British Manufacturing Industries Part 3

a.r.s.enious acid also acts as an oxidizing agent, in that it gives up its oxygen to the protoxide of iron, converting it into the peroxide; but the a.r.s.enic itself, which has lost its oxygen, is reduced to the metallic state, and being volatile, does not remain with the gla.s.s, but pa.s.ses off by the flues of the furnace. If too much a.r.s.enic is used, it sometimes renders the gla.s.s milky or cloudy.

Before describing in detail the method of mixing and founding gla.s.s, it will be necessary to mention the composition of the vessels in which the gla.s.s is made. They are called gla.s.s-pots, and differ in shape according to the different kinds of gla.s.s to be made in them.

Gla.s.s-pots are made of fire-clay (generally the best Stourbridge), which is a silicate of alumina, and here great care is taken to select that which contains least lime or iron. It is ground, then moistened and well kneaded together, and left to ripen, while a certain quant.i.ty of old gla.s.s-pot is ground fine and mixed with the fresh fire-clay.

Ma.s.ses about the size of two hands are kneaded separately, the object being to exclude all air bubbles, and to obtain a perfectly h.o.m.ogeneous lump. The bottom of the gla.s.s-pot is then laid, the ma.s.ses of fire-clay being pressed in with the greatest care, so as to avoid all cracks or places where air might enter during the slow process of drying.

The modern shape is round; though formerly certain gla.s.s-pots, called _cuvettes_, used in the purifying of plate gla.s.s, were square. Pots used in the manufacture of common crown and sheet window gla.s.s, generally speaking, are larger at the top than at the bottom; but whatever may be the shape of the pot, the method of its building is the same. The sides are carefully made of fire-clay, each piece being laid on by itself and kneaded like the bottom of the pot, so that it is slowly built up until it reaches the desired height. It is then dried very gradually, and the process is finished in artificially warmed chambers. Before putting it in its place in the gla.s.s-furnace, it is allowed to remain for some time in what is called a pot-arch, that is, an archway built of fire-clay bricks, along the side of which is a fireplace, by means of which the arch is brought up to a red heat; and after it has been heated sufficiently, is removed while red-hot and put into the furnace. Gla.s.s-pots are never allowed to cool, and with care they may last for several months. From this description of their manufacture, it will be clear that it is attended with considerable cost, varying from 5_l._ to 10_l._

There are three different kinds of ordinary pots for crown, plate, and flint gla.s.s; and of these the last is decidedly the most expensive, as its top is covered over, and presents the appearance of a dome with an opening in front, through which the materials can be introduced when the pot is charged, and from which, when made, the gla.s.s may be drawn, in order to be blown into shape by the workman. In gla.s.s-furnaces the pots are sometimes arranged in a circle, with their mouths opening into the gla.s.s-house; but now a different construction is sometimes employed, since other methods of heating the furnaces have been introduced. It is hardly within the scope of this article to enter into a description of gla.s.s-furnaces; suffice it to state, that they should be of such a construction as to yield the greatest amount of well-regulated heat for the smallest consumption of fuel, and this object seems to be best effected by the adoption of Mr. Siemens'

excellent principle of heating furnaces. For some years his process has been in use at the Thames Plate Gla.s.s Company's Works, where the saving of fuel has been very considerable, and the gla.s.s greatly improved, owing to the fact that impurities from the fuel employed cannot possibly find such easy entrance into the gla.s.s-pot. In any case, the construction of the furnace is such, as to be best adapted to the convenience of the workmen, according to the kinds of gla.s.s which they have to make. Differently arranged furnaces are used for bottles from those employed for crown and sheet gla.s.s.

It has lately come to my knowledge that flint gla.s.s, that is to say, the gla.s.s used for tumblers, decanters, and such like, is occasionally injured by the appearance in it of little opaque white spots. Some portions of gla.s.s of this character have been a.n.a.lyzed by me, when I found that these white spots were owing to the presence of a gla.s.s containing alumina. Now alumina raises the melting point of any gla.s.s of which it is a const.i.tuent. So, then, these white spots were due to the presence in the flint gla.s.s, which was perfectly clear, of a much less fusible gla.s.s which was only partly made when the flint gla.s.s was ready for working. On investigating the matter, it was found that the alumina came from the gla.s.s-pots, for when by my advice the faulty pot was withdrawn from the furnace and carefully examined, although it had been in work only six weeks, the bottom was honey-combed to a very considerable extent, showing that portions of the pot had been dissolved; and inasmuch as the fire-clay, of which the pots are made, contains a large quant.i.ty of alumina, it was not difficult to trace the source of these white spots which had rendered useless much very valuable gla.s.s. On inquiry it was found that the pots had been made entirely of new clay, and on reference to the book of workings, which was kept in the gla.s.s-house, it was also found that for some time, the gla.s.s-pots used in that establishment had been made of new clay, and that on a previous occasion a similar calamity had before happened.

In the records kept where pots were made, as has already been described, with a portion of old pot as well as new clay, no white spots had ever appeared in the gla.s.s. It is therefore manifest, that it is much safer to use a portion of old pot than to trust to pots made entirely of new clay.

Having considered briefly the manufacture of gla.s.s-pots, I shall proceed to the treatment of the materials to be employed. In making common window gla.s.s, ordinary sand, which does not contain any very large quant.i.ty of iron, may be used, the alkali employed being sulphate of soda, while the purifying material is either a.r.s.enic or black oxide of manganese. A small quant.i.ty of anthracite coal is added to the mixture, in order to a.s.sist in the reduction of the sulphate of soda, together with some lime. The materials are carefully mixed and placed in the furnace, where they are heated for some time, a process which is called "fritting." Its object is to perfectly dry the materials, so as to expel carbonic acid gas, which would otherwise cause swelling in the gla.s.s; but no combination must take place, to allow of silicates being formed, otherwise the alkali would melt first and attack the substance of the gla.s.s-pots, and part of it would be volatilized and lost. When this operation is completed, the fritt is put into the hot gla.s.s-pot, and submitted to the action of the heat of the furnace, until the gla.s.s is made, or "founded," as it is technically termed. In the case of sheet and crown gla.s.s, this process lasts from sixteen to seventeen hours, for it will be remembered that the top of the pot is open to the furnace, so that the flames pa.s.s over the surface of its contents. In this way the materials get heated more rapidly than when a covered gla.s.s-pot is used.

M. Gehlen gives as a good mixture for window gla.s.s:

Sand 100 parts.

Dry sulphate of soda 50 "

Quicklime 20 "

Carbon, as charcoal 4 "

Different makers have different mixtures. This by M. Gehlen is given as _about_ the proportions of the several const.i.tuents employed.

The charging of the pots is conducted in this manner: they are filled with lumps of fritt, and the heat of the furnace is raised as rapidly as possible, until, in about eight or nine hours the fritt has run down or melted into gla.s.s. More fritt is then added, which also melts, and from time to time this is repeated, till the pot contains a sufficient quant.i.ty. After about sixteen hours the whole has become converted into gla.s.s, and the surface of the molten ma.s.s is covered with liquid salt and sulphate of soda. This sc.u.m is called gla.s.s-gall or sandiver, and is carefully removed with iron ladles. Some broken gla.s.s, or cullet, is now thrown into the gla.s.s-pot, a little at a time, the object being to cause any salt which may remain in the pot to rise to the surface, which is then removed, and so the gla.s.s is in this manner purified, after it has been further heated for some hours, to expel gases.

When the gla.s.s is made, and its temperature so reduced that it is in a doughy or pasty state, it is then worked off by the blowers into either sheets or tables, as is desired. The blowing of sheet and crown gla.s.s is a work of considerable difficulty and labour, and one which cannot be successfully performed, except by a workman who has been brought up from boyhood in a gla.s.s-house. A quant.i.ty of the soft gla.s.s is collected or gathered on the end of a blowpipe, and the workman then blows into it, and distends it into a globular form. Now it is necessary, in making sheet gla.s.s, that that globular form should be elongated; the workman therefore holds his blowpipe, which is about five feet long, in a vertical direction, and the softened globe becomes pear-shaped. By dexterously swinging the blowpipe from side to side, which he does while standing on a plank placed over a sort of pit, and by causing it to rise on either side, he converts the pear-shape into a true cylinder, having rounded ends. When the cylinder has a.s.sumed the exact shape desired, he places his thumb on the end of the blowpipe, and holds the opposite end of the cylinder in the mouth of the furnace. The gla.s.s softens at the heated end, and the expanding air causes it to burst the opening. It is then shaped with a suitable tool, so that it is of the diameter of the cylinder. When the latter is cooled, a piece of hot gla.s.s is applied to its shoulder with a pontee, and is drawn out into a thread around it. This makes the gla.s.s hot. The thread of gla.s.s is removed, a cold instrument is applied rapidly, and the shoulder of the blowing is cut off. The gla.s.s is next detached from the blowpipe, and its ends removed, and it is then annealed for a short time, and cut down lengthways internally by a diamond. It is afterwards placed, with the long cut uppermost, in what is called a flattening kiln, that is, in a sort of oven or furnace heated to a high temperature and having a perfectly smooth stone floor; after a short exposure the gla.s.s softens, and a workman, with suitable wooden tools, opens it out where it was cut by the diamond, and causes it to lie flat upon the stone. It is then rubbed by a wooden tool, and in this way is flattened, removed from the flattening stone kiln, and placed in a hot chamber, in which it is allowed to cool slowly, for the purpose of "annealing."

Sheet gla.s.s, formerly called broad gla.s.s, was originally made on the Continent; but its manufacture, first established in this country by the introduction of foreign workmen, has extended to very large dimensions, and the quality of English sheet is now quite equal, if not superior, to anything that is produced abroad. The advantage which it possesses over crown gla.s.s is, that much larger sheets can be made, and this is very easily noticed if we examine the larger dimensions of common window panes compared with those which were formerly made. Even now the workmen employed in this cla.s.s of manufacture are generally Belgians. A sheet gla.s.s blower must be very strong, and have great skill in handling his blowpipe, for the cylinders which he blows are frequently sixty inches long, and their weight is very considerable.

Gla.s.s shades are blown by sheet blowers. These sometimes are very large, and require great skill. When their shape is to be that of a cylinder with a dome top, they are made as in the ordinary course of blowing a cylinder of sheet gla.s.s, but instead of one end being burst as described, they are simply detached from the blowpipe. When they have to be oval or square at their bases, they are blown into wooden moulds of the required form, which have their insides charred. The gathered ma.s.s of gla.s.s is placed inside such a mould, and is then blown into until it touches the sides. This is an operation requiring great strength and delicacy; strength to blow with sufficient force to bring the softened gla.s.s to touch the mould in all its parts, and delicacy to prevent the pressure from being so great as to cause the outside of the gla.s.s shade to receive marks on its surface from the mould.

The shaping of the molten gla.s.s into tables of _crown_ is different in detail. The globular ma.s.s formed by the first blowings is held by a workman vertically over his head. An a.s.sistant gathers a small quant.i.ty of soft gla.s.s from the furnace on the end of a pointed iron rod, and causes it to adhere to the flattened surface, at a point opposite to that to which the blowpipe is attached. The gla.s.s near the blowpipe, while hot, is touched with a cold instrument, and immediately cracks around its neck, detaching the blowpipe from the ma.s.s. The pointel is taken by the blower, and the opening formed by the removal of the blowpipe is placed opposite to what is called a "flas.h.i.+ng" furnace, that is, a furnace with a large circular opening in its front, and which is heated to such an intense degree, that it is impossible for a person unaccustomed to it to approach within several feet of the furnace-mouth. The workman generally wears a s.h.i.+eld or screen to protect the upper part of his body and face. The gla.s.s becomes softened by the heat, and the workman gives his pointel a rotary motion, somewhat similar to that which a housemaid gives to a mop when she trundles it; and as the gla.s.s softens, the opening gets larger and larger, until at last the softened ma.s.s instantaneously flashes out into a circular sheet, an operation which produces a very startling effect upon the eyes of anyone beholding it for the first time. The circular crown table thus made is detached from the pointel, and the ma.s.s of gla.s.s which caused it to adhere forms what is known by the name of the bull's eye. The table thus made is, like the sheet, placed in an annealing furnace, and there left for a proper length of time.

The manufacture of _plate_ gla.s.s is altogether different from that of crown and sheet. First of all, much greater care is taken in the selection of the materials, the sand used being of a purer kind than that employed in the manufacture of common window gla.s.s; the alkali is of a better quality; and more caution is taken in all the manipulative processes prior to the melting of the mixture. a.r.s.enious acid is more frequently used than manganese for the correction of the iron impurity. It has been noticed that in the plate gla.s.s-pots, there are grooves placed around their sides, and these are intended to receive metal claspers, by means of which the pot can be removed bodily from the furnace. In former times the gla.s.s was made in large pots, and then ladled out into smaller ones, of a square form called _cuvettes_, and in these it was left exposed to the heat of the furnace for a length of time, in order that it might be refined, by the rising of impurities to the surface and by the escape of air bubbles. The use of these cuvettes is now discontinued, and the pot in which the gla.s.s is founded is removed from the furnace and its contents poured upon the tables on which the plate is formed, by the action of rollers. A plate gla.s.s table is made of iron; its surface is smooth and of the size required to make a large plate, and it is placed upon wheels and run upon a tramway from one part of the gla.s.s-house to another, so as to be opposite to the mouth of the furnace from which the gla.s.s-pot has to be removed. Along the sides of this table, taken lengthways, moveable strips of iron are placed, rising above it to a sufficient height to secure the desired thickness for the gla.s.s plate, and on these strips runs a roller, so adapted that it can be made to pa.s.s pretty readily from one end of the table to the other. The contents of the gla.s.s-pot, when placed over the table by means of a crane and tilted up, fall out somewhat as a lump of dough would fall from a kneading trough if it were inverted, for it must be borne in mind that the gla.s.s in this process is not in a very fluid state. The roller is made to pa.s.s rapidly over the softened gla.s.s, and in this way spreads it over the table, until it comes in contact with the strips placed along the edge, which serve as gauges for determining the thickness of the plate. After the plate is formed, it immediately sets, and is removed while hot into an annealing furnace, which is always so placed that the gla.s.s can be transferred to it from the table with the least possible delay. In this furnace several plates of fresh-made gla.s.s are deposited, and are allowed to cool extremely slowly, in order that the gla.s.s may be properly annealed. When this process is completed, the plates are removed, the edges are trimmed off with a diamond, and one plate, bedded in plaster of Paris, is placed upon a flat stone receptacle; another plate, also coated on one of its sides with plaster of Paris, is made to adhere to a piece of machinery placed directly above the other plate, and is so situated, with respect to this latter, that the two surfaces are perfectly parallel one to the other.

It should be here mentioned, that the side of the plate which touches the table is always rough, and has no polish, while that over which the roller is pa.s.sed is slightly undulating, and has a bright polish similar to that of a sheet of blown gla.s.s, and which is technically known as "fire" polish. The machine to which the upper plate is attached is so arranged that, when set in motion, it causes it to move in just the same direction that a plate would do if moved by the human arm; this is therefore called an elbow motion. Boys stand by the sides of the two plates, and throw fine sand and water on the lower one, so that the opposed surfaces mutually grind one another, and when this process is completed on one side, they are reversed, and the same operation is performed on the other side. The plates have now the appearance of ground gla.s.s, and the surfaces are further ground by fine emery powder, which causes them to be much smoother and more ready for the final polis.h.i.+ng. Formerly this was entirely done by hand, women generally being the operators, and oxide of iron, called crocus, mixed with water, the material employed for polis.h.i.+ng. Now, however, a more rapid and perfect method is adopted by the use of machinery. A table is prepared which moves from side to side, giving to the plate a lateral motion; and above is a beam, in which holes are drilled at intervals, through which short iron rods, nearly an inch in diameter, pa.s.s. On these are padded iron buffers, covered on their under surface with leather; while, pressing down these rods, and therefore the buffers, are springs, which act with considerable force, but which are able to yield to pressure caused by any inequality over which the buffers may pa.s.s. The gla.s.s plate is fixed upon this table, and its upper surface is exposed to the action of the buffers, while oxide of iron, in a very fine state of division and mixed with water, is allowed to come upon its surface. The gla.s.s travelling from side to side is rubbed by the buffers in a lateral direction, and has also a longitudinal motion, so that every portion of it is rubbed equally. If any inequalities occur on the gla.s.s, the springs which press down the buffers give way and allow them to rise over it, and this process is continued for some time, until at last the plate receives the polish so characteristic of plate gla.s.s. It is then removed from the table and examined by skilled persons, and whatever defects can be removed by hand, are remedied.

Another kind of plate gla.s.s, called "patent rolled plate," is made by ladling out from a pot molten gla.s.s in the proper state of consistence. The ladle is brought over a small gla.s.s table, and a similar operation is performed to that already described. This patent rolled plate is sometimes made with grooves on one of its surfaces, or with patterns in imitation of diamond quarry glazing, and, in fact, with any designs, according to the taste of the manufacturer. These designs are all engraved upon the table, and communicate their patterns to the soft gla.s.s; but the smooth surface of such gla.s.s which comes in contact with the roller is slightly undulating, though polished. This method of gla.s.s making was invented and patented by Mr.

Hartley, the noted manufacturer, of Sunderland.

A lighter kind of plate gla.s.s, which is princ.i.p.ally used for glazing the better cla.s.s of pictures and engravings, and called "patent"

plate, is simply sheet gla.s.s polished after the manner of plate gla.s.s.

Crown gla.s.s, which only admits of being cut into small squares, is also used for picture glazing, but is more carefully prepared, and is called by the name of "flatted crown."

_Looking Gla.s.ses._--Plate gla.s.s is employed for making looking gla.s.ses, and two processes are now in use for silvering them, the first of which consists in applying a sheet of tinfoil saturated with quicksilver to one side of the gla.s.s. The operation is conducted as follows: on a perfectly smooth table a sheet of stout tinfoil is laid, and on it is poured quicksilver, which is distributed evenly over the surface with a hare's foot. When the whole sheet is amalgamated with the quicksilver, more of that substance is poured over it, until it flows quite freely. The gla.s.s plate to be silvered, having been made perfectly clean, is floated upon the surface of the quicksilver, an operation requiring care, and is then covered all over with weights, by which means the excess of quicksilver is pressed out, and the gla.s.s comes in contact with the amalgamated sheet of tinfoil, to which it adheres entirely. This ancient method of silvering gla.s.s has some advantages over the one next to be described. The colour of the plate is, according to artistic taste, better, and with care the plate will not lose its brilliancy for years. I have in my possession some old gla.s.ses, the silvering of which is very beautiful, except where it has suffered from mechanical injuries. Silver can be precipitated from a solution of nitrate of silver in several ways, and in some of these specimens was like a bright film. If a crystal of nitrate of silver be put into a test-tube with some bitartrate of lime, and the mixture be rendered ammoniacal and gently warmed (it being kept in motion during the experiment), its sides will be covered with a very brilliant deposit of metallic silver. Oil of cloves and grape sugar have also the power of reducing metallic silver from ammoniacal solutions of the nitrate, when gently warmed; but the mixtures must not be made too hot. In silvering plates of gla.s.s, they are first well cleaned, then placed in a perfectly level position, and the silvering liquid is poured over the surface, the room in which the operation is performed being kept sufficiently warm to a.s.sist the deposition. When enough silver has been deposited on the gla.s.s, the liquid is poured off and the plate dried, while the silver film is protected by being coated with a suitable hard varnish. The composition of the mixtures used by different persons is generally kept secret, though the chemical principle of the reduction of the silver salt is the same. Gla.s.ses silvered by this process sometimes lose their brilliancy, by becoming covered on their silvered side with small spots. It is however stated that this results either from a bad system of deposition, or from the film of silver not being sufficiently thick and solid.

_Flint Gla.s.s_, although called by this name, is not made from flint, but from the best sand, of pure and dazzling whiteness, obtained from Alum Bay, in the Isle of Wight, and from Fontainebleau, in France. The cost per ton is from 1_l._ to 1_l._ 15_s._, whereas the price of the sand used for making plate gla.s.s is about one-eighth of that amount. The alkali employed is generally extremely good carbonate of potash, whereas soda is used in the manufacture of the other kinds of gla.s.s which have been described. The addition of a small quant.i.ty of black oxide of manganese is sometimes necessary to correct the slight tint imparted by iron, which seems to be always present in minute quant.i.ties, even in the purest samples of sand. Oxide of lead in the form of red lead, in this sort of gla.s.s, takes the place of lime. The advantages derived from using the oxide are, that it makes the mixture more fusible, and also imparts that particular brilliancy and l.u.s.tre so peculiarly characteristic of well-made flint gla.s.s. In different works, various mixtures are made for the composition of the gla.s.s; but to give an idea of the proportions in which the materials are mixed, it will be well to quote the statement of M. Payen, who says that of the finest crystal flint gla.s.s, the following is the composition: sand, 3; red lead 2 to 2-1/4; carbonate of potash, 1-1/2 to 1-2/3. A little nitre or saltpetre is used as an oxidizing agent. The gla.s.s-pots employed in this branch of the manufacture are covered, so that the flames of the furnace do not come in contact with the materials, the object in thus isolating them from direct contact with the flame being to prevent the entrance of impurities, by which the colour might be injured. On account of the pots being covered, the materials take a much longer time to get hot, and require quite double the time in founding that sheet or plate gla.s.s does; the presence of oxide of lead materially a.s.sisting the rapidity of the fusion. When flint gla.s.s is ready for working, the time required to work off a pot of it is much longer than that which is required for a pot of crown or sheet; and it is a matter of considerable importance, that the furnace-man should so manage his fires as to keep the gla.s.s in a proper working condition, that is, he should not let it get too cold (therefore too solid) nor too fluid. Flint gla.s.s is worked off by the blower into wine-gla.s.ses, tumblers, decanters, and other suitable vessels. Let us take a wine-gla.s.s as an ill.u.s.tration of the method of working. A small quant.i.ty of gla.s.s is gathered on the blowpipe, which is much smaller than that used in making sheet, and is blown into a bulb, which may be slightly elongated or globular, the forms being given to it by the motion which the workman imparts to his blowpipe while he is blowing, or after he has blown, into the ma.s.s. In the case of a wine-gla.s.s, an a.s.sistant boy gathers a small quant.i.ty of gla.s.s on the end of a small pointel, or solid iron rod. This is placed on the side of the globe opposite that which is in connection with the blowpipe, which is then detached by touching the gla.s.s nearest it with a piece of iron, wetted with cold water: this causes a crack, and a gentle tap causes separation. The workman then moulds the opening made by detaching the blowpipe, in order to do which, he has to apply the gla.s.s often to the mouth of the furnace, to soften it. He then opens out the globe into the shape of a cup with a pair of small iron tongs, with legs uniform in shape, slightly tapering and smooth, and he uses a peculiar kind of scissors for tr.i.m.m.i.n.g the edges. The other parts of the gla.s.s are moulded with the tongs, accuracy of size being obtained by means of measuring compa.s.ses and a scale. The workman sits during this operation in a seat with arms, laying the pontee on them, and turning it, so as to make it move backwards and forwards with his left hand, while with the tongs in his right he gives the gla.s.s the desired form.

Before pa.s.sing on to a description of the manufacture and composition of coloured gla.s.ses, it is necessary that I should make a few remarks on the difficulties under which our English gla.s.s makers labour, owing to not paying sufficient attention to the scientific treatment of their mixtures. It has already been stated that gla.s.s is composed of a mixture of silicates, which are definite chemical compounds. Some are much more dense than others, and are therefore liable to sink, so that the gla.s.s taken from one part of the pot will be very different in composition from that taken from another part; besides this, it is found on examination, that other portions of the materials employed are present in such proportions, that they cannot possibly exist in the form of true silicates. M. Dumas, the distinguished French chemist, a.s.serts, and with truth, that gla.s.s ought to be a true chemical compound. This, however, does not seem to be the opinion here; and sufficient attention is not paid by English manufacturers to mixing their materials, so as to form definite silicates, the result being that gla.s.s is produced with a striated effect. This is easy to be seen in the common kinds, as in bottle gla.s.s; but owing to the more careful and prolonged fusion of the finer varieties, such as plate gla.s.s, this defect is to a considerable extent remedied, though not altogether overcome. In the French manufacture of plate gla.s.s, more attention has been paid to the chemical composition of the various silicates which enter into it. At St. Gobain, a plate gla.s.s, is produced which, on a.n.a.lysis, is found to contain definite silicates, and without any excess of material which does not enter into chemical combination; and the consequence is, that this gla.s.s is more perfect and h.o.m.ogeneous than that made in this country. No doubt this superior quality is owing to the fact, that the famous chemist, Gay-Lussac, devoted much of his time to a.s.sisting in the manufacture carried on at these works. We cannot over-estimate the importance of a scientific superintendence, not only of gla.s.s-works, but of all other manufactures in which chemical reactions take place; for although experience may lead a cautious observer to produce substances of nearly correct composition, yet the a.s.sistance of a scientific observer is of the greatest importance, because, what under other circ.u.mstances must be simply empirical, is under his guidance carried on according to definite and fixed laws.

Mention has already been made of how, in the case of mixing carbonates of soda and potash, the one a.s.sists the fusibility of the other, and this is more particularly true in the mixture of silicates in the composition of the ordinary gla.s.s. Silicates of soda and potash are separately much more infusible than a mixture of the two, and the addition of other silicates to them renders them more fusible still; silicate of lead, as has already been mentioned, causing the gla.s.s into whose composition it enters to fuse at a much lower temperature than it would do if that silicate were absent. Again, if the silicate of lead be present in too large proportions, and if great care be not taken in the manufacture of lead gla.s.s, the silicate of lead, from its greater density, will sink lower among the molten silicates, and will therefore cause a larger proportion of lead to be in the gla.s.s at the bottom of the pot than there is at the top. We often notice in tumblers and decanters of the cheaper kind, that there are very distinct striae running through the whole substance in some particular portion of the gla.s.s. Now this is owing to the greater density of the lead silicate, which sinks lower down in the collected ma.s.s of gla.s.s, and therefore imparts to it this peculiar effect. When a pot of flint gla.s.s is worked off, that which remains at the bottom usually contains more lead than that which is worked off in the earlier part of the day.

_Coloured Gla.s.ses._--It has been before shown that silica unites with metallic oxides; in fact, gla.s.s is nothing but a compound brought about by the union. With certain metallic oxides, silica forms coloured silicates or gla.s.ses; and these, when fused with colourless gla.s.ses, impart to them the colour of the silicate. Oxide of iron colours gla.s.s either green or yellow, according to the nature of the oxide; the silicate of the protoxide of iron being green, and that of the peroxide, yellow of a slightly brownish tint. Copper forms two oxides, the suboxide and the protoxide; the suboxide colours gla.s.s red, while the protoxide renders it green. Black oxide of manganese colours gla.s.s purple; but if large quant.i.ties be used, it makes it perfectly black. Sesquioxide of chromium imparts a beautiful green colour to gla.s.s, while oxide of uranium produces an opalescent effect of yellow with a tinge of green. This latter, by the way, has the power of reducing the ultra-violet rays of the spectrum to luminous rays, and, when held in the rays of a spectrum obtained by the electric light, produces an extremely beautiful effect, which is called fluorescence. A small quant.i.ty of the oxide of gold tints gla.s.s pink, but the colour becomes extremely rich and ruby-like, when a larger quant.i.ty of the oxide is employed. Oxide of cobalt in very small quant.i.ties yields, with silicic acid, an intensely blue silicate. This substance, carefully prepared in a special manner and ground to a fine powder, forms the well known water-colour pigment called smalt. Oxide of silver stains gla.s.s from a delicate lemon tint to a deep orange, in proportion to the quant.i.ty of the oxide employed.

With the exception of the last-named colouring material, the above mentioned are mixed together with the substances which form the gla.s.s, and are melted in the usual way in gla.s.s-pots, except that they are treated with considerably more care, in order that their tints may be true. Oxide of silver, however, is never mixed with the materials of which the gla.s.s is made, but is applied to the surface in the following manner: a solution of nitrate of silver mixed with some substance, such, for instance, as chalk, may be painted upon the parts of the gla.s.s which it is desired to stain, and these are heated to a dull red heat, in what is called a "m.u.f.fle." Wherever the oxide of silver, which is reduced from the nitrate by heat, comes in contact with the gla.s.s, the latter is stained more or less intensely, according to the quant.i.ty of silver present. Pure metallic silver may be melted with metallic antimony, and the ma.s.s ground to a fine powder in water. This powder, after being mixed with some Venetian red and gum water, is applied to the surface of the gla.s.s, which is, when dry, heated to a dull red heat in a m.u.f.fle, producing the yellow stain, which can be seen after the Venetian red and the excess of silver have been sc.r.a.ped off. The reason why silver, or oxide of silver, is not mixed with the gla.s.s materials and fused with them, is because it does not readily unite with oxygen, and, when it has done so, it loses its oxygen again at a high temperature, and becomes reduced to the metallic state; and inasmuch as metals have no effect whatever in staining silicates, gla.s.s made in this way would not have the yellow colour which it has, when the silver is heated upon its surface to a much lower temperature in a m.u.f.fle; for the temperature to which the const.i.tuents of the gla.s.s must be heated, so as to cause them to burn it in, would be so high, that the oxide of silver first formed at a lower temperature would be reduced to the reguline or metallic state.

Gold also, like silver, does not unite with oxygen readily, or remain in union with it at high temperature; therefore great care is required in the preparation of gla.s.s to be coloured by oxide of gold; the form in which it is used being generally that of the purple of Ca.s.sius, made by precipitating a salt of tin with a salt of gold. This substance is mixed with the gla.s.s to be coloured, and heated in a suitable gla.s.s-pot. Portions of it are gathered and allowed to cool, these being generally of a yellowish, brownish, and sometimes reddish tint, though they have not in any case the same beautiful red colour which they produce when applied, as will be immediately described, to the surface of white gla.s.s. A certain quant.i.ty of white gla.s.s is gathered from the gla.s.s-pot in the soft state with one of these pieces of gold gla.s.s; the whole ma.s.s is heated until both become soft, and is then blown and formed into sheet, which, on examination, will be found to consist mainly of white gla.s.s, with its surface thinly covered with the gla.s.s stained with oxide of gold, while the beautiful ruby colour, which the gold imparts to the gla.s.s, appears pure and distinct. If such gla.s.s as this be heated to too high a temperature, as when it is used in the manufacture of stained gla.s.s windows, the ruby colour is in part, and sometimes altogether, destroyed, for the oxide of gold loses its oxygen, and metallic gold is left behind, which does not yield a colour to the silicate. I have in my possession a piece of French gla.s.s of a pale sapphire tint, which, when heated in the oxidizing flame of the blowpipe, a.s.sumes a brilliant and intense ruby colour, showing that in the first condition, the gold is not in a state of oxidation sufficient to impart colour to the gla.s.s.

When the suboxide of copper is mixed and fused with the gla.s.s which it is intended to colour, the result is an opaque substance, almost like red bottle-sealing-wax, which is treated in a manner exactly similar to the gold gla.s.s; viz. it is coated with white gla.s.s, and blown and shaped into sheets, which owe their intense ruby colour to a thin film of the coloured gla.s.s closely adhering to the ma.s.s of the white upon which it is placed. Gla.s.s made in this way is called "coated," and sometimes "flashed" gla.s.s, and is extremely useful for ornamental purposes, for by the action upon the coloured surface of hydrofluoric acid, the ruby coating can be eaten away, and the white gla.s.s beneath left entire. If the backgrounds of the patterns be painted upon the ruby side with a material like Brunswick black, which is able to resist the action of hydrofluoric acid, and if the plate of gla.s.s, on its ruby side, be exposed to the action of the vapour of this acid, or to the action of the acid in solution in water, in a short s.p.a.ce of time the pattern will be eaten away; and if the Brunswick black coating be removed with turpentine, a sheet of ruby gla.s.s will be obtained with a white pattern etched upon it.

Owing to the powerful colouring properties which oxide of cobalt exerts, a very deep-coloured blue gla.s.s can be made, which can be treated like the red copper gla.s.s, and may be made to coat and cover in the same way the surface of plates of white gla.s.s. Purple gla.s.s, coloured with oxide of manganese, and green gla.s.s are also sometimes used as coating materials for white gla.s.s, but other colours are never employed in this way.

It is manifest that if different metallic oxides be used with the same gla.s.s, mixed tints will be produced, so that by mingling small quant.i.ties of oxide of cobalt and protoxide of copper, a blue gla.s.s having a greenish hue may be obtained. The revival of gla.s.s painting has caused manufacturers to turn their attention to these mixtures, in order to produce tints resembling those of ancient stained gla.s.s.

Messrs. Powell and Son, of Whitefriars, were the first to perform experiments on these mixtures, and after much laborious attention and patience their efforts have been crowned with great success, for they have been enabled to produce gla.s.s as beautiful in tint and in texture as the best specimens of ancient manufacture. Their example has been followed by others, such as Messrs. Hartley of Sunderland, and Messrs.

Chance and Co. of Birmingham.

While treating of the effect produced by different metallic oxides upon colour, it may be well to mention that the opaque gla.s.ses used for such purposes, as the enamelling of watch-faces, are made by mixing with the materials a certain quant.i.ty of a.r.s.enious acid (or white a.r.s.enic), in much larger quant.i.ties than when it is employed simply to correct the tint imparted to gla.s.s by the iron impurities in the sand. Oxide of tin also renders gla.s.s white and opaque, and a certain quant.i.ty of bone ash will produce a similar effect, though not in so satisfactory a manner.

_Gla.s.s painting_ first became general in this country at the time when the Early English style of architecture prevailed, and some of the best specimens were executed during that period. By the best specimens is not meant, that the figures painted upon those windows were artistically as correct as similar works of a later date, but that they were designed and executed in accordance with those principles, which should always govern the adaptation of a substance like gla.s.s to ornamental purposes. The earlier mediaeval artists depended for effect more upon the boldness of their outline, than upon the intensity of their shading or the delicacy of their manipulation.

The form of a thirteenth-century figure is merely indicated by a few bold and well drawn outlines, the features being formed by lines, the pupils of the eyes by simple well-shaped ma.s.ses of opaque pigment; and such a treatment as this was quite sufficient to convey what was, to the observer, more or less a symbolical, than a truthful representation of the Scripture history which they were intended to ill.u.s.trate. These artists remembered that windows are openings in a building, through which light has to pa.s.s, and they did not, therefore, like many of the later imitators, render them opaque by ma.s.ses of intense shadow, which perfectly obscure the colour of the gla.s.s upon which the picture is painted, and render the pa.s.sage of light through it simply impossible. The thirteenth-century gla.s.s painters, too, in the treatment of their shadows, bore this great principle in mind, and instead of daubing and stippling them on, usually indicated them with a thin wash of enamel colour, intensified in parts by lines crossing one another, and therefore called cross-hatching, through the interstices of which the light, although subdued, was able, in a measure, to pa.s.s.

But as the object of this article is not to discuss the merits of the various styles of gla.s.s painting, however much I might desire to enlarge upon it, I pa.s.s on to a description of the methods employed in the manufacture of stained gla.s.s windows. In the first place, after a design has been drawn, in which the effect of the window as a whole can be carefully considered, cartoons of the figures and ornament are made of the exact size of the intended painting. And here it should be noted, that all the lines should be extremely clear, precise, and well drawn, because it is from these that the workman, who is not usually himself an artist, has to convey on the gla.s.s the feeling of the artist. The cartoon, when completed, is laid down in pieces for convenience-sake on a table, and fastened with small nails. The gla.s.s-cutter then selects the various coloured gla.s.ses which are required to be inserted in their proper places, so as to carry out the design of the artist. For instance, a piece of white or yellow-tinted gla.s.s is cut to the shape of the face. If the figure be a small one, the hair also is included in this; and probably in the figure of a saint, the nimbus which surrounds the head may be included; while in larger figures, particularly in the earliest styles, the face was of gla.s.s of one tint, the hair of another, and the nimbus of one or more tints, different from either of these. Sometimes, in the later styles, the hair, after the face was painted and burnt in, was stained with the silver stain already described, so that when the gla.s.s was cleaned, it was of a yellow colour. However, not to enlarge more upon these points, which really belong more to the artistic than to the industrial part of window painting, let us proceed to the consideration of manipulative details. The outlines of the figures and ornament are painted with a substance called "tracing brown," made by mixing with a flux some oxide of iron, heating them together in a crucible and grinding the product to a fine powder, which is mixed with certain vehicles adapted to the particular use to which it is to be applied. Different fluxes are employed by different gla.s.s painters; some contain borax, because such fluxes fuse more easily, and therefore cause the gla.s.s which is painted to be exposed for a less time, and to a lower temperature, than when less fusible fluxes are used.

It is always satisfactory to an author, to feel that his articles have been of some use to those whom he hoped to benefit. Since this article was written a letter appeared in one of the architectural journals, complaining that the gla.s.s furnished by manufacturers to gla.s.s painters was of inferior composition to that which was used by the manufacturers of ancient stained gla.s.s windows. In fact, it was a.s.serted that modern gla.s.s was not made with due care, and that to this was owing the unfortunate disappearance of some of the painting and tracing of modern stained gla.s.s windows; but that this is not the case, is manifest to all who understand the manufacture of gla.s.s. The real reason why the colouring matter with which gla.s.s painters outline and shade their designs, has in many instances gradually come off from the surface of the gla.s.s, is, because the fluxes used for making it adhere to the gla.s.s are of such a composition, that they themselves have by the action of time become disintegrated.

Some time ago, a person engaged in the manufacture of the enamel plates used for railway lamps, on which are written the names of the stations, called upon me, and told me, that the enamel which he employed had become dark, spotty, and in many cases had peeled off from the gla.s.s. The reason of this is identical with that which occurs in stained gla.s.s windows, viz. that the fluxes that he used were not suitable for the purpose, considering that they had to withstand the action of the weather. From an a.n.a.lysis made of these fluxes (not of those last alluded to, but of those which have been employed in stained gla.s.s windows), it appears that large quant.i.ties of borax have been introduced; and, wherever this is the case, no reliance whatever can be placed on the permanency of pictures painted with such fluxes.

I have appended a few receipts for fluxes, which can be used with safety by any gla.s.s painter who will take the trouble to try them. But I must strongly advise that all those who are connected with the making of fluxes in any gla.s.s painting establishment, should master sufficient chemical knowledge to enable them to ascertain the behaviour of the materials, with respect to one another, as well as of the nature of the gla.s.s upon which they are employed; for very much indeed depends upon a correct knowledge of the character of the gla.s.s as to whether it be hard or soft, what it contains, and of the temperature at which the gla.s.s becomes sufficiently soft to form a firm and enduring union with the colours fluxed upon it.

RECEIPTS FOR FLUXES.

1.

Flint gla.s.s (powdered) 10 parts. } moderately White a.r.s.enic 1 " } hard.

Nitre 1 " }

2.

Red Lead 1 " } soft.

Flint gla.s.s (powdered) 3 " }

3.

Flint gla.s.s 6 "

Red Lead 8 "

(Mixed with four parts of the first flux, soft.)

The use of very soft fluxes is attended with this inconvenience, that the boracic acid contained in them is generally acted upon by moisture and becomes hydrated, and in this condition often causes the painting to peel away. Harder fluxes, although they have the disadvantage of necessitating the gla.s.s to be submitted to a much higher temperature for a longer time in the kiln or m.u.f.fle, are the best, and, with judicious management, can be used without any injurious consequences to the work on which they are employed. Lead fluxes, containing oxide of lead, are sufficiently fusible for all ordinary purposes, and are not liable to the same objection as fluxes containing borax. Suppose, then, it is desired to paint the outlines of a face, the gla.s.s is cut to the shape of the face in the cartoon; it is then laid upon it, and the painter, seeing the lines through the gla.s.s, is able to trace them with his brown paint upon its surface. He generally uses gum water as his vehicle, and puts on the shading also with the same mixture, though sometimes it is found necessary to use a substance which is not affected by moisture, as for instance, tar-oil. It is impossible, in the short s.p.a.ce of this article, to indicate those occasions on which one should be used in place of the other; a knowledge of this can only be obtained by consulting authorities in which details are more minutely given, or by watching the operations of the gla.s.s painter in his workshop. When the face is finished, it is removed, and another portion of the figure, say a piece of the drapery, is proceeded with in exactly the same way; and so, by a repet.i.tion of this process in all parts of the figure, it is completed, and looks very much like a puzzle, the parts being put together on the cartoon before the work is finished, in order to see that the whole is harmoniously treated. In shading the face, hands, and those parts of the drapery which require it, a gla.s.s easel is used, on which the figure is put together, and the parts made to adhere by wax, so that the artist is able, while painting, to form an idea by transmitted light of the effect which will be produced when the window is finished. The ornament is painted in a similar manner, but usually not with the same care in the details of its execution.

When all the gla.s.s is painted, it is fired in a m.u.f.fle, upon the proper construction of which a great deal depends. It is usually made of iron, and should not be more than 15 inches from its bottom to the top, though its width may vary. It is never well to have m.u.f.fles for firing gla.s.s for painted windows larger than about 2 feet wide, by 2 feet 6 inches deep. The top of the m.u.f.fle is usually slightly arched from side to side, and it is placed in the furnace on a tolerably thick stone floor, so that the bottom may not get too hot. The fire, which is lighted below, is allowed to play up its sides and over its top, the flue being so built as to draw the flames in that direction, for a top heat is the best heat for firing gla.s.s regularly. The m.u.f.fle is arranged with ridges in its sides, pa.s.sing from front to back parallel to one another on one side, and exactly opposite to corresponding ridges parallel to one another on the opposite side.

These metal ridges are intended to receive iron plates, and there is generally about an inch or rather less between the top of one plate and the bottom of another, when the m.u.f.fle is perfectly filled. The plates are covered over with perfectly dry powdered chalk or whiting, and the pieces of gla.s.s are laid upon them with their painted sides uppermost. When the plates are charged, they are put into a m.u.f.fle with an iron door, in the centre of which is a hole, and a conical tube with the base attached round it. It is larger than the opening at the other end, which projects some 6 or 7 inches from the surface of the m.u.f.fle-door at right angles to it. A second door is then placed at a short distance from the first, the tube pa.s.sing through a hole made for the purpose in it. The orifice is usually stopped by a piece of fire-clay, which can be removed at pleasure. The use of the tube is, to enable the manager of the kiln to look into the m.u.f.fle, from time to time, to see that the gla.s.s does not get too much heated. When the firing is completed, the fire is raked out and the m.u.f.fle is allowed to cool very slowly, and by this process the gla.s.s becomes annealed.