I. Consider a sphere of melted glass at uniform temperature exposed to cold air. Its outer layer starts to solidify quickly, while the inner layers still preserve the previous temperature. Thus the outer layer must hold a volume of glass corresponding to the initial higher temperature. As the process of cooling continues, the inner layers "try" to contract and solidify. But the outer layer, quite solid by that time, becomes an obstacle. Being "attracted" inwards it is subject to compression; compression strains are produced within. On the other hand, in the inner layers "pulled" by the outer one stretching strains appear.
The endurance of glass to compression is much greater than its endurance to stretching. In the first case it usually amounts to 60-125 kg per square milimeter, in the second -- typically 3.5-8 kg per square milimeter. Therefore a glass sphere can resist great loads, the more so that the ball shape impedes the disturbance of strain balance. However, if the surface of the sphere is damaged, the entire system breaks up into many wedge-shaped splinters because of the stretching strains produced within the glass.
II. The behaviour of asymmetric bodies, like glass tears ("Glastraenen" in German) is quite different. The glass tears are made by dripping liquid melted glass into cold water. Again the outer layers of the drops become solid very quickly and important compression strains are produced. Such a tear turns out to be very resistant to shock.
Nevertheless, if the strain balance is disturbed, e.g. by breaking the "tail", the tear immediately turns into powder. If, however, the tail is being gradually diminished, like by a slow and careful treatment with hydrofluoric acid, the disintegration only occurs when the treatment reaches the point at which the diameter of the tail (thicker at the base) decreases rapidly.
From: Dralle - Keppeler Die Glasfabrikation, I Band, II Aufl., 1930, p.241