Bolognese tear
Bolognese tears (also Bolognese glass tears or Batavian drops or Prince Rupert 's Drop in English) are small glass drops with a head that tapers into a tail. Due to the way they are produced, they are so under internal stress that the head of the teardrop has a high mechanical strength and can withstand hammer blows, but the entire drop shatters to glass dust when its tail breaks off.
history
In the Middle Ages , glass was heated, drawn out for a long time and then quickly cooled so that it shattered into very fine glass dust. Some doctors and pharmacists gave their patients water mixed with this glass dust under the name of glass water as medicine, but this had no medical effect. In 1625 the phenomenon that glass suddenly "bursts into a thousand tiny pieces" was examined more closely for the first time, and in 1642 the Batavian drops were reinvented together with the Bolognese glass bottle in Bologna .
Manufacturing
Hot, liquid glass is poured drop by drop into a container with water or oil to cool it down.
Thermomechanical justification
The thick end of a Bolognese glass tear can be loaded up to a certain pressure; but as soon as you damage the thin end even a little, the strong internal tension causes the tear to burst. The decay front spreads at about 1,600 m / s, which is higher than the burning rate of black powder . The strong tensions within the tear result from the rapid cooling during production, as the drop hardens from the outside in. The thermal stresses induced by the inhomogeneous cooling can still relax further inside above the vitrification , whereas relaxation is no longer possible after the vitrification. This inhomogeneous behavior leads to train residual stresses inside and compressive residual stresses outside.
Similar phenomena
Single-pane safety glass , which shatters into numerous small parts with a few acute angles when damaged, works on the same principle. Here, too, the preload is generated by a heat treatment.
Bolognese bottles also work according to this principle. These are thick-walled glass bulbs that are so stable that you can use them to drive a nail into a piece of wood. However, if you drop a nail into the bottle, it will burst. Here, too, the reason for this is the strong internal tensile stresses on the inside of the piston, which are discharged through crack propagation when the surface is damaged when the nail tip hits it.
Trivia
Bolognese tears found literary treatment in the story of the same name by Gustav Meyrink (around 1905) and in Peter Carey's novel Oscar and Lucinda , published in 1988 .
literature
- H. Aben et al .: On the extraordinary strength of Prince Rupert's drops. In: Applied Physics Letters . Vol. 109 (2016), No. 23, doi : 10.1063 / 1.4971339 .
Web links
- Precise instructions for making
- Video about the production and attempt at destruction (German)
- Researchers solve the riddle of the Bolognese tears
Individual evidence
- ↑ Secret of the Bolognese tears revealed - the unusual behavior of the glass drops puzzled for 400 years . In: Scinexx. The knowledge magazine . 2017 ( scinexx.de ).
- ↑ YouTube video with production, high-speed recordings, calculations, explanations (English)
- ↑ Narayanaswamy, OS; Gardon, Robert: Calculation of residual stresses in glass . In: Journal of the American Ceramic Society . tape 52 , no. 10 . Wiley Online Library, 1969, pp. 554-558 ( wiley.com [PDF]).