Thank you for very inspiring demo going to try this this morning! Also added tip of protective gloves, so glad I came across you on RU-vid. Look forward to meeting up again.
Thanks for your question. I have had this happen a few times early on. The culprit for this is usually poor adhesion due to old or poor quality foil or dirty glass. Another reason I've heard of is that there may be too much heat being used. And just in case you're wondering, my favourite foil is Edco. ;)
I usually use my iron set around 360 degrees, but if you find that everything melts too quickly the next lowest setting on mine is 310 degrees and that would work well for doing this technique too.
The iron would have been at about 360 degrees but you can always adjust it up or down depending on personal preference. The iron used here is the Hakko FX601. amzn.to/2u5wqz4 (affiliate link)
It's really a personal preference. I never really noticed a difference when using my Weller iron, so I just got used to using 60/40 for everything. I have a roll of 63/37 sitting in the closet where it has been for about 9 or 10 years. Since I started using the Hakko iron, I just stick with the 60/40 and adjust the temperature control.