Chapter XV Glazes and Glazing
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 useJ but the true craftsman will not be satisfied until he can prepare his own.
GlazesK 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.
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
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 Name | Chemical Name | Symbol or Formula | Equivalent Weight |
| White Lead | Lead Carbonate | Pb(OH)2 2PbCO3 | 258 |
| Zinc Oxide | Zinc Oxide | ZnO | 81 |
| Soda Ash | Sodium Carbonate | Na2CO3 | 106 |
| Niter | Potassium Nitrate | KNO3 | 202 |
| Whiting | Calcium Carbonate (Carbonate of Lime) | CaCO3 | 100 |
| Feldspar | Orthoclase | K2O, Al2O3, 6SiO2 | 557 |
| Kaolin or China Clay | Aluminum Silicate | Al2O3, 2SiO2, 2H2O | 258 |
| Flint | Silica | SiO2 | 60 |
| Borax | Sodium di Borate | Na2B4O710H2O | 382 |
| Boric Acid | Boric Acid | B2O33H2O | 124 |
For coloring, the following metallic oxides are used:
| Color | Chemical Name | Symbol or Formula | Equivalent 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 | Fe2O3 | 160 |
| Brown | Manganese Carbonate | MnCO3 | 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, SiO2, 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, SiO2 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, 2PbCO3, which, being dissected is found to be 3PbO, H2O, 2CO2. H2O is water and CO2 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, SiO2 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,.2Al2O3, 1.6SiO2
The amount of silica required in any bisilicate glaze may be found by the following equation:
SiO2 = 2(3Al2O3 + 1) / 2
Thus if the alumina content were.25 equivalent this would be expressed:
SiO2 = 2(.75 + 1) / 2
Or—
SiO2 = 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, Al2O3, 2SiO2, 2H2O, it will be seen that there is twice as much silica present in equivalence as there is alumina and therefore.2 kaolin will contain.2Al2O3 and 4SiO2. Subtracting, then, the 4SiO2 from the 1.6SiO2 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 | |
| K2O Potassium Oxide | .1 | |
| —— | ||
| RO | 1.0 | |
| 6. | PbO | .50 |
| CaO | .35 | |
| K2O | .15 | |
| —— | ||
| RO | 1.00 | |
| 7. | PbO Lead Oxide | .45 |
| ZnO Zinc Oxide | .10 | |
| CaO Calcium Oxide | .30 | |
| K2O Potassium Oxide | .15 | |
| —— | ||
| RO | 1.00 | |
| 8. | PbO | .35 |
| ZnO | .15 | |
| CaO | .35 | |
| K2O | .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 | } | Al2O3 | .2 | SiO2 | 1.6 |
| —— | } | |||||
| 1.0 | } | |||||
| PbO | .46 | } | ||||
| ZnO | .12 | } | ||||
| CaO | .28 | } | Al2O3 | .2 | SiO2 | 1.6 |
| K2O | .14 | } | ||||
| —— | } | |||||
| 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 K2O, one of alumina Al2O3, and six of silica SiO2. 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 |
| K2O | .1 |
| —— | |
| 1.0 |
And this made up into a bisilicate glaze would be:
| PbO | .6 | } | ||||
| CaO | .3 | } | Al2O3 | .2 | SiO2 | 1.6 |
| K2O | .1 | } | ||||
| —— | } | |||||
| 1.0 | } |
These items are extended in a horizontal line, a space being left on one side for the list of ingredients.
| PbO | CaO | K2O | Al2O3 | SiO2 | |||
| .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.
Part II — Matt Glazes
The texture of the matt glaze is always pleasing and the artist is not content unless at least some of his work can be finished in this way.
Matt glazes are not underfired glazes nor are they deadened by acid or sand blast. They are produced in two ways. First, by an excess of alumina which is believed to cause the formation of certain compounds in the glaze, and, second, by an excess of silica which produces a devitrified surface. It was mentioned in the last chapter that a glaze free from alumina will devitrify or become dull. This is undesirable when a glaze is intended to be brilliant but it may be controlled and turned to advantage in the production of a certain type of matt. The successful preparation of this silica matt is extremely difficult. In fact, in the studio kiln it is almost impossible. These small kilns are apt to cool with great rapidity whereas, in order to produce the silica matt the kiln must be cooled very slowly, hours and even days of cooling being sometimes necessary.
The alumina matt is more simple and its texture is quite satisfactory, being, in the opinion of some, the more pleasing of the two.
It was mentioned in the last chapter that the best bright glazes for low temperature work are bisilicates, having an oxygen ratio of 1:2. The alumina matt has an oxygen ratio of about 3:4. This is secured in the following manner. The RO content may consist of any of the bases used in bright glazes, the proportion of each being adjusted in accordance with the desired point of fusion. The alumina content is rather higher than in a bright glaze and should not fall much below.3 equivalent,.35 equivalent is even better. The silica is adjusted in accordance with the following equation:
SiO2 = 3(3Al2O3 + 1) / 4
Now if the alumina content be placed at.35 equivalent this would work out:
SiO2 = 3(1.05 + 1) / 4
Or:
SiO2 = 6.15 / 4 = 1.5375
But as such a complete fraction is not necessary it may be stated as 1.54 equivalent. The formula would therefore be:
RO, Al2O3.35, SiO2 1.54
The RO content should not be too fusible. Lead oxide is desirable up to about.5 equivalent and it is an advantage to use feldspar so that K2O may be introduced. Calcium oxide is also good but zinc oxide must be used sparingly as it is apt to suffer if overfired. The high content of alumina necessitates a good deal of clay and as this, if used raw, would make the glaze too plastic and cause it to crack, it is best to calcine a part of it, thus removing the combined water and changing the equivalent weight from 258 to 222. The calculation will then proceed as in the case of a bright glaze.
| PbO | .50 | } | ||||
| CaO | .35 | } | Al2O3 | .35 | SiO2 | 1.54 |
| K2O | .15 | } | ||||
| —— | } | |||||
| 1.0 | } |
| PbO | CaO | K2O | Al2O3 | SiO2 | |||
| .50 | .35 | .15 | .35 | 1.54 | |||
| Addition | .50 | White Lead | .50 × 258 = 129 | ||||
| Subtraction | .0 | .35 | .15 | .35 | 1.54 | ||
| Addition | .35 | Whiting | .35 × 100 = 35 | ||||
| Subtraction | .0 | .15 | .35 | 1.54 | |||
| Addition | .15 | .15 | .90 | Feldspar | .15 × 557 = 83 | ||
| Subtraction | .0 | .20 | .64 | ||||
| Addition | .15 | .30 | Calcined Kaolin | .15 × 222 = 33 | |||
| Subtraction | .05 | .34 | |||||
| Addition | .05 | .10 | Kaolin | .05 × 258 = 13 | |||
| Subtraction | .0 | .24 | |||||
| Addition | .24 | Flint | .24 × 60 = 14 | ||||
| Subtraction | .0 |
The mix, therefore, is:
| White Lead | 129 grams |
| Whiting | 35 ″ |
| Feldspar | 83 ″ |
| Calcined Kaolin | 33 ″ |
| Kaolin | 13 ″ |
| Flint | 14 ″ |
This will give a silky matt glaze, nearly white, maturing at about cone 1. If a lower fusing point is desired the white lead may be increased at the expense of the whiting or if the glaze prove too fusible the reverse will correct it. The flint may be omitted without damage.
The grinding of a matt glaze is of great importance. It is better to have it too coarse than too fine. Grinding for one hour on the ball mill should be ample and if the glaze be then strained through 120 mesh lawn all coarse particles will be arrested. A glaze that is too fine will crack and peel off or will curl up in the kiln.
More than half the success of matt glazes lies in the using. It is necessary that the coating of glaze be very thick or the true texture will not be developed. When the glaze is taken from the mill plenty of water may be used in order to wash the apparatus clean and to save all the glaze. This is set aside in a deep bowl to settle. After some hours the clear water is carefully drawn off with a siphon.
Half an ounce of gum tragacanth is put to soak in a quart of clean water. After twelve hours the gum will have swollen to a jelly-like mass. This is now worked vigorously with a Dover egg-beater or in a Christy mixer and again set aside. After another twelve hours the operation is repeated and the solution is a clear syrup of the consistency of thin molasses. A drop or two of carbolic acid or other germicide should be added to prevent decomposition. This mucilage should be prepared in advance. To the glaze batch from which the water has been removed a tablespoonful of the mucilage is added. If more of the glaze than the single batch has been weighed out then more mucilage will be necessary. The mixture is to be stirred very thoroughly and it will be found to thicken under the hand. It must be very much thicker than the bright glaze. In fact, the thicker it is the better, only that it must flow sufficiently so that the pottery may be covered with a smooth coating, avoiding lumps. Matt glazes do not correct their own faults in the kiln as bright glazes do. Every finger mark will show and, consequently, the glazing must be done with the greatest care. The process is the same as that described for bright glazes, except that as much glaze as possible is left on the ware. No more shaking should be done than will suffice to secure a smooth coating. It is well to place the pieces upside down to dry.
For the inside of the pieces a matt glaze may be used or a thin coat of clear glaze at the pleasure of the worker. If the latter, care must be taken that none of the inside glaze is allowed to run over the edge.
In firing, the pottery is sometimes placed on a stilt but this is not absolutely necessary. For a support a flat piece of burned clay may be used and this should be covered with an infusible wash to prevent any possibility of sticking. Equal parts of kaolin and flint make a good wash. The wash is worked up with water into a slip and applied with an ordinary brush.
Part III — Fritted Glazes
Fritted glazes, like raw glazes, are clear and brilliant and for most purposes the latter will suffice. Since, however, the aim of this work is to give as complete information as may be the fritted glaze will not be omitted.
A fritt is a melt or compounded glass and the purpose of it is to permit the use of certain ingredients which are not available in the raw state. As glazes are ground in water it is essential that the substances used be insoluble. This condition would prohibit advantage being taken of borax, boric acid, and soda ash, if it were not for the possibility of rendering these insoluble by the operation of fritting.
The following is an example of a fritted glaze:
| PbO Lead Oxide | .30 | } | |||
| ZnO Zinc Oxide | .15 | } | Al2O3 Alumina.15 | } | |
| CaO Lime | .25 | } | B2O3 Boric Acid.40 | } | SiO2 Silica 2.65 |
| Na2O Soda | .20 | } | |||
| K2O Potash | .10 | } |
This will be produced in accordance with the usual calculation by the mix:
| White Lead | .3 × 258 | = | 77 |
| Zinc Oxide | .15 × 81 | = | 12 |
| Whiting | .25 × 100 | = | 25 |
| Borax | .20 × 382 | = | 76 |
| Feldspar | .10 × 557 | = | 56 |
| Kaolin | .05 × 258 | = | 13 |
| Flint | 1.95 × 60 | = | 117 |
The borax contains the required amount of both soda and boric acid and the potash is supplied by the feldspar. Borax, being soluble, must be melted with certain other ingredients into an insoluble glass, thus:
| Fritt: | ||
| Borax | 76 x 2 = | 152 |
| Whiting | 25 x 2 = | 50 |
| Feldspar | 30 x 2 = | 60 |
| Flint | 50 x 2 = | 100 |
| —— | ||
| 362 |
These ingredients are weighed out in double quantity to guard against loss in melting and are fused either in the kiln or in a special furnace. A good fritting furnace is the No. 15, made by the Buffalo Dental Manufacturing Company. The charge is put into a plumbago crucible and when melted is poured out into water. This breaks up the fritt and renders it easy to grind. A similar crucible may be used in the kiln but as the fritt becomes very hard when cold and a crucible must be broken each time, the furnace method is better. If the fritt as given prove too sluggish to pour freely, the feldspar may be omitted, being added, of course, to the glaze mix. The melted weight of the fritt must now be calculated.
Borax contains in each equivalent 180 parts water. Whiting contains in each equivalent 44 parts carbonic acid. Both water and carbonic acid pass off in the melting, thus the 76 parts of borax will be reduced in weight to 40 parts, and the 25 parts of whiting will be reduced to 14 parts. Spar and flint undergo no loss. The fritt after melting will therefore be:
| Borax | 40 |
| Whiting | 14 |
| Spar | 30 |
| Flint | 50 |
| —— | |
| 134 |
And the final mix for the glaze will be:
| Fritt | 134 parts |
| White Lead | 77 ″ |
| Zinc Oxide | 12 ″ |
| Feldspar | 26 ″ |
| Kaolin | 13 ″ |
| Flint | 67 ″ |
This is ground on the mill as already directed and is ready for use.
Fritted glazes are better than raw glazes for certain classes of ware. They are usually whiter and less easily scratched. They are, moreover, better for use with underglaze colors and are, as a rule, more easily melted. It is never necessary to make a fritt for the preparation of matt glazes.
Part IV — Recipes
While the purpose of this work is not so much to put ready-made materials into the hands of the craftsman as to enable him to work out his own plans, it is recognized that there are some workers who lack the training and even the patience to do this. For these, the following recipes are given, but with the proviso that no recipe can be regarded as perfect for all conditions. Just as an untrained cook can spoil a dinner even when surrounded by cookery books, so the best of recipes will fail when unskillfully treated. One must be prepared to recognize the faults which are sure to develop and to correct them in an intelligent manner. The previous chapters should therefore be carefully studied, not alone for the information but because “the joy of the working” depends greatly upon the knowledge one has of the operations involved and a modest confidence in one’s own powers.
1. Bright raw glaze.
| Cone.06 | ||||||
| PbO | .60 | } | ||||
| CaO | .25 | } | Al2O3 | .15 | SiO2 | 1.45 |
| K2O | .15 | } | ||||
| Mix: | ||||||
| White Lead | 155 | |||||
| Whiting | 25 | |||||
| Feldspar | 55.7 | |||||
| Kaolin | 13 | |||||
| Flint | 45 | |||||
| Grind, with one-half pint of water, for one hour. | ||||||
2. Bright raw glaze.
| Cone 1 | |||||
| PbO | .45 | } | |||
| ZnO | .15 | } | Al2O3.20 | SiO2 | 1.60 |
| CaO | .25 | } | |||
| K2O | .15 | } |
| Mix: | |
| White Lead | 116 |
| Whiting | 25 |
| Zinc Oxide | 12 |
| Feldspar | 83 |
| Kaolin | 13 |
| Flint | 36 |
3. Bright fritted glaze.
| Cone.02 | |||||
| PbO | .25 | } | |||
| ZnO | .15 | } | Al2O3.15 | } | |
| CaO | .30 | } | B2O3.30 | } | SiO2 2.35 |
| Na2O | .20 | } | |||
| K2O | .10 | } |
| Mix: | |||
| Fritt | Glaze | ||
| Borax | 114 | Fritt | 117 |
| Whiting | 60 | White Lead | 64 |
| Soda Ash | 10 | Zinc Oxide | 12 |
| Spar | 56 | Spar | 28 |
| Flint | 78 | Kaolin | 13 |
| Flint | 60 | ||
| Grind as before. |
4. Matt glaze.
| Cone.02 | ||||
| PbO | .50 | } | ||
| CaO | .30 | } | Al2O3.34 | SiO2 1.48 |
| K2O | .20 | } |
| Mix: | |
| White Lead | 129 |
| Whiting | 30 |
| Spar | 111 |
| Calcined Kaolin | 22 |
| Kaolin | 11 |
5. Matt glaze.
| Cone 7 | ||||
| CaO | .75 | } | Al2O3.55 | SiO2 2.10 |
| K2O | .25 | } |
| Mix: | |
| Feldspar | 139 |
| Whiting | 75 |
| Calcined Kaolin | 55 |
| Kaolin | 13 |
For colored glazes add to any of the above:
| Blue: | ||
| Cobalt Oxide | 3 parts | |
| Slate blue: | ||
| Cobalt Oxide | 3 parts | |
| Nickel Oxide | 1 part | |
| Warm blue: | ||
| Cobalt Oxide | 2 parts | |
| Iron Oxide | 1 part | |
| Green: | ||
| Copper Oxide | 8 parts | |
| Blue green: | ||
| Copper Oxide | 8 parts | |
| Cobalt Oxide | 1 part | |
| Cool green: | ||
| Copper Oxide | 8 parts | |
| Cobalt Oxide | 1 part | |
| Nickel Oxide | 2 parts | |
| Olive green: | ||
| Copper Oxide | 6 parts | |
| Iron Oxide | 4 parts | |
| Orange brown: | ||
| Iron Oxide | 8 parts | |
| Red brown: | ||
| Iron Oxide | 8 parts | |
| Chrome Oxide | 1 part | |
| Zinc Oxide | 3 parts | |
| Yellow: | ||
| Uranium Oxide | 3 parts | |
The coloring oxides should be weighed out and ground with the glaze. Any of the colors may be mixed together in order to modify the hue obtained or the amount of each coloring oxide may be varied to give a stronger or weaker value.
Opaque tin enamel.
| Cone.02 | ||||||
| PbO | .40 | } | ||||
| CaO | .25 | } | { | SiO2 | 1.75 | |
| K2O | .20 | } | Al2O3.25 | { | SnO2.30 | |
| ZnO | .15 | } |
| Mixture: | ||
| White Lead | 103 | |
| Whiting | 25 | |
| Feldspar | 111 | |
| Zinc Oxide | 12 | |
| Kaolin | 13 | |
| Flint | 27 | |
| Tin Oxide | 45 | |
| Grind, with one-half pint of water, for 45 minutes. | ||
Part V — The Defects of Glazes
While it may chance that body and glaze and fire are so adjusted that faults do not develop, this state of things is rare. Besides, it is always possible that an occasional trouble may arise, hence it will be well to recount a few of the commonest defects with the method of cure. A cure is not necessarily specific. There may be a complication of causes but the remedy indicates the line along which relief will be found.
1. Crazing. Fine cracks appear in the glaze but do not penetrate the body. There are many causes. The body may be underfired or overfired. In the former case the crazing does not always appear at once and it grows worse upon standing. In the latter case the glaze is found to be crazed when taken from the kiln and it does not extend even after long standing. The glaze may be underfired. In this case the lines of the crack are broken and irregular, one often changing its direction without meeting another crack. In all these cases the remedy is obvious.
Crazing also occurs when both body and glaze are correctly fired but there is an inherent disagreement in expansion. In such a case a little flint added either to the body or to the glaze will tend to cure the trouble but it must be remembered that the addition of flint to the glaze is apt to render it less fusible and therefore while one craze may be cured another may be caused. The addition of flint to the body is the simplest remedy.
2. Shivering or peeling. This is the reverse of crazing and is caused by the glaze being too large for the body. It almost always appears immediately the ware is cooled. The symptoms are that edges or convex surfaces are pushed off and even the ware itself is shattered. The remedy is to decrease the flint in either body or glaze.
3. Blistering. Glazes, both bright and matt, are apt to develop blisters at times. These may be yet unbroken when the kiln is opened or they may have melted down to a small crater, a ring with a depression in the center. The cause of this fault is usually to be found in the body. All clays contain sulphur and when a clay is aged this develops an acid which rises to the surface of the ware when dried and causes a scum. The glaze attacks this sulphate scum and a gas is generated which boils out and causes the blisters. If old clay blisters and new clay does not it may be regarded as certain that this is the cause. A little barium carbonate added to the clay will help to effect a cure. About one per cent. is usually enough. Clay so treated, however, must not be used in plaster molds as the barium attacks the plaster. If the cause be not found in the clay it may exist in the glaze itself. Some glaze ingredients contain impurities in the form of sulphates and these will cause blisters.
4. The glaze flows, leaving bare places. It is too fluid, add a little clay and flint.
5. A matt glaze burns to a bright surface. Matt glazes must be used in a very thick coat. If too thin they will inevitably brighten. The fire may be too high. The fire may be “reducing,” that is, with insufficient air.
6. The glaze crawls or rolls up in lumps. Notice whether the glaze is cracked before burning. If so it will surely crawl. Too fine grinding is usually the cause of this trouble. Too much clay in the glaze may cause it, or a too porous body. A body which is underfired will almost certainly cause the glaze to crawl.
7. Pinholes appear in the glaze when cool. Too rapid cooling is the cause.
Part VI — Alkaline Glazes
The glory of the Persian and Egyptian blue is too alluring for potters to withstand. Though the pursuit of this glory leads one into all kinds of disasters and failures, the avenues of research that it opens add unending fascination to the study. Even one beautiful glowing pot out of twenty or more efforts is a stimulating achievement though it should not be thought that this is the usual proportion.
It is a continual source of astonishment that with a slight variation of glaze formula a positive green will swim into a vibrating blue. The addition or substitution of one substance or another in the glaze mix may be the key to an unexpected transformation and may give the potter a new palette of color.
The clay body has a very positive effect on alkaline glaze both in its composition and its color. This is especially true under a transparent glaze where the effect is considerable since the color of the glaze would be modified by the red or buff clay showing through.
If, therefore, the object of the potter is to obtain a brilliant “Persian” blue, a white clay body must be composed or a white engobe applied over the buff or red clay to hide the color.
The Persians and Egyptians used a coarse, sandy body high in silica and covered the roughness of the clay with a fine white engobe on which they painted their decorations in various colors. The whole was finally covered with the transparent alkaline glaze.
While the effect of colored clay under opaque glaze is less pronounced, it still makes sufficient difference to be considered.
The word engobe is French and refers to a thin coating of clay, also called a slip, laid over a colored body to change the color or over a coarse body to give a finer texture.
The engobe is usually composed of china clay, flint, and feldspar much as a white earthenware body is constituted but with a larger content of flint. Ball clay may also be used but the color is not so white.
The mixture of porcelain given on page forty will make an engobe suitable for many clay bodies. If it should crack on drying more flint should be added.
An engobe must, of course, be put upon the unburned or green clay ware and this should be leather hard, not dry. The body with the engobe may be burned before glazing or the glaze may be put upon the unburned ware and the whole subjected to one fire only.
The ingredients in alkaline glazes are soda-ash, whiting, feldspar, flint and oxide of tin. The following is an example of a fritted glaze:
| Na2O | .60 | } | |
| K2O | .10 | } Al2O3.10 | SiO2 1.30 |
| CaO | .30 | } | |
| Soda Ash | 64 | ||
| Whiting | 30 | ||
| Feldspar | 56 | ||
| Flint | 42 |
The entire batch is fritted and ground in a ball mill with the usual amount of water for fritt grinding, adding a tablespoonful of gum tragacanth mucilage to the batch after it is sieved. The glaze should be the consistency of heavy cream when used.
It is also possible to use an alkaline glaze in the raw or unfritted state. This necessitates grinding by hand in a mortar, but great care must be taken to mix the dry ingredients thoroughly before adding water and to stir the glaze constantly while pouring in the water, otherwise the soda-ash will cake and harden and be very difficult to break up. A batch of glaze can be ground by hand in fifteen or twenty minutes if done vigorously. It is then put through a 120-mesh sieve. The consistency is of importance. If too much water has been added and the glaze has become thin, it cannot be used successfully and should be discarded. Unfritted alkaline glaze does not keep well when moist but the ingredients can be ground dry and kept ready to be moistened as needed.
The following is an example of an unfritted alkaline glaze:
| Na2O | .59 | } | ||
| CaO | .21 | } | Al2O3.20 | SiO2 1.6 |
| K2O | .20 | } | ||
| Soda Ash | 62 | |||
| Whiting | 21 | |||
| Feldspar | 111 | |||
| Flint | 24 |
For color add the following oxides to a batch.
1. Egyptian blue, opaque—from 5 to 8 grams of black oxide of copper—16 grams of oxide of tin.
2. Persian blue, opaque—from 8 to 10 grams of black oxide of copper—16 grams of oxide of tin.
3. Sapphire blue—1 gram black oxide of cobalt.
4. Aubergine—9 grams black oxide of manganese.
The clear glaze without any coloring oxide can be used over any of the colored glazes. This is sometimes necessary when the colored glaze contains such a large proportion of coloring oxide as to show black on the surface.
The application of alkaline glaze is very important. Any of the three methods of pouring, dipping, and brushing can be employed. Brushing seems to give the best results but the glaze must be put on thick, in two or three coats, to give quality.
The firing is interesting and important because of the varied effects it develops from the same formula. The range of temperature is great, varying from cone.05 to 1, developing the alkaline glaze according to the result desired. If the biscuit is soft fired the color will be more intense; if hard fired, the color will be much lighter in value with a high sheen on the surface. An unfritted alkaline glaze burned to.05 develops a soft matt finish.
Where the color of a transparent Persian blue comes out olive green, too little glaze has been used on the piece or the buff of the clay has modified the color. Bubbles mean undeveloped glaze or sulphur in the clay or fuel. Black scum shows an excess of copper in the batch, or reduction in the fire. Sand paper surface proves too low firing or too thin a glaze.
If one desires to reproduce the underglaze Persian decoration the black outlines may be drawn with a black underglaze color mixed with clay. A little mucilage must be added to secure smooth working. The turquoise blue is copper oxide, the dark blue cobalt, and the purple manganese. The oxides must be diluted with white clay and used rather thin. The Rhodian red is a finely ground red burning clay mixed with a little flint. This red must be laid on quite thickly. It will probably be found necessary to fire the painted decoration to about cone.03 before glazing. The glaze may be either quite clear or slightly tinted. Another effect may be produced by using the black outline alone under a peacock blue or turquoise glaze.
A great many modifications and additions to this subject will suggest themselves to the potter as he works, and a continual study of the masterpieces of the Persians in the museums will prove the greatest inspiration.