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.
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.
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 }
} Al_{2}O_{2} Alumina .15 }
CaO Lime .25 } } SiO_{2} Silica 2.65
} B_{2}O_{3} Boric Acid .40 }
Na_{2}O Soda .20 }
K_{2}O 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 Formula
PbO .60 }
CaO .25 } Al_{2}O_{3} .15 SiO_{2} 1.45
K_{2}O .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 Formula
PbO .45 }
ZnO .15 } Al_{2}O_{3} .20 SiO_{2} 1.60
CaO .25 }
K_{2}O .15 }
Mix:
White Lead 116
Whiting 25
Zinc Oxide 12
Feldspar 83
Kaolin 13
Flint 36
3. Bright fritted glaze.
Cone .02 Formula
PbO .25 }
ZnO .15 } Al_{2}O_{3} .15 }
CaO .30 } } SiO_{2} 2.35
Na_{2}O .20 } B_{2}O_{3} .30 }
K_{2}O .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 Formula
PbO .50 }
CaO .30 } Al_{2}O_{3} .34 SiO_{2} 1.48
K_{2}O .20 }
Mix:
White Lead 129
Whiting 30
Spar 111
Calcined Kaolin 22
Kaolin 11
5. Matt glaze.
Cone 7 Formula
CaO .75 } Al_{2}O_{3} .55 SiO_{2} 2.10
K_{2}O .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 Formula
PbO .40 }
CaO .25 } { SiO_{2} 1.75
K_{2}O .20 } Al_{2}O_{3} .25 { SnO_{2} .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:
Na_{2}O .60 }
K_{2}O .10 } Al_{2}O_{3} .10 SiO_{2} 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:
Na_{2}O .59 }
CaO .21 } Al_{2}O_{3} .20 SiO_{2} 1.6
K_{2}O .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.
CHAPTER XVI: DECORATION
The necessity for some kind of decoration upon the clay will always be a
point of difference amongst artists. Some prefer the simple form with a
glaze treatment only, others consider that the surface should be broken
up by design. The question will not be debated here. The aim of this
hand-book is instruction and the individuality of the worker is to be
encouraged. Directions for executing the different treatments do not
imply that these elaborations are advocated. That must be left to the
inspiration of the worker.
Decorations may be applied upon the soft clay by incising, inlaying and
embossing; upon the dry clay or upon the burned pottery in color under
the glaze or with no glaze at all; in the glaze by the use of colors or
colored glazes; or over the glaze with colors and enamels. Each of these
methods possesses special features. Each has its own possibilities and
limitations and these should be mastered by the craftsman.
As in the production of form a well-planned design should be prepared.
The first sketch should be made on paper or on a slab of clay but the
fitting and final arrangement are best made on the piece itself.
Incising consists in the excavation of a shallow trench or trough on the
surface of the clay. The vase or jar having been finished should be kept
in a damp place so that the clay does not dry out completely. The design
may be made in India ink with a brush. A steel tool with a narrow chisel
end is used for cutting and care must be taken that the clay is in such
a condition of moisture as will admit of a clean trench being dug
without any rough or broken edges. The bottom of the trench need not be
very smooth but the edges should be sharp and the lines well defined. At
the same time a mechanical hardness of finish is to be avoided. The
plastic nature of the clay should be kept in mind and every surface,
though decided in character, should be soft and expressive. This result
can be secured by working over the cutting with a moist camel-hair
brush. The work must not be mopped so as to leave a woolly effect, but a
little sympathetic penciling will remove the hard lines of the tool.
There are two possible developments of incised work. The details of the
design may be excavated or the background may be cut out leaving the
drawing in relief.
In modeling embossments the piece should be a little softer than for
incising. It is important that in any clay work attached to a clay body
the same amount of moisture should be present in both parts. This is not
entirely possible in modeling upon forms which have already been shaped,
for if the form be as soft as modeling clay it will not bear to be
handled, while if the clay were as hard as the form it could not be
worked. A compromise is therefore necessary. The vase must be kept as
soft as possible consistent with holding its shape and the clay must be
as stiff as the working will allow.
As little water as possible should be used and the modeling should not
be brought to its full height at once. If the clay be laid on little by
little there is much less chance of cracking. Low relief is sometimes
produced by painting in slip but here even more care is necessary. The
slip should be laid on with a brush in thin coats, each coat being
allowed to stiffen before another is applied and the whole work being
kept moist.
An atomizer with clean water is useful in this regard. The work, being
kept on a whirler or turntable, is sprayed now and then with water and
thus prevented from becoming too hard.
When the slip work has been raised to the desired height the surface is
tooled over so as to remove the brush marks. This is the method which
has been brought to such perfection by the French artists and by them
named _pâte-sur-pâte_.
Modeled work is generally carried out in the same clay as that of which
the form is made and depends upon high relief for its effect. Slip
painting is usually done in a different color and if a light-colored
slip be used upon a dark clay, the latter is partially seen through the
coating in the thinnest places. This fact is made use of to accentuate
the shadow effects.
In using one clay over another great care must be taken to insure that
the fire shrinkage is the same. The white body already given, or indeed,
any light colored clay, may be tinted by the addition of under-glaze
colors. The dry color, if sifted very fine, may be added to the plastic
clay by thorough kneading and wedging but it is better to work up the
clay into a slip and to stir in the color. The tinted slip is then
lawned two or three times and dried out on plaster or used in the slip
state as the case may be.
A trial should be made before any important work is undertaken, both to
see that the color is right and to discover any discrepancy in
shrinkage. If a clay shrinks too much, a little ground flint may be
added. If it shrinks too little, a little ball clay will correct it. The
tint produced by the color is apt to darken in the kiln but the general
hue will be similar to that of the color used.
For some classes of work a native red clay gives admirable results. It
may be lightened by the use of kaolin and flint and darkened by adding
burnt umber. These colors are more satisfactory than greens and blues in
clay because the brown and red tones are natural, the others are
artificial.
If a good buff-burning clay be available, it forms the best possible
foundation for color work. Burnt umber will darken it and red clay may
be mixed with it, always having regard to the matter of shrinkage
already mentioned.
Very pleasing effects may be produced by inlaying one clay with another.
The pattern or design is first cut out as described under incising and
then the second clay is pressed, morsel by morsel, into the excavation.
The surface is cleaned off level with the body of the piece and the
whole may be either polished or glazed.
A plastic clay can be polished when leather hard and the finish will
remain after firing. Any tool of steel, boxwood or ivory will do the
work but a good supply of patience is needed so that the whole surface
may be uniformly treated.
For color decoration upon the pottery, ordinary underglaze colors are
used, either upon the unburned clay or upon the burned ware commonly
called biscuit. For use upon the clay, the colors should be mixed in
water, using a little molasses, sugar, glycerine or gum arabic to make
the color flow easily from the brush. Before burning, a little glaze
should be sprayed over the work with an atomizer. Any ordinary fusible
glaze will do. It is diluted with a good deal of water as only the very
thinnest coat is necessary. The spray should not be held long in one
place or the water will flow and smear the color. If the piece be turned
slowly around the clay will absorb the water as it is applied. If this
spraying be not done the colors will be apt to rub off after burning.
Under-glaze colors are not fusible and hence they come from the fire as
dry powders.
The work on the biscuit is much the same except that turpentine and fat
oil constitute a better working medium. When dry the spray should be
applied as before.
CHAPTER XVII: THE FIRE
Kilns and burning form the pivot upon which the art of the potter turns.
M. Doat has said, "A potter can no more express himself without his kiln
than can a violinist without his violin," and yet there are some who try
to make out by sending their work to some nearby pottery to be burned.
Let it be at once understood that he who finds it impossible to procure
and manage a kiln had best take to some other craft.
Kilns are of two types, open and muffle. In the open kiln the flames
pass through the firing chamber and the ware may be exposed to their
action, as in stoneware and brick; or it may be enclosed in the
fire-clay cases, called saggers, as in the many forms of pottery, dishes
or faience. The muffle kiln is a closed chamber which is surrounded by
flames but which is not entered by them. These kilns are used in the
manufacture of terra cotta and heavy enamel wares, and the portable
kilns made for studio use are of this type.
There are certain advantages to be gained in the use of either type of
kiln but inasmuch as the open kiln involves the use of saggers and as,
moreover, it must be properly constructed of fire-brick by a skilled
mason, it will be best to consider only the portable studio kiln.[M]
[M] These kilns are made in several sizes by the H. J. Caulkins
Company, Detroit, Mich.
It must not be expected that any kiln will give perfect satisfaction.
Neither built kiln nor portable kiln will do this, but either may be
relied upon to do excellent work in the hands of those who will take
trouble. A kiln of the proper size having been purchased, it must be
carefully installed. A good chimney is an absolute necessity and if one
can be built on purpose it will be best. It should be at least
twenty-five feet high with the bottom lined with fire-brick to a height
of six or eight feet. The portable kiln is set on iron legs which raise
it about one foot from the floor. This is not enough for easy work and a
platform of brick or stone, ten inches high, should be prepared. This
will greatly simplify the observation and management of the burners
which are beneath the kiln, and if it should make the inside of the
muffle hard to reach, it is easier to stand on a box to attend to the
kiln than it is to go on one's knees to the burner.
The kiln room should have a cement floor and should be both well drained
and well ventilated. At the window there should be a stout bench where
the work of preparation may be done and at a convenient spot there
should be shelves for stilts, cones, wash, stopping and all the minor
accessories of burning. If there is room for a barrel of oil it will be
a convenience, and if the room be fire-proof the insurance company will
not object.
The kiln having arrived it is mounted on the platform and the
asbestos-lined pipe is securely connected with the chimney. The inside
of the muffle is examined with care to see that no part has been jarred
in transit. The reservoir cans are filled with oil and a slow fire is
started. This should be allowed to burn very gently for an hour or two
in order to thoroughly dry out and season the kiln. It is a good plan to
make up a wash of equal parts of kaolin and flint and to brush this all
over the inside of the kiln. It should not be put on so thick as to
shell off from the walls but at the bottom a good coating may be laid.
This protects the walls of the kiln from the attacks of glaze and will
make them last longer.
In order to fill the kiln economically a number of props and bats must
be provided. Some of these are sent out with the kiln but one is always
needing odd sizes and extra pieces. The props are simply legs of burned
clay; they are of any height desired and should be thick enough to stand
alone. The bats are slabs of burned clay and they rest on the props to
form shelves. The bats must be thick enough to bear the weight of any
pieces which they may be called upon to support, but they need not be
very large as two or more may be used to bridge the width and length of
the kiln. Bats and props are best made of sagger clay to which has been
added about one-third of crushed fire-brick. Broken bats serve well for
this after the first supply has been secured. This crushed burned clay,
called grog, has a very important influence upon wares which have to be
heated again and again. The size used should be about what will pass
through a 16-mesh sieve, and if the dust be sifted out through a 48-mesh
sieve, the resulting ware will be stronger. That is, only the grog which
passes a 16 sieve but lies upon a 48 sieve should be used.
The relative proportions of clay and grog in the mix will depend
somewhat upon the nature of the clay. Three parts of clay to two of grog
by measure will be about right.
The first charging of the kiln should be with pieces of no great
importance. The temperature in different parts must be carefully
ascertained. In order to do this a number of pyrometric cones[N] are
prepared in groups of three.
[N] The pyrometric cones are fusible pyramids for testing heat. They
are made by Prof. Edward Orton, Jr., Columbus, Ohio.
Let us suppose that the work is intended to be carried out at a
temperature of Cone No. 01. The numbers run both ways from this. The
higher or less fusible cones are, 1, 2, 3, 4, etc., up to 36, and the
more fusible numbers are 02, 03, etc., down to 022. If the firing is to
be to Cone 01, numbers 02, 01 and 1 are selected and set upright in a
small strip of soft clay. Eight or ten of these groups of three cones
are to be prepared for the first firing, so as to test the kiln, one
group is placed in each corner, at the bottom, and another in each
corner on a shelf, which is arranged opposite the spy-hole in the door.
In the middle of this, where it can be well seen through the hole, one
of the groups of cones is placed. They must be set so that all three
cones are visible as the kiln is being fired.
The kiln is now filled up on both levels with pieces of pottery. To burn
an empty kiln is not a reliable test. On the first occasion the fire
should be started in the morning because no one can tell just how long
the burn will take. When this time is ascertained it is best to start
the fire so that the kiln will be finished by early evening. The cooling
then takes place at night and there is no temptation to open the door
too soon.
The fire is started slowly and the flow of oil is gradually increased as
the muffle begins to glow. The work here needs practice, nerve and
judgment. A good deal of smoke will be seen at the chimney at first but
this should disappear as the kiln grows red. If the fire be urged too
strongly at the beginning fuel will be consumed to no purpose, the only
result being the choking of the flues with carbon. As the red becomes
visible through the spy-hole, more oil may be supplied, but notice must
be taken that the smoke at the chimney does not increase. The ideal
firing is where there is no smoke but this cannot be reached until the
kiln is hot enough to cause the smoke to burn.
Persons who have burned kilns for overglaze work will find the method of
burning pottery very different. Instead of a fire brought as rapidly as
possible to the finishing point, there must now be a slow soaking burn
in which the heat shall have time to saturate the ware.
The cones in front of the spy-hole must be observed from time to time
and presently as the kiln reaches a bright cherry red, number 02 will
begin to bend at the tip and will gradually arch over until the point
touches the shelf upon which the cones stand. By this time number 01
will have begun to bend and when the point of this has touched the
shelf, the firing is over and the oil is shut off.
It requires some resolution to leave a kiln until morning but it is
conducive to early rising anyway. The kiln need not be quite cold but it
will help the kiln itself to wear better and the pottery will be better
if nothing is done until everything can be handled without gloves.
The cones are now taken out and a diagram is made of each level with the
bend of each cone accurately drawn. This diagram should be mounted and
hung on the wall for reference. It is not well to trust to memory. It
will probably be found, in the type of kiln we are discussing, that the
cones on the bottom have bent further than those on the shelf. That is,
the bottom is somewhat the hotter.
The variation in the kiln is not necessarily a disadvantage. It may be
utilized in burning wares of different kinds. For example, if the bottom
prove much the hotter, the biscuit ware may be placed below and the
glazed pieces on the shelf. In such case the shelf itself should be
washed with a good coating of clay and flint in order to protect it from
casual drops of glaze.
If a number of small pieces are being made, more than one shelf should
be set up. The legs may be just a little taller than the tallest of the
small pieces, but the art of placing or filling a kiln economically
consists in making selection of pieces which fit well together both as
regards height and shape. Thus, pieces which are large at the base may
be dovetailed in with others of which the base is smaller than the upper
part. In the case of clay ware the pieces may be set close together or
even piled one upon another. There is no danger of sticking unless the
ware is burned to complete vitrification. The glazed pieces must not, of
course, touch each other.
It will be seen, from these instructions, that there should be a good
assortment of wares from which to select. Economical firing cannot be
managed if a burn be attempted whenever a piece is ready, and patience
must be exercised so as to fill the kiln to advantage.
It is important that anyone attempting to burn a kiln should have some
understanding of the phenomena of combustion. Many things occur in the
firing which, without such an understanding, are not easily explained
but which become perfectly clear when considered in the light of simple
chemical science.
Combustion means oxidation or a combination between the elements of the
fuel, principally carbon and hydrogen, and the oxygen of the air. This
combination is a chemical action and as it proceeds heat is liberated.
With a given amount of a specific fuel and a given amount of air there
is always the same amount of heat, but the rate at which this heat is
given off varies with the time occupied in the operation. Heat may be
generated slowly which means a low temperature, or the same volume of
heat may be generated rapidly, occupying a much shorter time and
developing a higher temperature. From these statements it will be seen
that there is a difference between heat and temperature; heat means
volume, temperature means intensity. Thus the temperature derivable from
a given amount of fuel depends upon the rapidity with which it is
burned.
Combustion may be either complete or incomplete. In the former case
enough air is supplied to oxidize all the fuel with, usually, some
excess. The contents of the kiln are then bathed in the heated oxygen
and the condition of the burning is called oxidizing. When the
combustion is incomplete, on the other hand, there is a deficiency of
oxygen. The kiln is charged with hot carbonaceous gases and smoke, and
these, being hungry for oxygen, will abstract it from any substance
which may be present. This condition is called reducing because the
compounds which exist in clay or glaze are deprived of oxygen and thus
reduced to a lower state of oxidation.
In burning a kiln one should be able to produce either of these
conditions at will because there are certain wares which require one or
the other in order to secure the best results. To put the matter in a
nutshell, oxidizing conditions are induced by a strong draft and open
flues, reducing conditions are obtained by closing the air inlets and
using a liberal amount of fuel.
CHAPTER XVIII: HIGH-TEMPERATURE WARES
The subject of "Grand Feu Ceramics" has been so ably developed by M.
Taxile Doat in his admirable treatise[P] that it will be unnecessary to
go deeply into the matter, but in order that the reader may be aware of
what is involved, some description of the technique will be given.
[P] Keramic Studio Publishing Co., Syracuse, N. Y.
Hard-fired wares are divided into two classes, porcelain and stoneware.
The latter is called by the French, "Grès," an abbreviation of the name
"Grès de Flandres," the stoneware made in the low countries in the
sixteenth century. Both these wares are, technically, once fired, that
is, the body and glaze come to maturity at one and the same burning. The
biscuit ware is often given a low burn at first in order to facilitate
handling, but this leaves the body very porous and is in no sense a
maturing fire. The glaze is laid upon this porous ware, or upon the
unburned clay if preferred, and then comes the high fire or "Grand Feu"
of the French.
A mix for a porcelain body has already been given but if the ceramist
means seriously to attack the porcelain problem he will have to do some
experimental work for himself. The Georgia kaolin mentioned in the
recipe on page 40 is a good, plastic clay but it is slightly off color.
It may be necessary to improve the color by the use, in part, of another
kaolin such as the Harris clay from North Carolina.[Q]
[Q] The Harris Kaolin Company, Dillsboro, N. C.
Furthermore, in the preparation of a fine porcelain it is necessary to
grind the whole mix upon a mill. The mill used for glaze grinding will
answer every purpose and care must be taken that the grinding, while
carried far enough, be not too long continued. A certain amount of fine
grit in the body mass is necessary but only by constant practice can the
right point be reached. In making these experiments each step should be
faithfully noted in a handy book. The amount of water to a given weight
of clay and the duration of the grinding should be accurately observed
and written down. It is most unwise to trust to memory.
The process of casting may be used for porcelain as already described,
but the very best of workmanship is necessary. The hard fire to which
the porcelain is subjected reveals every error which has occurred in the
making. The same thing applies to wheel work. Not only is great skill
required in order to shape the tender porcelain clay on the wheel but
the very essence of the porcelain is its lightness, to produce which by
craftsmanship a long and arduous course of training must be endured.
Stoneware is free from many of these difficulties and, consequently one
who attempts the conquest of high-temperature wares is advised to begin
with this. Stoneware clay need not be a mixture. There are many clays
which can be used for the manufacture of grès with no more preparation
than that laid down for common clays.[R] It sometimes happens that a
clay will need the addition of a small quantity of flint or spar but
this does not amount to a difficulty.
[R] Stoneware clays may be procured from The Western Stoneware Company,
Monmouth, Ill.; H. C. Perrine and Sons, South Amboy, N. J.
Stoneware does not present the same manufacturing difficulties as are
found in porcelain. The clay is quite plastic and can be easily shaped
on the wheel; casting is scarcely a suitable process for this ware. The
essence of stoneware is strength and virility, just as that of porcelain
is lightness and grace. Each ware has forms suited to itself and it is a
mistake to depart from these essential characteristics.
After shaping and drying the technical manipulation of both wares
proceeds along the usual lines. The first fire is at a very low
temperature. The melting point of silver (cone 010) is enough in nearly
every case. This leaves the ware in a soft and porous condition but hard
enough to resist the action of water. The process of glazing has already
been described but the composition of the proper glazes differs from
that of low temperature glazes.
Porcelain is always burned in a reducing fire; stoneware may be burned
either reducing or oxidizing. The temperature at which the glaze is
burned is very high, it must be, in fact, the maturing point of the body
itself.
The simplest form of porcelain glaze is that represented by the formula--
K_{2}O .3 }
CaO .7 } Al_{2}O_{3} .5 SiO_{2} 4.0
Which is carried out in the following mixture:
Feldspar 167
Whiting 70
Kaolin 52
Flint 108
----
The glaze is ground for use.
The same glaze will also serve for stoneware but it will burn to a
brilliant surface whereas stoneware is better when finished with a matt
texture.
The following is a stoneware matt glaze:
K_{2}O .3 }
CaO .7 } Al_{2}O_{3} .7 SiO_{2} 2.6
Of which formula the mixture is--
Feldspar 167
Whiting 70
Calcined Kaolin 66
Raw Kaolin 26
----
The porcelain glaze is at its best when uncolored. The matt glaze will
be more interesting when used as a colored coating.
The following are a few suggestions for colored matt stoneware glazes.
To the glaze batch, 329 parts, add:
For blue:
Cobalt Oxide 2 parts
Nickel Oxide 1 part
Ground Rutile 10 parts
For brown:
Iron Oxide 6 parts
Nickel Oxide 3 parts
Ground Rutile 10 parts
For green:
Chrome Oxide 2 parts
Cobalt Oxide 1 part
Iron Oxide 4 parts
For dark red:
Iron Oxide 10 parts
Chrome Oxide 2 parts
Zinc Oxide 6 parts
Rutile has not before been mentioned. It is a crude oxide of titanium
and is exceedingly useful in high temperature work for producing odd,
mossy and crystalline effects.
These mixtures make no pretense to be complete, they are given as
suggestions only because if the artist-potter is to be successful he
must be prepared to compound glazes which are the expression of his own
individuality.
For burning high-temperature wares the kilns already described may be
used but upon purchasing it should be stipulated that the kiln is to
stand burning up to cone 11 or 12. Successful porcelain can be made at
cone 10 but better results are secured at cone 12, though, of course,
the wear upon the kiln is proportionately greater. Stoneware requires a
burn of about cone 9, higher or lower according to the clay used but
fine results must not be expected below cone 7 nor is it necessary to go
higher than cone 10.
CHAPTER XIX: CLAY-WORKING FOR CHILDREN
One of the modern developments of clay-working is the use of it in
elementary and high schools as a branch of manual training. In this,
clay meets the most exacting needs of the work for it affords a perfect
means of self-expression. Other arts interpose between the pupil and his
material a series of tools or appliances, more or less elaborate, which
constitute a barrier to the personal touch. Clay presents no such
obstacles. The ten fingers are all the tools that are necessary at the
beginning and, consequently, the personal equation in clay-working is
remarkably high.
In the kindergarten the children take to clay work as little ducks to
water and the interest is never lost. In this way, clay, instead of
adding to the labors of a teacher already overburdened by a plethora of
subjects, constitutes a real relief. The work is so interesting that it
moves along of itself and all that is needed is intelligent direction.
It is, of course, necessary that anyone attempting to teach clay-working
to children should have a knowledge of methods and principles. The
essence of power, especially in teaching, is reserve, but there is great
danger in expecting too much from small heads and hands. In the early
exercises the skill of the teacher should even be employed to conceal
her art. It is a mistake to place before elementary pupils work which is
far beyond their reach. Let the teacher make before the class something
which they themselves can do if they try and they will be encouraged to
greater effort.
A small cylinder is a suitable beginning exercise for several reasons.
The form is definite and the result may therefore be easily criticized
by the children themselves, the size of the piece may be readily adapted
to the small fingers and the simplicity of line enables the attention to
be concentrated upon the manipulation of the clay.
This cylindrical form may be made more interesting by the addition of
little feet or handles; by a simple line border incised along the upper
edge; or by dividing the surface into well-spaced panels. The planning
of the cylinder itself is a good exercise in rectangle proportion.
In order to enable the pupils to turn their work from side to side each
one should be provided with a piece of paper or cardboard the size of
the base of the pottery. The building is started upon this and, managed
in such a way, the clay does not stick to the table.
When the idea of pottery building, either by coils or by pieces welded
together, has been grasped, the children should be taught to think in
the solid. There is almost always a difficulty in making children see
that an outline drawing and a solid form may be alike in meaning. The
teacher should draw upon the blackboard a simple jar in elevation, the
plan, of course, will be a circle. The same thing is then made in clay
by both teacher and children and the results are compared with the
drawing. This will lead to the designing of the forms in outline by the
children themselves. These designs should be made the exact size of the
proposed pottery and if the outline be carefully cut out the line of the
paper may be applied to the work as a template. By such means the
children are led to produce accurate lines in the clay and control over
the material is secured.
There is always a temptation, when the clay sags or loses shape, to
diverge from the original idea and to allow the material to shape
itself. This inevitably leads to slovenly work and should be resisted
from the first. The paper template helps to correct such an impulse and
the pupil presently finds that the clay can be successfully controlled
if enough trouble be taken. There is much interest too in the cutting of
pottery forms from folded paper. A number of these forms may be pinned
on a screen and the children led to select the best in line and
proportion. Too much emphasis cannot be laid on the necessity for
showing the children fine examples of pottery, both ancient and modern.
The more primitive types, where the form and the decoration are so
perfectly adapted to each other and to the material, are full of
inspiration for the child potter as well as for the adult. When one is
fortunate enough to be near a museum, many illustrations will be found,
but good photographs or drawings are available for almost everyone.
Constant comparison and the exercise of choice will lead to a
development of taste, which must affect the child whether he later
becomes a producer or a consumer.
A flower holder is a good problem. It is a solid piece of clay two or
three inches in diameter and an inch thick. This may be round or square
in form and may have simple modeled decoration added to it. Quarter inch
holes are pierced at regular intervals, in fact, they themselves should
form part of the design. For the older children a shallow bowl of good
line with a flower holder to fit is an interesting problem. Other good
problems, which may be made more or less difficult according to the
grade in which they are given, are rose jars, bread and milk bowls,
incense burners, cylindrical jars, square fern dishes, candlesticks and
small lamp bases.
When working out decoration for pottery forms, it is well to have the
children make their designs with the modeling tool upon the clay itself.
If a piece of soft clay be rolled out flat upon the table it affords the
best possible medium for making clay designs. The pupil is at once put
in touch with the possibilities and limitations of the material. A
drawing made upon paper may have to be entirely changed before it is
suitable for use on clay. The soft surface can be smoothed over as often
as necessary and a new sketch made until a design is approved for
application to the pottery itself. In the chapter on decoration will be
found suggestions for clay treatment.
The making of tiles affords an interesting application of the principles
of design, but the instructions in the chapter on tile should be
followed in order to insure a workman-like product. If it is possible to
use plaster, the making of a decorated tile from which a mold can be
made and other tiles pressed is a good problem. Animal forms lend
themselves to the decoration of such tiles and are always interesting to
children.
While these chapters are especially devoted to ceramics in the sense of
burned and glazed pottery a few words upon modeling as related to school
work may be added here. Imitative modeling from cast or copy with its
development of animal and figure modeling, both from life and from
memory, is valuable in the acquirement of the power of manipulation and
control as well as in the cultivation of observation, imagination and
memory. In the best regulated schools the work of the grades is often
correlated in the study of some phase of human life. Facts are grouped
around some epoch or event in history or some country or clime in
geography. The children take up the clay while their minds are full of
the current subject and nothing more natural than that they should
illustrate the story by models.
Such work is to be thoroughly commended as truly educational, though it
does not fall strictly within the field of pottery and a few suggestions
may therefore be in order.
The modeling of animals or people for the sand table is full of interest
for the younger children. Such stories as "The Three Bears," "Chicken
Little" and "The Little Red Hen" immediately suggest themselves. For
children of about the fourth grade "Alice in Wonderland" offers a most
fascinating array of models. "The White Rabbit," "The Duchess," "The
Mock Turtle," "The Mad Hatter," grotesques of all sorts, seem a natural
outcome of this illustration work and the wise teacher will see the
possibility for developing the imagination in the modeling of mythical
creatures, such as dragons and gnomes, and in the personification of the
elements. There is also an unlimited fund of material in the tales of
knighthood and of fairyland.
With the older children, simple principles of design and composition
should be suggested. A paper weight is an interesting problem demanding
the adaptation of form to space.
Many of these things may be modeled in clay, dried and painted with
water color or one of the patent modeling clays which set like cement
may be used. If no supports have been left in the model it may be fired
when thoroughly dry.
Some of the best projects for sand table work involving modeling are
Eskimo Life, Indian Life, Farm Life, The Circus, and Fairy Tales.
Generally a suggestion is all that is necessary to call forth the most
original conceptions and once started the children will soon far
outstrip the teacher.
INDEX
A
Alhambra Vase, 16
Armenian Bole, 15
B
Ball-Clay, Tennessee, 40
Bases for Glaze, 143
Berlin Porcelain, 28
Bisilicate Glaze, 145, 146
Black Surfaced Ware, 13
Blistering of Glazes, 166
Building, Clay for, 69
Building, Faults in, 72
Building, Methods of, 70, 71
Built Pottery, 10, 68
Burning Tiles, 139
C
Case for Mold-making, 58
Casting, 129
Casting, Slip for, 129
Castor Ware, 13
Children, Pottery for, 194
Chinese Porcelain, 23
Clay, Colored, 36
Clay, Crude, 9, 37
Clay, Decoration in, 173
Clay, for Building, 69
Clay, for Tiles, 134
Clay, Preparation of, 37
Colored Glazes, 24, 143, 163
Colors, Underglaze, 25, 143, 177
Combustion, 186, 187
Cones, Pyrometric, 182
Copenhagen Porcelain, 27
Crawling of Glazes, 166
Crazing of Glazes, 165
Cups and Saucers, 124
D
Decoration, 173
Decoration of Tiles, 137
Defects of Glazes, 164
Devitrification, 152
E
Earthenware, Mixture for, 40
Engobe Ware, Oriental, 15
F
Famille Rose, 24, 26
Famille Verte, 24, 26
Feet for Vases, 105
Firing the Kiln, 179
Fitness in Design, 3, 4, 7
Flowing of Glazes, 145, 166
Form and Weight, 7
Fritt for Glaze, 158
Fritting Furnace, 159
G
Glaze, Bases for, 148
Glaze, Bisilicate, 144, 145
Glaze, Calculation of, 150
Glaze, Fritt for, 158
Glaze, Pinholes in, 167
Glaze, Porcelain, 191
Glaze, Recipes, 160
Glaze, Stoneware, 191, 192
Glazes, Blistering of, 166
Glazes, Clear, 141, 161
Glazes, Colored, 143, 163, 192
Glazes, Crawling of, 166
Glazes, Crazing of, 165
Glazes, Defects of, 164
Glazes, Flowing of, 145, 166
Glazes, Grinding, 151, 155
Glazes, Ingredients for, 142, 143
Glazes, Matt, 152, 191
Glazes, Nature of, 141
Glazing, 140
Glazing Tiles, 137, 138
Grès de Flandres, 21
Gum Tragacanth, 156
H
Hard Porcelain, 23, 189
High-temperature Wares, 188
I
Ingredients for Glazes, 142, 143
J
Jars for Storing Clay, 41
K
Kiln, Firing the, 179
Kiln, Portable, 180
Kilns, 179
L
Lawns, Silk, 41
Large Pieces, 107
M
Matt Glazes, 152
Methods of Making Tiles, 135, 136
Molds for Plates, 62, 63
Molds for Vases, 52
O
Oriental Engobe Ware, 15
Ox-blood Red, 24, 26
Oxidizing Fire, 187
Oxygen Ratio, 145
P
Pâte-sur-pâte, 14, 175
Pieces, Large, 107
Pinholes in Glaze, 167
Plaster-of-Paris, 45
Plaster Dishes, 46, 47
Plaster Head for Wheel, 65, 78
Plaster, Setting of, 45
Porcelain, Berlin, 28
Porcelain, Copenhagen, 27
Porcelain Glaze, 191
Porcelain, Hard, 23, 189
Porcelain, Mixture for, 40
Porcelain, Sevres, 25
Potter's Wheel, 74
Pottery, Built, 9, 68
Pottery, for Children, 194
Pottery, White-coated, 14, 168
Props and Bats for Kiln, 182
Pyrometric Cones, 182
R
Recipes for Glazes, 160
Reducing Fire, 187
S
Salt-glazing, 21
Saucers, Cups and, 124
Shivering of Glazes, 165
Size, Mold-makers', 44
Slip, 38, 39, 129
Slip for Casting, 129
Slip-painting, 175
Stoneware, 21, 188, 190
Stoneware Glaze, 191
T
Tennessee Ball-clay, 40
Tiles, 133
Tiles, Burning of, 139
Tiles, Clay for, 134
Tiles, Decoration of, 137
Tiles, Glazing, 137, 138
Throwing, 77
Tin Enamel, 164
Tragacanth, Gum, 156
Turning Tools, 100
U
Underglaze Colors, 143, 177
V
Vase Forms, Turning, 49-52
Vases, Feet for, 105
Vases, Molds for, 52
W
Weight and Form, 7
White-coated Pottery, 14, 168
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