The manufacture of optical glass and of optical systems; a war-time problem by United States. Army. Ordnance Dept & Wright F. E. (Frederic Eugene) 1877-1953

Author:United States. Army. Ordnance Dept & Wright, F. E. (Frederic Eugene), 1877-1953
Language: eng
Format: epub
Tags: Optical instruments, Optical industry, Optical glass, Glass manufacture
Publisher: Washington, Govt. Print. Off.
Published: 1921-03-25T05:00:00+00:00

In case the pot of glass were to cool more rapidly than usual so that for example & = 0.002° per second, then P 3 = -687.44 kg. cm. 2 Under these conditions the final tangential stress in the marginal layer is still compressive 665 — 687 = — 22 kg. cm. 2 and there is no tendency for radial cracks to form. Under these conditions of rapid cooling the glass is not only poorly annealed, but is not cracked transversely, and a "marble" or " onion" is the result.

It is a matter of factory observation that the pot walls at the margins of a cold pot of glass are cracked but little, and that it is in many cases difficult to find a pronounced crack on the outside of the pot without examination of the glass itself.

The foregoing explanation of the formation of cracks is not complete and does not account adequately for the fact that in a properly cracked pot of glass a master vertical plane fissure divides the part into halves. The formation of the other joint cracks, roughly perpendicular to the master joint-plane, is readily deduced from symmetry relations and the existing tensional shearing stresses.

Roberts 69 found that rapid chilling of the pot of glass at about 300° C., induced by removal of the insulation, or by turning off the gas and opening the dampers in the case of melts cooled in the pot arch, favored the formation of radial cracks. Tangential tensional forces are introduced by this procedure, and the general cooling schedule of the glass is thereby disturbed so that a mechanically weak spot might give way and thus introduce the radial cracks. Once radial cracks have begun to appear the tendency for the spherical cracks to form is diminished.

It may be noted that the plane cracks (joints), produced as described above, form slowly and with reference to the distribution of the shearing stresses involved; in other words they form with reference to the symmetry of the pot. Similar phenomena are of common occurrence in nature as, for example the columnar jointing of certain lava flows in which the joint columns are normal to the cooling boundary surfaces. For the formation of plane joint cracks, slow and fairly uniform cooling is essential. In the case of stresses developed by the sharp blow of a hammer, the distribution of the elastic stresses is entirely different, and characteristic wavy or conchoidal fracture surfaces develop in well-annealed glass or other homogeneous material. If, however, there exist other stresses, as in a block of poorly annealed, badly strained glass, in addition to those developed momentarily by mechanical means, the fracture surface resulting from the hammer blow is uneven and commonly hackly in nature; it is rough and torn as though the glass had been split across some internal structural lines, as in a block of wood.

69 Jour. Amer. Ceram. Soc., 2, 543-563, 1919.

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