need help melting glass

[zeocrash]

ok i have a set of soada glass test tubes, i have recently been trying to turn them into capsules full of pottassium permanganate. i do this by
filling the tube 1/4 full of pottassium permanganate
heating the tube half way up until it melts
pulling the 2 ends apart, so the tube will narrow out and seal

when the tube starts to cool, the glass cracks and i dont know how to avoid this problem
any advce on glass melting would be welcome

[a_bab]

Your glass could be crappy.
I made small ampoules using glass tubes and it works great. Try to use the kind of glass used in labware (pipettes, etc). Of course, use broken items ;)

The problem when sealing chems inside of a glass tube is to keep the chemical as cold as possible during the sealing process; another problem is that the air from the freshly sealed tube tends to expand due to the dilatation, possibly breaking the glass.

[knowledgehungry]

The problem is uneven cooling,I am not sure how to fix it. Maybe by putting it in some near boiling hot water you could slowly and evenly cool it, but other than that im really not sure.

[Arthis]

Cooling glass into boiling water may not change much. The glass is at several hundred °C, the change from 300 K to 373 K is only 25 %, not much.

About the cooling it self, a water bath is way too effective: a fast temperature change may cause some fractures I think.
The fact is that the end, larger, will cool down slower than the rest. Why not watering the end only ?

[Tuatara]

You need to do two things.

1/ heat an area considerably larger than the bit you are melting - to about 600C. Do this after you make your seal. This will help to anneal the stressed areas next to the seal.

2/ drop the whole thing immediately into a bucket of vermicullite so that it cools very slowly over an hour or so.

Given that you are trying to seal in KMnO4, you may want to move your seal further up the tube. You could also make your seal by squeezing the tube shut with two graphite rods (graphite wont stick to hot glass)