Glass (hot) Knives

Hey whats up, ive been usin glass knives for oil, not even blades and they keep breakin, im really careful with them barely heat them up fast, and once its the right tempeture i turn the torch down a lil bit, so the torch is like bareley even on, so on feb 2nd(saturday) i got a big thick glass knife and i barely had it on, and it cracked, didnt break but there was a crack across the whole thing, an my bro barely touched it after it was cooled down, and it just fell off.... sooo

does any1 have any suggestions? like everytime a knife breaks, i go to the head shop and they give me a brand new one for free but still, if its gonna break like that fuck i dont wanna go through that all the time....

i talked to the salesperson who sold it to me, and i told them exactly how i did it, and he said that i wasnt really doin anythin wroung, but i gotta be...

never used glass ones before...just use normal metal knives unlkess your worried that it could potentially be bad for you but i dont think it is. glass knives sound cool i would get them but sounds like they have some problems

well, turn it on the lowest of low settings to heat it up, then turn it a little higher after it gets a little warmer, i think it is from the shock of going straight into a flame, start low, aim higher as it warms up, that is my only advice.