« AnteriorContinuar »
it will be necessary, in order to procure the whole of the blue contained, to pursue precisely the same method over again.
Cobalt Blue, or Regulus of Cobalt. — 60 parts, cobalt ore; 50, potash; 25, sand; 10, charcoal. Work the same way as for regutus of zaffre.
To Refine Regulus Of Cobalt.— 50 parts, regulus of cobalt; 6, potash. Refiue as for regulus of zaffre; the operation of refining must be repeated until the scoria is of a bright colour and of a slight bluish hue; then spread the purified metal, finely pulverized, half an inch thick, on flat pieces of earthenware covered with flint; place in a reverberator)' furnace, and apply a moderate degree of heat for a few hours.
Glass Making.—The furnace in which glass is made is a large circular building capable of holding about six pots or vessels, in which the glass is melted. Fig. 1 is an ordinary arrange
ment of this furnace. It is built upon an arch, and the space underneath, included within the arch, is called the cave, as at A. This apartment can be closed by the doors C C, to regulate the draught, as all the air necessary for the
support of the fire must enter at these doors. On the upper part of the cave is placed a grate L>, to support the hre. The ashes pass through this grate and fall into a tank of water B. Around this grate pillars are constructed, which, being hollow, serve as flues F K. Resting upon these pillars is an iron dome G, and on this is built the principal chimney H. Between these pillars are placed the pots E E, which, from the peculiar construction of the furnace, receive the heat equally on all sides; for, as the flame ascends it strikes the dome, and is reverberated, taking the direction pointed out by the arrows. The pots are constructed in the form of a cylinder, with a hemispherical top, having a small aperture on one side for the admission of the materials, and their removal when formed into glass. When the pots are placed in the furnace, they are so arranged that their openings are on the outside of the fire; they are then built in by a temporary wall, except the orifices, so that no dust or smoke can enter so as to injure the glass. The materials for these various kinds of glass are placed in the pots, and exposed to the heat of the furnace for upwards of forty-eight hours, during one-half of which time the heat is gradually increased, and during the other half gradually decreased, until the metal, as the workmen term the glass, is in a fit state for working. During the time tne materials are in the pot, the workman takes out a portion, from time to time, on an iron rod, and examines it wheu J cold, to see whether it is free from air bubbles and of good colour. If the materials employed be very impure, there rises to the surface a scum, which is called sandiver or glass gall, and which resembles large flakes of snow.
Flint Glass is employed for making lenses, decanters, drinking glasses, and owes its capability of being thus easily fashioned to the lead contained in it. The following quantities form a very excellent glass:—Fine white sand, 300 parts; rod-lead, or litharge, 200; refined pea flashes, 80; nitre, 20; arsenic and manganese, a smaller quantity.
Crown Glass is a compound of silica, potash or soda, and lime. It is employed as a window glass, and contains no lead. The proportions for its formation are— Fine white sand, 100 parts; carbonate of lime, 12; carbonate of soda, 50; clippings of crown glass, 100.
Bottle or Green Glass is made of the commonest materials, in about the following proportions:—Sand, 100 parts; kelp, or impure soda, 30, wood ashes, 40; potter's clay, 100; cullet, or broken glass, 100.
l'LATE Glass.—Great care is required in the choice of materials, and the management of the process for this glass. The following proportions are used:— Finest white sand, 720 parts; best soda, 450; lime, 80; nitre, 25; cullet, or broken plate glass, 425.
Common Window Glass.—100 parts, saud; 35, chalk; 35, soda-ash, and a considerable quantity of broken glass or cullet.
Colours For Glass.—Oxide of gold is employed to impart to glass a beautiful ruby colour. Sub-oxide of copper gives a red colour. Silver, in all states of oxidation, gives a variety of beautiful yollow and orange colours to glass. Antimony, lead, and silver, in combination, are employed to produce the inferior yellow colour. The oxides of iron give to glass various shades of green, yellow, red, and black. Oxide of chromium gives a fine green, and oxide of cobalt a splendid blue. The colour most valued, next to that produced by gold, is the yellow communicated by oxide of uranium, and which has an appearance resembling shot silk. White glass or enamel is made by adding either arsenic or the oxide of tin to the melted metal. The various metals employed in colouring glass are also used in the manufacture of artificial gems, and by their means the colour and general appearance are well imitated.
Strass. — Pure caustic potash, 16 parts; white-lead, 85; boracic acid, 4J; arseuious acid, ^; finest white sand, 50. These materials are carefully selected, placed in a Hessian crucible and fused in a porcelain furnace for a day and a
night, then cooled very gradually. tlsed to imitate the diamond. Other precious stones are imitated by adding to tne strass the metallic oxides, as in colours for glass.
Soluble or Water Glass.—Mix well 200 grains of fine sand, and 600 of fine carbonate of potassa; fuse in a crucible capable of holding four times as much. Carbonic acid escapes; the silica and potassa combine and form glass. Pour out the glass, which i; commonly termed silicated potassa, on an iron plate. The compound formed in this manner is pure silica soap.
Hard Glass For Receiving Colour. —Best sand, cleansed by washing, 12 lbs.; pearlashes, or fixed alkaline salt purified with nitre, 7 lbs.; saltpetre, 1 lb.; and borax, J lb The sand being first reduced to powder in a mortar, the other ingredients should be put to it, and the whole well mixed by pounding them together.
Glazing Windows.—Crown glass is made in circular disks blown by hand; these disks are about 4 ft. diameter, and the glass averages about -^ in. thick. Owing to the mode of manufacture there is a thick boss in the centre, and the glass is throughout more or less striated or channeled in concentric rings, frequently curved in surface, and thicker at the circumference of the disk. Consequently in cutting rectangular panes out of a disk there is a considerable loss, or at least variety in quality: one disk will yield about 10 sq. ft. of good window glass, and the largest pane that can be cat from an ordinary disk is about 34 x 22 in. The qualities are classified into seconds, thirds, and fourths.
Sheet glass is also blown by hand, but into hollow cylinders about 4 ft. long and 10 in. diameter, which are cut oB and cut open longitudinally while hot, and therefore fall into flat sheets. A more perfect window glass can be made by this process, and thicker, and capable of yielding larger panes with less waste. Ordinary sheet glass will cut to a pane of 40 x 30 in., and some to 50 x 36 in. It can be made in thicknesses from ^ ju. to & in.
Plate glass is cast on a flat table and rolled into a sheet of given size and thickness by a massive metal roller. In this form, when cool, it is rough plate.
Ribbed plate is made by using a roller with grooves on its surface. Kough and ribbed plate are frequently made of commoner and coarser materials than polished plate, being intended for use in factories and warehouses.
Polished plate is rough plate composed of good material and afterwards polished on both sides, which is done by rubbing two plates together with emery and other powders between them. Plate glass can be obtained of almost any thickness from J in. up to 1 in. thick, and of any size up to about 12 x 6 ft.
In the glazing of a window the sizes of the panes, that is to say, the intervals of the sash-bars, should be arranged, if practicable, to suit the sizes of panes of glass which can conveniently be obtained, so as to avoid waste in cutting; this consideration is of more consequence in using crown and sheet glass than with plate glass. The woodwork of the sash should receive its priming coat before glazing, the other coats should be put on afterwards. With crown glass, which is sometimes curved, it is usual to place the panes with the convexity outwards. When the glazier has fitted the pane to the opening with his diamond, the rebate of the sash-bar facing the outside of the window, he spreads a thin layer of putty on the face of the rebate and then presses the glass against it into its place, and holding it there, spreads a layer of putty all round the side of the rebate, covering the edge of the glass nearly as far as the face of the rebate extends on the inner side of the glass, and bevelling off the putty to the outer edge of the rebate. The putty is then sufficient to hold the pane in its place, and hardens in a few days. The glass should not touch the sash-bar in any part, on account of the danger of its being cracked from any unusual pressure; there should be a layer of putty all round the edges. This precaution is especially necessary in glazing windows with iron or stone mnllions or bars.
Glass Painting and Staining. —The different compounds for painting glass are glasses of easy fusion, chiefly coloured with metallic oxides ground, and laid on the glass with spirits of turpentine. In the production and modification of glass colours much depends on the different preparations of the metals, on the small proportion of the metallic oxides employed in proportion to the vitreous mass, on the degree of fire and time of its continuance, and on the purity of each ingredient intended for vitreous mixtures; from hence difficulties arise which even a skilful operator cannot always remove, and which oftea frustrate his intention. Having made choice of the subject to be painted, correctly draw the same on a paper exactly the size intended to be on the glass, then place the different pieces in regular order on the drawing and trace the outlines therefrom on the glass; when the tracing is quite dry the ground colours may be washed in together with the dark and prominent shades, and also the stains required. Th? stains are laid on in various thicknesses, according to the depth of colour required, and when they are dry the glass is ready to be burned in a muffle or kiln constructed for the purpose. The panes of glass are laid on sheets of iron, or earthenware bats, the size of the glass, previously spread over with dried ground flin', to prevent the surface of the glass from being defaced. After the first burning the stain is washed off with warm water, which will bring to view every part of the subject, in fact, every shade according to the thickness of colour applied; to heighten the colour paint on each side of the glass, and burn it a second time. The glass will require from four to six firings, the exact number of firings depending on the subject, the degree of perfection required, and the manner of executioa; but after each burning, the pieces of glass will want less labour, some of the colours and stains being perfect at the first and second burning, and few require the utmost quantity. The proper degree of heat to which the glass must be exposed in the muffle is ascertained by taking out at different intervals small pieces of glass, arranged for the purpose, on which are laid similar colours to those being fired. After the glass is burned it requires great precaution in cooling, for if suddenly cooled it is apt to fly, consequently all sudden changes of temperature should be avoided.
Red Orange and Yellow Stains.— 12 parts, green vitriol calcined; 1, oxide of silver. The vitriol must be calcined to a reddish colour, and repeatedly washed with boiling water until it is completely freed from its acid, which will be known by the water being insipid to the taste, then triturate the silver and vitriol together in a mortar, after which grind them up with spirits of tar for use. Various temperatures in burning produce various coloured stains, the highest a red, a less an orange, and so on to a yellow; but to procure a very deep red, the colour must be laid upon both sides of the glass.
White Enamel For Painting Glass. — 3 parts, borax calcined; 2, flint; 1, oxide of tin; 1, Cornish stone. The basis of this enamel, which is in general opaque, may also be employed in assimilating the opaque natural stones. These ingredients must be well mixed up together, and fused in an air furnace in a crucible, the fire at first applied very gradually, and the whole repeatedly stirred with an iron rod. The mixture by this calcination, and by being kept for some time in fusion in an intense heat, acquires its fusibility and opacity.
Purple. — 1. 20 parts, prepared purple; 2J, enamel flux (2); 1, white enamel. 2. 20 parts, prepared purple; 10, blue process; 5J, enamel flux (2); 1, white enamel.
Rose Colour. — 20 parts, prepared rose colour; 1, white enamel. The purples and rose colours for glass painting are nearly the same mixtures as those used for porcelain painting, with the addition of a small proportion of flux and white enamel, the latter gives firmness to the colour; in the course of working the rose colour, if a very small
quantity of purple be added, the colour will be perceivably benefited.
Red.—1 part, terra de sienna; 3, enamel flux (2). The terra de sienna must be calcined over a slow fire until its colonr becomes of a dark red, aftei which washed several times in boiling water and ground with the flux for use.
Transparent Orange.—1 part, oxide of silver; 10, enamel flux (2); 10, enamel flux (3); 1, white enamel.
Yellow.—1 part, yellow, under glaze, p. 46; 3, enamel flux (2); J, white enamel.
Dark Brown.—1 part, highly calcined copperas; 3J, enamel flux (3).
Red Brown.—1 part, black; 1, red; 1, enamel flux (4).
Light Brown.—1 part, easy calcined umber; 3J, enamel flux (2).
Green. —1.5 parts, cornelian red; 1, prepared purple. 2. 2 parts, blue; 1, yellow.
Blue.—1. 8 parts, flint glass; 3, redlead; 1, potash; 1, blue calx; J, common salt. 2. 4 parts, borax; 4J, flint glass; 1, flint; j, potash; J, prepared purple; 1, blue calx. In preparing these blues, let the materials be calcined in an air furnace, and the whole mass kept in a state of fusion for some time, a fine blue glass enamel will be produced; the cobalt blue calx should be of the finest quality that possibly can be procured, and free from all impurities.
Black. — 1. 1 part, highly calcined umber; 2, calcined borax; 1, red-lead; 1, blue calx. 2. 1 part, manganese; 1, black flux. The best Turkey umber should be procured for the first process, and calcined at the most intense heat that can be produced in an air furnace, after which pound and mix up with the other materials; then calcine the whole together in an air furnace, the degree of heat will be sufficient when the whole mass is in fusion.
Black Flux, for glass staining.—15 parts, red-lead; 5, borax; 5, flint; 1| oxide of blue vitriol.
Indigo Blue.—1 part, precipitate of gold; 4J, enamel flux (4); J, white enamel. These ingredierfis are simply ground together for use. They produce a beautiful colour on glass, of a fine purple hue. This very expensive colour is adapted principally for painting the draperies of figures, and is very susceptible of being injured by a high degree of heat. Etching And Deadening Colour.—
1. 7 parts, red-lead; 2, calcined borax;
2, flint; 1, oxide of tin. 2. 8 parts, red-lead; 6, flint glass; 3, flint; J, green copperas. The materials of the last two processes must be finely mixed and calcined in an air furnace, each process separately, after which take 2 parts of No. 1 and 3 parts of No. 2, mix them together, and repeat the calcination again in an air furnace ; then pound and grind this frit for use, but be particular that it is ground very fine, for much depends on the particles being minutely mixed previous to using. The composition is afterwards laid on the glass with water, and a small quantity of refined sugar dissolved in spring water applied occasionally; the solution of sugar must be of the consistence of thick oil; should too large a quantity of the solution be added, and by that means condensate it too much, add a few drops of acetous acid to the menstruum, it will immediately regain a proper consistence, and not at all injure the colour. When the deadening is laid on the glass, the figures must be engraved or etched with a pointed instrument made of wood, bone, or ivory, suitable to the subject, and afterwards burned in a kiln or muffle appropriated for the purpose. It fires at a less temperature than stained glass, although in some instances it will do in the same kiln.
To Transfer Engravings on Glass.—Metallic colours prepared and mixed with fat oil, are applied to the stamp on the engraved brass or copper. • Wipe with the hand in the manner of the printers of coloured plates ; take a proof on a sheet of silver paper, which is immediately transferred on the tablet of the glass destined to be painted, being careful to turn the coloured side against the glass; it adheres to it, and so soon as the copy is quite dry, take off the superfluous paper, by washing it with a
sponge; there will remain only the colour transferred to the glass, which will be fixed by passing the glass through the ovens.
Annealing Glass.—This consists in putting the glass vessels, as soon as they are formed, and while they are yet hot, into a furnace or an oven, not so hot as to re-melt them, and in which they are suffered to cool gradually. It is found to prevent their breaking easily, particularly on exposure to heat. In large works., annealing is performed by passing the glass through the oven, by means of revolving trays constructed for the purpose.
Cutting Glass.—To cut glass vessels neatly, heat a rod of iron to redness, and having filled the vessel the exact height you wish it to be cut with oil of any kind, proceed very gradually to dip the red-hot iron into the oil, which, heating all along the surface, the glass suddenly chips and cracks right round, when you can lift off the upper portion clean by the surface of the oil. If a tube is required to be cut, notch the tube at the point where it is to be divided with the edge of the file, or of a thin plate of hard steel, or with a diamond; after which press upon the two ends of the tube, as if to enlarge the notch, or what is better, give the tube a slight smart blow. This method is sufficient for the breaking of small tubes. Many persons habitually employ an agate, or a common flint, which they hold in one hand, while with the other they rub the tube over the sharp edge of the stone, taking the precaution of securing the tube by the help of the thumb. For tubes of great diameter, employ a fine iron wire stretched in a bow, or, still better, the glass-cutter's wheel; with either of these, assisted by a mixture of emery and water, you can cut a circular trace round a large tube, and then divide it with ease. When the portion which is to be removed from a tube is so small that you cannot easily lay hold of it, cut a notch with a file, and expose the notch to the point of a candle flame; the cut then flics round the tube. A good plan of cutting glass is to make