COPPER OXIDE IX T H E B O R A S BEAD*
BY WILDER D. BANCROFT AiVD R . L. NUGEKT
Under the oxidizing conditions that have been described’ for manganese in borax melts, copper gives a beautiful, clear, blue glass. When the proportion of boric oxide in the melt is increased gradually, the blue first changes to a light apple green. With further increase of boric oxide, there is a separation of a reddish solid, which is presumably cuprous oxide. The same phenomenon occurs on changing a copper-borax bead from the oxidizing flame to the reducing flame. The natural assumption is that the change from blue to green is due to a reduction of a cupric compound to a cuprous one. With increase in the relative amount of boric acid, more of the cuprous compound is formed, until red cuprous oxide separates due to its insolubility in melts high in boric oxide. The first thing to be done in order to verify this assumption was to find a method of determining the ratio of cuprous to cupric compounds in borax glasses. The glasses were dissolved in a solution of ferric sulphate and sulphuric acid. The cuprous compound present in the borax glasses reduces the ferric salt to ferrous salt and this latter can then be determined by titration with permanganate. Since it is not convenient to weigh out ferric sulphate diiect, we started with ferrous sulphate and converted it into ferric sulphate. To 150 cc water there were added 50 cc sulphuric acid and I O g FeS04.iH20. The ferrous sulphate was oxidized to ferric sulphate by means of three percent hydrogen peroxide, the excess of the oxidizing agent being removed by boiling and the resulting solution diluted with an equal volume of water. The final titration was made with 0 . 0 2 5 normal permanganate. A synthetic mixture of o . o j 0 4 g cuprous oxide and 0.0496 g cupric oxide, containing therefore 0.0448 g cuprous copper, showed 0.00447 g cuprous copper by this method of analysis. When 0.05 g cupric oxide was heated in three grams of borax in the usual manner, analysis showed the presence of I j percent cuprous copper in the melt. With three grams of a mixture containing 4.6 mol percent sodium oxide-an excess of boric oxide-the percentage of cuprous copper rose to about 2 1 percent. With an excess of alkali-1.6 mols sodium oxide to I mol boric oxide-the percentage of cuprous copper dropped to about 4. This indicates a shift of the cuprous-cupric equilibrium t o the cupric side with the increase in alkalinity of the borax glass, which is similar to the behavior of the manganese oxides in borax glasses. These figures for copper have not * This work is part of the programme now being carried out a t Cornell University under a grant from the Heckscher Foundation for the Advancement of Research established by August Heckscher a t Cornell University. Bancroft and Kugent: J. Phys. Chem., 33, 481 (1929).
730
WILDER D. BANCROFT AND R. L. NUGENT
been checked as much as we should have liked and the absolute values may need correcting. There is no question, however, about the trend of the equilibrium with alkalinity. The green of copper glasses is evidently due to a mixture of cuprous and cupric compounds. This confirms and explains the results obtained previously by Sir Herbert Jackson.’ “If the proportion of cuprous oxide introduced into the glass be about 8 per cent., the whole of it dissolves in the glass a t the temperature of IOOO~C, at which the glass is made. If the glass be chilled quickly from this temperature, no colour except the almost unavoidable green colour due to the oxidation of part of the copper will be seen; the glass is a nearly colourless transparent one. On reheating this glass i t is possible to produce, according to the temperatures to which the glass is raised and the length of time during which i t is heated, comparatively large crystals or aggregations of crystals of red cuprous oxide, smaller crystals of the same form, or particles so small as not to be recognized as crystals under t h e microscope. Along with these are frequently obtained definite crystals of the yellow form, clouds of yellow particles, and if the reheating be gentle, the particles of yellow cuprous oxide are so small as to be irrevealable by the microscope, and what is obtained is a clear yellow transparent glass. Here, then, from two forms of one and the same oxide of copper, we have a range of colours associated with the proportions in which the two forms are mixed in the glass and with the size of their particles. What is the inner nature of the difference between the red and yellow forms, which may account for their difference in colour, is yet to be made out.” There is some evidence that there are not two forms of cuprous oxide; but that the color varies from yellow to red as the particles become coarser.* Sir Herbert Jackson implies that the green color is due solely to the cupric salt; but our experiments are conclusive that the green is always due to a mixture of cupric and cuprous salts. Our experiments do not show whether the green is due to a mixture of the blue of the cupric salt with the yellow or the red of the cuprous compound. Probably all possible gradations can be obtained by varying the temperature and time of annealing. The fact that the green is due to a mixture of cupric and cuprous compounds accounts for another observation by Sir Herbert Jackson. “The colours produced by cupric oxide in glasses and glazes need not be dealt with in detail, as there is much common knowledge about these. One or two points not in common knowledge may, however, be emphasised. I n a glass of the same composition, cupric oxide may give a very marked blue colour if the glass is made at a comparatively low temperature (IOOOO C. to I 100°C.) ; whereas with the same concentration of copper and the same glass made a t a higher temperature, about 13oo0C., for example, there is a very marked green shade in the blue. It is worth pointing out that the blue low-temperature glass is green while hot.” Nature, 120, 264 (1927).
* Bancroft: “Applied Colloid Chemistry,” 243 (1926).
COPPER OXIDE IN THE BORAX BEAD
737.
The higher the temperature the larger will be the proportion of cuprous oxide when the other conditions remain the same, because the tendency of cupric oxide to dissociate increases with rising temperature. We cannot a t present explain why the blue low-temperature glass is green while hot; but this is not a n isolated phenomenon. We know why the ruby is red and not green;‘ but we cannot explain why the red ruby is green at temperatures above about 300’. It is possible that the change from green to blue on cooling the copper oxide glass is due to oxidation of cuprous oxide at the lower temperatures. It is not denied that one can get green crystals or solutions of cupric salts. Cupric chloride is a case in point. Sir Herbert Jackson is wrong, however, in saying that copper sulphate with one of water is colorless. When one eliminates the scattered reflection, the salt is green. I n unpublished work by Mr. Rogers in the Cornel1 Laboratory i t has been shown that for cupric salts the radical Cu.3H20 is always blue while the radicals Cu.2HZO and Cu.HpO are always green. Nothing analogous to this has yet been discovered in the borax bead and the close parallelism between color and degree of reduction with the oxides of manganese and of copper makes it practically certain that the green color in copper glasses is due to the admixture of a cuprous compound and not to the presence of a green modification of a cupric compound. Sir Herbert Jackson has done one experiment which parallels our work to some extent, though the results are not so clear cut and there are no analyses. “A small quantity of cupric oxide does not dissolve in fused boric anhydride, but forms a white borate which is dispersed through the fused mass. [He means a colorless borate which makes the mass white.] The addition of an alkali will bring about solution and give a clear blue transparent glassy mass. The most striking example is to take boric anhydride and the alkali lithium oxide in, say, three different proportions, such as one molecule of lithium oxide t o one, four, and ten of boric anhydride. With 0 . 5 percent cupric oxide the first is a fine deep blue; the second is a paler blue; the third is still paler; and if the proportion of alkali oxide be lowered until there is only just sufficient to bring about complete solution of the cupric oxide in the mass, there is but little d o u r to be seen a t all.” The deepening of the blue color for the same amount of cupric oxide with increasing amount of alkali is in agreement with our results. The difference is that there was apparently no green formed with the smaller amounts of alkali and no precipitation of cuprous oxide in the absence of alkali. The concentration of copper oxide in Sir Herbert Jackson’s experiments was less than half what it was in our work and there may have been a difference in the rate of cooling. Granger2 found that increasing the copper oxide content changed the color from blue toward green. If we write the reaction 4CuO = 2Cu.20 0 2 , it is evident that the glass should be less green the more dilute
+
Stillwell: J. Phys. Chem., 30, 1441 (1926).
* Compt. rend., 157,935 (1913).
i32
K I L D E R D. BANCROFT 9 S D R . L. XUGENT
the solution. We have confirmed this result. Increasing the copper content in a borax bead changed the color from a light blue to a deep green and the bead was full of a yellow substance, presumably cuprous oxide. The question of the rate of cooling as affect’ingthe color comes up in reference to another matter. “I must not deal further with cupric oxide glasses except to mention that, unlike cuprous oxide glasses, copper glasses, and many other coloured glasses, such as gold glasses, selenium glasses, cadmium sulphide glasses, and opal glasses, cupric oxide glasses cannot be rendered colourless by sudden chilling, nor, indeed, can the tint of these glasses be modified to any noticeable extent in this way. An attempt to explain this difference which seems to divide colouring agents generally into two classes, would involve a very lengthy account of the various phenomena which have been observed, and would, moreover, be to a considerable extent little more than a re-statement of facts which would involve differentiation between the meanings of such terms as ‘solution,’ ‘chemical combination,’ and ‘dispersion,’ and would lead t’o much argument. I must content myself with this short and incomplete account of the modes of behaviour of copper and its oxides in glasses and glazes.” It would have taken fewer words to have said that we know that one group of colouring agents forms colloidal solutions and that the other group occurs in true solution either a s oxides or as compounds.’ I t is helpful to take one step forward whenever possible. These experiments enable us to understand the general behavior of cupric oxide in various glazes. Speaking broadly, cupric oxide gives a blue in alkaline glazes and green in lead glazes.* “With an oxidizing atmosphere, copper in alkaline glazes gives a beautiful azure blue known by the name of turquoise blue. I t becomes intense green in boracic or plumbiferous glazes.” There is no suggestion of any change in the nature of the oxide. When discussing Egyptian pottery Birch3 says: ”When the object had assumed the intended shape, the glaze was laid on. I t was composed of silica-probably a finely ground or triturated sand, and soda, to which were added certain metallic oxides to produce the colour required. For the fine celestial blue, which is still the admiration of all v h o view it, and scarcely rivalled after thirty centuries of human experience, an oxide of copper was employed. The green glaze, which, in many instances, seems to be the blue changed by the effect of time, is also stated to have been produced by another oxide of the same metal. The red glaze, but rarely seen, is conjectured to be a protoxide of copper; the violet, to be formed by a n oxide of manganese [Mn,O,], although capable of being produced by [colloidal] gold. Yellow was, perhaps, made with [colloidal] silver; the white glaze with tin, or a white earth. KO very recent analysis has, however, been made; and it is to be regretted that we are compelled to acquiesce in the conjectures of archaeologists, rather Bancroft: “Applied Colloid Chemistry,” 438 (1926). “A Treatise on Ceramic Industries,” 249 (1911). 3 “History of Ancient Pottery,” 48 (1873). 1
* Bourry:
COPPER OXIDE I N THE BORAX BEAD
733
than to adopt the tests of chemists. Of these colours the celestial blue [CuO] is the predominant one, the rest being occasional varieties, used for objects made in the Greek and Roman epochs, when foreign ideas and tastes had superseded the genuine national feelings.” In the next chapter, p. 90, Birch says: “The analysis, made in the Museum of Practical Geology, of the colours of the enamel employed in this brick [from Simrbd], shows that the opaque white was produced with tin, the yellow with antimoniate of lead, the brown with iron, the blue and green with copper. The flux and glazes consisted of silicate of soda aided by lead.” In the Introduction, p. 4, Birch says: “The desire of rendering terra-cotta less porous, and of producing vases capable of retaining liquids, gave rise to the covering of it with a vitreous enamel or glaze. The invention of glass has been hitherto generally attributed to the Phoenicians: but opaque glasses or enamels, as old as the Eighteenth dynasty, and enamelled objects as early as the Fourth, have been found in Egypt. The employment of copper to produce a brilliant blue-colored enamel was very early both in Babylonia and Assyria, but the use of tin for a white enamel, as recently discovered in the enamelled bricks and vases of Babylonia and Assyria, anticipated by many centuries the rediscovery of that process in Europe in the fifteenth century, and shows the early application of metallic oxides.’’ There are a number of interesting paragraphs in the article’ on ceramics. “ I t is surprising to note that some of the very earliest glazes were coloured glasses containing copper or iron (the green, turquoise, and yellow glazes of the ancient Egyptians and Assyrians). Marvellous work was wrought in these few materials, but the era of the finest pottery-colour dawns with the Persian, Syrian, and Egyptian work that preceded the Crusades. By this time the art of glazing pottery with a clear soda-lime glaze had been thoroughly learnt. Vases, tiles, etc., shaped in good plastic clay, were covered with a white, highly siliceous coating fit to receive glazes of this type, and giving the best possible ground for the painted colours then known. With this rudimentary technique the potters of the countries south and east of the Mediterranean produced, between the ninth and the sixteenth centuries of our era, a type of pottery that remains ideal from the point of view of colour; for, with nothing more than the greens given by oxide of copper and iron, the turquoise of pure copper, the deep yet vivid blue of cobalt, the beautiful uncertain purple of manganese, and in certain districts the rich red of Armenian bole, they achieved colour schemes that have never been surpassed in their brilliant yet harmonious richness.” “The art of making a pottery consisting of a siliceous, sandy body coated with a vitreous copper glaze seems to have been known unexpectedly early, possibly even as early as the period immediately preceding the 1st dynasty (4000 B.C.). Under the X I I t h dynasty pottery made of this characteristic Egyptian faience seems to have come into general use, and it continued in use down to the days of the Romans, and is the ancestor of the glazed ware
* Encyclopaedia
Britannica, 5, 706, 708,
710, 7 1 1 ,
726, 737 (1910).
WILDER D. BANCROFT AND R. L. NUGENT
734
of the Arabs and their modern successors. The oldest of Egyptian glazed ware is found usually in the shape of beads, plaques, etc.-rarely in the form of pottery vessels. The colour is usually a light blue which may turn either white or green; but beads of the grey-black manganese colour are found.” “Characteristic of the Parthian period is a coarse green glazed pottery of which the slipper-shaped coffins of the time were made. This glaze possibly contains a small amount of lead; in appearance it is not unlike the contemporary, translucent blue glaze of Egypt. The Egyptian glaze certainly spread in to western Asia, and we find the last specimens of it in the tiles from the destroyed city of Rhagac in Persia, which may be as late as the 13th century A.D. The lead glazes, unknown in Egypt till the late Roman period, may be of Asiatic origin, though this important point is by no means clear.” “There is abundant evidence that pottery was made in the Egypt of the Roman times and later with rich turquoise blue and yellow glazes, though the potters had learned to produce this glaze on a material containing more clay and less sand than that used in earlier days. We know also that they had learned that the addition of lead oxide to a glaze enabled such glaze to be applied on vessels formed from clay, which was sufficiently plastic to be shaped on the wheel. . . Oxides of copper or iron were added to the lead glaze, and the resulting green or yellow glazes were applied to plain vases or to vessels decorated with moulded reliefs. . . We have already spoken of the prevalent use of coloured glazes in all the countries of the nearer East-from Egypt to Persia-from remote times, either as the sole colour decoration or in conjunction with modelled or painted ornament. The fragments from Rai and Fostat include rich turquoise glazes (derived from the ancient Egyptians), deep and light-green glazes containing lead and copper, imitations of ancient Chinese celadon-green, a brownish-purple glaze, a coffee-brown glaze, and a deep cobalt-blue glaze.” For the majolica glazes “pigments were compounded from metallic oxides or earths. the yellow, from antimoniate of lead, which was mixed with oxide of iron to give orange; the green, from oxide of copper (the turquoise tint given to the Egyptian and Syrian glazes by oxide of copper is impossible with a glaze of lead and tin) ; and the greens were made by mixing oxide of copper with oxide of antimony or oxide of iron; blue from oxide of cobalt, used in the form of a blue glass; brownish-purple, from manganese; black, from mixtures of other colours; and the rare red or reddish brown, of Faenza and Cafaggiolo was probably the same Armenian bole that was used so magnificently by the makers of the Turkish pottery, but on the white enamel ground this colour was most treacherous and uncertain. It must be remembered that many of these colours owe their tint to the lead used in their composition, or to the grounds containingoxides of lead and tin on which they were painted.” There are many passages by Burton’ which emphasize the conditions for forming blue or green with copper, though not understanding the chemical changes accompanying the color changes.
.
.
“Porcelain,” 36, 38, 73, 75, 80, 82, 135 (I@).
COPPER OXIDE IN THE BORAX BEAD
73 5
“Oxide of copper with a flux or glaze rich in lead gives various shades of green; but when it is dissolved in a glaze consisting of alkaline or earthy silicates, it produces all those wonderful blue-green tints which the potter calls ‘Turquoise.’ This beautiful tint of this turquoise is, however, destroyed by the addition of lead oxide or by firing to a high temperature. On the other hand, if the green or blue glazes obtained from copper oxide are fixed in a reducing atmosphere the colour changes to a marvellous red, which may be either brilliant and vivid, as in the finest red Lang-yao glazes of the Chinese, or opaque and liver-coloured as in those tints more commonly known as Sang-de-Boeuj. I t has been customary to speak of these green, blue, and red tints obtained from oxide of copper as due to the formation of different silicates of copper. Such a view is hardly tenable, however, for there is nothing to show that the copper-compound exists in any different state of oxidation when it gives a green tint in a lead glaze or flux, or a blue tint in an alkaline mixture; while as to the red, copper-glazes, it seems probable that the tint may be due to the presence of metallic copper in a state of excessively fine subdivision, and not to an oxide of copper a t all. Speaking generally, too, it is wiser not to assume that any particular porcelain colour is due to the presence of a definite silicate of the metal, for it may mors probably be due only to the solution of some simple or complex metallic compound in a mixture of glassy silicates, borates, etc.” “Certain coloured glazes can only be obtained either with glazes of special compositions or under definite firing conditions. The famous glaze, first made by the early Egyptians on their highly siliceous bodies, which is known to us as turquoise, is obtained from oxide of copper. B u t this especial tint can only be developed in glazes rich in soda and lime, and, moreover, the fine blue tint which is its special distinction is destroyed at a high temperature. I n the same way oxide of manganese will produce a beautiful purple colour comparable in depth and intensity to this turquoise, but only in an alkaline glaze fired at a comparatively low temperature. The appearance of glazes of fine turquoise and rich purple on Chinese porcelain is positive proof that the body of such pieces must have been first fired to the biscuit condition to make the ware translucent, and that the glazes themselves were subsequently fired at a much lower temperature. It will be noted that this involves a departure from the ordinary Chinese method of porcelain-making, and yet we have ample evidence from some such pieces as are described on page 66, that this practice was followed as early as the fifteenth century, before any porcelains had been made in Europe at all. “Another coloured glaze, made in the same way, is coloured with a yellow made from antimoniate of lead (the Naples yellow of the painter), which is also incapable of enduring the temperature needed to fire the body of Chinese porcelain; yet there can be no doubt that this yellow glaze, described by the Chinese as ‘Mi-se’ or ‘millet-coloured,’ made its appearance at a very early period.’ Dr. Bushell states that this glsss was first invented under the Sung dynasty.
WILDER D. BANCROFT AXD R. L. SCGENT
736
“We may also mention that the Chinese made another yellow glaze, coloured with oxide of iron alone, but this was only invented at a later date when the porcelain body was exceedingly white and translucent, and the glazes highly refined. This is what is commonly known as ‘Imperial’ yellow, and was first used on pieces of excessive thinness which are either perfectly plain or have designs incised in the paste. “It will be seen, therefore, that many of these glazes, especially the turquoise, purple, antimony-yellow, and clear iron-yellow, afford indubitable proof that the Chinese first fired their porcelain to the biscuit condition, and then glazed it at a lower temperature, whenever it suited their purposes so to do.” “Originally, the enamel-colours appear to have been merely applied as thin ground-washes to enhance the value of the decoration painted in underglaze blue, but the brightness and the jewel-like quality of the enamels, as well as the precision and delicacy with which they could be handled, soon brought them into more extended use, and we get the earliest pieces of the decoration so well known under its French title of fanzzlk verte, because of the predominance of a fine green-enamel, made from oxide of copper with a lead flux, which shines with the greatest brilliancy. Chinese porcelain decorated with on-glaze colours in which green predominates as here described, is almost invariably called Wing’ porcelain by the dealers; but the true collector will not need to be told that ninety-nine per cent. of such pieces now to be met with are of much later date than the close of the hIing epoch.” The Chinese attribute to Lang t’ing-tso, Viceroy of Kiang-si, the province in which Ching-tbchh is situated, two of the most beautiful of all their glazes, both of which bear the name of Lang-yao. “The first and the rarest of these is of a blue-green color, known as apple-green, while the second is the famous blood-red, which collectors are agreed is the crowning achievement in all that class of copper-red glazes, best known under their French title of Sang-de-Boeuj. . I t may perhaps be added that the two glazes, the green and the red Lung-yao, are apparently identical in composition, the remarkable difference in appearance being due to the fact that the green was fired in an oxidising and the red in a reducing kiin-atmosphere.” “In the earliest glazed pieces that we know-say of Sung and Yuan times -the colouring substance was dissolved in the crude glaze and fired along with it. The first advance, said, by Dr. Bushel1 to belong to the later Sung times, was that of first biscuiting the porcelain and then firing at a lower temperature glazes of the alkaline type used by the Persian and Syrian potters so as to obtain the turquoise tint from copper and the violet-purple from manganese. This plan has been followed a t all subsequent periods, and in the reign of Xhng-hsi the turquoise-coloured glazes, distinguished by the Chinese as ‘peacock green’ and ‘kingfisher blue,’ according as the tone is more green or blue (a change which can easily be effected by a slight difference in the firing temperature), were produced with surpassing excellence.” The experiments which we have done with borates agree exactly with the results obtained empirically by the Egyptians, the Chinese, and their followers.
..
COPPER OXIDE IX THE BORAX BEAD
i3i
I t is possible of course to obtain a green from blue cupric oxide and the yellow of ferric oxide or lead antimoniate. Apart from this we get blue with alkalies and a t low temperatures, and green with lead oxide and a t high temperatures. At high temperatures cupric oxide will dissociate more readily to cuprous oxide with t,he consequent formation of a green color. K e have shown that a t the same temperature there is less cuprous oxide in the melt the more alkaline it is. The experiments with the oxides of manganese’ showed reduction was greater when lead oxide replaced sodium oxide and the same thing will hold for copper oxide. Laurie2has prepared a crystalline, blue, double silicate of lime and oopper, CaO.CuO.4Si02, which has an inversion point a t 8 joo-9ooo, the inverted material turning green a t about 900’. I t has not yet been shown that the green glass contains cuprous oxide; but one can predict with great safety that it will. There is really nothing new about all this. It is contained, implicitly a t least’, in some work by PercyJ3who is a bit hard reading because he reverses the ordinary nonienclature and calls cuprous oxide dioxide of copper, and cupric oxide protoxide of copper, although using t’he modern formulas. His atomic weights are different, because he takes the atomic weight of copper as 31.648. “Protoxide of copper, CLiO,is the oxide which forms the base in ordinary salts of copper. According to Berthier, i t melts at a white heat. It is as easily reduced as the dioxide, and by the same reducing agents. Favre and lIaument5 stat,e that when protoxide of copper is exposed to about, the melting-point of copper, oxygen is given off in a regular stream, which, having once ceased, d l not again occur, though the heat may be increased.l I n four experiments the loss of oxygen varied from 8.0 to 8.2 per cent. [as against nearly nine for the theoretical loss.] The product, which was melted, was black, and consisted of z CuzO CuO. I had long previously found that when protoxide of copper was exposed in a clay crucible t o a high temperature in a common assay furnace, i t became brown and formed a sintered mass; but I was not sure that the reduction was the simple effect of heat, and thought that probably the partial reduction might have been caused by thc gases of the furnace. . “Berthier heated the following mixture of protoxide of copper and silica:
+
..
3CuO
= 0.421
gramme.
4Si02 = 0.579 do.
The product was only semi-fused and blood-red in colour, which proves that the protoxide [CuO] had been reduced to the dioxide [CuzO]. “This experiment has been repeated by R. Smith. The proportions employed were I 160 grains of protoxide of copper and 840 of silica. The mixture Bancroft and Xugent: J. Phys. Chem., 33, 481 (1929). *Proc. Roy. SOC., 89A, 418 (1924). “Metallurgy,” 243 (1861). Berzelius: Jehresber., 1846, 184.
738
WILDER D. BANCROFT AND R. L. NUGENT
was exposed in a fine-grained crucible to a high temperature during two hours. The product was fritted, and very similar in appearance to that obtained by heating a mixture of dioxide of copper and silica. Its upper surface was black, and where& was in contact with the crucible it was orange-coloured. In order to be certain that the gases of the furnace in which the crucible was heated had not contributed to effect the reduction of the protoxide to the dioxide, the following experiment was made in a muffle, in which the atmosphere was oxidizing:3CuO = 580 grains zSiOz = 440 do. “The materials were then intimately mixed, and the mixture was exposed in a n uncovered platinum dish to a strong red heat in a muffle during 34 hours. A sound, such as is produced by the evolution of bubbles of gas from a thick liquid was perceived during the process. The dish with its contents was left to cool in the muffle. The product was detached in one piece from the platinum; it was somewhat compact, semi-fused, opaque, and brown-red; the upper surface was black and porous. The platinum dish was attacked where it had been in contact with the mass. About half the product was again exposed during 5 + hours in the same platinum dish in a muffle to a degree of heat approaching whiteness. No perceptible change occurred, except the blackening of the surface of the mass. From the preceding experiments it may be concluded that, under the influence of silica, protoxide of copper [CuO] is reduced to dioxide of copper [CulO]. . . . “The experiment was made by R. Smith of heating protoxide of copper [CuO] with silica and alumina. 3CuO = I Z O grains A1203 = 52 do. 2Si03 = 92 do. The mixture was heated in a fine-grained crucible in the same furnace, and during the same time, with the crucible in experiment 2, p. 244. The product was melted, compact, and of a greenish-orange colour like that obtained in experiment 2 [where the charge contained dioxide of copper, CueO]. . , “The following experiment on borate of copper was made by Berthier:CuO = 9.91 grammes. z B O ~= 14.72 do. The mixture melted easily without intumescence. The product was tenacious, opaque, and red-brown in colour, spotted with blue. It contained cavities in which were brilliant prismatic crystals, some red and others of the finest blue colour. Part of the protoxide of copper must have been reduced to dioxide. “The following experiment was made by R. Smith:3CuO = 240 grains 2BO3 = 140 do. The mixture was heated in an open Cornish crucible in a muffle, and in ahout twenty minutes fused easily a t a red-heat; the product was a dark, greenkh-
.
COPPER OXIDE IN THE BORAX BEAD
739
coloured glass when seen by transmitted light, but it was blue and iridescent on the surface. . . “Dioxide of copper heated with protoxide of lead. Experiments by R. Smith. Cu2O = jzo grains PbO = 1 1 2 0 do. The mixture melted at a low red heat; it attacked and traversed the substance of the crucible with great rapidity. The product is crystalline and reddish brown-black. CuzO = 7 2 0 grains zPbO = 2240do. The mixture melted as in the last experiment, and the product had nearly the same characteristics; its upper surface was coated with a black film having a semi-metallic lustre. It follows from these results that dioxide of copper is not in any degree oxidized when heated with protoxide of lead; for, otherwise, metallic lead would have been separated. “Protoxide oj copper heated with protoxide of lead. Experiments by R. Smith CuO = 400 grains PbO = ~ r z o d o . The mixture melted into a compact, hard, dull slag; its upper surface was black, crystalline, and metallic in lustre; the colour of its fractured surface varied from brown to black from below upwards. CuO = zoo grains 2PbO = i r z o d o . The mixture melted into a crystalline, shining, dark-green slag, much softer than the last, and more resembling fused protoxide of lead in appearance; its upper surface was smooth, black, and semi-metallic in lustre.’’ While Percy does not call attention to the fact, the green color in the experiment just cited shows that there is more cupric oxide when the ratio of plumbous oxide is increased. The significance of Percy’s work seems to have been overlooked hitherto. Mellorl says that “copper oxide dissolves in fused silicates producing blue or green glasses. The alkali silicates tinted by copper are blue.” The statement on the next page that the hydrated silicates are blue and the anhydrous ones are green is inaccurate as it stands, because dioptase and ‘idealized’ chrysocolla are both green, while CaO.CuO. 4Si0, is blue. Thorpe2 says that copper borate is readily obtained by treating a soluble borate with copper chloride or sulphate. It is blue, and used in certain oil paints and also in the colouring of porcelain.” Sir Herbert Jackson3 has been misled by the apparently striking analogy between the colors of cupric salts in aqueous solutions and those in glasses. “It would lead us into too much detail to do more than direct attention to the possible analogies between the action of water in solutions of cupric salts
.
“A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” 6,340 (1925).
* “A Dictionary of Applied Chemistry,” 1, 645 (1921). a
Nature, 120, 265 (1927).
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WILDER D. BANCROFT AND R. L. N U G E S T
and the action of the oxides of the alkali metals in glasses coloured by cupric oxide. The change from bluc to green just mentioned in the case of the lowtemperature cupric oxide glasses calls to mind the fact that a green solution of cupric chloride, which becomes blue when sufficient water has been added, becomes green again on heating it. h study of the changes of colour which can be produced in aqueous solutions and salts, and of the methods of modifying these colours, has been of great assistance to me in shortening the experimental work necessary to arrive at the compositions of a number of glasses in which it was desired to produce certain colours either with copper or colouring agents.” This seems to be a case of a bad working hypothesis being better than none. We can have green aqueous solutions and green hydrated saks which contain no cuprous salt. The green copper glasses always contain cuprous oxide, so far as we now know. There is, therefore, no real parallelism. Lasareff and Lazarev’ have compared the color of a blue borax glass with that of a copper sulphate solution. If we take the absorption for 590 mp as unity in both cases, the absorption for 4 7 0 - j j o mp is much greater in the case of the borax glass. The authors draw the conclusion that the color in the copper oxide glasses is different from the color of the ions and is not due to these. It is possible, however, to duplicate, in aqueous solutions some of the results obtained by Sir Herbert Jackson and by ourselves in fused melts. It has long been known2 that metallic copper can be obtained by using a high current density with a copper-wire anode in sulphuric acid. Cuprous sulphate is formed at the anode, which then decomposes to metallic copper and cupric sulphate. hlr. L. T’. Redman called our attention to the fact that, if we substitute a moderately concentrated solution of copper sulphate for the sulphuric acid, the deconiposition of the cuprous sulphate gives a brilliant green color at the anode by reflected light,. We have confirmed this; but have not analyzed t,he precipitate to see whether it is metallic copper, cuprous oxide, or a mixture of the two. By transmitted light the solution is blue. This system seems worth studying in more detail. When the temperature is allowed to rise, the quality of the precipitate changes and the color of the solution becomes a dark olive-green by reflected light. So far as one can judge by the description, this duplicates pretty closely a glaze obtained by Sir Herbert J a ~ k s o n . ~ “With many glasses made a t a high temperature, cupric oxide gives an olive-green colour. Without going so far as to say that the dusky shade in the green is invariably due to some reduction of the cupric oxide to the lower oxide of copper, there is evidence of this in certain instances which I have come across. To take one: in making trials for a glass which was intended to be of a green colour with only a slight tinge of olive in it, and of a sufficiently Compt. rend., 185,855 (1927).
* Fischer: Z. Elektrochemie, 9, 507 Kature, 120, 266 (1928).
(1903).
COPPER OXIDE IS THE BORAX BEAD
741
light shade t o enable the iight of a candle flame to be seen through a one-inch thickness of the glass, the furnace conditions happened t o change so that the glass was exposed t o a reducing atmosphere. The resulting glass was so black that a bright June sun !vas invisible through a piece of the glass one-fortieth of an inch thick. Such a state of affairs might be considered to come about by the glass being a mixture of red copper glass with a green cupric oxide glass, through which mixture but little light could be transmitted. Kow red glass owing its colour t o finely dispersed metallic copper is rendered colourless by fusing it and chilling it quickly. The black glass referred t o might therefore be expected t o become green if fused and chilled quickly; but it did not change from its intense black colour. This just gives a hint of the possibility of a cuproso-cupric compound being present in the glass analogous to, though not so definite as ferroso-ferric oxide, the well-known black iron scale. “hgain an analogy with solutions helps a little. If t o a colourless solution of cuprous chloride in hydrochloric acid there be addpd a transparent green solution of cupric chloride, the mixture turns black. -1lthough dusky greens and t,he black glass just referred t o might be accounted for by varying mixtures of red and green glass, the colour of this solution could scarcely be accounted for in the same way. Moreover in experimenting with red copper glasses, and studying the way in which the red colour can be prevented from developing by sudden chilling and can be produced by subsequent heating, I have repeatedly noticed that, instead of obtaining a clear colourless glass in bulbs made from the red glass and chilled quickly, the bulb has been sometimes of a dusky hue and sometimes of a definite neutral tint. As no other colouring agent but copper was present in these glasses, I am inclined t o attribute the neutral shade to a cuproso-cupric compound which is stable in the glass and is analogous t o the compound formed \Then the cuprous and cupric chloride solutions are mised, rather than t o a physical mixt,ure of red and green glasses.” This seenis to be equivalent to explaining one unknown in terms of another. There is no independent evidence of any cuproso-cupric oxide which might be black in glass and the argument starts from what we believe to be an erroneous assumption, that the green in copper oxide glazes is not due to the presence of cuprous oxide. There could not of course be any cuproso-cupric oxide in a concentrated hydrochloric acid solution, so Sir Herbert Jackson must be postulating a cuproso-cupric chloride in the aqueous solution. 11ellorl says: “Cuprocupric chloride is said t o be present in the browncoloured solution formed when cupric chloride in acetonn is allowed t o stand; when a hydrochloric acid solution of cuprous chloride is oxidized; when an aqueous solution of cupric chloride solution is reduced on or a t the cathode; when an aqueous solution of cupric chloride is electrolyzed. There is every sign that the alleged cuprocupric chloride is a mixture of the two chlorides.” Anhydrous cupric chloride is yel!owish-broxn, dissolves in methyl alcohol t o form a brown solution, and gives a yellow color with aqueous hydrochloric “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” 3, 159 (1923).
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WILDER D. BANCROFT AND R. L. NUGENT
acid. Nobody knows whether cupric chloride increases the tendency of cuprous chloride to hydrolyze. While we do not yet know the actual cause of the black color in the aqueous solution of cupric chloride, cuprous chloride, and hydrochloric acid, it is certainly not justifiable a t present to postulate a cuproso-cupric chloride. It should not be difficult to clear up this point. Granger found that glasses containing boric oxide and copper oxide had a very deep color and were almost opaque, the glass seeming blackish even by reflected light. We have shown how the new technique described in these two papers has simplified the question of the colors of manganese and copper oxides in glasses. It seems to us that all problems pertaining to the colors of glazes could be worked out easily and economically by studying the behavior of borax or phosphate beads. Of course the results would then have to be translated into terms of glasses; but that would be relatively simple so soon as one knew exactly what one was trying to do. In any soluble glass, it would be a simple matter t o differentiate analytically between metallic copper and cuprous oxide. One could even analyze for all three substances, metallic copper, cuprous oxide, and cupric oxide if i t were necessary. The direct experimenting with glasses is evidently not easy. Fuwal has made a great many experiments on copper oxide in glasses and his results, &s given in the abstract are both negative and misleading. On addition of metallic copper, cuprous oxide, or cupric oxide, he obtained colored glasses varying from light to dark blue. I n general cupric oxide gave the deepest colors and copper the palest colors. This cannot be true if equilibrium was reached because the composition of the copper products in a given glass depends only on the amount of copper, the temperature, and the oxygen pressure. Familiarity with the phase rule would have enabled him to avoid this error. Fuwa also added oxidizing agents and reducing agents. With Cu(N03)z 6H20-which he certainly did not have in the molten glass-he obtained a deeper blue than when he added cupric oxide alone. Addition of one percent potassium nitrate gave him a still deeper blue. Here again he could not have reached equilibrium. What he had done was to over-oxidize the melt. Addition of five percent potassium tartrate to a one percent cupric oxide melt gave reddish colors. He must have reduced beyond the green stage. Granger’s experiments2 are nearly as disappointing. He recognizes that cupric oxide gives a better blue with more alkali; but he does not find the difference between lead and soda glazes which all potters have found since the time of the early Egyptians. The most probable explanation for his results is that his glasses oxidized at aU temperatures, thus throwing his results out. This is made practically certain by one experiment in which a film of copper formed on the surface of a chill-cast piece of glass, while the mass of the glass was blue. Unless the metallic copper was formed by the reaction 2
Chem. Abs., 19,478 (1925). Compt. rend., 157,935 (1913).
COPPER OXIDE IN THE BORBX BEAD
7 43
of the molten glass with the metal mold-and he does not mention that possibility-the cupric oxide must have been pretty well dissociated at the high temperatures and his glass should have come out either green or red. Granger considers that the color depends on the ratio of cupric oxide to bases and x t on the silica content. This may be true; but his experiments do not justify the conclusion. Since he gets a blue when he should get a green, the change due to the silica content may be within his experimental error. He does find that presence of boric acid increases the tendency of the glass to turn green and he finds a similar effect with alumina, which we had not tried, our experimental work having been finished before we read Granger’s article. I . It is possible to determine cuprous oxide and cupric oxide in borate glasses by treating with acidified ferric sulphate and titrating the ferrous sulphate formed by oxidation of the cuprous oxide.
2. It would also be possible, though it did not come within the scope of this paper, to determine metallic copper and cuprous oxide in the same way and thus decide whether a given red color was due to copper or to cuprous oxide.
3. When copper oxide is used to color a boric acid glaas, there is some reduction t o cuprous oxide. The percentage of cupric oxide in the melt increases with increasing alkalinity and decreases with rising temperature. 4. When lead oxide is substituted for sodium oxide, the percentage of cuprous oxide increases.
5 . Cupric oxide gives a blue color to glasses; the green color is due to the presence of cuprous oxide. It is for this reason that copper oxide gives a blue in alkaline glazes and a green in lead glazes. Our results confirm those of Percy and of Sir Herbert Jackson, and are in agreement with those of the potters of all countries.
6. While green glazes can be and have been made with blue cupric oxide and yellow ferric oxide or lead antimoniate, the copper greens are not usually so made. 7. The blue of CaO.Cu0.4Si02 is in accord with the theory; but it has not yet been shown that the green color of the glass obtained by heating above gooo is due to presence of cuprous oxide.
8. While there is a very strong apparent analogy between the blues and greens of cupric salts in aqueous solutions and the blues and greens in glazes, this analogy is superficial and misleading. The green of cupric salts in presence of water depends so far as we now know, on there being not more than two molecules of water attached to the copper. In the glazes the green is due to the presence of cuprous oxide and is never, so far as we now know, a color of cupric oxide. Dioptase and chrysocolla appear to belong in one group and Egyptian blue, Ca0.Cu0.4Si02, in the other.
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F I L D E R D. BASCROFT AND R. L. N U G E S T
9. It should be both easy and economical t o make a preliminary study of all questions concerning the colors of glazes in borate beads. I O . So far as the color of glass is affected by aluminum oxide, stannic oxide, titanium oxide, arsenious oxide, zirconium oxide, cuprous oxide, calcium phosphate, gold, copper, selenium, tellurium, and cadmium sulphide, these substances appear t o be in colloidal suspension. So far as the color of glass is affected by cupric oxide, chromic oxide, cobaltous oxide, ferrous oxide, ferric oxide, manganic oxide, and antimony oxide, these substances appear to be in true solution. This statement is not in conflict with the fact that, under certain circumstances, it is possible to have ferric oxide and ferroferric oxides in colloidal suspension, as in the obsidians.
Cornell University
