Glass

"O NCE upon a time, so the story goes, some sailors overtaken by bad weather landed on a desert shore and lighted a big fire to dry and warm themselves and pass the night. There being no wood on this sandy coast, they made their fire of dry seaweeds and grass. The furious wind caused their fire to burn fiercely, and, behold, the next morning the sailors were greatly astonished to find in the midst of the ashes sundry lumps of a substance as hard as stone but as transparent as ice. If they had seen this substance at the water's edge, and in winter, they would certainly have taken it for ice, but, raking it out from under the ashes, they could not escape the conclusion that it was something else."

"And what was it?" asked Claire.

"It was glass, that precious substance that to-day gives us our window-panes, which enable us to keep our houses warm without excluding the daylight. The sailors scrutinized closely the deposit left after their fire had burned out, and they perceived that the great heat had caused a part of the ashes to fuse with the sand of the soil and thus produce the transparent substance in question. It was thus that glass-making was discovered."

"Was that long ago?" Jules inquired.

"This discovery, one of the most important ever made, dates so far back that only a very vague record of it has been preserved, and this record is probably a mixture of fact and fable. But whether it be fact or fable, the story teaches us at least one thing: sand melted with ashes produces glass. Now, what can the substance be that imparts to ashes the property of thus transforming the sand? What is there in ashes of such a potent nature as to bring about this wonderful change?"

"There is soda," Marie suggested, "that same soda that turns oil or tallow into soap."

"It was in fact the soda from the marine plants burned by the sailors that had brought about the melting of the sand and the formation of lumps of glass. Potash, which closely resembles soda in all its properties, acts in the same way when it is heated to a high temperature with sand. In both cases the result of the fusion is glass, more or less colored, finer or coarser, according to the purity of the materials used. The fine and perfectly colorless glass of our goblets, decanters, and flasks is obtained from potash and very white sand; window-glass, which is very slightly green, at least on the edge, is made of soda and pure sand; common bottle-glass, dark green in color, or nearly black, is made of very impure sand and ordinary ashes.

"The manufacture of window-glass is a very curious operation. In a furnace heated to a very high temperature are large earthen pots or crucibles filled with a mixture of soda and sand. When these two substances are thoroughly melted together the result is a mass of glass, red-hot and running like water. Each crucible is removed by a workman and his assistant, standing on a platform in front of an opening through which the crucible is withdrawn. This workman is called the blower."

"Why blower?"  asked Jules. "Does he blow?"

"Indeed he does, and vigorously, as you will see. His tool is an iron rod or tube with one end cased in wood to enable him to handle the metal implement without burning himself. The assistant heats the other end by passing it through the opening in the furnace, and then plunges it into the crucible. In this way he gathers up a certain amount of paste-like glass, which he molds into globular form by turning it around again and again on a block of wet wood. That done, he again heats the glass at the furnace opening, softens it, and passes the rod to the workman, the glass-blower.

"The latter first blows gently into the tube, and the mass of glass becomes inflated into a bubble exactly as soap-suds would do at the end of a straw."

"I can make beautiful soap-bubbles by blowing with a straw," said Emile. "Does the workman do it like that?"

"Yes, just like that. He blows through his tube into the mass of glass which, flexible and soft as long as it remains red-hot, swells into a bladder. Then the tube is raised aloft and the workman blows the glass above his head. The bladder becomes flattened a little by its own weight, at the same time gaining in width. The blower lowers the tube again and swings it to and fro like the pendulum of a clock, every now and then resuming his blowing with greater force. By the action of its own weight, which lengthens it, and the blowing, which distends it, the mass of glass finally assumes the shape of a cylinder.

"The completed cylinder ends in a round cap which must be got rid of. To accomplish this the end of the cylinder is held near the opening of the furnace to soften it, after which the top of the cap is punctured with a pointed iron. By swinging the tube this puncture becomes enlarged and the cap disappears. The cylinder, hardened now although still very hot, is next placed on a wooden frame containing a number of grooves or gutters for receiving the cylinders. With a cold iron the workman touches the glass where it adheres to the tube, and by this simple contact a break occurs along the line thus suddenly chilled, leaving the cylinder entirely freed from the tube.

"Notice, children, the clever device adopted by the workman for detaching the glass from the tube without shattering it. He merely touches the very hot glass with a cold iron, and that suffices to produce a clean break all along the line touched. Glass possesses this curious property of not being able to withstand a sudden change in temperature without breaking. Chilled suddenly, it breaks; heated suddenly, again it breaks. That is a warning to you when you wash drinking-glasses or other glass objects. Beware of hot water if these objects are cold, and of cold water if they are hot; otherwise you run the risk of breaking them instantly. When the cold or the heat acts only along a predetermined line, it is on that line, suddenly chilled or heated, that the rupture occurs. That is how, without the slightest difficulty, the workman separates the glass cylinder from the iron tube to which it adheres.

"This done, the next thing is to remove the cap that still terminates one end of the cylinder. To do this the workman encircles this cap with a band of very hot glass, and then touches with a cold iron the line thus reheated. Instantly a circular rupture detaches the cap. Thus there is left on the frame a glass muff open at both ends. To split this muff the workman draws lengthwise, from one end to the other, a red-hot iron point, and then touches the hot line with a wet finger. A cracking follows, and the muff splits open. It is next taken to a furnace where, after being softened sufficiently, it is laid open and flattened out with an iron rule on a cast-iron plate. The final result is a large sheet of glass which the glazier will cut later with a diamond point into panes of any desired size."

"That is a very curious operation you have just told us about, Uncle Paul," said Claire. "For my part, I should never have suspected that a pane of glass, so perfectly flat as it is, was first a glass ball blown out like a soap-bubble."

"And how are bottles made?" asked Jules.

"For bottles glass is both blown and molded. The tube, laden by the assistant with the proper amount of melted glass, is passed to the blower, who gives to the vitreous mass the shape of an egg ending in a neck. The piece is then resoftened in the furnace and put into an iron mold. By energetic blowing the workman inflates the glass and makes it exactly fill the mold. This operation leaves the bottom of the bottle still flat, but by pressure with the point of a sheet-iron blade this bottom is driven up inside and shaped like a cone. A band of melted glass applied to the narrowed opening of the piece gives the neck of the bottle. The seal that some bottles bear, for example where the word liter is inscribed, is made by attaching a small disk of glass while it is still soft, and stamping it with a mold made of iron suitably engraved."