familiar with the fine blue and brown glazes showing starry and radiating crystalline groups that are being produced at Copenhagen, and Sèvres, Röstrand and Berlin. Some three or four years ago, in a paper I had the honour to read before this Section, I drew attention to these crystalline glazes, as well as to others of a very different type which had been produced at the Rookwood Pottery in America, and by my brother, Mr. Joseph Burton, at our works at Clifton Junction. Since then, however, the whole subject of crystalline glazes and their production with scientific precision, so that they might be used as a certain and regular means of pottery decoration rather than as the occasional triumphs of the kiln, has been engaging a great deal of our attention, and if in certain directions the results are not yet all that might be wished, such a measure of success has attended our experiments as to warrant me in laying the results before you.

It is essential, first of all, that one should try to form some conception of what a pottery glaze is, and as to how it behaves when it is melted on a piece of pottery at the requisite temperature of the kiln, and under the firing conditions which control production on a large scale.

The popular idea of a glaze, as of a sheet of window glass, is that of a uniform transparent substance, which may be either coloured or colourless, and which is melted on the surface of the pottery so as to form an impervious, protective layer. Such a conception is, however, very wide of the truth. In the first place, a glaze like any other form of glass is not, either physically or chemically, a uniform substance. Though it may be possible to produce glaze to which one can give an approximate chemical formula, no chemist would suggest for a moment that such a formula represented anything more than the ultimate percentage composition of a mixed mass which might be compounded in many different ways. A glaze, like any other form of glass, is really a complex of various silicates, very loosely combined, even if they are combined with each other.

Undoubtedly, the correct view of the nature of glaze is that which regards it as an alloy in which the various silicates play the same part as the separate metals in a metallic alloy. With the same ultimate chemical composition, the particular silicates that will be formed in any given glaze will differ within very wide limits according to the temperature and

duration of firing, and the relation between the glaze and the kind of pottery on which it is applied. With a glaze and body of the same ultimate chemical composition produced under firing conditions which are fairly constant, it is possible to obtain constant results, but if any of these conditions vary from time to time, as they will in manufacturing processes conducted on a large scale, then, of course, the results will vary too. It has been found by the accumulated experience of generation after generation of potters, that certain types of glaze have fairly wide limits of stability within which they will produce satisfactory results; while there are other types of glaze which have such narrow limits of stability that they are of very little use for commercial work, their limits being narrower than those commonly met with in every-day practice.

Thus it is that after the experience of centuries, we find certain types of glaze associated with certain types of pottery throughout the civilised world. The universal use of a felspathic glaze on hard porcelain; of tin enamel on faïence or "Delft;" of salt

glaze on stoneware; and of lead glazes on earthenwares made after the English fashion, are the common illustrations that will occur to everyone. It would, of course, be possible to use glazes of other types on each of these varieties of pottery, but in every case the glaze which has become as it were, common or traditional, is the one which experience has proved to fulfil most completely all the conditions of that particular branch of manufacture. Another feature of pottery glazes which marks them out from glass and from the enamels used on metals, is the fact that the glaze is melted or produced on a bed of silica and mixed silicates analogous in composition to the glaze itself. When a transparent enamel is melted at a low temperature on a sheet of metal, there is practically no chemical action between the molten enamel and the bed on which it lies. On the other hand, when a pottery glaze of any type is melted on a piece of ware, there is a decided and in some cases a strongly-marked chemical action between glaze and ware. One of the well-marked properties of complex silicates is the readiness with which, at high temperatures, they will dissolve into each other, and in nothing is this more clearly shown than in the way which a melted glaze attacks the surface of the pottery on which it is fired.

One of the simplest experiments in a chemical laboratory is to dissolve clay by

heating it up with a sufficient quantity of lead oxide. At a fairly low temperature the lead oxide melts and dissolves the clay just as perfectly, though not so rapidly, as hot water would dissolve lump sugar. Many other metallic oxides would behave precisely like lead oxide, in fact it is only a question of getting any particular oxide in a molten or vaporous condition to enable this action to take place. In pottery manufacture this solvent action of melted oxides upon clay has been used as a means of producing glazed pottery for many centuries. The common

mediæval green and yellow glazed pottery made over the whole of Western Europe was produced in this manner. When the vessel had been shaped in any common clay it was dusted over with powdered lead ore (generally galena, the native sulphide of lead), and the clay vessel thus coated was placed in the potter's kiln and fired. The first result of the firing would be to drive the sulphur out of the lead ore, which the increasing heat then slowly roasted into oxide of lead. This oxide of lead in its turn melted and attacked the clay body, now become red hot. In this way, and at one operation, the ware was hardened from clay into pottery, and its surface was coated with a brilliant glassy compound of lead oxide, alumina and silica. The well-known saltglaze of stonewares is produced by an entirely analogous method though with very different materials, and at a much higher temperature. In this case the clay vessels are put into the kiln without any glazing substance upon them. They are then fired to a white heat, at which point the silicates present in the clay itself commence to fuse. When this temperature has been reached, wet common salt is thrown into the kiln and is rapidly decomposed with the formation of vapours of oxide of sodium and hydrochloric acid. The oxide of sodium vapour coming in contact with the white hot stoneware melts some of the clay substance, just as the lead oxide would do, and forms a glass of soda, alumina, and silica on the surface of the ware, which is the well-known salt-glaze.

It might be imagined that while vaporised or melted, metallic oxides are thus capable of attacking and dissolving the surface of a piece of pottery and forming an actual glass with it, natural or artificial silicates, such as the felspar used in glazing hard-paste porcelain, or the fritted glazes used on English earthenware and porcelain would not necessarily have the same effect. Experience proves, however, that

they have this effect. Even molten felspar, which is already a complex silicate of potash, soda and alumina, or of soda, lime and alumina, when it is melted at the high temperature of the porcelain furnace, actually dissolves some of the clay substance. A microscopic examination of a thin slice of hard-paste porcelain, prepared exactly as one would prepare a rock section, shows three clearly marked layers — (a) an outer skin of clear glaze; (b) inside that an intermediate "felted" layer; (c) the body of the ware itself. The intermediate felted layer is clearly a mixture of glaze and body where the molten glaze has attacked the body, and dissolved some of its constituents. The same effect is shown, though generally to a less marked degree, with the ordinary fritted glazes used on English earthenware. When these glazes are fired at a high temperature on earthenware, and the fire is unduly prolonged so as to give a longer time for the action to take place, it is found that their solvent action becomes quite pronounced. I have here a tile of ordinary English earthenware which has been submitted to a prolonged firing, and the glaze has so attacked the body of the tile that it has eaten into it in places, leaving certain portions of the edge exposed almost like bits of hard rock on the edge of a precipice from which all the covering soil has been washed away.

Incidentally, it may be remarked that even those scientific men who have studied the chemistry of pottery most carefully, appear to have overlooked the influence of this solvent action of the glaze upon the body, and the consequent formation of an intermediate layer, with the cracking of glazes which is known technically as "Crazing." A moment's reflection will convince anyone who is familiar with the various kinds of pottery that it is precisely the hard-fired varieties, in which the formation of this intermediate layer is strongly marked, that are least liable to this defect. In the case where a glaze has been melted on pottery at so low a temperature that it is practically unable to dissolve any of the body substance, crazing takes place most readily.

It follows from what has been said that one should always look upon a piece of glazed pottery as exhibiting a gradual progression from the outer skin of what is technically called the glaze to the true body or clay substance, with an intermediate layer within which body and glaze are in various states of

transfusion. As to the exact relation which the intermediate layer bears to the true glaze itself, the three controlling factors are-(a) the chemical nature of the glaze; (b) the chemical composition of the body, and (c) the degree and duration of the firing to which they have been subjected.

Having clearly established why a pottery glaze behaves, when it is melted, like any other fluid, there is no difficulty in imagining a molten glaze as capable of dissolving certain other substances which may be presented to it. Thus if the clay substance contains a considerable quantity of free oxide of iron, as all the common red clays do, a molten glaze of suitable composition would dissolve that oxide of iron; and ifthe iron oxide were present in sufficient quantity, the glaze might even become saturated with it. Everyone who has studied the formation of crystalline bodies knows that one of the readiest methods of obtaining such substances in a perfect condition is by dissolving them in fluids and allowing them to crystallise out as the menstruum becomes more concentrated. The same results can be obtained to a less perfect degree when the solvent is saturated at a high temperature and allowed to cool slowly. Under such circumstances, when the solution is cooled, it is no longer able to retain all the dissolved substance, and if this substance be a crystallisable one, it will separate out in the crystalline form. These conditions can be obtained perfectly in the process of firing pottery glazes, and it so happens that the earliest piece of crystalline glaze which has ever been described was formed in this simple way. There is, in the British Museum collection, a Staffordshire tyg of the 17th century, the body of which is of common red clay, glazed by firing on it powdered galena in the manner already described. When the lead oxide formed from the galena was melted in contact with the clay at a bright red heat, it dissolved some of the silica and alumina of the clay so as to form a lead glaze. This lead glaze must also have dissolved in its turn some of the oxide of iron contained in the clay, and on cooling, this oxide of iron, being in excess of what the glaze could retain in solution, has crystallised out, and the glaze is filled in its under layer with brilliant sparkling crystals, giving it an appearance very m ich like that of the beautiful mineral known as Aventurine." In recent years this particular fo..n of crystalline glaze has been largely developed at the Rookwood pottery in America, and some

specimens which they have produced are quite remarkable, and at the same time singularly beautiful, from the silky golden sheen of the crystals een under a layer of clear yellow glaze. V. hen glazes of this type are produced on suitable shapes, the natural flow of the glaze down the side of the vase tends to arrange the crystals in lines, with their long axes in the direction of the line of flow, with the result that in specimens that have been slowly cooled, so that the changes should have time to take place perfectly, a beautiful striated effect is produced, very similar to that given by the asbestiform threads in the mineral "Crocidolite" or tiger-eye stone of South Africa. By the kindness of the authorities of the Victoria and Albert Museum I am able to exhibit here to-night, some of the most perfect specimens that I have ever seen from the Rookwood pottery, while I have also a group of vases showing crystalline glazes of the same type from my own factory. I have spoken as if the crystals obtained in this way might be simply pure oxide of iron which had crystallised out from the glaze as it cooled, but in all probability the crystals are not simply oxide of iron, but small crystalline plates apparently belonging, by their appearance and optical properties, to that large group of minerals known as Micas." This being the case, we must look upon the crystals, whatever may be their ultimate chemical composition, as one of the complex silicates formed in a glaze which separates out from the other silicates with which it is associated, in such a way as to be visible to the naked eye. We must therefore regard these crystalline glazes not as being entirely different from the clear and apparently homogeneous glazes which the potter generally obtains, but as glazes constituted like all other glazes, save that some of the mixed silicates have separated out from the rest in a crystalline form. Another point which may just be mentioned in passing, is that although these crystalline glazes were obtained in the first place accidentally by means of the oxide of iron which the glaze had dissolved from the body, they can just as well be obtained by adding the oxide of iron to the glaze mixture itself before firing. Other oxides, particularly those of chromium and uranium, readily yield glazes of a similar character. It is now nearly ten years since my brother first turned this knowledge to account in the production of our Sunstone glazes, many of which are exhibited here. These can be obtained in a variety of colours;

thus we have many shades of green, of yellow and of brown, while at a hard fire, approaching to that of true porcelain, they develop a strong steely-blue colour. A microscopical examination of our sun-stone glazes proves that they are very similar in their optical properties to those of the Aventurine or tiger-eye glazes already described. The crystals are small hexagonal plates having the general character of "Micas."

The decorative effect of these sunstone glazes, though rich and subdued, is yet very striking, for in addition to the clouds of brilliant golden crystals which are disseminated through the green, olive, brown or yellow glazes, we have found that the mixtures which give the best crystalline effects have peculiar colour qualities such as have never been obtained in pottery glazes of the ordinary type, so that in colour alone they are a decided addition to the potter's palette.

We must now enter on the consideration of the better known crystalline glazes that have been produced of late years on the hard-fired porcelains and stonewares of the Continent. In this case the crystals appear as radiating needle-like or starry groups, sometimes white -when they recall the patterns traced on the window pane by frost-sometimes brilliantly coloured blue or green, or at other times having a fine bronzy sheen recalling that of burnished metal. In this case there is no question of any extraneous oxide dissolved by the glaze setting up a crystallisation. The glazes which give these wonderful crystalline forms are the ordinary felspathic glazes used on true porcelain, to which have been added artificial frits carefully compounded so as to be in effect tri-silicates of zinc, or of zinc and potash. At the high temperature of firing, which in this case approaches 1,350°C., all the constituents of the glaze melt to an apparently uniform glass, but, on cooling, some of the silicates, probably the silicate of zinc, or a silicate of zinc and alumina separate out from the remainder, and assume these beautiful crystalline forms. We have found on our own works that the same effects can be produced whether the silicate of zinc be added to the glazing mixture as an artificial substance obtained by melting together at a very high temperature the requisite proportions of oxide of zinc and silica, or as the natural mineral silicate of zinc, known as Willemite." There is a very singular fact in connection with these crystals which may throw some light on their constitution, and that is, when

the general colouring oxides used by potters are added to these glazes the crystals behave differently with oxide of copper CuO. than they do with oxide of cobalt Co2Oз. or oxide of iron Fe2O3. If the glaze contains oxide of copper, the crystals separate out as white colourless needles, but in the other cases the crystals absorb a large proportion of the oxide of cobalt or oxide of iron contained in the glaze, becoming, in the first case, of a brilliant cyanine blue, and in the latter case of a fine yellow bronze colour almost like yellow metal.

[A number of specimens of crystalline glazes of various colours from Copenhagen, Röstrand, and Sèvres were exhibited.]

Considering that these crystalline glazes were first developed in Europe on true porcelain, it is rather remarkable that the Chinese and Japanese do not appear to have produced such effects even accidentally. Probably this is due to the fact that, so far as we are aware, they have never used oxide of zinc as an ingredient of their glazes. Quite recently Mr. Bernard Moore drew my attention to a few specimens of modern Japanese porcelain in which small cryptocrystalline patches are strongly marked. I have here on the table one of these pieces which I found some time ago in London, and as from the peculiar tone of the glaze I am convinced that the glaze contains zinc, I think that there can be no doubt that the wide-awake Japanese are already turning their attention to the production of crystalline glazes like those on the modern porcelains of Europe. Their results at present are very rudimentary, but in a few years' time European potters may have to look to their laurels in this scientific development of pottery decoration also.

Hitherto it has always been considered by the continental potters who have been working with this type of crystalline glaze, that it was impossible to obtain good results at a lower temperature than 1,250° to 1,350° C., and that consequently the crystalline effects could not be produced on earthenwares, the glaze of which is fired at so much lower a temperature. After a series of researches extending over several years, it has, however, been proved at our works that crystalline glazes identical in type, in colour, and in decorative effect, can be produced on English earthenware at temperatures even as low as 1,000° C. I am able to show you to-night a number of vases decorated in this way with crystalline glazes, and all on the body of our English earthenware. Moreover, by

treating tiles of this kind exactly as one would treat a piece of mineral in order to obtain a thin section, I hope to show you on the screen with the lantern, slices of crystalline glaze which will demonstrate how perfectly developed and symmetrically arranged the groups of crystals are.

Although we have succeeded for the first time in producing crystalline glazes similar to those on the hard-paste porcelains of the Continent, at temperatures much lower than they have ever been made before, we were faced with the same difficulty that the Continental potters experienced, from the capricious way in which the crystals are developed, even when every imaginable care has been taken to ensure regular and perfect conditions. In all the Continental work these glazes have been used all over the piece, leaving the crystals to develop very much as they may. Were it possible to produce these crystal growths exactly where and as one wishes on the surface of a vase, a dish, or a plate, it would be possible to turn them to the highest decorative account by combining them with painting, or relieving them with other colours. As it is, the glazes are too capricious to admit of such perfect control; we are too much at the mercy of the kiln, or as one ought to say, we do not know sufficiently how to govern the conditions which determine the formation of the crystals, to use them just as we would. You will have observed in all the examples of vases, photographs of which have been thrown upon the screen, and in all the specimens actually displayed here upon the table to-night, how capricious the results are, indeed I believe in many of the Continental factories it is possible to produce only a small percentage of really fine examples, so that there still remained a wide field for the exercise of the patient skill of the English potter.

I have now to bring before your notice an entirely novel and most striking variety of crystalline glaze, that was discovered in our researches in this direction about twelve months ago, and the whole science of which is so well known to us, that it is as perfectly under control as the production of the commonest coloured glaze used on garden pots or tiles. The glaze in question is in itself of a bright yellow colour, but it is shot through and through with lines, groups, or patches of brilliant golden, prismatic crystals. These crystals have very much the same optical character as those found in the sunstone glaze already referred to, but they are

considerably larger, and are so brilliant that they make the vase or piece of pottery on which they are applied shine like burnished copper, or like a piece of fine Japanese gold lacquer. Scientifically and technically, the most singular feature of these latest crystalline glazes is the effect on them of a gradually increasing temperature. The crystals are developed in their full perfection and brilliance only when the glaze is matured at a temperature of from 1,000°C. to 1,030°C. When the firing temperature is increased, the crystals are reabsorbed by degrees into the glaze, and between 1,050°C. and 1,070°C., while the crystals are much smaller, the glaze generally assumes a beautiful purple tone. Above 1,070°C. the crystals are entirely reabsorbed, and the glaze once more becomes of a brownish-yellow colour which is more or less opaque.

[A number of vases which showed all these transitional stages produced by varying degrees of fire were exhibited.]·

It would be impossible to find a clearer or more perfect exemplification of the statements made in the early part of this paper as to the real nature of pottery glazes. Here, at one stage of the firing, some of the silicates are in such a loose state of combination that they will sepa rate out from the general body of the glaze in thin, prismatic, crystalline plates. If the firing be pushed further, these crystalline separations are dissolved at a higher temperature by the other silicates of the glazes, until, at a higher temperature still, they form an apparently homogeneous compound, in which no traces of crystallisation are apparent. It is possible, however, to regulate the firing of a modern pottery kiln so perfectly that we can produce exactly the kind or degree of crystallisation we require, so that these crystalline glazes, along with the sunstone glazes, can be used in conjunction with other colours, i.e., they take their place in a decorative colour scheme with other pottery colours of an entirely different character. The first and most obvious method of turning them to account is that of using them on pieces decorated with various coloured clays. Here are a number of vases which have been shaped in a dark buff clay; patterns have then been painted on them in white slip, and the crystalline glaze being laid over the whole piece we get a very simple form of decoration, which while it enables the crystals to display their beauty, gives them an added interest and variety which is all to the good artistically.

Reverting now for the moment to the idea