Written By Aubrey Whymark 2007-2017
Many years back I was excited to find a webpage on thermal cracks by Prof. B. Bourdin. 

Glasses & Thermal Shock

Some glasses, such as borosilicate glasses. are less subject to thermal stresses and therefore more resistant to thermal shock. Tektites are actually quite a durable and high quality glass, but atmospheric re-entry has clearly subjected them to thermal shock.
ABOVE: The Author of the site included this photo of a glass flask with oscillating cracks. The image was originally in Bahat, D. 1991. Tectonofractography. Springer Verlag, Berlin. The oscillating cracks in the glass were formed by pouring cold water into a heated flask. Similar cracks may develop when heated glass slides are dipped into a cold liquid.
The cracking reminded me a great deal, albeit in an imperfect way, of the meandering grooves so often found on Philippinites and Billitonites, particularly the more spherical specimens under 100g. The so called meandering grooves occur preferentially on one side - the anterior. It is the anterior that is suddenly heated during atmospheric re-entry and then suddenly cooled once the cosmic velocity is lost.
I believe that the crack development in tektites is a complex play of cooling/contraction cracks (better developed in larger specimens) and thermal cracks (better developed in smaller specimens). The cracks themselves would have been paper thin when the tektite landed. They were subsequently enlarged into U-grooves by chemical etching in the ground.

Thermal cracks certainly look like an interesting area of study. With xmas coming up I'm going to visit the shops and buy some good quality glass baubles - then hopefully make them crack!

Update: My first attempts at destroying baubles have been thwarted in that I can only find plastic ones! I managed to find a glass candle holder with a fairly rounded base though. I heated this up until it exploded. This wasn't the intention, but I think I let it get too hot. I wanted cracks, not catastrophic failure, but I was able to glue it back together. The cracks were all pretty much straight lines, with possibly a hint of a wavy line in places.

Safety First!

If you experiment with heating and cooling glass then ensure you wear full face and hand protection. Take extreme caution - eyes don't grow back!
ABOVE: A drinking glass heated until it exploded! The same drinking glass, partially glued back together.
UPDATE: OK, not only did I manage to get some glass spheres, but I also managed to get some solid glass spheres! They lasted approximately 2 hours; then I destroyed them. See what happened on my Bauble Destruction page!