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:

SiO_{2} = 3(3Al_{2}O_{3} + 1)/4

Now if the alumina content be placed at .35 equivalent this would work out:

SiO_{2} = 3(1.05 + 1)/4

Or:

SiO_{2} = 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, Al_{2}O_{3} .35, SiO_{2} 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 K_{2}O 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 } K_{2}O .15 } ---- } Al_{2}O_{3} .35 SiO_{2} 1.54 RO 1.00 }

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

.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.