Glass Safety

GLASS : SAFETY. The problem of overcoming the danger of "flying" and splintering when glass is broken has, in the case of sheet glass, been overcome in two ways. The first consists in embedding a mesh of wire in the glass; the second by preparing what may be described as a sandwich of two glass sheets with one of celluloid between.

Experiments on the preparation of wired glass were made as far back as 1855 by Newton in England, but the first successful commercial manufacture appears to have been by Siemens in Dresden in 1891 based on Tenner's process. The Shumann process was developed between 1892 and 1903 in America and is known as the "solid process." Mild steel wire mesh (wire o.5–o.6 mm. diameter), free from dust, grease, rust or other impurity such as would develop bubbles is normally employed, although, with the view to an approximation between the thermal expansion of the wire and the glass, a nickel (34%) steel has been tried in France. The glass sheet must be sufficiently thick to have strength to withstand the internal stresses set up between glass and metal because of the difference in contraction and expansion. The dia grams 1 and 2 illustrate two methods employed for making wired glass. In (I) (Appert system) two rollers are arranged at different fixed levels over a table travelling right to left. Molten glass cast before A is rolled out so that its upper surface adheres to the wire mesh previously placed at the desired height above the table. A second casting in front of B is rolled out by that roller to cover the mesh. In process (2) (Shumann), the roller A rolls out the glass on the iron table ; it is followed by B which has deep corrugations or rings penetrating the glass so that the mesh, W, fed from a slide, is pressed into the glass and the process is com pleted by a smooth-surface roller C which follows B.

Wired glass is used largely for building construction. In case of the glass being broken either by a blow or by fire the frag ments are held together by the mesh.

Laminated Glass.

Several processes of preparing non-shat terable glass (also termed composite glass) by the use of a layer of transparent cellulose product are used but a description of the best known and established, the Triplex process, the original invention of which is due to Benedictus in France, will sufficiently indicate the principle on which these compound glasses are formed. The sheets of glass to be reinforced must be smooth and flat ; and although by some companies drawn sheet glass is now being employed, the best type is prepared from thin, well polished plate, the thickness of sheet normally employed being slightly less than s in. so that the composite sheet is about 4 in. thick, or similar to ordinary plate glass. After thorough cleaning and drying, one surface is spread with a uniform layer of gelatinous adhesive material which, when dried (in dust-free air) is from o•o 16 in. to 0.02 in. thick. A very thin coating of collodion is then given to the gelatinous layer, when dried, being only about 1 o that of the first layer. Two glass sheets so treated are then paired, cut to size and shape (for motor windshields, goggles, etc.) and a sheet of transparent celluloid 0.5 mm. thick is also cut a little less in area than the sheets between which it is to lie. The prepared film is cut back about s in. from the edges of the sheets so as to leave a covered area equal to the cellulose sheet. The latter is dipped in a softening bath which renders the outer surfaces slightly plastic and immediately then laid between the prepared surfaces of the glass sheets. The sandwich is placed in a hot water heated hydraulic press operating at a total pressure up to i oo tons as the result of which the three sheets, two of glass and one of cellulose become firmly united. The gap between the edges of the glass sheets is finally filled with a preparation of bitumen to seal in and preserve the celluloid layer. Other successful commercial processes for the manufacture of laminated glass require no edge protecting seal.

When such compound glass is struck a heavy blow, the cracks radiate from the centre of attack but the "splinters" remain firmly adhering to the intermediate celluloid layer. It is, therefore, finding popular use in automobiles, particularly for windshields, the breaking of which is responsible for many accidental injuries. It is also retardent but not proof against rifle and revolver shots and has been used in Armoured Car (q.v.) service in America.

(W. E. S. T.)