened. A shallow box may be procured and made water-tight and the slip, when poured into it, will thicken much more rapidly than in the barrel, but it is better to have some shallow plaster dishes as the plaster itself absorbs the water and thickens the clay. Instructions for making these dishes appear in the chapter on plaster.

These directions will suffice for the preparation of a natural clay but it is sometimes desired to prepare a white body either of earthenware or porcelain. These bodies do not exist in nature and therefore a mixture must be made. The ingredients are kaolin or white porcelain clay, ball clay or plastic potters' clay, ground quartz or flint, and ground feldspar.[B]

A suitable mixture for earthenware is—

and for porcelain—

The earthenware will be creamy in color and porous at an ordinary fire. The porcelain will need a hard fire and will be white and translucent. It is, however, non-plastic and hard to work. The preparation of these mixtures of course necessitates a pair of scales but otherwise the treatment of the mix is the same as that of natural clay. Washing is not necessary but the clay must be powdered, mixed with the flint and spar, and sprinkled into water as already described. In place of the wire sieve, however, a silk lawn of 120 meshes to the inch should be used.

The lawn is simply a fine sieve and is named because of the material (also called bolting cloth), with which it is covered. Have a carpenter make a box without a bottom. Cypress or oak should be used and this should be a full half inch thick. Four strips of the same thickness are also to be provided. The box may be of any convenient size; eight inches square and four inches deep is about right. The sides should be fastened together with brass screws to avoid rust and a piece of lawn is strained tightly across the bottom and secured with copper or brass tacks. A strip of coarse muslin folded and laid along the edges will help to prevent the lawn from tearing, the tacks being, of course, driven through both muslin and lawn. Then the four wooden strips are set upon the muslin and secured with brass screws. The completed lawn is then a tray of which the bottom is formed of lawn. The strips of wood beneath serve to protect the lawn when placed on a table as well as to assist in holding it firmly.[D]

For storing clay in the plastic state there is nothing better than stoneware jars. These may be had of any size and a tinman should make close-fitting covers. Earthenware covers do not fit tight and are always getting broken. A little water is poured into each jar and a support provided for the clay so that it does not rest in contact with the water. Under any conditions clay will slowly harden so that not too large a stock should be kept. Slip, on the other hand, keeps well so long as some water is always on the top and it is not a long process to stiffen it into clay.

CHAPTER VI
Mold-making and Plaster

Plaster is almost a necessity to the potter and therefore something should be learned about it. Even if one does not use molds there are numberless purposes for which plaster is convenient. For stiffening slip into clay, and for absorbing water from glazes, shallow dishes of plaster are used, and for holding work either in making or drying, plaster bats or round slabs are always in demand.

It is best to purchase the finest quality of potters' plaster by the barrel.[E] It will keep indefinitely if stored in a dry place. The necessary appliances are:

• One or two large jugs for mixing, or a metal can with a spout.
• A metal spider or frying pan.
• Six feet of rubber machine belting, six inches wide, or similar strips cut from linoleum or enameled cloth.
• Two or three thin pieces of steel of various degrees of flexibility (scrapers).
• Handy knives, called vegetable knives.
• A small painter's brush.
• Two or three fine sponges.

To begin with, a size of soft soap and water is prepared. Put a quart of water into a kettle and add a piece of soap the size of an egg.[F] Simmer for an hour or until the soap is entirely dissolved and then set aside to cool. When cold the size should be of the consistency of maple syrup. This size is used whenever plaster is to be kept from sticking to a form or surface, and it has also the merit of causing clay to stick to plaster. For example, if a mold is to be taken from a clay model no size should be used, but if a plaster form is used as a foundation for clay ornament it should be well sized first. The size is laid on with a brush and wiped off with a sponge. Another sponge is then used with clean water and the sized surface is washed, all superfluous water being removed. Size is then applied a second time and washed off as before. A third application is sometimes necessary, or until the sized surface rejects water like grease does. On the last sizing, water is not applied, but the surface is polished with the sponge containing size. If the surface to be prepared be of wood or metal a single coat of size will often suffice, but if it be of plaster three or four applications are often necessary.

The first lesson may well be the manufacture of a plaster bat. The frying pan is first sized and set upon a level table. Let us suppose that a quart of water will fill it to about an inch in depth. This amount of water is put into a jug and two pounds and three-quarters of dry plaster is weighed out and allowed to trickle through the fingers into the water. This proportion has been found to be best for ordinary mixings. A smaller quantity of plaster to the quart of water will result in a very soft bat; a larger quantity will be proportionately harder. After the plaster has soaked up all the water it will take, that is in about two minutes' time, the hand is plunged in and the whole stirred to a smooth cream. All lumps must be broken up and the air bubbles removed as far as possible. Continue stirring gently and presently the mixture will be felt to grow thicker. The psychological moment arrives when the plaster forms upon the hand a white coating which cannot be shaken off. The creamy liquid is then poured into the frying pan which is gently shaken to level the surface.

If the plaster has been poured at the right moment it will set smoothly with a mat surface like sugar icing. If poured too late it will be stiff and difficult to level, and if poured too soon it will curdle on the surface and water will be seen above the plaster. A little practice will show the right moment. The jug should be washed out immediately while the plaster is soft. In the place used for plaster work a tub should be provided in which all vessels and tools can be washed, for, if allowed to flow down the waste pipe of a sink, the plaster will speedily choke the outflow.

After standing for some ten minutes, more or less, the bat in the frying pan will grow warm. This is the sign of a combination between the plaster and the water and shows the completion of the setting. The pan is now taken by the handle and, holding it upside down, the edge is rapped smartly on a brick or stone. This will cause the contents to fall out and there is a smooth disc which is one of the most useful of appliances. The edge will need to be scraped and the bat can be set aside until needed. It will be good practice to make a half dozen of these.

This process of mixing and pouring plaster is the same for all operations and the instructions will not be repeated, but when the student is told to "pour plaster" it will be presumed that this experiment has already been made.

The next step is the making of a plaster bowl or dish for the purpose of drying out slip or glaze. A convenient size should be determined upon as it is best to have all the dishes the same. Upon any flat, smooth surface a mound of clay is reared which shall be the size and depth of the inside of the proposed dish. About twelve inches in diameter and three inches deep is a good size, though fourteen inches is not too large for the former dimension. This mound should be made as nearly circular as possible and the clay finished as smoothly as may be. The rubber belt is then set around the mound in the form of a hoop leaving a space of two inches between the clay mound and the rubber hoop. The rubber is fastened either by tying with string or by binding the overlapping ends with clothes pins. A roll of soft clay is laid down where the belt joins the table and pressed down outside to prevent leakage. Enough plaster to fill the space within the belt is now mixed and poured, covering the clay mound to a depth of at least one inch. When the plaster has set the rubber is detached, the whole turned over and the clay dug out. We have now a circular plaster dish three inches deep but we have only one. The trouble of rebuilding the clay is unnecessary a second time because a "case" or reverse can be made from which as many dishes as may be necessary can be formed.

The dish is carefully smoothed and trimmed. The sharp edge is removed and the inside is dressed with fine sandpaper to a perfectly smooth surface. Size is now applied to the inside and upper edge until a bright slippery surface is obtained. The rubber belt is now bound closely around the dish and plaster is poured to a depth of about one and one-half inches on the edge. This, of course, makes a depth of four and one-half inches in the center. When this new plaster has set in turn the rubber is removed and the two castings can be easily separated by inserting a knife at the junction. The knife should be gently driven in with a hammer. Obviously it is now possible to make a number of dishes from the reverse thus obtained, by simply binding the rubber belt around each time and pouring plaster as at first. The original mold having been sized is no longer absorbent but must be kept in case additional reverses are needed. The molds or dishes must be thoroughly dried out before being used.

The molding of a vase form is more elaborate but not really difficult. Even if one does not intend to produce pottery by molding there is always an advantage in having a number of simple forms upon which to make experiments.

The vase to be molded is first drawn to exact size upon paper and a plaster model is turned on a lathe. This can be done equally well on the potters' wheel and the method is as follows: A plaster bat is saturated with water and set upon the wheel so as to run true when the wheel is revolved, and is cemented to the wheel head by a little slip. A few deep scratches are made on the face of the bat and a cylinder, either of the rubber belt or of stiff paper, is rolled up and set on end in the center of the bat. The size of the cylinder should be a little larger every way than the proposed vase. Plaster is now mixed and poured to fill the cylinder. It will adhere to the bat below by reason of the scratches. When the plaster has set, the cylinder is unfastened and removed and the turning may begin. To turn plaster well involves a good deal of practice but it is better to spoil three or four plaster cores in the learning than to spend a long time on one for fear of damaging it.

The board support and turning stick described on page 100 are used in turning plaster as well as clay. The turning stick is held in the left hand and the point is pressed into the board. All this is, of course, made ready before the plaster is poured. The turning tools are here illustrated. They are not sold in the stores but can be made by any machinist. The head or cutting blade consists of a flat piece of steel through the center of which is a shaft or pin which is driven into a handle. The head may be of any shape but the triangle and the circle will meet every need. The tool is held in right hand and braced against the turning stick, the stick and tool being moved together by raising or lowering the left hand which holds the butt of the stick.

While the plaster is still soft the round tool is used and the rough form is rapidly turned. Then as the setting of the plaster proceeds and it is found to grow harder, the triangle tool should be used and the shape gradually wrought out with the point. Finally by using the circle tool for concave lines and an edge of the triangle tool for convex lines the form is perfected. The surface is to be finished and the tool marks removed by using, free hand, a flexible scraper which is bent by the fingers and thumb to fit the lines of the form, and a final smoothing is given by fine sandpaper, the wheel being revolved all the time. At the top of the form a small cylindrical piece is left, called the "spare" which represents the thickness of the mold substance, and for the bottom a small piece is turned in the shape of a truncated cone. The small end of this should be the same diameter as the base of the vase. These are shown in the illustration (Fig. 6).

It will be obvious that in the directions given above the base of the vase is not finished off and therefore the form must be cut off from the bat, either by a knife or saw, and the base is then finished by hand, or by setting the form upside down in a clay cradle—called a "chum"—and turning the base true. The form is now ready for molding.

The plaster vase is laid upon its side on a piece of soft clay and a thin bat or plaster slab is cut to fit the outline. This template should fit with reasonable accuracy but need not be absolutely exact. A pair of these will be required, one to fit each side of the form. These slabs or sheets of plaster are always useful and if a sheet of glass is kept handy any excess of plaster left from a mixing may be poured on to it. This upon setting is easily detached and will present a smooth face where it has rested on the glass. The pair of templates must include, in their outline, both the spare and the foot piece but should not extend beyond either of these. The outside diameter of the mold is now to be determined and the templates cut to this dimension so that the two together, with the vase between them, constitute a longitudinal section of the mold.

The vase must now be divided accurately into two halves by a line running from top to bottom. There are several ways of doing this. While the form is still on the bat a diameter of the bat may be drawn and a perpendicular erected from each end of this diameter. These perpendicular lines will, of course, mark the center of the vase on each side; or after the vase has been cut off another method is possible. With a pair of dividers find the center of both the top and the bottom of the vase. Mark each with a small hole or the point of a pencil. Now lay the vase on its side on the clay cradle upon a glass sheet or other level surface and raise or depress one end until the two centers are exactly the same height from the glass. Take this height in the dividers and, sliding one of the compass legs along the glass, gently scratch the plaster vase with the other or upper point. If the two centers have been accurately adjusted this scratch line will be the exact center of the form.

Some soft clay is now built up on each side of the vase and the templates are pressed down upon it, one on each side until the upper face of each corresponds with the scratched line. The vase is now seen to be buried as to one half in a plaster surface, and plaster poured on this will give a half mold. There is yet, however, nothing to confine the plaster and it would flow away as fast as poured. Two end plates are necessary and these must rise in a half circle above the bed formed by the templates. The part below may be of any shape but must be high enough to cause the diameter of the half circle to coincide with the plane of the templates. Two pieces of cardboard, wood, or rubber belt are now bound to the sides, the apertures at the top and bottom, caused by the curve of the end plates, are stopped with clay and the whole presents the appearance of a vase, only half of which is visible, lying in a shallow trough. All the fitting should be carefully done but the tying up is not yet. The whole is now taken apart and well sized. Vase, foot piece, templates and end pieces are all to be sized thoroughly in the manner described. They are then put together again and bound around with twine. It is necessary now to make provision for the proper fitting of the halves of the mold. This is done by providing knobs and hollows which fit together. These are technically known as "natches" and will be referred to as such. Take two pieces of moderately stiff clay each about the size of a cherry. Roll them into neat balls and cut them in two with a thin knife. Lay each of the halves, flat side down, upon the templates, two on each, placing them in pairs opposite to each other. Affix two or more of these on the inner face of the bottom end plate. Now mix and pour the plaster. This should be poured to the height of the top of the end plates and, after pouring, shake this well down by dipping the fingers into it, so that no bubbles may cling to the surfaces below. As soon as the plaster has become firm but while it is still soft remove the string and the side boards, pull off the pieces of clay and with a straight, thin piece of wood scrape off the surplus of plaster by following the line of the end plates and thus making a half cylinder.

As soon as the plaster has become warm the whole may be turned over and the templates and end plates removed. The four half spheres of clay will be found embedded in the face of the plaster and these, being removed, will leave four hemispherical depressions. The vase can now be gently detached from its bed and the first half of the mold is completed. A little dressing will be necessary. All overhanging edges and rough places should be finished off and the hollow natches smoothed with a piece of muslin on the end of a finger.

The second half is simple. Replace the vase in the half mold, set the foot piece in its place, replace the end plates with the diameter on the line as before but with the semi-circular edges upward, and set two or three clay natches on the bottom one. Size, bind up, pour and scrape off as before, thus completing the two halves of the mold in cylindrical form. It only now remains to make the bottom for, at present, the mold is open at both ends.

The two halves with the vase inside are bound very tightly together with twine and set on the table bottom upwards. The clay natches in the bottom are taken out and the hollows smoothed. The foot piece is taken out and the rough places dressed. The bottom end of the vase is now visible and this, together with the end of the mold, is sized. A strip of stout paper is bound around the mold, projecting about an inch above the end and plaster is poured to fill it. When this is set the paper is peeled off and the edges of the mold are dressed smooth. The bottom may now be detached by inserting a thin knife at the junction, the mold opened and the form taken out. The mold is now in three parts which may be put together at will and used for casting the vase in clay.

CHAPTER VII
Cases and Working Molds

The mold described in the previous chapter is called, technically, a "block mold" and is not, as a rule, used for making the clay ware. The reason for this is that molds will wear out more or less rapidly and to repeat the process of making new ones from the original form would be tedious and expensive.

From the block mold a reverse is made, called a "case," and from this, in turn, working molds are made in any required number. While it is possible to use the block mold as a working mold, and, if only a few pieces are required this is quite sufficient, yet, as it is often necessary to have a number of molds, the student should understand how to make a case.

A case may be defined as a mold from which a mold is made. If one can imagine the visible half of the vase form as it appears in making the mold, with the templates and ends cemented into one piece, one has a conception of one half of a case. The problem is to make this with permanent but movable ends so as to have a convenient form from which half molds may be easily made.

The ends are joined to the body by means of offsets and the first step is the construction of these. One half of the block is taken and laid upon its back, being supported by clay so that the face is level and steady. An offset plate is now cut to fit each end. To make these a piece of plaster is selected or made which is true and smooth on both sides. The plates are cut of the same width as the mold and are beveled at the upper edge so as to rise slightly from the mold face. The curve at the end of the mold is cut out to fit and beveled in like manner. Then two end plates are fitted. These should be about two inches higher than the offset plates and are square at the top. Upon each of these two or three clay natches are set, being placed low down near the face of the mold. The mold and plates are well sized and bound together with side walls just as in the making of the mold. Plaster is poured to a height sufficient to well cover the natches and left to set hard. No shaping is necessary. When well set the end plates and offset plates are removed but the vase mold and the case are left attached together. The other half of the mold is prepared and run in the same way, the same offset plates and end plates being used with such slight refitting as may be necessary. The work is now examined and all rough places and scraps of adhering plaster are removed. The two halves of the case, the half molds being still attached, are set up on end, back to back, being separated by a thin piece of plaster or a strip of cardboard which should extend two inches above the top. The top ends are now sized, the natch holes having been smoothed off, a band of paper is tied around and plaster poured on top to a depth of about one inch. When set the whole is turned over and the operation is repeated on the other end. After the final setting the ends are easily removed and by the insertion of a thin knife driven by a light blow, the molds and case are separated. Each half case is now laid on its back and the proper ends are fitted in place. It only now needs the usual side walls to be tied on and molds can be made with ease just as the original block mold was made.

It now remains to make a case of the bottom mold. The bottom piece of the block mold is taken and sized and with a strip of paper bound around it, plaster is poured. The two are detached when set and the case is finished. It consists of seven pieces; three are used in each half and one for the bottom.

Thus equipped it is possible to make any number of working molds and if the case should wear out or be damaged, a new one can always be made from the block mold. The block mold itself, having been sized, is no longer absorbent and cannot be used for making vases. The working molds should be thoroughly dried before using and they will last longer.

Flat ware, such as plates and saucers, is made on, not in, a mold. The diameter of the plate having been decided upon, a block of plaster three inches wider is run. This is placed on the center of the wheel or jigger and in it the face of the plate is turned. This must be sunk below the level of the block and when finished, must appear as though the plate itself were embedded in the plaster. One half of the thickness of the edge is shown in such a way that there is no under cutting. Just outside of this edge the plaster is turned so as to slope gently up to the level of the block.

Without removing the block from the wheel the face of the plate is well sized, a band of belting is arranged, of the same diameter as the edge of the slope and plaster is poured to a depth of three inches. Out of this the back of the mold is turned as shown in the illustration (Fig. 12).

The top of this as it lies upside down is shaped with a straight, almost upright slope which enables the mold to be set securely in the wheel head. Around the exposed edge of the original block, three or four natches are now bored or cut. They should be placed at irregular distances so that there will be no doubt as to the putting together of the sides of the case. If two circular pieces of plaster have to be set together and held by natches there should always be either this irregular spacing or some distinctive mark, because if this be not provided for, two or three trials will always be made before the correct fitting is found and these trials wear out the natches very quickly.

The back of the plate mold and the edges of the block are now sized and plaster is run to the level of the highest part of the mold but no higher (Fig. 13). When this is set, the two halves of the case can be separated and the mold taken out. Now when the halves of the case are fitted together there will be a cavity the exact size of the mold. This can be filled again and again with plaster, a new mold being formed each time.

In order to use these molds a special head must be provided for the wheel. The regular head of the wheel should be detachable and in its place an iron frame called a prong is fitted. This consists of a collar either with a hollow cone or a screw to fit the shaft of the wheel, and from this radiate four short arms. In order to use this a circular block of plaster some two or three inches thick is poured on a table or slab and just as this is setting, the prong, upside down, is pressed into it just below the surface and held there until the plaster is hard enough to support the weight of the iron. When hard, the whole is lifted and the prong with the plaster attached is set in position on the wheel. This now forms a rough plaster head and it must be turned true. In this head a circular depression is to be turned which will exactly fit the back of the plate molds. If the recess should wear larger as it will if much used, a new head can easily be run. The same principle can be applied to the making of molds for saucers.

Cups and bowls are molded from the outside. A block of plaster about one inch thicker than the height of the proposed cup is taken and centered upon the wheel. Out of this the piece is to be turned, upside down, leaving a ledge or platform, the outside diameter of which is the size of the mold. The rubber belt is tied around this and the mold poured. If for casting this will suffice, but if it is intended to make the cups upon the wheel the outside of the mold must be turned to fit a wheel-head which is hollowed to receive it. The making of the cups is described in Chapter XI. A bowl is simply an enlarged cup.

CHAPTER VIII
Building by Hand

The production of pottery by hand is a form of modeling but with the important difference that while pieces modeled by art-school methods are not intended to be preserved in the clay itself, built pieces are destined for the fire. It is therefore necessary not only that a special clay be used but that the work be such as will hold under the strain of the burn. The composition of the clay has been dealt with in another chapter and it is presumed that the worker has decided upon the proper mix or has procured a suitable clay.

There are two possible treatments of built pottery; the work may be finished by fingers and tools only or it may be placed upon the wheel and turned to a true surface. In the latter case the result is much the same as if the piece were thrown on the wheel as will be described. The principal point of difference is that while building needs less practice than throwing, turning a built piece is much more difficult and tedious than turning a thrown one. It is almost impossible to build with sufficient accuracy for the work to run true, and a great deal of time is consumed in filling hollows and removing lumps. These do not appear obtrusive when the work is held in the hand, but if it be revolved upon a fixed center every slight irregularity appears to be accentuated. On the other hand the charm of built ware lies in the subtle plastic quality which belongs to no other material or method.

For very large pieces such as tree pots the combination method is useful but these should be built on the wheel itself and kept true as the work proceeds. Then a slight turning at the finish, when the clay is leather hard, will produce a satisfactory result.

The clay for building should be rather soft as it is apt to dry quickly on handling. The work may be done either with coils or pieces.

A plaster bat should be made with a low dome in the center. This bat may either fit the wheel or not, depending upon the plan adopted. The dome is to raise up the bottom of the vase and form a foot. The table may be covered with a piece of oil cloth or may be kept slightly damp. The first attempt should be to build a cylinder as this form is easy to construct and to keep true, so that the attention may be devoted to the manipulation of the clay.

It is first necessary to roll out the clay into cords which should be a little thicker than the proposed walls are to be. These cords should be as uniform as possible and should be rolled quickly to avoid undue hardening. It is best to roll them as required. The domed bat is made quite damp and upon it should be marked the diameter of the cylinder to be built. A roll of clay is taken, one end laid in the center of the bat and the rest is coiled around it in a spiral line. When the disc so formed has reached the proper size, the coils are gently rubbed over with the fingers until they have thoroughly united and the lines of the spiral have disappeared. The clay disc may now be turned over and the rubbing continued on the other side. The circle is cut true and a new coil is laid on the outer edge thus making a shallow circular tray. In raising the walls it is best to pinch off the roll of clay when one circle has been completed and the new roll should be begun at another point so that all the joints will not be at the same place. This plan is better than coiling a long roll in a spiral for in this case one side of the piece will be higher than the other.

After three rolls have been laid in position the wall, both inside and out, should be worked like the bottom so that the rolls will disappear and the clay be welded uniformly together. This should be done without water or with as little as possible. The use of water is very tempting. It makes the clay so smooth and seems to help but it will inevitably make the work sloppy and will tend to soften the walls.

After three or four rolls have been worked in, the piece should be laid aside for some hours to stiffen. If this be not done the weight of the second building will cause the work to sag and fall out of shape. For this reason it is well to have two or three pieces in hand at once so that there need be no waiting. When the cylinder is of sufficient height it should be allowed to become quite stiff and then the irregularities should be corrected with a little soft clay which is worked into the joints. The whole surface may now be gone over with tools and brought to the required finish. As soon as the clay is hard enough it should be removed from the damp bat and placed upon a dry one to become dry.

In the method of building by pieces no rolls are prepared but the clay is taken, pinch by pinch, each morsel being pressed into place as the work goes on. This plan is somewhat more plastic in effect and is well adapted to free-hand work; the resulting pottery, however, is generally thicker and heavier.

The craft of building is not mastered until the lines of a drawing can be successfully followed. The clay is apt to choose its own way and the result will be very different from what the potter intended. The design should be carefully worked out on paper, full size if possible, and the clay form should be compared with the drawing as the building goes on. A profile may be cut in cardboard and this, applied to the clay from time to time, will verify the line, but all such mechanical aids should be used sparingly as the value of this work depends largely upon the sense of freedom and self-expression which belongs to it.

The thickness of the clay walls is a matter of great importance. A small piece should not be so thick as to feel clumsy and heavy, nor should a large piece be so thin as to lose the sense of strength and solidity.

It may be found on drying the ware, that cracks, especially in the bottom, are developed. The cause of this may be in the clay. A clay which is too plastic or too fine in the grain will surely crack. Such a clay may be opened or meagered by the addition of ground flint or fine grog. The cause may, on the other hand, be in the building. If the welding of the coils or pieces be imperfectly done, cracks are sure to result. If the bottom be too thick it will crack. A great strain is put upon the bottom in drying. The clay must be able to shrink and while the side walls are able to settle down on themselves, the bottom is pulled in every direction by the sides. The bottom should be made quite thin in the center and thicker toward the edges. This will help to avoid cracks. A bad crack cannot be successfully mended. It is best to break the piece and begin again. To burn it means the loss of the clay but the clay will be saved if the damaged work be withheld from the kiln. A small crack on the edge is also hopeless. A crack showing on the edge of a piece is a bad fault. A small crack in the bottom may be mended by dampening the place carefully and pressing in a little stiff clay.

CHAPTER IX
The Potter's Wheel

Much of the glamour of the potter's art is associated with the wheel. Poets have sung its praise and artists have delighted in its rhythmic motion, but alas! the wheel as a commercial method of manufacture is doomed to extinction. It cannot compete with the precision and speed of machinery. It devolves, therefore, upon the artist potter to maintain the wheel in its rightful place as, par excellence, the potter's tool.

No clay worker's studio should be without a wheel, but the particular form of wheel depends upon the nature of the circumstances under which it is to be employed.

The simplest wheel is that used by the Chinese. A circular plate with a heavy rim is set upon a spindle so that it will revolve freely and run steadily. As the workman sits or kneels upon the floor the surface of the wheel is about at the floor level. Around the periphery and upon the upper surface four holes are sunk and the workman, inserting a short stick into one of these, gives the wheel a rapid motion. Then while it is revolving by its own momentum the clay is centered and shaped. As the motion is lost the stick is again inserted and the wheel spun. This method, of course, involves much skill on the part of the workman.

In the next form, one which is only adapted, however, for crude experimental work, the wheel is set upon the frame of a sewing machine and operated by the treadle. A beginning may be made upon such a wheel but the operator will soon wish for something better.

A common factory form and one which is well adapted for studio work is the kick wheel. The wheel head is set at the top of a spindle and in the upright shaft there is a crank to which is attached a horizontal moving treadle. This is worked continuously by the left foot, the weight of the body being supported by the right. The action is strenuous and scarcely fitted for persons of other than robust physique but it can be used successfully after practice. This wheel is made by the manufacturers of potter's machinery.

Another form of the kick wheel is used in Europe and is, in fact, the original wheel used by the French and German potters in the seventeenth century. The head is set on a spindle as usual but instead of the crank there is a large heavy disc on the bottom of the shaft and revolving in a horizontal plane. This is within reach of the foot and the operator, being seated, imparts a rapid motion by pushing, usually with the ball of the right toe. The momentum is kept up by the weight of the disc and there is a great advantage in that the foot need not be in continuous motion. On the other hand it is difficult to acquire sufficient speed and power for the work.

There are several forms of machine wheels which are entirely satisfactory but which need the application of power. If a gas engine or a water motor or electric current be available, every effort should be made to obtain a wheel of this description. The prime motion is imparted to a short horizontal shaft which moves at a constant speed. Then the operator, seated in comfort, regulates the speed of the wheel itself by pressure upon a treadle. No action is required but a simple pressure, light for a slow speed and heavy for rapidity. Where the electric current is available, nothing could be better. Self-contained motor-driven wheels are available but are rather expensive.

One more plan may be mentioned in which the wheel is simply a vertical lathe with a belt and handle to be turned by an assistant. This may be convenient for some but it is not always possible to secure help at the moment when the wheel is to be used. Moreover the cost of labor would soon pay for a mechanical wheel.[G]

Whatever type of wheel is selected it should be arranged with a head which can be removed. There are two methods of constructing this; the head may be screwed on to the spindle, or the latter may terminate in a cone-shaped plug upon which the wheel head is made to fit as in the illustration (Fig. 14). The latter plan is to be preferred as the head can be removed more quickly and is not so likely to work loose. Several heads for the wheel can then be provided, one for regular work, one for making plates, one for finishing and so forth.

The regular operation performed upon the wheel is termed either throwing or turning according to the industry in which it is employed, but in this description the word "throwing" will be used because the subsequent operation in which tools are employed is best described as turning.

The best head for the wheel to be used in throwing is made of hard wood or brass because the ball of clay can be easily centered upon a smooth surface. This, however, involves that the work shall be cut off with a wire and removed while soft. This is commonly done by professionals but is beyond the skill of the beginner. It is best, therefore, to use a head like that illustrated for plate making and to have a number of specially shaped plaster bats to fit the recess (Fig. 17). Then when the piece is formed, the bat with its burden can be set aside for the work to harden.