Slip (ceramics)

“ Large pieces, for example, frequently have to be thrown in several parts, fastened together, and afterward finished. Some pieces the thrower shapes over a plaster-of-Paris mould; and some he shapes inside the mould, reversing the process and pressing the clay against its confining surface. The danger in thrown ware comes from an uneven pressure on the clay resulting in a lack of solidity; the pieces are not always equally firm at all points and in consequence sometimes crack.
"Can pieces of any design be thrown?" inquired Theo.
"No, only round pieces such as cups, mugs, vases, jars, or bowls. ”

“ Cracks or a regular network of craquelage are generally produced on new ceramics by the same principle as they are obtained on oil paintings, namely, by producing artificially a difference in the shrinkage capacity of two superimposed layers. In oil painting it is the layer of pigment and of varnish, in the case of pottery the two layers are represented by the baked clay and the glaze. ”

“ Because of the practice of slipping and polishing only half the surface at a time, there develops around the shoulder a narrow line of dried slip, which is naturally lighter in color than the worked areas. (This line may be seen encircling the olla in pl. 21, c) . After the polishing of the upper and lower areas has been completed, the potter’s attention turns to this line, and since dry slip cannot be polished, the rubbing stone is either wet with the tongue or dipped into the liquid in the slip-container before being used on the line. ”

“ The least will belong to ware pressed with stiff clay gently fired; the greatest, to that cast with liquid slip and brought to the vitrified state. In these last, the shrinkage is greater in height than in width, a fact explained by the weight of the upper portions acting vertically to assist the closer contact of the particles in the under-structure, when the same opposes their free action in an horizontal direction. In making the models, care should be taken to bring the contraction to a common centre, or if there are several, to strengthen sufficiently the connecting parts. ”

“ For such purposes hair spaces, copper-thins, and brass-thins are provided. The thickness of hair spaces varies according to the size of the type; the name is given generally to any cast space thinner than the 5-to-em. Copper spaces are 1⁄2-point in thickness and brass spaces are 1-point thick, the difference in the metal used being for easy distinction of the thicknesses. Copper and brass spaces, because of greater durability, are superior to cast metal hair spaces for hand composition.
When metal thin spaces are not at hand it may be necessary to resort to pieces of paper or thin card. ”

“ In grinding the colours upon the stone, and managing them upon the pallette, care should be taken not to use an iron knife, the steel or iron that grinds off, in mixing with the colours spoils their brightness and vivacity; flake-white and white-lead, yellow-oker, lacque and light-red, suffer greatly by it, it gives them a dull and dirty cast; Naples-yellow suffers most of all from it; its vivacity is entirely destroyed by the iron’s touching it. Horn, ivory, or tortoise shell knives, or [Pg 54] wooden spatulas are fitter for all manner of painting; they will affect no colours ”

“ Niello is more than simply a drawing on metal. That would come under the head of engraving. A graver is used to cut out the design on the surface of the silver, which is simply a polished plane. When the drawing has been thus incised, a black enamel, made of lead, lamp black, and other substances, is filled into the interstices, and rubbed in; when quite dry and hard, this is polished. The result is a black enamel which is then fused into the silver, so that the whole is one surface, and the decoration becomes part of the original plate. ”

“ It is also used in coining, where the impression of a die is to be made upon a piece of metal, and in the same way in producing the impression of a seal upon wax or other substance adapted to receive it. When soft and light materials, such as cotton, are to be reduced to a convenient bulk for transportation, the screw is used to compress them, and they are thus reduced into hard dense masses. In printing, the paper is urged by a severe and sudden pressure upon the types, by means of a screw. ”

“ In the thirteenth century appear plates of metal, generally silver or gold, covered with a delicate chiselling in bas-relief, and clothed with colour by means of a coating of various transparent enamels through which the pattern is seen. And, lastly, in the fifteenth century, we find plates of metal, gold or copper, coated with a thick covering of enamel, on which the design is painted, [1] In these successive processes, we perceive a tendency to the concealment and subordination of the metal that forms the groundwork of the enamel. ”

“ In most of the figures showing examples in metal, the appendages have to be secured to the primary mass by screws, rivets, or solder, whereas in clay they may be moulded into the primary mass. This tends to secure a more unified appearance; but in metal, the junction of the handle and the primary mass is often made a decorative feature of the design and gives added interest and variety to the project. ”

“ Repousse or embossing involves practically the same principle as modeling in clay or wax, the only difference being that metal is used as the material and that different tools are employed. In this, as in clay or wax work, it is desirable to bring certain parts of a design into relief; to do this with metal the work must be placed on a substance which will give some resistance and yet allow each blow of the hammer or tool to make an impression. The substance commonly used for this purpose has the following composition, in the proportions given:
Black pitch 1 lb. ”

“ When large pieces are made, the European method is to pour the necessary thickness of slip over the inside of the mould, against the side of which it is kept by means of forcing air into the interior, after covering the surface, or exhausting the air through the mould. When sufficiently dry to support its own weight, the piece is fired.
Fig. 23.—A Potter in Palestine.
The other method of forming the wares is technically called “throwing” upon the potter’s wheel, and is suitable for all circular{71} vessels, or those with modifications of the circular shape. The process is very simple. ”

“ When cold, the sheet or disc of glass may be cut to the shape required, either, as in the old days, by running a hot iron slowly along the proposed line of fracture, in which case a crack will follow the iron, or by scratching it with a diamond and then bending it so as to break along the line of the scratch. ”

“ The mass in a tough but plastic state, a toughness and plasticity in combination with which there exists no approachable parallel—having been so rendered by simply plunging it into boiling water, becomes of as easy manipulation as clay. Indeed the material in this stage being so like clay, we can scarcely point to any better illustration than the porcelain button manufacture, which being familiar to most, there can be no need to detail. Treated thus like clay, the moulds may be filled by the gross, and the buttons afterwards submitted to vulcanization. ”

“ After the surface has been completely covered in this manner once, and none too carefully, the rim is coated with the slip, applied by the forefinger of the right hand, and gone over with the stone. The vessel is then wiped with a cloth upon
PLATE 21
a. Applying orange-red slip with a cloth mop; the beginning of the stroke may be seen just above the lap of the potter. b. Polishing a bowl with a small stone after red slip has been applied. The marks of the stone are visible at first, but quickly disappear as the slip dries. c. Women working with the polishing stone. ”

“ Yet you cannot suppose it is in reality easier to draw a line with a cutting point, not seeing the effect at all, or, if any effect, seeing a gleam of light instead of darkness, than to draw your black line at once on the white paper? You cannot really think [Y] that there is something complacent, sympathetic, and helpful in the nature of steel; so that while a pen-and-ink sketch may always be considered an achievement proving cleverness in the sketcher, a sketch on steel comes out by mere favor of the indulgent metal ”

“ An artificial disproportion between the shrinkage of the clay and the glaze is usually obtained by modifying the quality of either the one or the other. Does the clay shrink more in the firing than is desired, the ceramist generally mixes it with non-shrinking elements such as powdered brick, or even another kind of clay which he knows must shrink less on account of its composition, although it may not be suitable in colour and quality. ”

“ Marwood replied. "Many materials have been tried with varying degrees of success—plaster-of-Paris, alabaster, steel, gun-metal, and brass. Of course what is necessary is a strong, firm, absorbent material. Clay moulds break too easily, and also become saturated with water and lose their shape; metal moulds, on the other hand, while most useful in making wares decorated with fine, raised designs such as the Wedgwood figures, fail to seep up the superfluous water. ”

“ As to the methods of producing lustred pottery, the chemical investigations practised by Riocreux, Brogniart, Carand, and others, have shown that the metals used to produce the characteristic reflex which gives the ware its name were copper and silver, entering into the composition of an extremely thin glaze extended over the surface of the pottery, and employed, sometimes together, and sometimes separately. ”

“ A great deal depends upon the care and time bestowed on the preparation of the work, sometimes as many as ten coatings being given to the wood, and these are successively rubbed down with pumice stone and glass paper, care being taken not to lose the sharpness of carved ornaments. This application of gold leaf is termed mechanical gilding, and is used for gilt furniture, picture frames, or other decorations. Within the last ten years the gold has been applied to the more richly carved furniture in a powder. ”

“ When completely cut, the enamelling substance, which is generally sulphate of silver, is placed upon the design in just sufficient quantities, and the whole piece of work is then put into a furnace and heated to such a point that the enamel melts and fills all the cuttings of the design, while the metal itself remains uninjured. ”

“ On the contrary, the room was of dazzling brilliance and beauty, for it was lined throughout with an exquisite metal that resembled translucent frosted silver. The surface of this metal was highly ornamented in raised designs representing men, animals, flowers and trees, and from the metal itself was radiated the soft light which flooded the room. All the furniture was made of the same glorious metal, and Scraps asked what it was.
"That's radium," answered the Chief. ”

“ The tops of the ridges print, the valleys are the spaces which are to appear white in the impression. The second figure, a cross-section of an7 intaglio plate,—an engraving on copper we will say,—shows no hills and vales, but a flat surface with a number of V-shaped cuts filled with ink. When engraving on a copper plate, we cut with the graver into the metal every line of our design that is to appear black. Wherever we want a white space we are careful to leave untouched the polished surface of the plate. ”

“ Subsequent shrinkage allows its removal from the mould. After a partial drying, the ware is dressed or “shaved.” The process is a very delicate one, especially in the finer kinds of ware, in which a finely polished surface is necessary. The piece is placed on a lathe, and cut to the necessary thickness, and receives its ornamental lines, or has the mouldings applied. The handles are then attached, and, after drying, the piece is ready for the kiln.
The moulding of porcelain requires very great care, on account of the fragility of many of the pieces. ”

“ An irregular contraction is the source of most of the defects attending the ceramic manufacture, and it is worth explaining the causes, of which there are three. I have already mentioned that natural clays, which have remained in a damp soil for ages, contain materials in a hydrous state, i.e. combined with water, which sometimes increases their bulk considerably. These are unstable compounds, and may be destroyed by thoroughly drying them. Some other materials used in pottery may be artificially combined with water, as would be the case, if ground in it for an unnecessary length of time. ”

“ Constructional features.-Features of vessels resulting from construction are infinitely varied and often highly suggestive of decoration. Constructional peculiarities of the clay utensils themselves are especially worthy of notice, and on account of their actual presence in the art itself are more likely to be utilized or copied for ceramic ornament than those of other materials. The coil, so universally employed in construction, has had a decided influence upon the ceramic decoration of certain peoples, as I have shown in a paper on ancient Pueblo art. ”

“ Other silicates are also transformed by artificial processes into fibrous forms, such as “glass,” which is fused and drawn or spun to a continuous fibre, and various “slags” which, in the fused state, are transformed into “slag wool.” Lastly, we note that a number of metals are drawn down to the finest dimensions, in continuous lengths, and these are woven into cloth or gauze, such metallic cloths finding valuable applications in the arts. Certain metals in the form of fine wire are woven into textile fabrics used as dress materials. ”

“ Now as to certain general considerations in relation to the appropriate and logical use of ceramics in the construction and adornment of buildings, exterior and interior. In our northern latitudes care should be taken that ceramics are not used in places and in ways where the accumulation of snow and ice render the joints subject to alternate freezing and thawing, for in such case, unless the joints are protected with metal, the units will work loose in time. ”