PART I

Much of the fascination of pottery making centers in the glaze. At one time a great deal of mystery appeared to surround the composition and use of glazes, but if one will take the trouble to learn, much of this may be dispelled. Some knowledge of chemistry is desirable if an understanding of the theory of glaze-making is to be acquired, but a good deal may be learned even without this knowledge. Only such simple instruction as can be assimilated by ordinary intelligence will be attempted here, as an exhaustive treatment of the subject would be long and tedious.

It is possible to purchase glazes ready for use[J] but the true craftsman will not be satisfied until he can prepare his own.

[J] The Roessler & Hasslacher Chemical Company, 709 6th Avenue, New York City, manufacture glazes according to the recipes of the author, and also chemicals for use in the laboratory.

Glazes[K] belong to a class of chemical compounds known as silicates; that is, they have silica as the characteristic ingredient. Clear glazes are compound silicates of lead, zinc, lime, potassium, sodium, aluminum and boron. Matt glazes are characterized by certain of these ingredients being present in excess; and stanniferous or tin glazes are, as the name implies, rendered opaque by the use of oxide of tin.

[K] It is admitted that glazes are not chemical combinations but solid solutions, but the principle is more easily understood when the analogy of chemical action is adopted.

The commonest type of glaze is that which is made from ready prepared, commercial substances. These are called raw glazes as being made from raw materials or materials which need no preparation.

It is possible to mix a glaze in a druggist's mortar by hand, using fine sieves, but if the best results are to be secured, a small mill must be used for grinding. The best form of mill is the ball mill or jar mill. This consists of a porcelain jar which is set in a frame and made to revolve upon its axis in a horizontal position. It is about half filled with porcelain balls and these as they roll against each other perform the grinding. These mills may be purchased ready for use, either as a single jar to be worked by hand or a battery of two or more revolved by power.[L]

[L] Paul O. Abbé, 30 Broad Street, New York City.

A good pair of scales is a necessity and it will be found convenient to use metric weights which need no calculation into pounds and ounces. Suspended scales are not as easy to use as the form known as counter scales or balances. They should have movable pans which are usually nickel plated. Upon these the materials can be placed direct without the use of pieces of paper, which are always troublesome and inaccurate. There should be a graduated bar on the front for the adjustment of weights of five grams and under. This avoids the use of small weights which are always being mislaid and lost. Dealers in chemical supplies keep these scales in stock and the cost is about eight dollars. A set of weights must also be procured from one hundred grams to five grams inclusive. These need not be of the accurate adjustment which are used in analysis. A good inexpensive grade is sufficient.

The ingredients for glazes are given in the following list:

Commercial Chemical Symbol or Equivalent Name Name Formula Weight

White Lead Lead Carbonate Pb(OH)_{2}2PbCO_{3} 258

Zinc Oxide Zinc Oxide ZnO 81

Soda Ash Sodium Carbonate Na_{2}CO_{3} 106

Niter Potassium Nitrate KNO_{3} 202

Whiting Calcium Carbonate CaCO_{3} 100 (Carbonate of Lime)

Feldspar Orthoclase K_{2}O,Al_{2}O_{3},6SiO_{2} 557

Kaolin Aluminum Silicate Al_{2}O_{3},2SiO_{2},2H_{2}O 258 or China Clay

Flint Silica SiO_{2} 60

Borax Sodium di Borate Na_{2}B_{4}O_{7}10H_{2}O 382

Boric Acid Boric Acid B_{2}O_{3}3H_{2}O 124

For coloring, the following metallic oxides are used:

Color Chemical Symbol or Equivalent Name Formula Weight

Blue Cobalt Oxide CoO 80

Blue and Green Copper Oxide CuO 79

Gray and Brown Nickel Oxide NiO 75

Brown and Yellow Iron Oxide Fe_{2}O_{3} 160

Brown Manganese Carbonate MnCO_{3} 115

Under-glaze colors may also be used for coloring glazes, the color being ground with the glaze batch.

It is not absolutely necessary to commit the formula and equivalent weight to memory. They will soon be remembered as use becomes second nature.

A glaze is usually expressed as the chemical formula. In this there are three divisions given, each of which expresses a distinct function. On the left hand are the bases, the foundation of the glaze. These indicate the type, such as lead glaze, a lime glaze, an alkaline glaze, etc. All glazes being silicates, this is the usual way of distinguishing them. In the center are the alumina and boron oxide. These regulate the behavior of the glaze in the fire. They make it viscous or sluggish as it melts and prevent a too rapid flow. The alumina is infusible, the boron is fusible, but boron cannot be used in a raw glaze for reasons to be presently explained. At the right stands the silica, the dominating factor with which all the other ingredients combine, and which controls the behavior of the whole as regards the fitting of the glaze to the body.

The very simplest form of glaze is a bisilicate of lead, represented by the formula PbO, SiO_{2}, or one equivalent of lead oxide and one of silica. The term "equivalent" means that the mixture is calculated, not upon the actual weight of a substance but upon its equivalent or unit weight. Thus the equivalent weight of lead oxide, PbO, being 222, in order to produce the formula in actual weight 222 grams or pounds must be weighed out. It does not matter what weights are used so long as they are the same for all.

In like manner the equivalent weight of silica is 60 and as flint is pure silica, the formula PbO, SiO_{2} would be produced by weighing--

Litharge or Lead Oxide 222 parts Flint or Silica 60 parts

Litharge is not, however, a convenient substance to use. It is very heavy and does not mix well in water. The most usual substance for the introduction of lead oxide is white lead. This is not lead oxide but it changes to lead oxide when burned. White lead bears the formula Pb(OH)_{2}, 2PbCO_{3}, which, being dissected is found to be 3PbO, H_{2}O, 2CO_{2}. H_{2}O is water and CO_{2} carbonic acid, both of which pass off in burning. Both, however, are weighed when the white lead is put on the scales and therefore the equivalent weight of white lead is 258 and not 222.

The mixture for practical purposes then would be--

White Lead 258 parts Flint 60 parts

Which, when ground and spread upon the ware would be a very fusible glaze of a yellowish tone.

This was spoken of as a bisilicate of lead because the measure of the silica, also called the acidity of a glaze, is calculated upon the oxygen contained in the base and the silica respectively. PbO contains one molecule of oxygen, SiO_{2} contains two. Hence the relationship of the oxygen in the base to the oxygen in the silica is as one to two. This is called simply the "oxygen ratio" and is of great importance in determining the behavior of a glaze. While this simple bisilicate of lead will be a glaze under certain conditions it is found to possess two faults. 1. It is too fluid under fire. The glaze will run down a vertical surface and leave the upper edge of the piece bare. 2. If subjected to a long slow fire it will lose its gloss and become devitrified. This devitrification is often seen in commercial work and appears as a dull scum in patches and around the edges of the ware. It is, in fact, a crystallization of the silica which separates out, as salt does from an evaporated brine. Both these faults may be corrected by the addition of a little alumina to the glaze. A whole equivalent of alumina would be too much, in fact it is found in practice that .2 equivalent is sufficient for most lowfire glazes. In order to maintain the oxygen ratio and to keep the glaze as a bisilicate the silica content must be raised. Alumina contains three molecules of oxygen so that the total amount of alumina is multiplied by three and the silica brought to the equal point thus:

PbO, .2Al_{2}O_{3}, 1.6SiO_{2}

The amount of silica required in any bisilicate glaze may be found by the following equation:

SiO_{2} = 2(3Al_{2}O_{3} + 1)/2

Thus if the alumina content were .25 equivalent this would be expressed:

SiO_{2} = 2(.75 + 1)/2

Or--

SiO_{2} = 3.50/2 = 1.75 equivalent

Now in order to produce this as a mixture it would be possible to introduce the alumina in the pure state, but pure alumina is expensive and clay which contains alumina is cheap so that clay is generally used to supply the alumina. Clay, however, contains silica as well, and therefore allowance must be made for this. On referring to the formula for kaolin, the purest form of clay, Al_{2}O_{3}, 2SiO_{2}, 2H_{2}O, it will be seen that there is twice as much silica present in equivalence as there is alumina and therefore .2 kaolin will contain .2Al_{2}O_{3} and 4SiO_{2}. Subtracting, then, the 4SiO_{2} from the 1.6SiO_{2} needed there will be 1.2 left to be supplied in the form of flint. The mixture therefore is--

White Lead 1.0 × 258 = 258 Kaolin .2 × 258 = 51.6 Flint 1.2 × 60 = 72

This is a glaze of the same character as that first given except that it no longer flows unduly from the higher places nor will it devitrify in a long-continued fire. The alumina will have counteracted both these evils.

A glaze with only lead oxide as the base is not, however, desirable for general use. The color is yellowish and the lead oxide is apt to destroy the hue of any colors which are used with it. The available bases may be classified under three heads. 1. The metallic oxides, lead and zinc oxides. 2. The alkaline earths, the oxides of calcium and barium. 3. The alkalies, potash and soda. Barium oxide is not often used and soda cannot be used in raw glazes because there is no convenient substance which contains it. As glazes are always ground in water only insoluble ingredients can be employed without preparation. Potash is found in feldspar which is insoluble and while there is a so-called soda feldspar it can rarely be obtained of sufficient purity.

In arranging the bases with which to compose a glaze it is desirable to use one at least from each class, but it must be borne in mind that however many bases are introduced the total must always be unity. This unit is, for the sake of brevity, described as RO. For example the following groups may be set forth:

1. PbO Lead Oxide .7 CaO Calcium Oxide .3 --- RO 1.0

2. PbO .6 CaO .4 --- RO 1.0

3. PbO Lead Oxide .5 ZnO Zinc Oxide .2 CaO Calcium Oxide .3 --- RO 1.0

4. PbO .6 ZnO .1 CaO .3 --- RO 1.0

5. PbO Lead Oxide .6 CaO Calcium Oxide .3 K_{2}O Potassium Oxide .1 --- RO 1.0

6. PbO .50 CaO .35 K_{2}O .15 ---- RO 1.00

7. PbO Lead Oxide .45 ZnO Zinc Oxide .10 CaO Calcium Oxide .30 K_{2}O Potassium Oxide .15 ---- RO 1.00

8. PbO .35 ZnO .15 CaO .35 K_{2}O .15 ---- RO 1.00

The reason for the unit rule is that if one formula is to be compared with another there must be a uniform basis upon which to work and, furthermore, it makes no difference whether the silica combines with one, two, three, or four bases, the chemical action is the same and, so long as the sum of the bases is kept at unity, the same amount of silica will be required.

If two glazes be taken as an illustration this will be made clear:

PbO .6 } CaO .4 } --- } Al_{2}O_{3} .2 SiO_{2} 1.6 1.0 }

PbO .46 } ZnO .12 } CaO .28 } K_{2}O .14 } ---- } Al_{2}O_{3} .2 SiO_{2} 1.6 1.00 }

Both of these formulae are bisilicates and each being properly fired, will stand, without crazing, on the same body.

The use of the formula is to give an insight into the composition of the melted glaze. It takes no account of volatile ingredients or losses in the fire but for this very reason it must be translated into the substances to be weighed before use can be made of it.

Of the ingredients given on pages 142, 143, some contain but one item of the formula, others contain several, as in the case of kaolin already cited. Feldspar, of the variety known as potash feldspar and named by mineralogists, "orthoclase," is a very useful ingredient in raw glazes, being, in fact, almost the only source of potash. The formula, page 142, shows that a molecule or equivalent of feldspar contains one molecule of potash K_{2}O, one of alumina Al_{2}O_{3}, and six of silica SiO_{2}. This fact is taken into account in calculating the mixture or batch weight.

Base No. 5 (page 148), is as follows:

PbO .6 CaO .3 K_{2}O .1 ---- 1.0

And this made up into a bisilicate glaze would be:

PbO .6 } CaO .3 } K_{2}O .1 } ---- } Al_{2}O_{3} .2 SiO_{2} 1.6 1.0 }

These items are extended in a horizontal line, a space being left on one side for the list of ingredients.

PbO CaO K_{2}O Al_{2}O_{3} SiO_{2}

.6 .3 .1 .2 1.6 Addition .6 White Lead .6 ---------------------------------------- Subtraction .0 .3 .1 .2 1.6 Addition .3 Whiting .3 ---------------------------------------- Subtraction .0 .1 .2 1.6 Addition .1 .1 .6 Feldspar .1 ---------------------------------------- Subtraction .0 .1 1.0 Addition .1 .2 Kaolin .1 ---------------------------------------- Subtraction .0 .8 Addition .8 Flint .8 ---------------------------------------- Subtraction .0

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.

White Lead .6 × 258 = 154.8 parts by weight Whiting (calcium carbonate) .3 × 100 = 30.0 " " " Feldspar .1 × 557 = 55.7 " " " Kaolin .1 × 258 = 25.8 " " " Flint .8 × 60 = 48.0 " " " ----- 314.3 Batch of Glaze

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 glaze as 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.