Before attacking the somewhat complicated subject of the nature and
composition of glazes, it will be well to take up again the thread of
the mechanical processes that are involved in the making of a piece of
porcelain.
The materials that enter into the glaze are reduced to the finest powder
in mills similar to those in which the china-stone and flint are ground
for the preparation of the paste. If any substance soluble in water,
such as borax or salts of the alkalis, enter into the composition of the
glaze, these must be first partially fused in combination with the other
materials to form a _frit_, a kind of imperfect glass. These frits,
which enter so largely into the composition of soft-paste porcelain, are
formed with the object of bringing the soluble constituents into an
insoluble form before mixing with water to form the slip. There are
indeed other practical reasons that render a preliminary partial fusion
desirable.
The finely ground elements of the glaze, mixed in due proportion, are
worked up with water to form a creamlike slip into which the vessel to
be glazed is now dipped. In China, in many cases, the glaze-slip is
blown upon the surface in the form of a spray. This is done by means of
a bamboo tube, covered at one end by a piece of silk gauze, through
which the liquid is projected by the breath of the operator (French,
_insufflation_); in other cases the glaze may be painted on with a
brush. In China, as we have mentioned, the glaze-slip is generally
applied to the raw surface of the thoroughly dried but unbaked ware, but
in other countries there is, almost without exception, a preliminary
firing of greater or less degree to produce a biscuit.
We shall restrict the use of the word glaze to the vitreous coating
applied directly to the surface of the raw paste or of the biscuit to
enhance the decorative effect of the ware, and with the more prosaic
object of allowing the surface to be easily kept clean. In the case of
porcelain this coating is always more or less transparent.[16] There is
here no necessity for concealing the natural white colour of the paste.
In the case of many kinds of pottery, however, as in the ‘enamelled
fayence’ of Delft and Italy, the glaze is rendered opaque by the
addition of oxide of tin, so that the ill-favoured ground is concealed
by a white shiny surface which may be made to resemble closely the
natural surface of porcelain. A glaze of this kind is often called an
enamel, but as we are not concerned with such an expedient we shall
confine the use of that word to the various forms in which a vitreous
decoration, whether translucent or opaque, is _superimposed upon the
glaze_ and fused into it, more or less thoroughly, by a subsequent
firing in a muffle furnace.
The English word ‘glaze’ is only another form of the word ‘glass,’ and
we may say at once that, in composition at least, there is often little
difference between the two substances. The French word for ‘glaze’ is
_couverte_ or _vernis_; the last term applies well to the thin skin of
glaze found on Greek pottery. The Chinese have several expressions, but
it is a curious fact that the characters with which most of these terms
are written contain the radical for ‘oil,’ and indeed the word ‘oil’
itself is often used in the sense of ‘glaze.’
Mr. Rix puts it well when he says that the glaze is to the enameller of
porcelain what his canvas is to the painter; while in the case of a
decoration ‘_sous couverte_,’ the glaze corresponds to the varnish
which, while protecting his work, gives brilliancy to the colouring
(_Journal of Society of Arts_, vol. xli.). It is, moreover, the vehicle
by which the design is harmonised and rendered mellow. The effect is
produced at once and endures practically for all time.
The hardness and fusibility of glazes differ widely, and they are
conditioned by the nature of the wares that they cover. It is evident
that there must be a close relation between the fusing-points of paste
and glaze, and that the latter should be the more fusible of the two.
The difference of melting-point should, however, not be too great. The
melted glaze should rather, by penetrating into the already softened
paste or by a chemical action upon its surface, form a more or less
uniform mass with it. In cooling, the contraction of the glaze should
follow that of the subjacent paste. This is a most important point; any
discordance may lead to splitting, cracking, and ‘crazing.’
The beauty of the surface of porcelain depends on the fact that the
glaze has become intimately united with the paste during the long
exposure of both to a high temperature. We should not be conscious, in
regarding a fine specimen of porcelain, of a greater or less thickness
of glass covering an opaque substance; we should rather see in it the
polished surface of ivory or of some precious marble.
It would seem that it was the beauty of the glassy surface, enhancing
the brilliancy of the colouring, rather than any practical advantage
connected with its use, that first led to the application of glaze to
pottery. The turquoise and green glazes of the Egyptians (the colour is
derived from a silicate of copper along with soda and sometimes lime)
were known to the men of the Early Empire. They were applied to a
fritlike mass of sand held together by silicate of soda, to which the
name of porcelain has sometimes been very wrongly given. Objects of
steatite, of slate, and even of rock crystal were sometimes covered with
a coloured glaze of this kind, but it was never applied to the clay
vessels in daily use. These were made, then as now, from the unctuous
clay of the Nile bank. For this restriction there was a very good
reason, namely that a glaze of this nature, composed chiefly of alkaline
silicates, will not adhere to a base of ordinary clay. It was not until
Ptolemaic and Roman times that, by the discovery or adoption of a glaze
containing lead, the ancients were enabled to glaze their pottery. So in
Assyria, the employment of glazes was almost confined to the decoration
of the surface of brickwork, the bricks being of a loose and somewhat
sandy texture.[17]
In these glazes, and indeed in much earlier examples from Babylonia,
both tin and lead have been found. The respective virtues of the
silicates of these metals were doubtless appreciated, that of tin to
form a white opaque enamel hiding the material below, and that of lead
to enable the glaze into which it enters to adhere to a paste formed of
a plastic clay.
With the Chinese the aim was rather æsthetic than practical. They sought
by means of the marvellous glazes that cover their ancient porcelain to
imitate the surface of natural stones; their early celadons were in a
measure intended to take the place of the precious green jade, so highly
esteemed by them.
At the time when the manufacture of porcelain was first introduced from
China there were (apart from the salt-glazed stoneware, which lies quite
outside our inquiry) three classes of glaze in general use either in
Europe or in the nearer East:–
1. Glazes consisting essentially of alkaline silicates without either
lead or tin. Such glazes could only be applied to a fritty silicious
base, and in India and Persia their employment seems to have been a
survival from Egyptian and Assyrian times.[18]
2. Opaque enamel glazes, the opacity being due to the presence of tin; a
considerable amount of lead also is generally found in these glazes. We
are not concerned here with the obscure origin of this group, but in the
sixteenth century this enamelled fayence was in general use for the
better class of table-ware. It includes the Italian majolica, the French
fayence of Nevers and Rouen, and above all the earthenware of Delft.
3. The oily-looking lead glazes with which the common earthenwares were
covered. These were essentially the glazes of the Middle Ages in Europe,
and their employment could probably be traced back to the lead-glazed
ware sparingly used by the Romans. We have already noticed the use of a
similar glaze in Egypt as far back probably as Ptolemaic times.
There were practical objections to all these glazes. It is true that at
Delft, by the use of the tin enamel, a ware could be turned out closely
resembling, in external aspect, the blue and white porcelain of China,
but the enamel was soft and would in time chip off at the edges, showing
the dark earthy clay beneath. On the other hand, the alkaline glazes of
the East were not much known in Europe; they can only be used upon a
very tender and treacherous base. In India and Persia, however, a ware
thus glazed still competes with the hard porcelain of the Far East. In
spite of the great objections to the glazes of our third class, those
containing lead–objections arising from their softness and from the
danger of poisoning to those employed in their manufacture–their use
has tended rather to increase. Not only is lead the principal
constituent of the glazes still universally used for common pottery, but
it forms an important element in the glaze of our finer earthenwares as
well as in that of those bone pastes which rank with us as porcelain.
The glaze which had been brought to perfection by the Chinese at an
early period differs from all those yet mentioned by its hardness, its
high fusing-point, and in its chemical composition. Speaking generally,
the glaze of porcelain differs in composition from the paste which it
covers only sufficiently to allow of its becoming completely liquid at
the extreme heat of the furnace; and just as the paste of Chinese
porcelain has a wider limit of variability than that made in Europe, but
is on the whole of a ‘milder’ type than the latter, so we find that
while the glazes of the Chinese are as a whole less refractory and not
quite so hard, there is still a wide range of variation in these
qualities.
If, then, we theoretically regard porcelain as a compound of a silicate
of alumina with an alkaline silicate of the same base, we may say that
the glaze of porcelain is formed by the latter body alone, that it is,
in fact, merely a fused felspar. But as in the case of the paste, so in
the glaze there is generally present an excess of silica, derived from
the quartz contained in the petuntse or pegmatite, and this silica
enters into combination with some other bases which are present in the
constituents of the glaze, thereby increasing its fusibility and
modifying the contraction in cooling. The most important of these
additional bases is lime, so that the more fusible type may be called a
calcareous, as opposed to a more refractory or purely felspathic glaze.
As much as 21 per cent. of lime has been found in some Chinese glazes,
the amount of alumina being proportionately reduced.
There is more or less lime in the glaze of most kinds of European hard
porcelain, but the exceptionally hard and refractory paste made at
Sèvres since the time of Brongniart is covered by a glaze of
corresponding hardness from which that earth is absent. This hard paste
has, however, of late been replaced in part by one of a milder type, and
with this latter a calcareous glaze has been adopted even at Sèvres, the
object of the change being, as we have said, to allow of a more
brilliant decoration.
There is a perceptible difference in the aspect of these two types of
glazes after firing. The hard, non-calcareous glaze has a slightly milky
look. The softer calcareous type is more brilliant, and approaches in
transparence and limpidity to the lead glazes of soft porcelain. A glaze
of this last kind was used at Sèvres for a few years after the first
introduction of the hard paste, and perhaps also at Dresden in quite
early days.
The principal objection to a hard refractory glaze, such as that so long
in use at Sèvres, arises from the difficulty of properly incorporating
the enamel colours with its body. The restriction of the number of
pigments that can be employed, both under and on the surface of the
glaze, in consequence of the high temperature at which the latter melts,
is another drawback. The dulness, the ‘painted on’ look of so much of
the decoration on European hard paste porcelain, is in great measure a
consequence of the employment of a glaze that is only softened at a high
temperature. As an example of a medium type of glaze we give the
composition of that used at Berlin in 1836. This consisted of kaolin, 31
per cent.; quartz, 43 per cent.; gypsum, 14 per cent.; and ground
porcelain, 12 per cent. A glaze long in use at Dresden is of a very
similar character. Felspar, it will be seen, does not enter into its
composition, and such a glaze can contain but little potash or soda.
With this we may contrast the hard glaze of Sèvres, composed simply of
ground pegmatite, a rock consisting mainly of felspar. This glaze yields
on analysis 74 per cent. of silica, 17 per cent. of alumina, and as much
as 8 per cent. of potash.
The glaze on Chinese porcelain is prepared by mixing certain special
varieties of petuntse with an impure lime, prepared by burning limestone
with dry fern as fuel. It contains, as we have seen, from 15 to 21 per
cent. of lime, 5 to 6 per cent. of alkalis, 11 per cent. of alumina, and
66 per cent. of silica.
We give these examples to illustrate the principal types of glazes used
for hard paste porcelain. It will be noticed that the constituents are
drawn from widely different sources.
The glazes of soft paste porcelain always contain a large amount both of
lead and of potash or soda, so that they approximate in composition to a
flint glass. The alkalis, generally introduced as carbonates,
necessitate a previous fritting of part at least of the materials.
Boracic acid plays an important part in the glaze of most modern English
wares: it is generally introduced in the form of borate of soda or
borax. This acid replaces in part the silica, just as in the paste the
glassy materials are replaced by bone-earth.
