Читать книгу Glass and Glass Manufacture - Percival Marson - Страница 6

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The main essential and peculiar property of glass is its transparency. When subjected to a gradually increasing temperature, glass becomes softened, and whilst hot it is plastic, ductile, and malleable, in which state it can be cut, welded, drawn, or pressed. A thread of glass can be drawn so thin and fine that it can be twisted and bent to a remarkable extent, showing that glass is flexible.

The above properties shown by glass while softened under heat permit it to be shaped and formed by a variety of methods, so that in the manufacture of the different kinds of glass we find goods pressed, blown, drawn, moulded, rolled and cast from the hot metal. Upon cooling, the form given to them is retained permanently.

Another property of glass is its conchoidal fracture and liability to crack under any sudden change of temperature. Advantage is taken of this peculiarity in dividing or cracking apart glass when necessary, during the stages of the manufacture of any glass article.

By permission of Melin & Co. HORIZONTAL CRACKING-OFF MACHINE

If a glass worker, in making an article of glass, desires to detach or cut apart certain sections, he applies a cold wet substance, such as an iron file wetted with water, to any portion of the hot glass, which causes it to fracture at the point of contact with the cold metal, and a slight jar is then sufficient to break the two portions apart. This method of chilling heated glassware to divide it is applied in the mechanical process of cutting up the long cylindrical tubes of glass into short sections for use as miners’ safety lamp chimneys. Wherever it is desired to cut them through, a narrow section or line round the cylinder is first heated by a sharp, hot pencil of flame projected from a burner against the rotating cylindrical tube of glass at equidistant short sections, and the divisions chilled by contact with a cold, steel point, or the heated area may be gently scratched with a diamond point, when a clean, sharp fracture results exactly where the chill or scratch has been applied and spreads round the whole circumference in a circle, giving neat, clean-cut divisions. In cutting narrow tube and cane, the fracture caused in the structure of the glass by scratching its surface with a steel file or diamond is sufficient to cause it to break apart without the application of heat.

A piece of hot glass will weld on to another piece of hot glass of similar composition. The glass maker uses this method of welding for sticking handles on to jugs, etc., during the process of making table glassware.

The density of glass varies according to its composition. Certain classes of lead and thallium glass for optical work are of very high density. The specific gravities of such glasses may vary from 3·0 to well over 4·0. In soda-lime glasses the density is less and approaches 2·4. Ordinary crystal glass approximates to a specific gravity of 3·1.

The elasticity and thermal coefficient of expansion of glass can be regulated within normal limits. Glasses are now manufactured which can be perfectly sealed to copper, iron, nickel, and platinum wires.

Glass, if kept heated for any length of time at a temperature just short of its softening or deformation point, becomes devitrified and loses its transparency, becoming opaque and crystalline. In this state it has much of the nature of vitreous porcelain and is totally different to manipulate, being tough and viscid on further heating. This devitrified state may occur during glassmaking, where the metal is allowed to remain in the pot or tank furnace for a considerable time under low temperature. Small stars or crystals first develop throughout the glass and continue to grow until it becomes a stony, white, opaque, vitreous mass. “Réaumur’s Porcelain” is a glass in a devitrified state, and is used for pestles and mortars, devitrified glass being less brittle than ordinary glass and similar to vitrified porcelain.

Glass can be toughened to an extent which is surprising. Bastie’s process consists of plunging the finished glass article whilst hot into a bath of boiling oil, which toughens the glass so much as to make it extremely hard and resistant to shocks, losing most of its brittle nature. Strong plates of glass are produced by a process of toughening under pressure. These plates of glass are used for ship porthole lights and in positions where great strength is required. Toughened or hardened glass is of great value in the production of miner’s lamp glasses and steam-gauge tubing. Glass, when hardened, is difficult to cut even with the diamond, and difficulty is experienced in finding suitable means to cut it into shapes to suit commercial requirements.

“Prince Rupert drops,” or tears, exhibit the state in which unannealed glass physically exists. These are made as a curiosity by dropping a small quantity of hot metal from the gathering-iron into a bath of water and then taking the pear-shaped drops out quickly. These pear-shaped drops of glass will stand a hard blow on the head or thicker portion without breaking, but, if the tail is pinched off or broken, the whole mass crumbles and falls to powder. This well illustrates the latent stresses or strains apparently in a state of tension and thrust within the structure of unannealed glass.

Glass is not a good conductor of heat. This accounts for the necessity of slow cooling or annealing glassware, and also applies when re-heating glass, which must be done slowly and evenly to allow time for the conduction of the heat through the mass gradually. Glass is a non-conductor of electricity, and is used to a considerable extent in the electrical trades for insulation purposes. Most glasses are attacked slightly, but not readily, by water and dilute mineral acids. Continued exposure to a moist, humid atmosphere causes slight superficial decomposition, according to the stability and chemical composition of the glass. Old antique specimens of glass show the superficial decomposition caused by long continuous exposure to atmospheric moisture. Many antique specimens have been known to collapse instantly upon being unearthed. The first change in antique glass is exhibited by a slight iridescence forming on the surface, gradually increasing towards opacity afterward disintegration sets in, until it finally collapses or crumbles to powder. Glasses high in lead are readily attacked by the acid vapours met with in the atmosphere, but the harder soda-lime glasses are more resistant. An excess of boric acid, soda, or potash also renders glass subject to disintegration and decay.

Hydrofluoric acid attacks all silicate glasses, liberating silicon fluoride. Use is made of this acid reaction in decorating glasswares in “Etching,” by exposing the surface of glass to the fumes of hydrofluoric acid gas in some form.

The most permanent glasses are those containing the highest proportion of silica in solution, but the available heat necessary to decompose such highly silicious mixtures is limited by the present known refractory materials which can be procured for constructing the furnaces. Quartz glassware is a highly silicious glass. It is now made and used in the manufacture of special chemical apparatus and laboratory ware such as crucibles, muffles, etc., which have to withstand severe physical and chemical tests. This quartz glass possesses remarkable features in its low coefficient of expansion and resistance to heat changes. It is highly refractory. Articles made of this glass can be heated to red heat and plunged directly into cold water several times without fracturing. Several varieties of quartz glass are now manufactured, and a new field for investigation is presented in applying the features and properties of this glass for use in chemical processes.

In a purely physical sense glass is a supercooled liquid, the silicates are only in mutual solution with each other, and they appear to be constantly changing. Glass cannot be described as a homogeneous or definite chemical compound. Many of the after effects and changes which occur in glass, and the formation of crystals in the devitrification of glass tend to prove the above assertion. The colour changes which take place when ruby and opalescent glass is re-heated, and even the change in colour of glass going through the lehr, cannot be explained unless in the above sense of viewing these remarkable changes. Glasses with an excess of lime in their composition are more subject to devitrification than lead glasses or those of moderate lime content constructed from more complex formulas. The presence of a small proportion of alumina in glass prevents this tendency to devitrification and ensures permanency. Those glasses which have the highest silica content, and which have been produced at the highest temperatures, show the greatest stability in use. Bohemian glasses of this type contain as much as 75 per cent. silica, and are produced in gas-fired regenerative or recuperative furnaces, where the heat approaches 1,500° Centigrade. Such glass is much sought after for enamelling on, being harder and less easily softened by the muffle heat firing on the enamels used. Taking two corresponding glasses of the same basicity, or proportion of silicic acid to the bases present, those formulae which have the greater complexity of bases produce the more fusible glasses. A multiple of bases constituting a more active flux than a single base content, it follows that a compound mixture of silicates fuses or melts at a lower temperature than the respective simple silicates would. These facts are useful in constructing commercial formulae for glasses.

Glasses containing lead oxide as an ingredient are subject to reduction when exposed to flames of a carbonaceous nature. The carbon partially reduces the lead oxide to its metallic state, forming a black deposit. On this account, lead glasses cannot be used in blow-pipe working with the ease with which soda-lime glasses can be worked, without reduction taking place. English crystal glass, which contains a high percentage of lead, is usually melted in hooded or covered pots to prevent the carbonaceous flames of the furnace reducing the lead and otherwise destroying the clearness of the glassware. Soda-lime glass and others without the presence of lead can be melted in open pots without any fear of reduction. Modern gas-fired recuperative furnaces, in which more complete combustion of the carbon takes place, can now be used for melting lead glasses in open pots, thus presenting a great saving in the fuel required to melt and produce such glass, besides permitting the use of a cheaper form of pot. This cannot be done with the ordinary English coal-fired furnaces.

Advantage is taken of the reducing action of the coal-gas flame when producing lustre and iridescent glassware. A small proportion of easily reducible metal, such as silver or bismuth, is introduced into the glass and first melted under oxidising conditions. It is then reduced in after-working by flaming, which deposits the metal in a thin sheen upon the surface of the glass, where it comes in contact with the reducing flames. An example of this effect is shown in Tiffany lustre ware, in which silver chloride is used and reduced within the glass, giving a pretty coloured iridescence on the surface, due to the reflection of light from the particles of metal deposited under the surface.

“Aventurine” is a form of glass in which copper and iron oxides are introduced under reducing conditions during melting. The glass is then allowed to cool slowly. The metallic copper tends to separate out in small spangled crystals, which give a pretty sparkling effect. The use of strong reducing agents with very slow annealing is necessary to produce this effect. Copper and gold ruby-coloured glass presents other instances of partial precipitation of the metal by reduction within the glass. According to the extent of reduction, the glass ranges in colour from yellow, ruby, to brown.

The manganese silicate is readily affected by oxidising or reducing conditions, the purple colour being present under oxidising influences and a greenish-grey colour under reducing conditions. In using manganese as a decolorizer, the glass maker may have added too much of it to his glass, in which case it shows too prominent a purple colour. To destroy this excess of colour he pushes either a little strip of green willow wood or a clean potato to the bottom of the pot of metal. The reducing action of the carbonaceous gas involved takes out the excess of purple colour by partially reducing the manganese present to a colourless state.

The colour of glass is gradually affected in course of time by sunlight. This change in colour is often noticeable in old windows, the glass having developed a yellowish green tint in course of time from the action of the solar rays.

Glass which has been incompletely fused or not sufficiently melted to give a complete solution of the materials present is in a weakened state of cohesion and is liable to disintegration. The presence of undecomposed sulphates, chlorides, or borates in the glass also tends to early disintegration. A continual exudation and crystallisation of salt takes place upon the surface until the glass wholly disintegrates away to a white powdered salt.

Glass is a poor conductor of heat. When a piece of glass has been expanded under the influence of heat, and is rapidly cooled, the superficial outer portions become intensely strained and contracted upon the interior portions, which retain the heat longer. Under these conditions of cooling, glass is apt to “fly,” or collapse and fall to pieces, owing to the outer portions giving way under the great strain. These stresses or strains are relieved in the process of annealing, under which they are gradually eased by a slow and regular cooling from the heated condition. Certain glasses, the composition of which shows considerable differences in the density of the respective bases present, are more subject to this defect than those in which the bases are of more even density and homogeneous in character. Such glasses should be “de-graded” and re-melted in order more thoroughly to diffuse and distribute the denser portions throughout the mass. In de-grading glass, the hot glass is ladled out and quenched in cold water, dried, and re-used as “cullet.”