The thing is, we don’t, or rather we can’t unless we choose to rely on specialized equipment. And what better-specialized equipment can we rely on other than a kiln made for the purpose? However, since we are avoiding using the kiln, we’ll explore the other possibility of relying on a blowtorch to generate the required heat for our experiment.
Depending on the type of blowtorch you have at hand, you will be able to achieve temperatures of about 1100 degrees to 1900 degrees, which should be more than enough to melt down the glass for you. It now comes down to application and whatever other problems one might run into in the process of melting down the glass with a blowtorch. It cannot definitively be said whether using a blowtorch to work on a wine bottle will suffice to do the job, but we will melt one part of the bottle time, going by the achievable temperature. Would that suffice for your purpose?
It might be possible to flatten a wine bottle without a kiln if you heat it with a microwave oven for a while and then continue the process with a blowtorch, it will, at the very least, be a long task, and the success of it is not guaranteed.
