Vitrification

“ Oxidation must be complete before vitrification is started as otherwise blisters will be raised due to imprisoned carbon dioxide. The important points in vitrification are to make the required temperature within a reasonable time and to maintain a uniform distribution of heat throughout the kiln. ”

“ In conclusion, as the Glass of Lead hath the property of running through the crucible, and carrying with it the matters which it has vitrified, it follows, that, when the vitrification of the imperfect metals is effected by its means, all those vitrified matters together penetrate the vessel containing the fused metalline mass, disappear, and leave only the Gold and Silver perfectly pure, and freed, as far as is possible, from all admixture of heterogeneous parts. ”

“ It is particularly remarkable, that although the plain and shore are covered with fragments, yet these are almost entirely fragments of the primary rocks. I state this for the purpose of obviating a supposition that may be adduced to nullify the argument which I am about to derive of a previous intention in the builders to vitrify their work, from their having neglected to use that rock on which the building was erected, and which was not adapted for the purposes of vitrification. ”

“ It is therefore essential that, for the manufacture of stoneware, [115] a clay should contain a large proportion of refractory material which will form a 'skeleton,' the interstices of which will be filled by the more fusible silicates produced by the firing.
It is generally found that of all the fluxes present in vitrifiable clays, soda and potash compounds—the so-called 'alkalies'—and all lime compounds are the most detrimental, as in association with clay they form a material with a very short range of vitrification. Magnesia, on the contrary, accompanies a long vitrification range. ”

“ By regulating500 the fire in a particular manner, they cause these ashes to assume a certain degree of concretion, or vitrification, by which means they are formed into solid cakes of a grey colour, interspersed with many white and black spots. This substance, which in consequence of the vitrification does not become moist in the air, is broken into fragments, and sent to every part of Europe under the name of soda, for the use of the glass-houses, soap-boilers, dyers, and for other purposes. ”

“ It is a well-recognized fact that successive firings to a temperature approaching the fusion point, or long continued heating near that temperature, will gradually produce vitrification, which brings about a very dense mass and close texture, and entirely alters the properties of the brick. ”

“ Porosity may thus be regarded as the opposite of vitrification; porous goods being relatively light and [40] soft whilst vitrified ones are dense and hard. For some purposes, porosity is an important characteristic: for example, building bricks which are moderately porous are preferable to those which are vitrified. The manufacture of porous blocks for the construction of light, sound-proof partitions, etc. has increased rapidly of late. They are made by adding sawdust or other combustible material to the clay. The added substances burn out on firing the goods in a kiln. ”

“ If the heating is stopped before the fusion has begun, the clay will be porous and comparatively soft, but as more and more material fuses, the mass (on cooling) becomes harder and less porous, as the fused material occupies the pores and sets to a dense, firm glassy mass. The amount of vitrification, or partial fusion, which occurs is, therefore, of great importance in some industries, as by stopping it at an appropriate stage articles of any desired degree of porosity, translucency or strength may be obtained. ”

“ The Physical causes of liquation or melting of Mettals, etc.For vitrification is the last work of fire, and a fusion of the Salt and Earth, which are the fixed elements of the composition, wherein the fusible Salt draws the Earth and infusible part into one continuum, and therefore ashes will not run from whence the Salt is drawn, as bone ashes prepared for the Test of Metals. Common fusion in Metals is also made by a violent heat, acting upon the volatile and fixed, the dry and humid parts of those bodies ”

“ The former is also called vitrifiable, and the second unvitrifiable Earth; because, when Earth is melted by the force of Fire, it becomes what we call Glass, which is nothing but the parts of Earth brought into nearer contact, and more closely united by the means of fusion. Perhaps the Earth, which we look upon as incapable of vitrification, might be fused if we could apply to it a sufficient degree of heat. It is at least certain, that some Earths, or stones, which separately resist the force of Fire, so that they cannot be melted, become fusible when mixed together. ”

“ The presence of a considerable proportion of iron oxide results in the formation of red ware, which is necessarily of a porous nature, as the fluxes in the clay are such that they will not permit of its being heated to complete vitrification without loss of shape. ”

“ As ordinary clays are never quite free from metallic [156] compounds other than alumina, this formation of a fused portion—technically known as vitrification (p. 37) —occurs at temperatures depending on the nature of the materials present, so that a wide range of products is obtained, the series commencing with the entirely unfused pure clay (china clay) , passing through the slightly vitrified fireclays, the more completely vitrified ball clays to the vitrifiable stoneware clays and ending with materials so rich in easily fusible matter as scarcely to be worthy of the name of clays. ”

“ The colour of burned ware depends upon the iron compounds in the clay—these producing buff, red, brown or black (usually termed 'blue') articles—on the presence of finely divided calcium carbonate (chalk) [20] which can destroy the colouring power of iron compounds and produce white ware, and on the treatment the clay has received in the kiln. A clay which is white when underfired will usually darken in colour if heated to vitrification, and one which burns red in an oxidizing atmosphere may turn blue-grey or black under reducing conditions. ”

“ Secondly, That nevertheless this Colour [pg 365] may be easily over-powr'd by those of divers other Mineral Pigments (if I may so call them) so that with a glass of Lead, you may Emulate (for Instance) the fresh and lovely Greenness of an Emerald, though in divers cases the Colour which the Lead it self upon Vitrification tends to, may vitiate that of the Pigment, which you would introduce into the Mass. ”

“ The great heat requisite to effect the vitrification of the pudding stone, is an additional argument against this hypothesis, as it could not have been produced by any quantity of wood capable of entering into such a wall, unless the wood had predominated to an extent incompatible with any idea we can form of its architecture. It is not indeed easy to conceive a plan capable of producing these effects, and certainly none more feasible than the suggestion of Mr. ”

“ And this is called, calcining of them, by Chimists. Thus Iron and Copper by heating and quenching do turn all of them by degrees into Scoria, which are evidently vitrified Substances, and unite with Glass, and are easily fusible; and when cold, very hard, and very brittle.
Fourthly, That most kind of Vitrifications or Calcinations are made by Salts, uniting and incorporating with the metalline Particles. Nor do I know any one calcination wherein a Saline body may not, with very great probability, be said to be an agent or coadjutor. ”

“ One variety manifests itself on the surface of the glass before it reaches its working temperature, but if the glass be heated to the highest temperature of the flame it will disappear except in the portion at the edge of the heated part. The glass seems to work all right, but [Pg 3] an ugly crystallized ring is left at the edge of the portion heated. This kind appears most frequently in old glass which was originally of good quality, but has in time been superficially altered, probably by the loss of alkalies. The other variety of devitrification does not appear when the glass is first heated ”

“ Long experience at Saint-Gobin has proved that one part of dry carbonate of soda is adequate to vitrify perfectly three parts of fine siliceous sand, as that of the mound of Aumont near Senlis, of Alum Bay in the Isle of Wight, or of Lynn in Norfolk. It is also known that the degree of heat has a great influence upon the vitrification, and that increase of temperature will compensate for a certain deficiency of alkali; for it is certain that a very strong fire always dissipates a good deal of the soda, and yet the glass is not less beautiful. ”

“ The glass being, at first, colorless, a drawing is made upon it, and the painting is laid on with mineral substances, the vehicle being a volatile oil, which soon evaporates. The sheets of glass are then exposed to a powerful heat, until they are so far melted that they receive the colors into their own substances. Enamel painting is done on the same principle. This is a time of great anxiety to the artist, as with all possible care, valuable paintings, both in glass and enamel, are frequently spoiled in the proving, or vitrification. ”

“ By many lines of evidence we are led to believe that obsidians in course of time suffer devitrification, in other words they pass from the vitreous into a crystalline state, but as the changes take place in a solid mass they require a very long time for their achievement, and the crystals produced are only of extremely small size. A dull stony-looking rock results, the vitreous lustre having entirely disappeared, and in microscopic section this exhibits a cryptocrystalline structure, being made up of exceedingly minute grains principally of quartz and felspar. ”

“ In ordinary glass we have a familiar example of a liquid which has been cooled to a temperature at which crystallization takes place with very great slowness. If, however, glass is heated, a temperature is reached, much below the melting point of the glass, at which crystallization occurs with appreciable velocity, and we observe the phenomenon of devitrification. ”

“ I know other vitrifications of divers colours hitherto unknown, and indeed most secret, not to be communicated to every one indifferently: but he that knows how to reduce metals into a true glass, retaining the colour of its metal, is indeed the inventor of a very great secret ”

“ Another Experiment.
Heat some blue vitriol (sulphate of copper) in an iron ladle till all the water contained in the crystals is driven off, and the color changes to a gray. Take the lumps out without breaking them, and lay the dried blue vitriol on a plate. If this be moistened with water steam is produced; and if a slice of phosphorus is then laid on the sulphate of copper it ignites, demonstrating again that the condensation of a liquid produces heat. The addition of the water restores the blue color, thus proving that water was necessary to the composition of blue vitriol. ”

“ Vit′rify, to make into glass.—v.i. to become glass.—ns. Vitrī′na, a genus of land molluscs forming a connecting-link between the slugs and true snails—the glass-snail; Vit′rine, a show-case made of glass and used to protect delicate articles. [L. vitrum, glass—vidēre, to see.]
Vitriol, vit′ri-ol, n. the popular name of sulphuric acid: a soluble sulphate of a metal—green vitriol=sulphate of iron, blue vitriol=sulphate of copper, white vitriol=sulphate of zinc.—v.t. Vit′riolāte, to convert into vitriol.—n. Vitriolā′tion, the act or process of converting into vitriol.—adjs. ”

“ At common temperatures it is hard and brittle, in thick pieces; in thin plates or threads, flexible and elastic; sonorous when struck; fracture conchoidal, and of that peculiar lustre called vitreous; at a red heat, becoming soft, ductile and plastic. Besides glass properly so called, other bodies are capable of entering into vitreous fusion, as phosphoric acid, boracic acid, arsenic acid, as also certain metallic oxides, as of lead, and antimony, and several chlorides ”

“ A beam of white light falls upon a plate of colourless glass, and it passes freely through it, losing but little of its intensity; the largest portion being lost by reflection from the first surface upon which the light impinges. If the glass is roughened by grinding, we lose more light by absorption and by reflection from the asperities of the roughened surface; but if we cover that face with any oleaginous fluid, as, for instance, turpentine, its transparency is restored. We have thus direct proof that transparency to light is due to molecular condition. ”

“ The phenomena just described in reality constitute the natural limit to the range of bodies which can be obtained in the vitreous state: as we approach this limit the glass requires more and more rapid cooling through the critical range of temperature, and is thus more and more liable to devitrify during the manufacturing processes, until finally the limit is set when no industrially feasible rapidity of cooling suffices to retain the mass in the vitreous state. ”

“ Therefore, if one of these elements, heat, becomes predominant in any body whatsoever, it destroys and dissolves all the others with its violence. This defect may be due to violent heat from certain quarters of the sky, pouring into the open pores in too great proportion to admit of a mixture suited to the natural temperament of the body in question. Again, if too much moisture enters the channels of a body, and thus introduces disproportion, the other elements, adulterated by the liquid, are impaired, and the virtues of the mixture dissolved. ”