Pottery decoration under the glaze

by M. Louise McLaughlin

THE STATE OF THE WARE IN WHICH THE PAINTING SHOULD BE APPLIED.

For this method of decoration the ware should be in the “green” state. The more moisture it contains the better, and it will be in proper condition if procured as soon as it leaves the hands of the molder, or as soon as it has become sufficiently dry to retain its shape. Pieces can be kept in this state a considerable length of time, by placing them in a box which has been lined with plaster of paris. This lining should be about an inch thick, and if the inside is occasionally sprinkled with water, it will remain moist, and keep the clay in good condition. A box of this kind is really invaluable for all work, where it is necessary to keep clay moist. Within certain limits the clay is improved rather than injured by keeping in this way; the effect upon it being that it becomes tougher and more plastic. If kept too long, it becomes “rotten,” and the plastic quality is lost, but it will keep a reasonable length of time, even two or three months. If the box can not be had, the pieces may be covered by a rubber cloth, which should be kept from touching them by a frame of wood or galvanized iron wire, damp cloths having been placed around them to produce sufficient moisture.

It is necessary to exercise this care in keeping the ware moist, in order to prevent the clay, which is applied to the surface in painting, from cracking during the process of drying. It must always be borne in mind that the material made use of in painting is simply clay, which has been artificially colored, and, as clay, becomes subject to the rules which govern all work of this kind, whether it is making pottery, modeling, or painting faience.

To produce depth and richness of effect, the painting should have a heavy impasto, and the high lights should be laid on in such a manner as almost to present the appearance of modeling in relief. When this is done, there is a tendency in the applied clay to crack in drying, if the conditions are not right. In applying one piece of clay to another, it is requisite that both should be of the same consistency, to ensure equal contraction. In this case, however, the clay is used as paint, and must be of such consistency as will permit its manipulation with the brush. Its application, then, to a body of clay that has lost much of its moisture, is attended with some risk, and the fact that the coating of clay is comparatively thin, alone renders it possible to defy one of the conditions which govern the use of this material. When the piece of ware, upon which this painting is applied, has previously become too dry, the moisture in the coating of clay is very quickly absorbed into the body, and this causes it to shrink unequally with the body, and thus to crack. When the body and the applied clay are more nearly in the same condition, and the former still retains considerable moisture, it does not absorb that of the clay on the surface so rapidly, and they shrink and dry together.

The drying of the clay used in painting can, however, be retarded by the use of gum tragacanth, which may be mixed with it. It would, perhaps, be better always to take the precaution to use this medium, although when, as remarked before, the body of the ware is still quite soft, and the condition of the atmosphere such as to prevent too rapid drying, the painting may be executed simply with the use of water.

Just here a caution as to the use of gum tragacanth may be given, and that is, that it is better not to mix the gum with more of the colored clay prepared for painting than is to be used at once. When mixed only with water, the clay after becoming dry, can be easily rubbed down again with water; but if gum tragacanth has been used in the mixture, and it has been allowed to dry upon the palette, the gum will not re-dissolve readily under the action of water and the palette knife, and the mixture will be full of small particles of undissolved gum, which will render it unpleasant to work with. To prepare gum tragacanth for use, it is dissolved in water, to which may be added a drop of salicylic acid to keep it from turning sour.

The clay, which is to form a body for the colors, must be dissolved in water until of a proper consistency for painting. It should be a little thicker than the diluted clay used by potters and technically called “slip.” As to the quality of the clay, the desideratum is that it shall be colorless and not inclined to crack in drying. The clay used in the manufacture of white granite ware is probably the best clay for this purpose, generally accessible. This can be mixed with all the colors without injury to the tint of any. Parian clay produces the most beautiful effects, and, where it is possible to procure it, forms the best medium for this painting. More care must, however, be exercised in its use, as it is very liable to crack in drying, and from the translucency produced in the body by the action of the fire, must be laid on more thickly than clays which have greater opacity, to produce the effect desired.

In the case of yellow, which does not always stand the fire well, it is best to mix the color with a yellow-tinted clay to ensure a successful result. C. C. clay, which is a cream-tinted white, will effect this. If a darker yellow is desired, clay which in firing acquires a decided yellow-tint may be used.

A light cream-tinted clay, such as C. C., or “stilt” clay, may be used with yellows therefore, with advantage, with browns and blues without disagreeable effects; but in the case of scarlet, pink, or crimson and greens, nothing but white clay must be used, if purity of tint is desired. The reason for this is that the presence of iron in the yellow-tinted clays injures the colors mentioned.

The clay can be procured from the pottery in broken pieces, or, better, in a semi-pulverized state from the trimming in making the wares. When in this state it is easy at any time to dissolve enough for use. It is, perhaps, better to prepare it just before it is to be used, as the longer clay stands, after it has been mixed with water, the more compact it becomes. When it has just been dissolved, it still retains air among its particles, and this renders the mixture more porous and less liable to crack. It soon dissolves in water and can be stirred until smooth. It may be strained through a sponge, alternately allowing the sponge to absorb the diluted clay and then squeezing it. Probably the most convenient way is to rub the clay in a mortar until all the lumps are dissolved. If a very small quantity only is needed it may be rubbed down upon the palette with the palette knife as it is wanted.

A palette or a slab of glass may be used for mixing the colors and clay. The latter is the best, as it can be procured of large size, and as in this kind of work a considerable quantity of paint is necessary, there should be ample room to mix the colors. A slab of French plate glass, at least twenty by twenty-four inches, may be procured. This can be imbedded in plaster of paris, which will make it stronger, as well as furnish a white surface upon which the colors will show to better advantage.

The powdered colors as procured are not perfectly pulverized, and must be well rubbed down with a muller and palette knife. It is even more necessary in this than in other kinds of painting upon pottery that the color should be well ground. If not rubbed down sufficiently before being mixed with the clay, particles of color which are scarcely apparent before firing, will show when the work is glazed, forming specks in what should be, a perfectly uniform tint. If the color is mixed with the clay before it has been perfectly dissolved, it will not afterward be possible by any rubbing down to remove the specks from the mixture.

The colors, after being thoroughly ground, can be mixed with clay in such proportions as may be desired. The white clay takes the place of white, and is used to lighten every tint, as well as to form a body for the colors. It is a good plan, where experience has rendered it possible to do it with judgment and certainty of result, to mix the various tints in water with clay, in quantities, and then allow them to dry. When dry, they can be pulverized and the colored powder can be put away in bottles for future use. This saves time when painting, and also ensures a like result from the use of a mixture which has already been tested and found to be satisfactory. It is only, however, after considerable experience in mixing colors, and verifying their results, that this can be safely done.

As in other kinds of underglaze painting, it may be given as a general rule, that the colors are intensified in firing, in this, however, much more than in other methods. All the stronger colors are intensified to a degree difficult to realize before experience has taught the artist what to expect. They all become several shades darker, and, in the case of blues, blacks and greens, it is more difficult to obtain delicacy, than depth of tint.

In the use of scarlet, crimson and yellow, a considerable amount of the color in proportion to the clay must be used for the darker portions to give sufficient depth. Only enough clay must be taken to give body to the paint, and the mixture should then be applied very thickly. The painter must also bear in mind the fact that the stronger color will overpower the weaker in mixtures, even when the latter is in such degree as to give the color to the mixture before firing. After firing, the stronger color will appear in excess. In making such mixtures, therefore, the stronger color should be used very sparingly, if it is desired that the other should predominate in the tint when fired. Otherwise, the colors may be found after firing to be entirely different from what was intended.