plate of glass used to keep dust from settling upon a painting.» Byzantine glass has a long record, its two best periods being the 6th and the 10th centuries. It advanced considerably after the fall of the Roman Empire, probably on account of the aid obtained by securing the craftsmen who had worked at Rome. The great work of the Byzantine glass-makers for some time was in mosaics for church use. In the 10th and 11th centuries personal ornaments were a feature of production, in some of the methods earlier practised at Rome. They also used the Roman methods of pressing glass cameos. They were experts in the art of enameling and gilding of glass, but as the Byzantine decorative motives were employed at other eastern glass-making centres there has always been some difficulty in identifying their work. We know little of what they did in glass for domestic purposes. Byzantine glassmaking passed out when Venetian came in.

Venice.-Glass-making has been a Venetian industry since the end of the 11th century. In mid-12th century Venice employed mosaic craftsmen. Half a century later other processes, those practised by Greek and Byzantine glass workers had been adopted there. Up to the last decade of the 13th century the art was practised within the city itself, but in 1291 it was decreed that the larger furnaces be demolished and the seat of the industry removed to nearby Murano. This law remained in force for nearly five centuries. About the beginning of the 14th century glass "lanterns" for galleys and lighthouses were made at Murano, also optical glass, in mass, for use by the makers of spectacles. Mosaic glass, beads, glass for windows and mirrors, vessels for ornament and domestic uses, were all separate branches, each conducted under its own code of laws, binding upon master and workman.

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Manufacturing secrets were closely guarded, and the export of raw materials absolutely prohibited. "Crackled" glass, variegated "marbled" glass, imitations of tortoiseshell and various chalcedonic effects were among the varieties then made. The glass known as millefiori came into use in the 15th century, and was long continued. The ancient Roman processes of manufacture had presumably been followed as there was a noticeable similarity in effect and appearance. The famous vitro di trinaalso called siligrana — glass of Venice also corresponded in some of its features with the more ancient products known as "lace," "filigree" and "reticulated"; but the Venetian workman carried this class of glass-making to its highest point of perfection. This was undoubtedly the finest development of the art of manipulating threads of glass. Up to the middle of the 15th century the forms adopted by Venetian glassworkers were of the massive order and bore a resemblance to the silver work of western Europe. Colored glass-largely blues and greens with decorations in enamel colors and gold tracery, with occasionally figure subjects, was produced at this time. Decorations in gold leaf, both worked into the substance and spread over the surface, were also well-practised forms of ornamentation. Later more classical outlines in form were adopted, and by the middle of the 16th century had reached a very high standard of elegance and beauty.

At this time special privileges were granted

by the Venetian Republic to those who practised the art of glass-making and preserved it as a Venetian industry; but punishment, even to the death penalty, awaited those who carried its secrets to other countries.

The glass known as "avanturine was a Venetian invention of the early part of the 17th century, and its manufacture remained a secret for at least two centuries. It has the appearance of myriads of atoms of burnished copper diffused through a mass of transparent amber. It was largely used by the Venetians in conjunction with other means of decorating glass. It has still many uses of an ornamental nature.

The full period of fame for Venetian glass can be reckoned in centuries, and with the possible exception of cameo glass, every variety of manufacture known to the industry seems to have been produced in some form, from beads and "burning glasses" to table wares and chandeliers. In the 17th century it reached its zenith, Venice supplied the world with its finest glass. The task, however, was a heavy one, and extensive distribution of the product, combined with other circumstances, forced western Europe - hitherto its best market → into the competition, which by the middle of the 18th century had wrested from Venice much of its best trade. It is to this period we may assign the foundation of what is now, and has been for at least a century and a half, the principal factor in the flint glass industry, viz., cut glass. As Venice declined, the new aspirants to precedence in glass manufacturing began their effort to advance "wheel-cutting" upon glass, as a substitute for the more fanciful materials which had served Venice so well in the centuries just passed. With "tablewares» as its staple, cut glass then began its revolutionizing influence. In the thirties of the 18th century one of the Murano manufacturers obtained permission to re-establish the glassmaking industry in Venice itself, and with the additional protection of certain patent rights he succeeded in introducing a variety of production somewhat different to that hitherto practised. Among his successes were mirrors, with frames of glass ornamented by methods which have been practised in several glass-making countries since that time, and are even now quite familiar in some. Chandeliers ornamented with flowers and foliage-all made in glass and a continuation of earlier Venetian motives all contributed to the success of his endeavor.

A century later, laudable efforts to revive the glories of Venetian glass were made along some lines of the old-time productions, and fortunately for the industry these efforts succeeded, and operations have since been uninterruptedly continued. Several of the earlier Venetian motives have been further developed, notably the form of decorating with "glass applied to glass" at the furnace. This feature remained long a Venetian characteristic, but in the eighties of the last century both French and English glassmakers adopted it, fruit and flowers with their foliage and grotesque animal and reptile creations supplying the motives.

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France. As Pliny mentions the "glass of Gaul France may be credited with about 20 centuries of association with glass-making. Vessels and fragments attributed to the 2d and 3d centuries have been found in Nor

mandy, and in the period between 486 and 752 glass-making was practised in several parts of the country. Greek workmen were employed there in the 7th century. There is 9th and 11th century evidence also. In the 13th, 14th and 15th centuries glass-makers worked at Poitou. Provence had its glasshouses as early as the 13th century, and they were quite important by the 16th. Window glass was made in Normandy in the early part of the 14th century. In the 15th century it was a custom in France for the proprietors of glass works to become practical in the art themselves, whether they worked at the furnace or not, hence the "gentilhommes verriers." In 1556 glass-works were established in Lorraine, upon a site which has not even yet been abandoned. Presumably Venetian modes of production were adopted at the foundation, for as soon as history begins to record its progress we get evidence of operations identical with those which carried France along in its glass-making till it eventually-in the 18th century-dispossessed its mentors of a great part of their western trade. In 1664 the disturbing element of warfare arrested the progress of this establishment, but did not stamp it out, and when peaceable times came again preparations were made to restart the furnace fires. On 17 Feb. 1767 land was donated by royal decree to the promoters of the company, on condition that they erect a factory, homes for workmen and a church. Such success accrued from this incentive that after 20 years of effort to advance the art of glass-making the directors of the establishment were enabled to present to the Royal Academy of Sciences at Paris the first pure crystal glass ever made in France.

The government - delighted by this achievement granted several thousand acres of forest land to the company in those days wood fuel was used for melting glass-that all the world might know that it was the first in the French nation to fashion articles in pure crystal glass.

In 1788 three furnaces were in operation and more than 400 people employed in their working, an exceptionally large number in those days when small factories were the rule. In the 17th and 18th centuries there were works at La Rochelle and Nantes. Vessels for domestic purposes and for ornament were made; and enameling was one of the early forms of decorating, family and city "arms" and mottoes furnishing the motifs.

White and colored glass was made, including opalescent and marble effects. The secret also seems to have been known of patterning glass with studs formed in dies and welded on the vessel at the furnace. Much use has been made of this means of ornamentation in quite recent years. Mirror making was practised at Paris and at Cherbourg toward the end of the 17th century. These manufactories were united and the joint production was very large. About 1690 the process of casting glass was invented, and thus it became possible to produce very large plates. In 1693 these interests were transferred to Saint Gobain, a great centre of the plate-glass industry of our own time.

There is scarcely a variety of glass or a decorative motif that has not been practised by the French glass-worker, and in some branches they possess the largest establishments and employ the greatest number of people.

Belgium. The period when glass-making was introduced into Belgium cannot be fixed for certain, but whenever it occurred it was under Venetian influence. Mirror-making and "glass of crystal in the Venetian manner» are mentioned in 16th century history. Toward the middle of the 17th century a "gentleman glass-maker" from Murano had a patent granted him to make glass at Brussels. The terms of this patent implied an intention to substitute for imported glass a real home-made article.

An example of glass, mounted in silver, in the peculiar "windmill" fashion of this period, is also mentioned. This same verre au moulin example had an engraved pattern upon it. Engraving upon glass became a well-practised art later in the century, and continued to be a feature of Belgian glass. Silver mounted glass has been such an important factor in the industry during the last 200 years that an early example is worth mention.

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In 1825 an organization calling itself the Société des Manufactures de Glâces, with offices in Brussels laid the foundation of a glassmaking establishment from the operations of which the industry has since gained material benefit in many ways. The manufactories were located in the neighborhood of Liege Saint Lambert and Jemeppe-now the principal seat of Belgian glass-making, but later were supplemented by establishments near NamurJambes and at Charleroi. Though most of the departments of the glass industry of a century ago were operated in connection with the orig. inal foundation, window glass was one of the earliest to be advanced to large proportions there, and claims were established to association with the great expansion which took place in the production of this commodity and its accessories in its many varieties. Among the early products were glass tiles, prism lights, vault lights, glass door signs and the many uses of plate glass. The different classes of glass production are now distributed over different centres and every demand is provided for; table glass, in crystal and colors; ornaments of many kinds; candle, oil, gas and electric lighting requisites; bottles and jars for the pres ervation of fruits and meats; coal-mine lamp chimneys; toughened glass, in vessel and slab form and the blanks used in the manufacture of rich cut glass.

British Isles. The period when glassmaking was first introduced into Britain has been assigned to all the Christian centuries up to the seventh, each probability being supported by some historic note or tradition. Furnace remains and fragments suggestive of actual glass-making practices in the 4th century have been unearthed, and there are traditions of window glass-making in very early periods History tells us that in the 10th century "colored windows in churches were the work of high ecclesiastics." There is no positive evidence however, in way of record, till mid-15th century times, when "glass windows were added to the homes." Thomas Charnock tells us that glass-makers were "scant in the land" at that time. In the early days of the reign of Queen Elizabeth-1558-1603 — an Italian was "making Venice glasses at ye Crotchet Friars in London," and this may be taken as the starting point of British glass-making other than window glass, which was in general use at that

time -as an established industry. It may be presumed also that Venetian methods of production were employed, followers of those methods craving permission "to come to England, make glass and teach the art." By 1589 there were 15 glass-making establishments in England. Progress in the industry was rapid that in 1615 a law was passed prohibiting the importation of foreign glass; but five years later this law was made easier by an order to admit "rare and curious glasses," probably for the sake of instruction in the art.

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About 1620 glass-making was introduced into Scotland again under Venetian supervision - and 12 years later into Ireland.

Mirror and spectacle plate glass was being made in England in 1634. Flint glass was introduced in England in the early part of the 17th century, and by 1673 was in general use; a London manufactory producing it "as clear, ponderous and thick as crystal." The use of plate glass for many purposes, and flint glass for table wares and ornaments, became general about this time, and manufactories were established throughout England, Ireland and Scotland. Window glass, in its various forms of crown, sheet and plate filled demands not only of the building trades, but of the interior decorator, the cabinet-maker, the mirror-maker, the coach-builder and the ship-builder; and was linked up with the optical branches of glass manufacture, and in accessories for some sections of the flint glass industry. The advent of flint glass had a revolutionizing influence upon the whole trade and opened out many roads for its development and expansion. Table wares became its staple and "cutting" the most extensive form of decoration. The ancient arts of enameling, gilding and engraving upon glass were practised by themselves and in conjunction with "cut" designs; some of the patternings of that time requiring a second process when the limitations of the cutter's wheel had been reached.

The forms in use- for table wares were very few in number, and all the establishments adopted the same shapes and decorations, being guided mainly by the home furnishings of their period. All through the Georgian times the same class of form obtained, and till well into the 19th century the only appreciable difference was in the elaboration of the several decorative motives.

Toward the middle of the century there was an all-round advance in range and form, variety of patterning and elaboration of treatment, the more highly skilled branches of craftsmanship being encouraged to meet the tastes and fashions of the time.

The dinner-table and sideboard needed additional articles and the mantel-shelf and cabinet called for more choice ornaments. Cutglass furniture for the beautifying of the palaces of Eastern potentates was in demand; chairs, settees, tables, bedsteads and even balusters for the stairways had to be provided, as also elaborately constructed fountains and lamp stands. A pioneer example of this class is the glass fountain -made for the 1851 exhibition still to be seen in the Crystal Palace, London. The 1851 (London) and succeeding international exhibitions very materially assisted the glass industry and many forms of production developed into large and continuous

business from a single exhibition specimen. New departments of the industry about this time added largely to its wealth and importance. Hand and machine etching upon table-glass were introduced in the late fifties and early sixties, respectively; ornamental glass for table use and home decoration advanced enormously and brought with it extensive use for colored glass. Gas and coal-oil similarly benefited glass-making, without materially reducing the extent of candelabra and candlestick production.

Association with other industries as the silversmith also brought much benefit to the glass interests. Sculpturing of glass, after the manner of the bas-relief marbles of ancient Greece, was introduced in the sixties. The first specimen was in clear flint-glass and its decorative motif that of Greece in the 4th century B.C. Following this was the effort to revive an art not known to have been practised since Roman Empire times and believed to be of Athenian origin. This endeavor was to reproduce the "Portland Vase" in its original materialglass. The effort was successful in every sense and resulted in opening the road to several new features in glass ornamentation.

In the late seventies a decorative motif akin to glass sculpturing resulted from efforts to reproduce, in glass, some of the ancient examples of carved rock crystal. This effort too was entirely successful and resulted in the establishment of another new branch of the industry. In the early eighties, enameling and gilding upon flint-glass was successfully revived, as also was the intaglio style of cut-glass decoration, now sometimes called "stone-cut." The process of iridising the surface of glass has been practised in England since 1880. In that year its application to very deep shades of blue and green glass produced effects very closely resembling the antiques of Egypt and Rome. The same material and process, used upon a surface "crackled" in the 16th century Venetian manner, produced "Scarahæus glass."> The manufacturing of flint-glass in Ireland declined about 1835.

JOHN A. SERVICE.

GLASS, Chemical Properties of. The surfaces of glass objects, though apparently stable under the conditions of commercial economic uses, are subject to certain chemical changes in favorable circumstances. All glasses containing alkalis, particularly if the proportion is unduly large, will give up a minute portion of that constituent to pure cold water standing against the surface for a prolonged period. If the water is heated the chemical action of solution progresses faster. This is quite noticeable in the case of cheap window glass used in greenhouses, which soon loses its polished surface on the under side, where it is continually moist and subjected to considerable heat. When the water touching the glass is very hot, as in a steam boiler under pressure, the action is quite rapid, as illustrated by the clouding of the glass gauge tubes of high pressure boilers. A piece of Bohemian (potash) glass placed in very hot water under high pressure will dissolve completely in a few hours.

Water containing alkalis exerts a chemical action on all forms of glass in which silica is a component, abstracting this substance by solution, and later carrying away the released alkalis and basic substances. On the other

hand water containing a percentage of acid is very slow to attack silica glass, the acid seeming to reinforce the silica and oppose its separation. Strong acids, however, will decompose slowly glasses which have an excessive content of basic components, or in which there is a relatively large proportion of boric or phosphoric acids. Strong hydrofluoric acid and phosphoric acid are both destructive of glass surfaces, the former to such a marked degree that it is used to etch patterns upon polished glass. Deterioration of the surface also occurs where carbonic acid gas and moisture are both present in the air. The moisture which condenses on the glass begins the dissolving process, and the carbonic acid is absorbed by this alkaline solution, uniting with the alkali present to form a salt. In the case of a soda glass, a dry crust of crystalline soda carbonate spreads over the surface giving it a cloudy appearance. An attempt to wipe this off with a dry rag results in scratching the surface with the sharp soda crystals. It should be gently washed away with warm water, allowing plenty of time for the crystals to fully dissolve. In potash glasses the cloudy appearance is not seen because the potash carbonate is hygroscopic and remains continually in a liquid form. In both cases the alkaline skin or scum becomes a breeding place for certain bacteria or fungi which act destructively upon the glass. A percentage of boric acid in the glass mixture tends to prevent this bacterial action, and boric acid glasses are found to be notably resistant to atmospheric deterioration.

Glasses prepared for optical purposes are submitted to approximation tests as to their atmospheric resistance by placing them in a continuous current of moist air at a temperature of 175° F. for a period of several days. The durable glasses are unaffected. Those less durable exhibit varying degrees of dimming of the surface. This, however, is an arbitrary method, and it has not been proved that it really determines the relative economic endurance of the glasses tested.

Glass is also affected by prolonged exposure to direct sunlight. The action is most pronounced as to the ultra-violet rays, which produce a discoloration especially noticeable in glasses containing manganese: these glasses become brownish or purple-brownish. Similar phenomena of discoloration have been noted in glass subjected to the action of radium. The chemical sensitiveness of glass to the action of light is of considerable economic importance in the manufacture of photographic dry-plates. Glass which has once carried a strongly contrasted negative cannot be coated with emulsion a second time, for the image of the first exposure remains in some degree in the glass surface, and reappears in the second exposure.

The most resistant of all varieties of glass is a borosilicate glass containing some magnesia. After heating a few times, however, this glass has a tendency to become slightly milky and finally opaque. Its rival for many purposes is vitrified silica, which resembles glass in many respects, but, containing no alkali, is free from the deteriorations which the alkalis invite.

Chemical and Physical Properties, Colors, etc.- How long sand and soda served by themselves for the production of glass no one can now

tell, yet while these elements are always the prime essentials scientific discoveries have from the earliest times so directed the artificer of this most useful commodity that no quality or variety of material and no color or shade of color is now impossible of production. There is scarcely a mineral that has not in some form been employed in glass-making, either for body or color. Sand, flints and various rocks have provided the silica, but sand is now almost universally used, as being more free from the impurities which affect glass in substance or color-the most readily obtained and needing the least preparation. When rocks were used they were calcined, pulverized and freed as far as possible from impurities. There are different grades of glass-making sand. The first bed of the required quality mentioned in glass-making history was situated at the mouth of the river Belus, near Mount Carmel. The ancient Phoenician glass-makers used this sand, and the supplies were drawn upon extensively as late as the 16th century by the Venetian glassmakers for their finest productions; though for inferior kinds of glass they could use the sand of their own lagunes. The finer grades of glass require a purified sand and this is obtained by washing and burning out of the earths, metals and vegetable matter which affect glass in various ways if not removed before the melting process takes place. Iron is nearly always present and has a tendency to give a green cast to the glass. The proportion of sand in glass is rarely less than 50 per cent of the full mixture and not often more than 75 per cent. Outside these bounds the molten glass becomes difficult to manipulate. Where a more permanent glass is required an increased proportion of silica fused at a higher temperature will produce it, but at the sacrifice of other qualities. Various alkalis and alkaline earths are used to fuse the sand. Soda and potash are the chief solvents for nearly all the varieties of glass made. The ashes of wood, various plants and seaweed have also answered the purpose. In France, during the Middle Ages, the alkali used was produced from fern. There were alkali yielding plants in the lagunes of Venice. In the 15th century the dregs of wine provided Spain with a suitable ash for the purpose, and the early Chinese "made mirrors from pebbles and a material obtained from the sea and reduced to ashes." A suitable soda can be extracted from sea-salt. Glass made from potash is more limpid than that made from a vegetable ash The soil of Egypt, where the first glass is said to have been made, contained abundance of soda and sand. An excess of alkali makes glass liable to decompose through the effects of time and atmospheric influence.

Arsenic, alumina, barium, iron, lime, lead, magnesia, strontium, zinc are also elements in glass-making. The simple glasses are composed principally of silica and alkali; the mixtures varying according to requirements, and embrace the several kinds of window and bottle glasses. Ordinary window glass contains sand, soda and lime. Crown glass has potash added. Plateglass contains sand, soda or potash-lime, alumina and iron. Bottle glass has sand, soda, alumina and iron. The compound glasses are composed of silica and alkali, with more or less metallic substances. Flint glass is the most important of the compound varieties. Various

recipes are employed in making it, but a wellproved one embraced in its composition sand, red lead, refined ash, saltpetre, arsenic, manganese and borax. These combined ingredients form the "batch" or "frit." The proportions are roundly three of sand, two of lead, one of ash and about one-fifth of one of saltpetre. The arsenic, manganese and borax are added in varying very small quantities, slightly graded in order named. Lead is the most important metallic substance in the compound glasses. It is used in the form of litharge and red oxide. Both are produced in the furnacing of pig lead. Both are active fluxes and glasses made with them require less alkali in proportion to silica than those made without them. The red lead, after it has been ground in water and dried to a very fine powder, is now the more generally used.

For optical glass and the strass from which artificial gems are made, an increased proportion of lead is required, density being the chief aim, though as with an excess of alkali its lasting power is affected. Density, refractiveness and pellucid brilliance are the qualities imparted to glass by the use of red lead. A refined ash ready for use is now obtainable. Saltpetre assists in driving off the globules of air in the liquid glass. Arsenic and borax are purifying aids. Manganese is used to neutralize the colors imparted by the other elements in glass. The aluminous earth frequently combined with manganese may be removed by a washing process. The aim is to make flint-glass absolutely colorless. It is considered "high" in tone when it assumes a pink cast; and "low" when of a green cast. The high tone may disappear during the working out of the metal from the melting pot, or it may be reduced by a very uncertain and not to be commended - trick known to experts, but once low it remains low to the bottom of the pot.

Flint-glass is always liable to get low in color if kept in fusion too long. A "proof" is taken of the color of each pot of metal in the furnace as soon as the fusion is complete and the rough scum which always arises has been skimmed or raked from the top of the molten mass. The true color can only be correctly gauged by observing it through the thickness

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fracture, lengthways of the proof, not through its surface.

The oxide of some particular metal is generally employed to produce particular colors in glass. Metallic substances so employed include antimony for ambers, purples, reds and yellows; copper for ambers, blues, greens and reds; cobalt for blues and greens; chromium for greens; gold for finest ruby reds; iron for ambers, blues, browns, greens, reds and yellows; nickel for blues and greens; silver for ambers and yellows; tin for reds; uranium for greens and yellows. Self-colors are usually made separately, but shades of all can be produced by superimposing one color glass upon another — or casing as it is familiarly called and as many as six different colors in one article have been successfully united. A further variation in shade may be obtained by changing the quantities of the separate colors used. Ruby colored glass made from gold is essentially a "casing" material, not for use by itself, heat being an obstacle in the way of controlling the tint. Though the first melting

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of the "ruby" ingredients may show only a golden yellow tint-in which stage it is gathered into lumps and annealed - when reheated for casing purposes the desired ruby tint develops and continues to deepen in color by continuous heat. A vessel made from uncased ruby would be scarcely transparent, unless very thin. Gold is capable of producing a variety of tints in glass, including - besides rubyshades of blue, orange, green and yellow. Copper is also capable of producing ruby tints, and a brownish red is obtained from iron. Sometimes ruby glass is semi-opaque, or what the glass-maker calls "muddy and some peculiar freaks have occasionally developed from this cause. Some of the freaks in this connectionfreaks because they cannot be controlled in the fabrication show a muddy red exterior which changes to a dull but nearly transparent green when light is passed through it. Opaque and opalescent glasses are obtained from the use of oxide of tin, or from phosphate of lime, in the ingredients.

Fully opaque glass resembles porcelain, and while all the colors can be made opaque, chalk white, as a ground for other colors, is most frequently in use. Opaque white is used as an enamel, in the fashioning of glass vessels for painting upon, and, when united to another color, for cameo carving. When used for the last named purpose the usual "white" formula must be altered to agree with that of the glass with which it has to be united. Transparent and opaque glasses differ in density in the substance and the union of "hard" and "soft" glasses invites trouble after annealing; especially so when the abrasion of one of the parts is taking place as in cameo carvingruinous cracks often developing from the slightest initial fracture- - as a scratch from any sharp-pointed instrument. Opalescent glass turns from clear to opaque by fire means, and can be controlled. The transition occurs during the process of fashioning the glass vessel, and the opacity can be arrested and largely controlled at the will of the artificer. When gathered from the melting pot the metal is transparent; a partial cooling influences the action of the opacity-producing chemicals in the ingredients and makes possible both transparent and opaque effects in the one article.

There are numerous recipes for coloring glass, but success with any largely depends upon the skill shown in adjusting the quantities of the several ingredients and in regulating the heat of the furnace during fusion; variations in temperature very materially affecting the shade of color. Various colors have been obtained from the same metal and various metals have produced similar colors. Amber tints can be obtained from antimony, arsenic, copper, iron, manganese, and silver. Black and browns from charcoal, iron, manganese and zaffre. Greens from copper, cobalt, chromium, iron, nickel and uranium. Purples from antimony and manganese. Reds from antimony, copper, gold, iron, and tin. Yellows from antimony charcoal, iron, manganese, silver, and uranium. Opaque white from antimony, arsenic, phosphate of lime; with sometimes a mixture prepared from tin and lead. The flint-glass "batch" is usually employed for colored glasses. The presence of iron, copper and other metals, in varying proportions, renders colored glass