Lustre Ware The word 'lustre', in its connection with pottery, has not always been used in its present limited sense, but rather as an equivalent of our word 'glaze'. It has been applied to the glossy surfaces of the Greek, Roman, and Etruscan pottery, although the gloss upon these is comparatively slight. Nevertheless they are lustrous as compared with unglazed tile or brick. In a more restricted sense the term may describe any metallic deposit on a glazed surface where the decorative effect depends upon, or is enhanced by, reflected light. But even this definition would be now-a-days disallowed by the common understanding of the word, which applies it almost invariably to the special iridescence of the Persian, Arabian, Hispano-Moresque, and Italian majolica. This iridescence, the means by which it has been and is obtained, and more especially the means by which I have myself endeavoured to obtain it, form the subject of the following paper. It is a subject which may be treated either historically or technically, but the two lines of treatment intersect at many points, and it is at these points, if at any, that what I have to say may be of interest. I will, therefore, before describing my own experience, give the shortest possible retrospect of the history of the subject, although in so doing I shall no doubt be going over ground already familiar to many now present.
All investigations seem to converge towards the conclusion that Egypt or Assyria were the earliest nurseries of the art of decorating pottery, which afterwards attained its maturity in Persia. The glazed and coloured terra-cottas from the palace of Darius, brought from Susa by M. Dieulafoy, and now in the Louvre, show that those who produced them possessed all the materials and almost all the processes known to their successors. But there is no apparent knowledge of the lustre process either in these or any other ceramic productions of remote antiquity, although an accidental stain of copper lustre has been detected on an Egyptian vase in the British Museum. The earliest lustres known are those found on fragments dug from the ruins of Persian cities, more especially those of the city of Rhe, or Rhages, to which a very great antiquity has been assigned. They have been generally accepted as antedating the destruction of the city 600 years ago. But although Sir R. Murdoch Smith speaks of a possible age of 2000 years for some of them, this is very conjectural; and until we can find proof of the process being in existence in Persia previous to the Arab conquest in 641, its Persian origin cannot be considered beyond a doubt. However ready we may be to ascribe a Persian parentage to the art of the Arabs, we cannot shut our eyes to the fact that the area of Arab conquest in the ninth century is almost exactly co-extensive with the distribution of the manufacture of lustres, so far as it is known to us, in the twelfth. Before the Arab conquest, decorated Persian ware is known to have existed by surviving samples, all without lustre. As soon as our authentic record of lustre-making begins, it is found throughout the conquests of the Arabs, and nowhere else. In 1154, Edrisi, the Arabian geographer, speaks of it as being then made at Calatayud, in Spain, which was at that date a recent conquest of the kings of Arragon from the Saracens. A century before (1040), Nassairi Khosrau testifies that they were made at Cairo. These are, I believe, the two earliest trustworthy testimonies to the manufacture of lustred pottery.
Perhaps the safest judgment as to the origin of the process in modern times is that which makes the Arabs the distributors, east and west, of a knowledge transmitted to them from Egypt or Assyria. It is stopped in its journey eastward at the borders of the Persian empire, and is conspicuously absent from China and Japan. Its non-appearance in the manufactures of these countries may be accounted for by the nature of the materials already in use. I have tried to get lustres on Chinese and Japanese ware, but have always failed; the glazes appear singularly refractory. Other causes may have helped to prevent the Persian or Arabic process going eastward. In the other direction, the course of the Arabs, from Cairo to Tangier, has been said to be traceable by the glazed and decorated wall tilings of their buildings, especially the mosques. No doubt, this is in some sense true, but it has been more than once told so as to convey a false impression that the Saracen invaders of Africa built tile-kilns at every station of importance, and that pottery factories were at work in Spain, if not during the time of the Abbasides, at any rate very soon after the establishment of the Caliphate of Cordova. Wall tiles, beautifully decorated, were placed by the historical imagination on the walls of the great mosque at that town, and by implication at Seville and Toledo also. But the tendency of more recent investigation is to ascribe all the surviving examples of Arab wall tiling in Spain to a much later date. However, there might have been wall tiles in the mosque of Abd-el-Rhaman of native manufacture, as fragments of decorated pottery, supposed by some to have been made in Spain about the year 969 A.D. occur in the Museum at Grenada. The construction of the mosque was still going on in the time of the vizier Almansor, who melted up the bells from the shrine of Compostella to make lamps for the mosque, at which he used to work with his own hands. This was in 985. But, if we judge by contemporary descriptions of buildings, these great mosques, and others, such as the palaces of Az-zahra, at Cordova, were marvels of decoration in marble, gold, and ivory, but were entirely without wall tiling. Moreover, mosques of the same period, at Cairo and elsewhere, are entirely without tile decoration.
The next landmark in the history of the subject is the erection of the Alhambra by the Moorish kings of Granada. The old tiles with which its walls are covered are genuine native azulejos of the date of the completion of the building, about 1350. They must be distinguished from those placed in the building when it was restored by Charles V. in the sixteenth century. They belong to the same group of manufactures as the great jars which were found full of coin under the building. The well-known one, of which there is a copy by Deck at South Kensington, is still in the Alhambra, and there is a similar one in the museum at Madrid. These and one or two others are the oldest surviving examples of the practice of lustre in Spain. There does not seem to be any need to assume that they were imported from Cairo or Persia, and we may probably ascribe their fabrication to Malaga. The Alhambra tiles may have been made there too, although in view of the comparatively simple operations involved in the making and firing of the latter, and the vast quantity required, it might be more reasonable to suppose they were made on the spot. It would have been far easier to build a kiln on the works at Granada than to carry all that weight of tiles over the Sierra Tejada. But the pots were quite another thing. They were made for exportation as well as native consumption, and Ibn-Batoutah, an Arab traveler, found the manufacture in full work at Malaga in 1350.
Time, or rather the want of it, prevents my making more than a passing allusion to the lustred wares of Sicily. There are some examples of what has been called Siculo-Arabian ware to distinguish it from the dark blue pottery covered with small diaper ornament, which is ascribed to Saracenic or Moorish potteries at Calata-girone (about thirty miles from Syracuse), probably of the period of the maturity of the Hispano-Moresque. I believe there is nothing antedating the Norman Conquest of Sicily. But there is documentary proof of the exportation of faience to Sicily from Barcelona, in 1528, which suggests that the manufacture never became firmly rooted. However, we must remember that Sicily had been under the Spanish rule, or misrule, from 1479, and hunting Saracens was a part of their political economy. The desirability of extending the export trade may have accelerated the suppression of the Siculo-Arabs. I am rather surprised that the near coincidence of this date of the Spanish in Sicily with the appearance of lustres in Italy should not have caused the ascription of its Italian origin to these particular potters and no others. It would have been at least as plausible as the other theories.
One of these had its origin from the existence of bacini, or bacili, with which the walls of certain churches at Pisa and elsewhere are decorated. Marryat, the historian of pottery, ascribed a Saracenic character to these, and framed a theory to account for them which is so picturesque that I wish it were true.
In 1115, the Pisans made a crusade against Nazaredek, the Saracen King of Majorca, whom they overcame after many mishaps by land and sea. They liberated no less than 30,000 Christian captives from his dungeons, and returned home with shiploads of Saracen prisoners, gold and silver, embroi- dered garments, and decorated crockery, to say nothing of two large porphyry columns, which they afterwards gave to their allies, the Florentines, and which stand on each side of Benvenuto Cellini's Baptistry gates to this day an eternal testimony to the truth of the story. The decorated bacini in the walls of the churches at Pisa were supposed to have been placed there as trophies, and, indeed, continued to be thought so until Mr. Fortnum's examination of them in 1868 settled the matter. He made a close inspection of those on six churches at Pisa, and found only one fragment of Saracenic origin. Marryat had the tale of the crusade-which, no doubt, is good history-from Sismondi, but Sismondi says nothing whatever about the cargo of lustred pots. His chief authority is Laurence of Verona, who wrote a poem about the Crusade. It does not seem to contain any allusion to the bacini or kindred subjects, but then my failure to find it in 3000 Latin hexameters is far from conclusive. However, what I rely on is that I have seen no citation from them in Mr. Fortnum's writings. Mr. Fortnum has been over the ground, and had there been a word about the bacini in the poem, he would certainly have found it out. The passage in Marryat's history conveys the impression that he did not look behind Sismondi, and indeed that his conjecture was a mere passing surmise, chiefly based on the bacini in the church walls themselves.
Another conjecture, which would have been just as good as the foregoing if the dishes had been Oriental in character, ascribes their origin to the Saracen merchant ships captured by the Pisans at Syracuse or Palermo, at the time of their alliance with the Normans in their invasion of Sicily in the tenth century.
Another, the latest I have met with, is that of M. Mely, who tells us that at Tchakindji in the Caucasus, the armorial bearings of Genoa and Pisa occur on mediaeval buildings, and that the mosque of Erivan in that neighbourhood is richly decorated with tiles. He suggests that this may have been the point of contact between West and East which caused Italian pottery. But there is no difficulty in finding ways in which the Italians might have learned technical secrets from the Saracens. The difficulty is to connect the actual art, as it appeared first in Italy, with any one of the sources to which it has been ascribed.
Majorca has certainly had the preference hitherto. But we have no actual record till 1442, when one Giovanni di Bernardi, of Uzzano, in a treatise on navigation, speaks of 'the war of Majorca and Minorca, which had then a very large sale in Italy'. To my own thinking, there are two things which account for the prominence given to Majorca. One is the story of King Nazaredek and the conjecture Marryat founded on it about the bacini. The other is the name majolica, applied to lustred ware at first, and afterwards to all Italian faience. But when we come to look for the first use of the word in the sense of pottery, we are referred to Julius Gesar Scaliger in the sixteenth century, who says that the recent skill of the Majorca potters has made such clever imitations of Indian (Chinese) pottery, that it is hard to tell them apart, and that these are called after Majorca, all but a letter or so. An antiquity which this record is not entitled to creeps round it in the mind of any unobservant reader when, in close proximity, he reads the line of Dante, in which he speaks of ''Tisola di Cipri e maiolica'. I feel certain that many who have seen this connection of two writers born over 200 years apart, must have run away with the idea that Dante called earthenware majolica.
There is also the testimony to the importation of Spanish-Moorish ware from Valencia to Italy of Escolano, a Spanish writer, who says that Pisa was the port to which the Moors exported their faience, in exchange for that of Italy. This is a very odd statement. If the merchants of that date ( before 1600) really brought faience back from Italy, it must have been something of great value, some article of luxury. It is utterly inconceivable that they should bring back freights of cheap serviceable earthenware to a country famous for its crockery since the days of the Romans, when Murviedro was Saguntum. On the other hand, if rare and highly decorated work came from Italy, where is it now? It is well known that the bulk of the fine samples of Hispano-Moresque were found in Italy, having been exported to the Italians as a favourite article. There should have been a few, at least, of corresponding samples in Spain of imported Italian ware. However, it is noticeable that testimony of Spanish exports to Italy at an early date should aso bear witness to the importation to Spain of Italian ware presumably of equal value. It is also a fact of much importance that, while all this testimony is limited to showing that there was a commercial reciprocity between the Spanish ports and Pisa, the first records of lustred pottery in the north of Italy begin, not at Pisa, but at Pesaro.
Pesaro is a clay centre, and a seat of pottery manufacture from a remote antiquity. Passeri, the earliest historian of Majolica, was Vicar-general of Pesaro, and although he writes with some natural partiality for the town, I believe his conclusions are generally respected, especially as he had opportunities for inquiring into the earlier manufacture which no longer exist. He places the decoration of the mezza-majolica (as the ware came to be called) as early as the end of the trecento period, and the characteristic madreperla lustre towards the end of the quattrocento. He also claims for Pesaro the honour of having produced ruby lustre in 1480. If some of the mezza-majolica at the British Museum is rightly dated, there is no reason why this should not have been the case. At any rate, Pesaro is where we have to look for the Moors, who came and taught the Italians lustre.
Now, Pesaro is a long way from Pisa, and the Apennines are between. It is quite as near Venice, and is a seaport. For every one communication between Pesaro and Pisa in the Middle Ages, there would be a hundred between Pesaro and Venice. And between Venice and Syracuse there would be much more frequent communication than between Valencia and Pisa. If we must have Arabs to teach the Pesarese lustres, why not go to Sicily for them. The fact is, that a mistaken historical clue guided the inquirer to Pisa first, in the case of the twelfth century bacini, and it is difficult to get away from it. If another instance of bacini-that of the church of Lucera, in Apulia-had attracted attention to the same extent, we probably should now find it equally difficult to get rid of the Moors or Arabs of Calata-girone. For my own part, I do not believe either had anything to do with it, beyond the impulse the circulation of their ware gave to the ingenuity of one of the cleverest races in the world in an age of increasing artistic activity.
I have lately had the satisfaction of finding that this opinion is also expressed in the work of M. Theodore Deck, on 'Ceramics'. M. Deck's opinion ought to have great weight, as he is certainly the most eminent practical potter who has written on the history of the subject.
I have dwelt upon this historical evidence, because it is just one of the points at which the lines of history and technical inquiry intersect. I will now give my version of the other side of the matter.
The technical distinctive feature of Hispano-Moresque ware is that it is invariably on a white ground of glaze rendered opaque by oxide of tin. All the mezza-majolica, the early Pesarese ware, is on the white ground described by Passeri, which had, according to him, been in use since 1300, perhaps earlier. It is not a tin enamel, but a white slip covered with a mixed glaze of lead and alkali, marzacotto. Not a single dated example of lustre appears in Italy on a tin enamel till near the end of the fifteenth century, either at Pesaro or elsewhere. Now I can testify, and so can every potter who has ever made lustre, to the facility and certainty with which it can be produced when tin is present in the glaze as compared with other glazes.
That the Italian potters should learn from a Moorish source, and yet get so little clue to the superiority of this white ground that no experiment in its use should survive, and no tradition be handed down, is most improbable. But it becomes more improbable still at the next step in the history of majolica when, about 1475, or perhaps rather earlier, the tin glaze begins to supersede the white slip. The course the Italian potters took then was to use it not as the glaze par excellence susceptible to lustre, but as a whiter substitute for the white slip, glazing it over with marzacotto precisely as they had glazed the old ground. It might be difficult to pronounce certainly on this point from inspection alone, but there is other testimony. Piccolpasso, in his very explicit description of the Gubbio process and Italian pottery generally, in 1548, leaves no room for doubt. He notices the necessity for glazing the tin glaze thinner than the white ferra di Vicenza, or slip. But there is not the slightest hint that the old marzacotto glaze was ever discarded.
It may be objected that the tin glaze was unknown, or unobtainable. There is strong evidence to the contrary even as to its use by Italian potters, and the tendency of recent research is to place the date of its adoption earlier and earlier. But apart from this, it was in use in Italy for another purpose long before. Luca della Robbia had had time to mature his method of covering terra-cotta with it, and to place his two great bas-reliefs above the doors in the Duomo at Florence by the year 1438. So that, however much we strain dates, there is a long period during which the tin enamel was known in Italy, and yet the makers of the mezza-majolica persisted in the use of a ground of their own, while, had they been acting under Moorish instructions, they must certainly have been looking and longing for the more manageable Moorish ground. If Luca got his knowledge (as Jacquemart supposed) from the potteries of Faenza or Caffaggiolo, it makes the combination of circumstances still more puzzling. Faenza is a two days' journey (about seventy miles) for a horseman from Pesaro, certainly not more-and the interchange of employe's was probably not uncommon.
I believe what I have said would hold good equally whatever view is taken of the exact date of the first Pesarese lustres. The question is a little complicated by the fact that the date of the lustre is not of necessity that of the piece. An unlustred piece of faience, 300 years old, could be put through the kiln now, and no human penetration could discover the date of the lustre. Bacili made at Pesaro in 1450 may have been lustred at Gubbio in 1550.
I leave this part of the subject with many things unsaid, in order that I may not be forced to omit the next landmark in the history. The bottega at Gubbio, and its connection with Giorgio Andreoli, has been considered as the most important of all from an artist's point of view. Maestro Giorgio was a sculptor, a pupil of Luca Della Robbia, who took to pottery and worked at this bottega, which had already produced some fine lustre work, perhaps as fine in all decorative respects as any which followed. The free use of ruby distinguishes this from the mezza-majolica, and the Maestro Giorgio productions may be again distinguished from it by a varied character in the ruby, and a finer white ground. The aim evidently is a transparent ruby to be used as a pigment. To my own thinking, the work throws doubt on its suitability for this purpose. However, there can be none that in his time the colour attained its greatest brilliancy. The dates range from 1418 to 1537, the later examples being by his son and other contemporaries, who carried on the work a little longer. But all lustres disappear from Italian decorative work about 1550, so that a period of 60 years covers the whole production. I have before spoken of the longer lifetime of the Hispano-Moresque process, but, in both cases, the disappearance was only that it ceased to be used on elaborate work; as certainly in Spain, and possibly in Italy, the practice has never completely died out.
It is more difficult to account for its disappearance so long before the collapse of the arts in the eighteenth century than for the vigorous and successful attempts to out-do the Moorish pottery in the age of Renaissance. It may satisfy some to say that a change of fashion did it; but this is only substituting one phrase for another, and very nearly means nothing at all. A plausible surmise is that its use became incompatible with the careful and dexterous brushwork on fresh-dipped enamel, which reached to such perfection at Castel Durante and Urbino. The greater an artist's success in manipulation, the less is he disposed to incur the risks of an uncertain firing. A lustre-kiln may spoil the whole outright, or even if the lustres themselves are successful, may only come into existence at the cost of elbowing their neighbours out of it. So long as the material was used for its own sake, and no attempt was made to go beyond its natural limits, all went well, but as soon as the artists began to be discontented with the restrictions its use put upon their opportunities of showing their own dexterity, the lustre colour had to give way. It may have been this, or possibly the demand may have been diminished by the disappointment consequent on local mishaps. The death of a furnaceman of special skill might cause a suspension of work, or, what is more likely, a substitute might be found whose failures might lead to a belief that the process depended on some secret which his predecessor had kept to himself. Whatever the cause, or complication of causes, they vanish in 1550, and do not re-appear for 300 years.
The process is not described by Brongniart, who was the great technical authority on pottery of fifty years ago; and Salvetat, who was his successor, makes only a very speculative allusion to its possible character. And in the catalogue of the Great Exhibition of 1851, which is a sort of death register of the arts of antiquity, not a hint of lustred pottery appears. The modern revivals begin with those at the Ginori factory at Doccia, near Florence, and those of Carocci at Gubbio, of which Mr. Fortnum speaks very highly. There were some of these in the 1862 Exhibition in London. I have never seen any myself. The best I have seen are those of Cantagalli, at Florence.
In spite of the Doccia and Gubbio reproductions, an impression continued to prevail that the process was a secret. I used to hear it talked about among artists, about twenty-five years ago, as a sort of potters' philosopher's stone. At that date the attempts to reproduce it in England had met with only very partial success, although an Italian had gone the round of the Staffordshire potteries showing how to do it. Even now it is sometimes spoken of as a secret by newspaper writers. My attention was attracted to some very interesting work of Massier, of Cannes, in the last Paris Exhibition, by a newspaper paragraph headed 'Re-discovery of a Lost Art'.
In fact, re-discovery appears to have dogged the footsteps of the lustres from the beginning. I re-discovered them myself in 1874, or thereabouts, and in the course of time some of my employe's left me, and re-discovered them again somewhere else. I do not think any rediscoveries of this sort contributed in any way to the very general diffusion of the process in the potteries at this moment. Very likely some of them have an earlier record than mine, but the only one I chanced upon when I was in Staffordshire was that of the late Mr. Clement Wedgwood, who showed me a number of experiments which would have been successes if the glaze had been suitable, and a small sample shown me by the late Mr. Colin Campbell. As far as the technical difficulties of simply evolving a copper or silver lustre go, I see no reason why ( as in the case of the Arabs and Italians) every discovery should not be totally unconnected with every other. But there was one thing the Italians found out, when they reproduced the Moorish firings, namely, how to make a strong, and beautiful, and original use of their materials. It may be that the less we say about the modern parallels of their case the better.
Perhaps we may now make a new departure, and consider that the process is as well known as any other process in the arts; at any rate, I will contribute what I can to make it so, by telling all I know of it myself. I got nothing from Piccolpasso, as I did not see the work till long after, nor from any printed information, except the chemical manuals I had read in youth. The clue was furnished by the yellow stain of silver on glass. When overfired this shows iridescence, which is often visible on the opaque yellow visible from the outside on stained glass windows. I tried the stain on Dutch tiles, and found them unsusceptible in the glass kiln, but, in a small gas muffle, I found that both copper and silver gave a lustre when the gas was damped down so as to penetrate the muffle. I pursued my investigation, and, after an interruption, occasioned by setting the house on fire and burning the roof off, I developed the process in Chelsea. This was 1873-4, since which time it has not varied materially, although I have tried many experiments, with a view to improving it.
As we now practise it at Fulham, it is as follows: The pigment consists simply of white clay, mixed with copper scale or oxide of silver, in proportions varying according to the strength of colour we desire to get. It is painted on the already fused glaze with water, and enough gum arabic to harden it for handling and make it work easily-a little lamp black, or other colouring matter, makes it pleasanter to work with. I have tried many additions to this pigment, of infusible white earths such as lime, baryta, or strontia, and other metallic oxides, but without superseding the first simple mixture. Any infusible clay will answer the purpose, though we have always used kaolin, as the least fusible. In Deck's work on pottery he gives several receipts for lustre pigments, only one of which seems to me to belong to the true process of lustre. The others all contain sulphur, which is not necessary, though it may work very well. The sulphur lustres are akin to the old Swansea lustre, which only requires to be burnt at a low heat without smoke. The sulphur evaporates and leaves a metallic deposit which is not oxidated, or only partly so, by the access of air after the sulphur vapour has left the kiln. I believe all the lustres included in the colour-maker's lists are of this nature, but the results produced in modern ware do not tempt the investigator. The prettiest one I have seen is Burgos lustre, which, however, contains gold. The only ingredient containing sulphur mentioned by Piccolpasso would be the small quantity of vermilion (that is, if cinabrio means vermilion), which he adds to his receipt for oro. Piccolpasso's recipes are for the diluent clay only, as he says nothing of either copper or silver. But he had them from hearsay, and if he really tried to produce lustre with them without any addition of metal, it quite accounts for no lustres ever appearing at Castel-Durante, where he was master potter. Indeed, it raises the question whether he was not hoaxed by Maestro Cencio, Giorgio's son, who is supposed to have given the information.
The ware, when painted, is packed in a close muffle, which is then raised to a very low red heat, so low, when the ordinary tin enamels are employed, as to be only just visible. A charge of dry wood, sawdust, wood-chips, or, indeed, any combustible free from sulphur, is then introduced into the muffle through an opening level with the floor, a space having been left clear under the ware for its reception. As soon as it has blazed well up, the opening is closed. The flare then chokes down and the combustion of the charge is retarded, the atmosphere in the muffle consisting entirely of reducing smoke. The test pieces will soon begin to show a red or yellow stain, the pigment itself looking black, until it is wiped off to show the stain. This operation must be repeated until the tests look right, when the fires should be drawn and the muffle left to cool.
The difference between this operation and Piccolpasso's is chiefly in the use of the closed muffle, which is rendered necessary by the difference in fuel. The sulphur from coal or coke would injure the glazes where there was no lustre, and would interfere with the process itself. In the Italian process, where wood is the fuel, the wood is packed in a perforated sagger, into which the smoke from the furnace is choked back by closing a damper, or by simply increasing the volume of smoke from the furnace by heaping on brushwood. But the principle of the operation is the same in both cases, and the dangers are the same. The firing may be vitiated in either by any of the following causes. There may be too great heat, or too prolonged heat; the smoke may be too dense, or too attenuated, or not long enough maintained, or the reverse. If more than one of these factors is wrong at the same time, the harm done will be in proportion. Even when the conditions are most closely observed, the results will show unexpected variations. It is impossible to secure uniformity throughout a muffle. Consequently, the size of the ware must be small in proportion to that of the muffle, or a vase might be overdone at the top and underdone at the bottom, while a number of small pots in the same space would have turned out very well, a few of the top ones being uniformly overdone (and perhaps little injured), and a few of the bottom ones underdone, and only wanting a second similar firing. This also makes a longer and slower firing necessary with larger ware, and this means more risk.
The different sorts of copper lustre may be classified thus:
1. Opaque metallic copper deposited on the surface of the glaze. The oxide is in this case probably reduced at the moment of deposit. Nearly the same result takes place in the common lustre of the potteries, where the sulphur of the sulphide of copper is driven off by a low heat.
2. Combination of copper suboxide with the glaze without reduction to metal. This is to all intents and purposes the same thing as when glass containing copper is flashed and becomes ruby. The harder the glaze is, and the higher the temperature, the less likely is a deposit of metallic copper.
3. The result of prolonging heat without smoke on No. 1. The deposited copper is thus slowly absorbed into the glaze, becoming ultimately red without lustre, but passing through every intermediate stage.
4. The result of increasing the reducing agent on No. 2. In this case the oxide already in combination is brought back to the state of metal. I believe that all the best lustre should be classed with this or No. 3. Silver lustres show the same results, but at a lower temperature. So when both lustres are fired together, we may expect Nos. 3 and 4 of silver lustre, with Nos. l and 2 of copper.
The ugliest results are when the glazes are overcharged to the point of opacity. But accidents of this sort may be taken advantage of when the designer foresees the result. For instance, great blotches of opaque pae yellow on an inky black background may be very ugly, when an arabesque of fine lines of the same yellow on the same ground might be rather pretty.
I have said that the tin glaze is the most susceptible to lustre, but it does not necessarily give the finest results. The Gubbio lustres are really on superposed marzacotto, and possibly the exceptional beauty of some Persian lustre may be due to what is often called a siliceous glaze, which is what I call an alkaline glaze, as all glazes are siliceous. A film of such a glaze over the tin would almost elude any possible means of detecting it, and yet would scarcely be penetrated by the lustre colour, so thin is it.
Ihe best of the first lustres I made on Staffordshire ware were on ironstone or granite. The body was repellent in colour, but the glaze particularly good. Latterly, we have used the common opaque white made with tin. It has also been ugly in colour, being, I believe, made so by the addition of cobalt, to make it whiter, just as the house-painter spoils his beautiful white chalk with French blue. I have tried many experiments with glazes, but I am inclined to think that the way they are fired in the glost oven has as much to do with their adaptability for lustre as their chemical composition.
I have also tried in this past 20 years a vast number of experiments, with the idea of adding to the first simple process of the Arabs. To save others needless work, I will enumerate a few, with my recollection of their results.
1. Reduction by other agents than carbonaceous smoke: by ammonia, bysteam in contact with reducing fuel, by coal-gas, by vapour of water and glycerine or spirit. None of these gave any new results.
2. The use of copper and silver colours as enamels, or under glaze, and their subsequent reduction by any of these agents. Sometimes there were good results, but the colour was always patchy.
3. The deposit of copper or silver from vapour of the chlorides, ammonium chlorides, or iodides, those portions of the glaze being protected which were to remain white. These experiments might be repeated with advantage. A similar one was the painting of the pattern in a susceptible glaze on a refractory one, and its exposure to vapour containing copper or silver. The suboxide of copper itself vaporises under certain conditions, which is the cause of the flown red colour occurring on many examples.
I have, of course, tried endless modifications of the ordinary process, such as using special woods for smoking, sawdust, shavings, paraffin, and other combustibles. Any of these answer the purpose, the application being slightly varied. But nothing material has come of any of these experiments, and the process remains substantially the same as at first. I believe that if there had been any new opening for the application of chemistry, although I might not have followed the clue successfully, I could hardly have missed it altogether.
In conclusion, I may say that I believe we have learned all there is to know of the chemical and mechanical side of the art, as it was known to the ancients. What remains to be discovered in order to produce original work, equal to that of the Renaissance, is not a technical mystery, but the secret of the spirit which animated the fifteenth century not only in Italy, but all through Europe. We have got the materials and many more, but the same causes that forbid the attainment of new beauty with the new ones, have stood between us and the revival of old beauty with the old. In saying this, I do not suppose myself to be going outside a universally accepted truth, or, at any rate, one that is very rarely questioned. Some day there may be a new imagery and a new art. In the meanwhile I can only say that if anyone sees his way to using the materials to good purpose, my experience, which I regard as an entirely chemical and mechanical one, is quite at his disposal.