GLAZES AND LUSTRES
"I began to think that if I should discover how to make enamels I
could make earthen vessels and other things very prettily,
because God had gifted me with some knowledge of drawing."
--PALISSY.
It is in this department of potting, with its surprises, difficulties, and
disappointments, its rare but exciting successes, that for most potters
the greatest interest lies. To those of a scientific bent it is perhaps
the summit of the craft, but the artist groping amidst formulas and
methods may take heart. The finest work in pottery was not produced by
scientists alone and does not depend altogether upon the quality of its
paste, its unique colour, or strange lustre. The last word, the form,
decoration, and craftsmanship, is with the artist.
It is beyond the scope of a book of this description to enter into a
necessarily long and complicated account of the different processes
concerned in the composition of glazes. It will be sufficient to indicate
their leading characteristics plainly enough to enable the student to
start experimenting. None can afford to miss opportunities for
experiment, just as surely as none can afford to be always experimenting.
For a full description of materials and glazes and their manufacture such
books as those of Drs. Shaw, Furnival, and Hainbach are recommended.
Putting aside salt glaze we find a countless number of both lead and
leadless glazes. They range from the thin silicious coating of the
ancients up to the rich alkaline glazes of the Persians and Chinese; from
the raw galena of peasant pottery to the rich Majolicas and fine hard
glazes of modern commerce. Salt glaze is obtained by the vaporizing of
salt inside the kiln at a great heat. The sodium oxide so formed combines
with the silicates in the clay to form a very thin coat of refractory
glass, intimately connected with the body. Porcelain glazes, though not
differing so much in composition from the ordinary _fine_ earthenware
glazes, are extremely hard, being compounded of kaolin, felspar, and
quartz, with possibly limestone and ground sherds. It has in common with
the salt glaze the close union with the body, so that when fractured the
line of demarcation between glaze and body is indeterminable.
With a few minor exceptions the following list comprises the ingredients
of all colourless glazes:
Kaolin
Quartz
Cornish stone
Felspar
Fluorspar
Flint
Sand
Barytes
Bismuth
Gypsum
Limestone
Nitre
Borax
Bone-ash
White lead
Red lead
Zinc oxide
Tin oxide
Salt
Soda
Potash
These materials must be free from all trace of iron. They are pulverized
and some are calcined or oxidized. Then they are mixed in varying
quantities to form the glaze mass. This mass is easily fusible when lead
or borax is present in large proportions, more infusible or harder the
more silica it contains, and very refractory if alumina is present in any
quantity. The silica forming the glassy part of the glaze is stiffened by
the presence of alumina, which stops any tendency to run.
Lead is very largely used as a powerful flux at low temperatures but is
unsuited to hard glazes. Borax and boracic acid are important constituents
of leadless glazes. They are used to replace some of the silica, than
which they are more fusible. Matt or non-reflective glazes are opaque and
less vitreous than the glassy glazes. They do not flux or run.
All these minerals are finely ground before mixing. Then those insoluble
are mixed and _fritted_; that is, fused in a crucible or fritting furnace
to a greater or less degree, according to the hardness of the glaze. If
fused into a glass, the melted mass is poured into water to facilitate the
next process, which is its reduction to a fine powder by re-grinding. Then
the completing ingredients are added and the mass coloured by the addition
of metallic oxides. Of these the chief are:
Iron
Copper
Nickel
Antimony
Cobalt
Chrome
Manganese
Titanium
and the more precious metals, in various forms, as oxides, carbonates,
sulphates, and nitrates soluble in the glaze at great heat.
These metals impart the many varied colours found in pottery. Zinc oxide
is used to brighten a glaze or to stabilize colour. Tin oxide, which is
insoluble at great heat but remains in suspension, gives opacity.
This is no more than a skeleton outline of the intricate processes often
used in the fabrication of a glaze. The manifold minerals, metals, oxides,
acids, and alkalies are used in a variety of ways by the modern chemical
potter.
To all this seeming complication is added the question of pastes and
bodies. There then arises the great problem of fitting one to the other.
Salt glaze and porcelain excepted, the finished pot presents three
stratas. Outside is the glaze, next the body, then inside the glaze again.
If the coefficient of expansion of these three layers differs, in other
words, if the glaze does not fit, the result is _crazing_, that bugbear of
the potter.
This crazing, which has been followed up and developed into their
delightful _crackle_ by the Chinese, may show itself at once or only after
a lapse of months. It appears as a minute network of fine cracks over the
entire surface of the pot. It is often not unsightly, but sooner or later
it must cause devitrification. The glaze after a time assumes an
evanescent iridescence followed by a dull smoky appearance; finally,
perhaps not for many years, it decomposes and peels off.
With low-firing natural clays rich in silica and iron, the craze is not of
much consequence. The body itself at a moderately high temperature becomes
non-porous. With hard short bodies containing lime or chalk it may have
quite disastrous consequences. Water placed inside will eventually
percolate through leaving a network of grey lines all over the pot and
completely spoiling its appearance.
It will be readily granted that, whether porous or non-porous, a craze is
most undesirable on any piece of pottery that may be used for food or
drink.
It is here that the commercial potter is such an admirable fellow. Many of
the glazes on modern tablewares are perfect for their purposes. Sometimes
only a little more fire is needed to stop the nuisance, but a bad craze
usually needs more attention. The glaze requires stiffening, and the
addition of ground flint or quartz, China stone or clay and felspar
introduces alumina and silica and raises the fusing point. The
substitution of borax for a portion of the silica can also be tried and
will permit the use of slightly lower firing point.
If the glazes are bought ready mixed, the body must be altered instead.
Refractory China clays should be replaced by more fusible clays or some
reduction made in the amount of infusible materials. The addition of
ground sherds or flint will have a contrary effect should the glaze peel
or crack at the edges, as it may do on a very silicious body. In working
with natural clays on a moderate scale it will be found best to mix "fat"
or rich natural clays with those of a more porous or hard nature. A few
graded mixtures submitted to a thorough trial should soon show when a
sympathy has been established between the body and glaze.
The receipts given on pages 183 and 184 will make good colourless glazes
without fritting if thoroughly ground in a mortar and passed through a
sieve. Numbers 3 and 7, when calcined, will give much more even results
and they can be coloured by the addition of the oxides named. But simple
as it sounds, the washing, grinding, fritting, re-grinding, and sieving is
a long and laborious process demanding machinery, and on that account is
unsuited to schools or potters of moderate means. Glazes like Nos. 1 and
2 will do quite well for elementary work but unless the appliances are to
hand the manufactured article will have to be relied on for more finished
and ambitious work.
If, indeed, you are already in possession of a good receipt for a fine
colour and glaze, one quite worth while, so much the better. Mix it and
feel the joy of the Compleat Potter unafraid of spoiling his own good
shapes with a faulty or unknown glaze.
Admittedly, to get anywhere in an original direction systematic research
is essential. One must keep on experimenting, keep on hoping, and keep on
taking notes; but at the start let us not be too impatient or independent
if we wish to produce good pots.
There is often among young potters a false pride that prevents them using,
and among old potters acknowledging the use of, the manufactured article.
Why this should be is a little difficult to understand. A painter might
far more reasonably be ashamed to use modern tube colours or a
stained-glass craftsman as logically insist on making his own glass, as a
potter in the twentieth century refuse to avail himself of the wonderful
range of glazes that modern research has placed at his disposal. These
resources should be used intelligently, not mechanically, or by the
book--artistically, inventively, secretly, if you will, but they should be
used--until the multitudinous experiments have borne fruit and repeated
trials convince you that at last you possess some gem of research worth,
as well it may be, the months of patient toil engendered in its
production.
The various receipts are given on pages 183 and 184 without analysis of
the composition of the paste or body to which they were applied. The first
group have been used on common earthenware clays with complete
satisfaction. They are to be considered as points of departure for future
experiments in which they may be modified at will, and not regarded as a
contribution to the science of glaze making.
In colouring it will be found that combinations of cobalt, iron, and
copper oxides give an interesting range of simple blues or greens; iron
and manganese browns; and so forth. The colour mass or stain is ground
fine and lawned, and from about 2 to 7 per cent mixed with the colourless
glaze mass, according to the depth of colour required. The ordinary
under-glaze colours may be used to stain glazes, the percentage being
fixed by small trials. For the rare colours--turquoise, crimson, or
purple--a more complicated process is necessary and only perfected after
many trials. The ingredients of these fine colours are naturally kept
secret by their fortunate possessors.
It must be noted that a glaze suited to one body may peel or run off an
unsuitable one. Then a colourant is affected differently by a lead or an
alkaline base in the glaze. Again, copper and iron oxides may help to flux
a glaze, whilst cobalt or nickel will exert a contrary effect. Cobalt,
being a strong colourant, will need a sparing use, whereas a similar
percentage of iron will merely tinge the glaze mass. And so _ad
infinitum_.
It is self-evident that any attempt to emulate the vast range of the
modern ceramic chemist is doomed to failure. To a craftsman the
fabrication of one fine individual glaze or lustre is an achievement of
which he may be proud, and for which he will find abundant and varied
uses. In this connection it is encouraging to the craftsman to learn from
so high an authority as W. Burton, Esq., F.C.S., that it is impossible to
obtain with purified oxides the fine tones got by the Orientals with
impure materials. Further, that the simple glaze of the Persians--a
mixture of clean white sand with soda or wood ash or potash--is still the
best for under-glaze painting. Although tastes differ so widely,
invariably it will be found that more and more heat will be the cry.
Imperceptibly this leads to the desire for hard, cold, "fat" translucent
glazes, neither matt nor glossy. And on the summit, far out of reach,
stand the wonders of the Old Chinese.
LUSTRES
There are several kinds of lustres, but the true lustres possess a pearly
iridescence in addition to their colour. The copper and silver _lustres_
of the eighteenth-century Staffordshire potters were thin metallic films
over the whole surface of the glaze. Gold or silver solutions were used.
Only where the gold was fired on a white clay is there any iridescence,
and then hardly so pronounced as to deserve the name of lustre. The
bismuth and other lustres made by the modern potter are combinations of
metallic oxides and resinates dissolved in ethereal oils. These are
painted on the glaze, transparent or opaque, but having almost invariably
a lead base, and then fired at a dull-red heat. The medium disappears, and
the metal in a finely divided state is deposited on the surface of the
glaze. This, however, gives a painted look very different from the lovely
irradiance of the Persian Hispano-Mooresque or Italian work.
With these the lustres were fired in a reducing atmosphere, one
supercharged with combustible gases, the metals decomposed and fixed to
the semifluxed glaze. The manipulation was not infallible and was attended
with much risk; but the successful pieces are unrivalled, and according to
Piccolpasso "paid in gold." De Morgan, who more nearly approached the
works of the Italian masters than any other modern potter, used a very
similar medium and method with his fine lustres. In the last few years
lustres have been brought to such perfection, in preparation, application,
and firing, that accident is practically eliminated. Glorious colours and
gorgeous effects are obtained. Yet it may be said without senseless
adulation of the merely mediæval that nothing has been produced superior,
or even akin in spirit, to the work sent out from Persia, Spain, or the
botega of the inimitable Maestro Georgio of Gubbio.
Lustres may be bought ready to apply. They are then painted on the glazed
pot, which is re-fired at a dull-red heat in the ordinary oxidizing
atmosphere. Once the painted look has been contrasted with the lustrous
appearance of the _reduced_ effects there is only one kind of lustre for
the artist. Fortunately, perhaps, their preparation is not easy, and the
correct method of reducing is a difficulty to be overcome by practical
experiment alone. Hainbach gives many practical receipts for lustres that
are not beyond the scope of a craftsman. The reducing atmosphere can be
obtained in an open kiln by the introduction at the right moment of any
combustible giving smoke free from all traces of sulphur.
In firing with a muffle kiln the introduction of coal-gas free from
sulphur is a matter attended with some risk at the necessarily low
temperatures required by the lustres. It should be approached with
caution, and each mixture will have a varied firing point, the correctness
of which is established only by trials.
Lustres applied with skill and restraint enhance the most beautiful glaze,
but in unskilled hands they inevitably vulgarize and cheapen. The fresh,
unsophisticated renderings of the Persians or the Moors and the virility
of the Italians should be studied, but not merely imitated before working
in this medium.
[Illustration: FIG. 55--PAINTING OUTFIT.]