CHAPTER V

THE GLAZE AND HOW TO APPLY IT

Clay that is simply baked, without a glazed coating, will not hold water perfectly.

One can imagine what a blow it must have been to the early potter when he found that this was the case. Some say that he used wax at first to close the pores of his pottery, and later—perhaps by the overheating of a kiln—glazed pottery was discovered. Pottery that is soft will develop a semi-glazed surface when overfired, and it is probable that some such accident suggested the use of the glaze.

The first glaze was doubtless a pure silicate of soda. Oxide of lead added to this made it more fusible, but it was not as hard or durable.

What is known as biscuit is baked clay porous and without gloss—for example, a flower-pot.

Glossy pottery has a very thin layer of glaze upon it. The Samian ware of the Greeks furnishes examples of this finish. Glazed pottery is covered with a perceptible coating of glass.

Enamelled ware, or pottery with a mat-glaze, has a glazed coating made opaque with oxide of tin. This finish is used on some of the most beautiful art pottery. Glazes may be coloured with certain metallic oxides without losing their transparency.

One is often confused by hearing the terms enamel and glaze carelessly used. Enamel should be used to describe a vitreous coating that is opaque, and glaze a glossy, transparent surface—both may be coloured. The term mat-glaze is an exception to this rule. This means an opaque glaze with a glossy finish.

The materials of which glazes are composed are about the same as those that enter into the composition of clays with a few additions. There is in glazes, as in clays, the play of soft and hard materials, or the flux and frit. Flint, aluminum, or china clay forms the frit or hard, refractory portion; potash, soda, lead or borax the melting part or flux. The addition of clays gives durability.

In preparing the flux, it is melted like glass and then crushed to powder. The frit must first be melted, then dropped from the crucible into water before it can be powdered.

The making of glazes is not often done by amateurs, and unless one is planning to start pottery-making as a business, and requires large quantities of glazes, it is best to buy them from a reliable dealer. They come in the form of powder—soft glaze, hard glaze, and stannifère, which is a hard glaze with some oxide of tin added, to make it opaque. The soft glazes are used on ware which fires at a low heat, and is therefore not durable. A large proportion of hard glaze must be added to make the preparation applicable to a pottery which is fired at an intense heat, while if one wishes a mat-glaze the stannifère is also used.

The colouring materials may be bought of the same dealer. They are as follows: Antimony for yellow, cobalt for blue, copper for green, chrome for green, manganese for brown, and iron for brown. Red oxide of iron colours a purplish red, and carbonate of copper makes a gray green that is very beautiful. A ground glass or stone slab, on which to mix the glaze, and a palette knife, will be required; also a small quantity of gum-arabic and of gum tragacanth, a small teacup, and a measuring-glass. These, with two or three soft paint-brushes of various sizes—one that is an inch wide, one smaller, and another larger—will be enough of an outfit to start with.

The gum-arabic should be dissolved in water to the consistency of honey. It is used in the first coat of glaze to bind the glaze and prevent it from rubbing off. Quite a little of it may be prepared at a time and kept in a covered glass jar until needed. The gum tragacanth is bought in small quantities—five cents’ worth at a time. Cover this amount with one pint of water and let it stand over night. In the morning, strain it through a fine sieve and put it away in a glass jar till it is needed.

If possible, very large pieces of pottery should be fired before they are glazed—in the biscuit, as it is called. Small and medium-sized pieces may be glazed on the green clay—the term by which unbaked clay is known among potters. The piece should have dried thoroughly for several days, until it is light gray in colour, and what is known as bone dry. It is decorated, if decoration is required, and then glazed. All vessels that are used to hold or measure glaze should first be dipped in water to prevent waste from the glaze clinging to them. The hands should be washed thoroughly after working with glazes, as some of the materials used are poisonous.

For the inside of most pieces a transparent glaze is used, whether the outer glaze is to be transparent or opaque.

All of the odds and ends of transparent glaze, no matter what the colour, that are left after each glazing, are poured into a large bowl or other vessel which is kept for the purpose. The mixture of all colours in this combination of glazes makes a neutral tint which harmonizes most agreeably with the outer glaze, whatever its colour.

It is applied as follows:

_To Glaze the Inside of a Piece of Pottery_

Take a small cupful of transparent glaze, and, holding the piece of pottery over the large vessel containing the liquid, pour the cupful of glaze into it, rolling it around the inside of the piece quickly but carefully, so as to have it cover the entire inner surface. Then turn the piece deftly and suddenly bottom up, so as to empty it into the large vessel without letting any of it drip over onto the outside of the piece. Should this happen, by accident, rub it quickly off with the fingers.

_The Outside Glaze and How to Apply It_

The most satisfactory finish for the outside of fine pottery is a mat or opaque glaze in any soft dull shade of green, brown, blue, yellow, or the red obtainable with red oxide of iron. For an art pottery, composed of fire-and blue-clay, which requires a strong heat, the following glaze is applicable:

_Gray-Green Mat-Glaze_

=Mix=

1 tablespoonful of soft glaze,

½ tablespoonful of stannifère, and

½ tablespoonful of China clay,

together on a stone or glass slab with a palette knife, adding ¼ teaspoonful of gum-arabic and enough water to make the consistency a little thicker than thick cream. About ¼ teaspoonful of carbonate of copper added to this mixture (and ground thoroughly into it with the palette knife) will make a light gray-green. For deeper shades increase the quantity slightly. The tint appears much lighter than it will when fired; indeed, in the colouring of glazes, as in painting on china, the worker needs a great deal of faith, for until the magic of the kiln brings out the colours one would never guess what they were to be.

In applying the glaze, place your piece bottom up on a table, or other flat surface. Dip a soft, flat paint-brush into the bowl of glaze, and beginning with the bottom, paint it on in short strokes in every direction—what an artist would call cross-hatching—and overlapping slightly like the shingles on a house. The bottom receives but one coat of glaze, as it is liable to stick in firing and be uneven if it has more than one. Next start at the sides, near the bottom (as the piece stands upside down), and paint down for an inch or two all around. The piece is then set right side up, providing, of course, that the bottom is dry, which it will be undoubtedly, and the rest of it is glazed. From time to time stir up the glaze from the bottom, that it may be thoroughly mixed.

The edge especially should be carefully covered, and the outer glaze may even be brought over inside the piece a little. Before applying the second and third coats (for the piece receives three) two teaspoonfuls of gum tragacanth, well mixed according to directions, are added to the glaze. In putting on the second coat, the piece is again placed upside down on the table, and beginning where the sides join the bottom, the glaze is applied as before. The sides and top edge only are glazed this time. The third coat, which is put on when the second is dry, is begun at the top edge, covering it well, and is ended gradually and unevenly half way down the sides.

_Pale-green Mat-Glaze_

To make a lighter shade of green, with just a hint of yellow to soften it, add to half the quantity of the glaze first mixed as much again of the uncoloured glaze and a slight sprinkling of yellow.

It will be seen from these directions how much the colouring of glazes is like the blending of pigments for a picture. It is a delightful field for experiment, and the element of chance is supplied by the kiln, which often does unexpected and interesting things to one’s colours and glazes—leaving here, a touch of brown about the rim to relieve an expanse of green; there, a metallic tinge almost like lustre; and again the biscuit peeps through the glaze, giving a warmer tone to the edge of a decoration.

In mixing the glaze for a piece that has been fired in the biscuit, make it a little thicker than that for use on the green clay—about the consistency of whipped cream. It is applied somewhat differently, too. A potter would tell you to "rag it on"—that is, put it on with firm, short strokes, using the ends of the hairs of the brush instead of the flat part. Let each coat dry well before the next is applied. Three or four coats will be necessary, except for the bottom, which receives but one. After the first coat, gum tragacanth is added, as in glazing on green clay. The fourth coat need not entirely cover the piece, but the top should be carefully glazed. If for any reason the pottery has to be re-fired, it should be reglazed, but two coats only will be necessary, and the glaze need not be quite so heavy as that used on the biscuit. Do not be discouraged if your pieces need a second or even a third firing, for often the most beautiful results are obtained by re-firing. Quality—that combination of richness, and depth of colour and texture—rarely comes with the first firing.

Pottery for Beauty and Use